polymers and light: fundamentals and technical applications

Fundamentals and Technical Applications

With Contributions of Stefan WeigelMichael P Schluumlsener and Jens A Andresen

W Schnabel

Polymers and Light

Innodata
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W Schnabel

Polymers and Light

Each generation has its unique needs and aspirations When Charles Wiley firstopened his small printing shop in lower Manhattan in 1807 it was a generationof boundless potential searching for an identity And we were there helping todefine a new American literary tradition Over half a century later in the midstof the Second Industrial Revolution it was a generation focused on buildingthe future Once again we were there supplying the critical scientific technicaland engineering knowledge that helped frame the world Throughout the 20thCentury and into the new millennium nations began to reach out beyond theirown borders and a new international community was born Wiley was there ex-panding its operations around the world to enable a global exchange of ideasopinions and know-how

For 200 years Wiley has been an integral part of each generationrsquos journeyenabling the flow of information and understanding necessary to meet theirneeds and fulfill their aspirations Today bold new technologies are changingthe way we live and learn Wiley will be there providing you the must-haveknowledge you need to imagine new worlds new possibilities and new oppor-tunities

Generations come and go but you can always count on Wiley to provide youthe knowledge you need when and where you need it

William J Pesce Peter Booth WileyPresident and Chief Executive Officer Chairman of the Board

1807ndash2007 Knowledge for Generations



W Schnabel

Polymers and Light

The Author

Prof Dr W SchnabelDivison of Solar Energy ResearchHahn-Meitner-InstitutGlienicker Str 10014109 BerlinGermany

Library of Congress Card No applied for

British Library Cataloguing-in-Publication DataA catalogue record for this book is availablefrom the British Library

Bibliographic information published bythe Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publica-tion in the Deutsche Nationalbibliografie detailedbibliographic data are available in the Internet athttpdnbd-nbde

copy 2007 WILEY-VCH Verlag GmbH amp Co KGaAWeinheim

All rights reserved (including those of translationinto other languages) No part of this book maybe reproduced in any form ndash by photoprintingmicrofilm or any other means ndash nor transmittedor translated into a machine language withoutwritten permission from the publishersRegistered names trademarks etc used in thisbook even when not specifically marked as suchare not to be considered unprotected by law

Composition K+V Fotosatz GmbH BeerfeldenPrinting betz-druck GmbH DarmstadtBookbinding Litges amp Dopf GmbH HeppenheimCover Adam Design WeinheimWiley Bicentennial Logo Richard J Pacifico

Printed in the Federal Republic of GermanyPrinted on acid-free paper

ISBN 978-3-527-31866-7

All books published by Wiley-VCH are carefullyproduced Nevertheless authors editors andpublisher do not warrant the information containedin these books including this book to be free oferrors Readers are advised to keep in mind thatstatements data illustrations procedural details orother items may inadvertently be inaccurate

Preface XIII

Introduction 1

Part I Light-induced physical processes in polymers

1 Absorption of light and subsequent photophysical processes 511 Principal aspects 512 The molecular orbital model 713 The Jablonski diagram 1014 Absorption in non-conjugated polymers 1015 Absorption in conjugated polymers 1216 Deactivation of electronically excited states 13161 Intramolecular deactivation 13162 Intermolecular deactivation 14163 Energy migration and photon harvesting 16164 Deactivation by chemical reactions 2117 Absorption and emission of polarized light 22171 Absorption 22172 Absorption by chiral molecules 23173 Emission 2618 Applications 30181 Absorption spectroscopy 301811 UVVis spectroscopy 301812 Circular dichroism spectroscopy 321813 IR spectroscopy 35182 Luminescence 37183 Time-resolved spectroscopy 381831 General aspects 381832 Experimental techniques 391833 Applications of time-resolved techniques 4118331 Optical absorption 41

V

Contents

18332 Luminescence 44References 45

2 Photoconductivity 4921 Introductory remarks 4922 Photogeneration of charge carriers 50221 General aspects 50222 The exciton model 52223 Chemical nature of charge carriers 54224 Kinetics of charge carrier generation 55225 Quantum yield of charge carrier generation 5723 Transport of charge carriers 6024 Mechanism of charge carrier transport in amorphous poly-

mers 6425 Doping 6626 Photoconductive polymers produced by thermal or high-energy

radiation treatment 6927 Photoconductive polymers produced by plasma polymerization or

glow discharge 70References 70

3 Electro-optic and nonlinear optical phenomena 7331 Introductory remarks 7332 Fundamentals 74321 Electric field dependence of polarization and dipole moment 74322 Electric field dependence of the index of refraction 7833 Characterization techniques 79331 Second-order phenomena 793311 Determination of the hyperpolarizability 793312 Determination of the susceptibility (2) 81332 Third-order phenomena 823321 Third harmonic generation 833322 Self-focusingdefocusing 843323 Two-photon absorption (TPA) 853324 Degenerate four-wave mixing (DFWM) and optical phase

conjugation 8634 Nonlinear optical materials 87341 General aspects 87342 Second-order NLO materials 893421 Guest-host systems and NLO polymers 893422 Orientation techniques 92343 Third-order NLO materials 9335 Applications of NLO polymers 96351 Applications relating to telecommunications 96352 Applications relating to optical data storage 99

ContentsVI

353 Additional applications 100References 101

4 Photorefractivity 10341 The photorefractive effect 10342 Photorefractive formulations 10543 Orientational photorefractivity 10744 Characterization of PR materials 10845 Applications 110

References 112

5 Photochromism 11351 Introductory remarks 11352 Conformational changes in linear polymers 115521 Solutions 115522 Membranes 12253 Photocontrol of enzymatic activity 12354 Photoinduced anisotropy (PIA) 12355 Photoalignment of liquid-crystal systems 12656 Photomechanical effects 130561 Bulk materials 130562 Monolayers 13357 Light-induced activation of second-order NLO properties 13458 Applicationss 136581 Plastic photochromic eyewear 136582 Data storage 137

References 139

6 Technical developments related to photophysical processesin polymers 143

61 Electrophotography ndash Xerography 14362 Polymeric light sources 146621 Light-emitting diodes 1476211 General aspects 1476212 Mechanism 1506213 Polarized light from OLEDs 1546214 White-light OLEDs 155622 Lasers 1566221 General aspects 1566222 Lasing mechanism 1586223 Optical resonator structures 1596224 Prospects for electrically pumped polymer lasers 16263 Polymers in photovoltaic devices 16264 Polymer optical waveguides 167641 General aspects 167

Contents VII

642 Optical fibers 1686421 Polymer versus silica fibers 1686422 Compositions of polymer optical fibers (POFs) 1696423 Step-index and graded-index polymer optical fibers 170643 Polymer planar waveguides 170644 Polymer claddings 170

References 171

Part II Light-induced chemical processes in polymers

7 Photoreactions in synthetic polymers 17771 Introductory remarks 177711 Amplification effects 178712 Multiplicity of photoproducts 178713 Impurity chromophores 180714 Photoreactions of carbonyl groups 18272 Cross-linking 183721 Cross-linking by cycloaddition of C=C bonds 184722 Cross-linking by polymerization of reactive moieties

in pendant groups 186723 Cross-linking by photogenerated reactive species 188724 Cross-linking by cleavage of phenolic OH groups 19273 Simultaneous cross-linking and main-chain cleavage

of linear polymers 19374 Photodegradation of selected polymers 196741 Poly(vinyl chloride) 196742 Polysilanes 19875 Oxidation 19976 Singlet oxygen reactions 20277 Rearrangements 202

References 205

8 Photoreactions in biopolymers 20781 Introductory remarks 20782 Direct light effects 2118 21 Photoreactions in deoxyribonucleic acids (DNA) 2118211 Dimeric photoproducts 2128212 Other DNA photoproducts 214822 Photoreactions in proteins 2148221 Chemical alterations by UV light 2158222 Formation of stress proteins 2168223 Effects of visible light ndash photoreceptor action 2178224 Repair of lesions with the aid of DNA photolyases 219823 Photoreactions in cellulose 221824 Photoreactions in lignins and wood 221

ContentsVIII

83 Photosensitized reactions 222References 228

9 Technical developments related to photochemical processesin polymers 231

91 Polymers in photolithography 231911 Introductory remarks 231912 Lithographic processes 2319121 Projection optical lithography 2339122 Maskless lithography 235913 Resists 2369131 Classical polymeric resists ndash positive and negative resist

systems 2369132 Chemical amplification resists 2399133 Resists for ArF (193 nm) lithography 2429134 Resists for F2 (157 nm) lithography 245914 The importance of photolithography for macro- micro-

and nanofabrication 24692 Laser ablation of polymers 248921 General aspects 2489211 Introductory remarks 2489212 Phenomenological aspects 2489213 Molecular mechanism 250922 Dopant-enhanced ablation 250923 Polymers designed for laser ablation 251924 Film deposition and synthesis of organic compounds

by laser ablation 252925 Laser desorption mass spectrometry and matrix-assisted laser

desorptionionization (MALDI) 254926 Generation of periodic nanostructures in polymer surfaces 256927 Laser plasma thrusters 25693 Stabilization of commercial polymers 257931 Introductory remarks 257932 UV absorbers 2589321 Phenolic and non-phenolic UV absorbers 2589322 Mechanistic aspects 259933 Energy quenchers 260934 Chain terminators (radical scavengers) 262935 Hydroperoxide decomposers 265936 Stabilizer packages and synergism 266937 Sacrificial consumption and depletion of stabilizers 267

References 268

Contents IX

Part III Light-induced synthesis of polymers

10 Photopolymerization 275101 Introduction 275102 Photoinitiation of free radical polymerizations 2761021 General remarks 2761022 Generation of reactive free radicals 27610221 Unimolecular fragmentation of type I photoinitiators 27610222 Bimolecular reactions of type II photoinitiators 27910223 Macromolecular photoinitiators 27910224 Photoinitiators for visible light 281102241 Metal-based initiators 282102242 Dyeco-initiator systems 284102243 Quinones and 12-diketones 28510225 Inorganic photoinitiators 287103 Photoinitiation of ionic polymerizations 2881031 Cationic polymerization 28810311 General remarks 28810312 Generation of reactive cations 290103121 Direct photolysis of the initiator 290103122 Sensitized photolysis of the initiator 291103123 Free-radical-mediated generation of cations 2921031231 Oxidation of radicals 2921031232 Addition-fragmentation reactions 2941032 Anionic polymerization 29510321 General remarks 29510322 Generation of reactive species 295103221 Photo-release of reactive anions 295103222 Photo-production of reactive organic bases 296104 Topochemical polymerizations 2981041 General remarks 2981042 Topochemical photopolymerization of diacetylenes 2991043 Topochemical photopolymerization of dialkenes 301

References 302

11 Technical developments related to photopolymerization 305111 General remarks 305112 Curing of coatings sealants and structural adhesives 3071121 Free radical curing 30711211 Solvent-free formulations 30711212 Waterborn formulations 3091122 Cationic curing 3091123 Dual curing 310113 Curing of dental preventive and restorative systems 312114 Stereolithography ndash microfabrication 313

ContentsX

115 Printing plates 3161151 Introductory remarks 3161152 Structure of polymer letterpress plates 3171153 Composition of the photosensitive layer 3171154 Generation of the relief structure 317116 Curing of printing inks 318117 Holography 3191171 Principal aspects 3191172 Mechanism of hologram formation 3211173 Multicolor holographic recording 3211174 Holographic materials 3221175 Holographic applications 323118 Light-induced synthesis of block and graft copolymers 3241181 Principal aspects 3241182 Surface modification by photografting 328

References 329

Part IV Miscellaneous technical developments

12 Polymers in optical memories 337121 General aspects 337122 Current optical data storage systems 3381221 Compact disk (CD) and digital versatile disk (DVD) 3381222 Blue-ray disks 340123 Future optical data storage systems 3411231 General aspects 3411232 Volume holography 34212321 Storage mechanism 34212322 Storage materials 3431233 Photo-induced surface relief storing 345

References 345

13 Polymeric photosensors 347131 General aspects 347132 Polymers as active chemical sensors 3491321 Conjugated polymers 34913211 Turn-off fluorescence detection 35013212 Turn-on fluorescence detection 35013213 ssDNA base sequence detection 35213214 Sensors for metal ions 35213215 Image sensors 3531322 Optical fiber sensors 3531323 Displacement sensors 354133 Polymers as transducer supports 355

References 356

Contents XI

14 Polymeric photocatalysts 359141 General aspects 359142 Polymers as active photocatalysts 3591421 Conjugated polymers 3591422 Linear polymers bearing pendant aromatic groups 361143 Polymers as supports for inorganic photocatalysts 362

References 364

Subject Index 365

ContentsXII

Light can do a lot of quite different things to polymers and light is employedin various quite different technical applications related to polymers that have be-come beneficial to humans and are influencing the daily lives of many peopleThese applications include photocopying machines computer chips compactdisks polymer optical fiber systems in local area networks and printing platesThere are many other very useful practical applications Since these are com-monly dealt with separately in monographs or review articles the idea arose tocomprehend and combine in a single book all important developments relatedto polymers and light that concern industrially employed practical applicationsor show potential for future applications Actually I first contemplated writing abook dealing with both physical and chemical aspects related to the interactionof light with polymers and to the synthesis of polymers with the aid of lightwhile I was lecturing on certain topics of this field at the Technical Universityin Berlin and at Rika Daigaku (Science University) in Tokyo However I onlystarted to immerse myself in this extensive project when I retired from activeservice some time ago Upon retrieving and studying the salient literature I be-came fascinated by the broadness of the field The results of this project are pre-sented here for the first time In referring to the different topics I have tried todeal with the fundamentals only to the extent necessary for an understandingof described effects In attempting to be as concise as possible descriptions oftechnical processes and tools have had to be restricted to a minimum in orderto keep the extent of the book within reasonable limits To somewhat compen-sate for this flaw a rather comprehensive list of literature references also cover-ing technical aspects is presented at the end of each chapter

Writing a monograph implies that the author can both concentrate on thesubject in a quiet office and rely on the cooperation of an effectively functioninglibrary Both were provided by the Hahn-Meitner-Institute HMI and I am verygrateful to the management of this institute especially to Prof Dr M SteinerScientific Director Chief Executive for giving me the opportunity to work onthis book after my transfer to emeritus status Special thanks are due to ProfDr H Tributsch head of the Solar Energy Research Division of HMI for appreciat-ing my intention to write this book and for providing a quiet room The HMIlibrary under the direction of Dr E Kupfer and his successor Dr W Fritsch has sub-

XIII

Preface

stantially contributed to the preparation and completion of the manuscript bydelivering necessary resources and executing many retrievals The latter yieldedmost of the literature citations upon which this book is based In this context Iwish to express my special gratitude to senior librarian Mr M Wiencken whohas performed an excellent job Other people who proved very helpful in thisproject are Mr D Gaszligen who has kept the computer running and Mrs PKampfenkel who has scanned various figures

The personnel of the publisher Wiley-VCH worked carefully and rapidly onthe editing of the manuscript after its completion in the summer of 2006 Thisis gratefully acknowledged

Last but not least credit has to be given to the efforts of the authorrsquos familyMy wife Hildegard has accompanied the progress of the project with encourag-ing sympathy and moral support and my two sons Dr Ronald Schnabel andDr Rainer Florian Schnabel have given substantial advice The latter has criti-cally read all chapters of the manuscript

Berlin November 2006 Wolfram Schnabel

PrefaceXIV

The technological developments of the last decades have been essentially deter-mined by trends to invent new materials and to establish new technical meth-ods These trends encompass the synthesis of novel polymeric materials andthe employment of light in industrial processes To an increasing extent techni-cal processes based on the interaction of light with polymers have become im-portant for various applications To mention a few examples polymers are usedas nonlinear optical materials as core materials for optical wave guides and asphotoresists in the production of computer chips Polymers serve as photo-switches and optical memories and are employed in photocopying machinesand in solar cells for the generation of energy Moreover certain polymeric ma-terials can be utilized for the generation of light

On the other hand light serves also as a tool for the synthesis of polymersie for the initiation of the polymerization of small molecules a method whichis applied in technical processes involving the curing of coatings and adhesivesand even by the dentist to cure tooth inlays

Obviously the field related to the topic polymers and light is a very broad oneA principle of order derived from the distinction of photophysical from photo-chemical processes may help to steer us through this wide field Hence photo-physical and photochemical processes are addressed in separate parts of thisbook (Part I and Part II) where both fundamentals and related practical applica-tions are dealt with Regarding pure photophysical processes that are not com-bined with chemical alterations of the polymers (Part I) separate chapters aredevoted to fundamentals concerning the interaction of light with polymersphotoconductivity electro-optic and nonlinear phenomena photorefractivity andphotochromism (Chapters 1ndash5 respectively) Important technical applicationsrelated to photophysical processes in polymers are dealt with in Chapter 6These applications include xerography light-emitting diodes (LEDs) lasers solarcells optical wave guides and optical fibers

In Part II fundamentals of light-induced chemical processes are discussed bymaking a distinction between synthetic organic polymers (Chapter 7) and biopo-lymers (Chapter 8) Also in Part II important technical applications related tophotochemical processes in polymers are dealt with separately in Chapter 9Here important practical applications such as photolithography which is a nec-

1

Introduction

essary tool for the production of computer chips and laser ablation are coveredMoreover one section of Chapter 9 is devoted to the stabilization of commercialpolymers a very important subject regarding the long-time stability of plasticmaterials

The light-induced synthesis of polymers is the topic of Part III While the var-ious modes of photoinitiation of polymerization processes are discussed inChapter 10 related technical applications are treated in Chapter 11 The latterinclude curing of coatings and dental systems printing plates (used to printnewspapers) holography (important for data storage) and the synthesis ofblock-and-graft copolymers

Finally Part IV reviews miscellaneous technical developments that do not fitneatly into the scheme of the preceding parts These concern in particular theapplication of polymers in the field of optical memories treated in Chapter 12which refers also to currently important data storage systems (compact disksdigital versatile disks and blue-ray disks) Moreover the application potential ofpolymers in the fields of photosensors and photocatalysts is outlined in Chap-ters 13 and 14 respectively

Introduction2

Part ILight-induced physical processes in polymers

To open the way into the wide-ranging fields covered in this book some ele-mentary facts essential for an understanding of the material covered are out-lined at the beginning Since books [1ndash6] are available that comprehensivelytreat the principles of the interaction of light with matter the aim here is topresent the salient points in a very concise manner Nevertheless in citing typi-cal cases close adherence to the actual subject of the book has been sought byreferring to polymers wherever possible

11Principal aspects

Photons are absorbed by matter on a time scale of about 10ndash15 s During thisvery short time the electronic structure of the absorbing molecule is alteredwhereas the positions of the atomic nuclei in the molecule vibrating on a timescale of 10ndash12 s are not changed There are two prerequisites for the absorptionof a photon of energy h by a molecule (1) the molecule must contain a chro-mophoric group with excitable energy states corresponding to the photon en-ergy according to Eq (1-1)

h En E0 1-1

En and E0 denote the energies of the excited and the ground state respectivelyTypical chromophoric groups are listed in Table 11

(2) The transition between the two energy states must cause a change in thecharge distribution in the molecule ie a change in the dipole moment Interms of quantum mechanics absorption of a photon is possible (allowed) ifthe transition moment M has a non-zero value Since M is a vector composedof three components parallel to the three coordinates [Eq (1-2)] at least onecomponent must have a non-zero value

M Mx My Mz 1-2

5

1Absorption of light and subsequent photophysical processes

The higher the value of M the more efficient is the absorption As described byEq (1-3) M is composed of three integrals

M

vvdv

edpede

ssds 1-3

where v e and s are the vibronic electronic and electron-spin wave func-tions of the absorbing molecule respectively The asterisk denotes ldquoexcitedstaterdquo dp is the electronic dipole moment operator dv de and ds refer tothe three respective coordinates d= dxmiddotdymiddotdz

The three integrals in Eq (1-3) are the basis of the so-called selection rules whichdetermine whether a transition is allowed or forbidden v

vd2 is the Franck-Condon factor and

ssds applies to the spin properties of the excited and the

ground states If any of the three integrals in Eq (1-3) is zero the correspondingtransition is forbidden ie a final probability could only result from a second-orderapproximation This applies eg to the forbidden transitions between levels of thesinglet and the triplet system The magnitude of the Franck-Condon factor deter-mines the probability of transitions with respect to molecular geometry The rulestates that the transition probability is highest if the geometries of the ground andexcited states are equal A more detailed treatment of these aspects is beyond thescope of this book and the reader is referred to relevant monographs [2ndash4]

The probability of the occurrence of an electronic transition is given by the(dimensionless) oscillator strength f which is proportional to the square of thetransition moment [Eq (1-4)]

1 Absorption of light and subsequent photophysical processes6

Table 11 Typical chromophoric groups [4]

Chromophore Typical compound max

(nm) a)max

(L molndash1 cmndash1) b)Mode of electrontransition

Ethene 193 104

Ethyne 173 6103

Acetone 187271

103

15 n

Azomethane 347 5 n

t-Nitrosobutane 300665

10020

n

Amyl nitrite 219357

219357

n

a) Wavelength of maximum optical absorptionb) Decadic molar extinction coefficient (log I0I = cd)

f 875 102EM2 1-4

Here E is equal to EnndashE0 (given in eV) A large value of f corresponds to astrong absorption band and a short lifetime of the excited state The maximumvalue is f = 1

Experimentally the absorption of light is recorded as a function of the wave-length or the wave number =ndash1 by measuring the change in the intensityof a light beam passing through a sample of unit path length (1 cm) For ahomogeneous isotropic medium containing an absorbing compound at concen-tration c (mol Lndash1) the light absorption is described by Eq (1-5) the Lambert-Beer law

A lg10I0I cd 1-5

where A is the absorbance (extinction optical density) and I0 and I denote thelight intensity before and after absorption Equivalent denotations for I0 and Iare incident and transmitted radiant flux respectively (L molndash1 cmndash1) is thedecadic molar extinction coefficient at a given wavelength The Lambert-Beerlaw does not hold at high light intensities as experienced eg with lasers Theoscillator strength f is related to the measured integrated extinction coefficientd by Eq (1-6) where and have to be given in units of L molndash1 cmndash1 and

cmndash1 respectively

f 23 103c2mNe2F

d 432 109 F

d 1-6

Here c is the velocity of light m and e are the mass and charge of an electronrespectively and N is Avogadrorsquos number The factor F which reflects solvent ef-fects and depends on the refractive index of the absorbing medium is close tounity max the extinction coefficient at the maximum of an absorption band isa measure of the intensity (magnitude) of the band and an indicator of the al-lowedness of the corresponding electronic transition

12The molecular orbital model

Changes in the electronic structure of a molecule can be visualized with the aidof the molecular orbital (MO) model [3 4] Molecular orbitals are thought to beformed by the linear combination of the valence shell orbitals of the atomslinked together in the molecule The combination of two single orbitals of twoadjacent atoms results in two molecular orbitals one of lower and the other ofhigher energy than before combination The low-energy orbital denoted as thebonding orbital is occupied by a pair of electrons of antiparallel spin The high-energy molecular orbital is called an antibonding orbital It is unoccupied in the

12 The molecular orbital model 7

ground state but may be occupied by an electron upon electronic excitation ofthe molecule

There are different kinds of molecular orbitals bonding and orbitals non-bonding n orbitals and antibonding and orbitals and orbitals arecompletely symmetrical about the internuclear axis whereas and orbitalsare antisymmetric about a plane including the internuclear axis n orbitalswhich are located on heteroatoms such as oxygen nitrogen or phosphorus arenonbonding and are of almost the same energy as in the case of the isolatedatom A pair of electrons occupying an n orbital is regarded as a lone pair onthe atom in question

The simple MO model is based on several assumptions For instance and orbitals are assumed not to interact Moreover molecules are described by lo-calized orbitals each covering two nuclei only Delocalized orbitals involvingmore than two nuclei are thought to exist only in the case of -bonding in con-jugated systems

When a molecule in its ground state absorbs a photon an electron occupyinga or n orbital is promoted to a higher-energy or orbital In principlethe following transitions are possible n and n As


Fig 11 Molecular orbitals (not to scale) and electronictransitions induced by the absorption of a photon

can be seen in Fig 11 the orbital energy increases in the series n

According to the differences in the orbital energies the electron transitionsindicated in Fig 11 correspond to light absorption in different wavelength re-gions This is illustrated in Table 12

It follows that under conveniently practicable conditions (gt 200 nm) photonabsorption initiates transitions of n or electrons rather than those of elec-trons

Commonly molecular orbitals are classified as occupied (doubly) singly occu-pied and unoccupied The acronyms HOMO and LUMO denote the frontier orbi-tals ie the Highest Occupied and the Lowest Unoccupied Molecular Orbitalrespectively SOMO stands for Singly Occupied Molecular Orbital (see Fig 12)


Table 12 The correspondence of electron transition and optical absorption

Electron transition Absorption region(nm)

Extinction coefficient(L molndash1 cmndash1)

100ndash200 103

n 150ndash250 102ndash103

(Isolated -bonds)(Conjugated -bonds)

180ndash250220ndashIR

102ndash104

n (Isolated groups)(Conjugated segments)


1ndash400

Fig 12 Classification of molecular orbitals with respect to electron occupancy

13The Jablonski diagram

Photon-induced excitations of molecules also include vibrations of nuclei Thisfact can be visualized with the aid of the Jablonski diagram (see Fig 13)

The diagram shows the various energy states of a molecule and further indi-cates the transitions related to the formation and deactivation of excited statesHere photon absorption leads to electron transitions from the ground state S0

to the excited states S1 S2 etc Electron release occurs when the photon energyexceeds the ionization energy EI This is not the case within the wavelengthrange of UV and visible light ie = 200ndash800 nm (h= 62ndash16 eV)

14Absorption in non-conjugated polymers

Figure 14 shows absorption spectra of the typical unconjugated linear polymerspresented in Chart 11

Due to the fact that electronic excitations also involve vibronic and rotationalsublevels (the latter are not shown in Fig 13) the absorption spectra of mole-cules consist of bands rather than single lines It is notable that the maxima ofthe absorption spectra shown in Fig 14 are located in the UV region They re-flect spin-state-conserving electronic transitions ie transitions in the singletmanifold upon photon absorption molecules in the singlet ground state S0 are


Fig 13 Jablonski-type diagram Abbreviations and acronymsAbs absorption Fl fluorescence Phos phosphorescenceIC internal conversion ISC intersystem crossing

converted into molecules in an excited singlet state Sn At long wavelengths(low photon energies) photon absorption generates S1 states At shorter wave-lengths S2 and higher states are excited In the case of polymers containing car-bonyl groups the absorption bands located at long wavelengths correspond ton transitions with low extinction coefficients ie low values of the transi-tion moment At shorter wavelengths transitions with larger transitionmoments are excited In this connection the readerrsquos attention is directed to Ta-ble 12 which indicates the relative orders of magnitude of the extinction coeffi-cients of the different electron transitions

14 Absorption in non-conjugated polymers 11

Chart 11 Chemical structures of poly(vinyl acetate) PVAcpoly(methyl methacrylate) PMMA polystyrene PSt poly-(methyl vinyl ketone) PMVK poly(phenyl vinyl ketone) PPVK

Fig 14 Absorption spectra of non-conjugated polymersAdapted from Schnabel [7] with permissionfrom Carl Hanser

15Absorption in conjugated polymers

In recent years various aromatic polymers with conjugated double bonds so-called conjugated polymers have been synthesized and thoroughly investigatedwith regard to applications in the fields of electroluminescence (organic light-emitting diodes) and photovoltaics (energy conversion of sunlight) Figure 15presents typical absorption spectra of conjugated polymers (see Chart 12)

The maxima of the absorption spectra of conjugated polymers are located inthe visible wavelength region

Certain phenomena observed with conjugated polymers cannot be rational-ized in terms of the model described in Section 11 This concerns above allthe generation of charge carriers with the aid of UV and visible light and theconduction of photogenerated charge carriers A rationale for these phenomenais provided by the exciton model which was originally developed for inorganicsemiconductors and dielectrics [9ndash11] According to this model the absorption


Fig 15 Absorption spectra of conjugated polymers Adaptedfrom Shim et al [8] with permission from Springer

Chart 12 Chemical structures of poly(14-phenylene vinylene) PPV and three PPV derivatives

of a photon by a conjugated polymer promotes an electron from the groundstate to an upper electronically excited state which takes on the quality of a qua-si-particle resembling a hydrogen-like system and can be considered as an elec-tronhole pair The electron and hole are bound together ie they cannot moveindependently of one another in the medium Significantly however excitonsare considered to be able to diffuse and under certain circumstances to dissoci-ate into free charge carriers This aspect is also treated in Section 222

16Deactivation of electronically excited states

161Intramolecular deactivation

In condensed media vibrational relaxation (internal conversion) is usually so fastthat molecules excited to vibronically excited states S1v S2v etc relax to the lowestexcited singlet state S1 before they can undergo other processes Further intramo-lecular deactivation processes of S1 states (see the Jablonski diagram in Fig 13)may be radiative or non-radiative There is one radiative deactivation path result-ing in photon emission termed fluorescence and two non-radiative processes com-peting with fluorescence internal conversion (IC) to the ground state and intersys-tem crossing (ISC) to the triplet manifold The latter process involves a change inelectron spin ie a molecule excited to the singlet state having solely pairs of elec-trons with antiparallel spins is converted into a molecule in an excited triplet statepossessing one pair of electrons with parallel spins Triplet states are commonlyformed via this route The direct formation of triplet states from the ground statethrough photon uptake is strongly spin-forbidden In other words S0T1 transi-tions are very unlikely ie the respective extinction coefficients are very low Inanalogy T1S0 transitions are also spin-forbidden which implies that the life-time of triplet states is quite long and significantly exceeds that of S1 states Tripletstates can deactivate radiatively The emission of photons from triplet states istermed phosphorescence Both luminescence processes fluorescence and phosphor-escence cover a variety of transitions to the various vibronic levels of the S0 state(see Fig 16) and therefore yield emission spectra with several bands instead of asingle line as would be expected for the sole occurrence of 0-0 transitions Fig-ure 17 presents as a typical example the emission spectrum of poly(25-diocty-loxy-p-phenylene vinylene) DOO-PPV (see Chart 12) [12]

Since fluorescence is emitted from the non-vibronically excited S1 state (seeFig 16) and absorption involves higher ie vibronically excited S1 states themaximum of the fluorescence spectrum is shifted to lower energy (higher wave-lengths) relative to the absorption maximum (Stokes shift) The maximum ofthe phosphorescence spectrum is located at even higher wavelengths since phos-phorescence originates from the non-vibronically excited T1 state which is of low-er energy than the corresponding S1 state (see Fig 13) The emission spectrum

16 Deactivation of electronically excited states 13

presented in Fig 17 features three bands at 215 eV (577 nm) 198 eV (626 nm)and 18 eV (689 nm) which may be attributed to the zero-phonon (0-0) the one-phonon (1-0) and the two-phonon (2-0) transitions respectively

162Intermolecular deactivation

Energy transfer from electronically excited molecules to ground-state molecules ofdifferent chemical composition represents a highly important intermolecular de-activation path In general terms energy transfer occurs according to Eq (1-7)from a donor to an acceptor the latter frequently being referred to as a quencher


Fig 16 Schematic depiction of transitions occurring duringabsorption fluorescence and phosphorescence

Fig 17 Emission spectrum (full curve) and part of theabsorption spectrum (dotted curve) of DOO-PPV Adaptedfrom Lane et al [12] with permission from Wiley-VCH

D A D A 1-7

This process is energetically favorable in the case of exothermicity ie if the ex-citation energy of D exceeds that of A E (D) gt E (A) A typical case concernsthe stabilization of polymeric plastics If an electronically excited macromoleculeP transfers its excitation energy to an additive A according to Eq (1-8) hydro-gen abstraction [Eq (1-9)] is inhibited and the macromolecule remains intact

P A P A 1-8

P RH PH R 1-9

There are two major mechanisms by which energy transfer can occur (1) Thedipole-dipole (coulombic) mechanism also denoted as the Foumlrster mechanismoperating through mutual repulsion of the electrons in the two molecules It ischaracterized by relatively large interaction distances ranging up to a molecularseparation of 5 nm (2) The exchange mechanism also denoted as the Dextermechanism according to which a transient complex is formed on close approachof the partner molecules

The dependence of the rate constant kET of intermolecular energy-transferprocesses on the distance R is given by Eqs (1-10) and (1-11) [13]

Long-range interaction kET k0DR0R6 1-10

Short-range interaction kET k0D expR 1-11

Here kD0 is the unimolecular decay rate constant of the excited donor and R0 is

the critical distance between D and A at which the probabilities of sponta-neous deactivation and of energy transfer are equal Typical R0 values are listedin Table 13 which also includes values for self-transfer [14] The latter processis of relevance for down-chain energy transfer (energy migration) which is re-ferred to below

In principle energy-transfer processes from both singlet and triplet exciteddonors to ground-state acceptors are possible [see Eqs (1-12) and (1-13) respec-tively]


Table 13 Typical R0 values (in Aring) for aromatic chromophores [14]

Naphthalene Phenanthrene Pyrene Anthracene

Naphthalene 735 1316 2897 2316Phenanthrene 877 1443 2172Pyrene 1003 2130Anthracene 2181

DS1 AS0 DS0 AS1 1-12

DT1 AS0 DS0 AT1 1-13

Commonly singlet energy transfer takes place by the dipole-dipole mechanismwhereas triplet energy transfer occurs by the exchange mechanism since the di-pole-dipole mechanism is spin-forbidden in this case

If electronically excited chemically identical species are generated at a highconcentration for example at high absorbed dose rates or during the simulta-neous excitation of various chromophores attached to the same polymer chainannihilation processes according to Eq (1-14) can become important

M M M M 1-14

M denotes a highly excited species that can emit a photon differing in energyto that emitted by M or can undergo ionization or bond breakage Annihila-tion is a self-reaction of excited species that may be singlets or triplets

163Energy migration and photon harvesting

A polymer-specific mode of energy transfer concerns energy migration in linearhomopolymers ie in macromolecules composed of identical repeating unitsSince all of the repeating units contain identical chromophores excitation en-ergy can travel down the chain provided that the geometrical conditions are ap-propriate (large R0 for self-transfer) and the lifetime of the excited state exc islonger than the energy-hopping time h ie exc gt h There are various path-ways that may ensue following the absorption of a photon by a certain chromo-phoric group Figure 18 shows besides the energy migration process energytransfer to an external acceptor molecule and light emission

Actually monomer emission needs to be distinguished from excimer emissionThe latter process originates from a transient complex formed eg in the caseof aromatic compounds by the interaction of an excited molecule with a non-ex-cited chemically identical molecule leading to an excited dimer denoted as anexcimer (see Scheme 11) In linear macromolecules bearing pendant aromaticgroups this process corresponds to the interaction between neighboring repeat-ing units as demonstrated in Scheme 11

Excimers can usually be detected by a shift of the fluorescence emission maxi-mum to a wavelength longer than in the case of monomer emission

After down-chain energy migration in linear polymers had been evidenced bytriplet-triplet annihilation and enhanced phosphorescence quenching [15ndash17]the idea arose to guide electronic excitation energy along the chain to definedsites where it might serve to initiate chemical or physical processes Obviouslysuch a mechanism is relevant to photon harvesting processes employed by naturein photosynthetic systems operating on the following principle which is also re-


ferred to as the antenna effect [18] a large number of chromophores collectphotons and guide the absorbed energy to one reaction center As regards syn-thetic polymers early studies on photon harvesting were devoted to linear poly-mers composed overwhelmingly of repeating units bearing the same donorchromophore (naphthalene) and to a very small extent the acceptor chromo-phore (anthracene) acting as an energy trap [15 19] Relevant work concerninglinear polymers has been thoroughly reviewed by Webber [13] Very interestingrecent studies concerning multiporphyrin systems of various nonlinear struc-tures have been reviewed by Choi et al [20] and are considered below In thecase of the linear polymers mentioned above practically all photons are ab-sorbed by naphthalene moieties upon exposure to light in the wavelength range290ndash320 nm As illustrated in Scheme 12 excitation energy taken up by anaphthalene chromophore migrates down the chain and eventually reaches ananthracene trap

This process is evidenced by the anthracene fluorescence which is quite dis-tinct from that of naphthalene The quantum yield of anthracene sensitization


Fig 18 Pathways of excitation energy in a linear macromolecule

Scheme 11 Excimer formation (a) general description (b) in polystyrene

13S ie the number of sensitized acceptors per directly excited donor can beobtained from Eq (1-15)

13S 1 13IDID0 1-15

Here I(D)0 and I(D) are the donor fluorescence intensities in the absence andin the presence of the acceptor respectively 13S values varying between 01 and07 have been found by examining in aqueous or organic solvents a variety ofpolymers having naphthalene and anthracene groups attached to the main


Scheme 12 Mechanism of photonharvesting Illustration of thetransport of excitation energy byself-transfer through donor moieties(naphthalene) to an acceptor trap(anthracene)

Chart 13 Chemical structures of repeating units bearingnaphthalene and anthracene groups contained in copolymersemployed in photon-harvesting studies [13]


Cha

rt1

4C

hem

ical

stru

ctur

eof

ade

ndri

tic21

-por

phyr

inar

ray

cons

istin

gof

20Z

npo

rphy

rin

units

atta

ched

toa

Zn-

free

porp

hyrin

foca

lco

re[2

122

]

chain in different modes (see Chart 13) The largest 13S values were found incases in which excimer formation was unlikely [13]

Obviously excimer formation represents a serious obstacle to energy migra-tion since the excimer site itself functions as a trap and after excitation ismostly deactivated by emission of a photon rather than by energy transfer to aneighboring donor moiety (exc lth) Moreover any effect on coil density exertedby the choice of temperature or solvent can dramatically effect the efficiency ofenergy trapping


Chart 15 Chemical structure of a dendritic multiporphyrinarray consisting of four wedges of a Zn porphyrin heptameranchored to a Zn-free porphyrin focal core [22]

The light-harvesting multiporphyrin arrays synthesized in recent years seemto mimic natural photosynthetic systems much more closely than the linearpolymers of the early studies As outlined in the review by Choi [20] strategiesfor the synthesis of multiporphyrin arrays of various architectures have been de-veloped These comprise besides ring- star- and windmill-shaped structuresalso dendritic arrays With the aim of a high photon-harvesting efficiency com-bined with vectorial energy transfer over a long distance to a designated pointdendritic light-harvesting antennae have proved to be most promising A typicalexample is the system shown in Chart 14 It consists of a total of 21 porphyrinunits ie 20 PZn Zn-complexing porphyrin moieties which are connected viadiarylethyne linkers to one centrally located Pfree unit ie a non-complexing por-phyrin moiety The quantum yield for the energy transfer PZnPfree is13ET = 092 [21]

The structure of another large dendritic system is depicted in Chart 15 Itconsists of four heptameric Zn-porphyrin segments acting as energy donorsThey are anchored to a central Pfree moiety acting as the acceptor [22] Photonabsorption by the PZn moieties at = 589 nm or 637 nm results in very effec-tive PZnPfree energy transfer (13ET = 071 kET = 104109 sndash1) as indicated by astrongly increased light emission from the Pfree moieties

164Deactivation by chemical reactions

Triplet excited molecules formed in condensed media are liable to undergo bi-molecular chemical reactions since their long lifetimes permit a large numberof encounters between the reaction partners The hydrogen abstraction reactionEq (1-16) of triplet excited carbonyl groups is a typical example

C O RH C OH R

1-16

Singlet excited molecules are usually relatively short-lived and therefore are notvery likely to undergo bimolecular reactions In many cases however chemicalbond cleavage competes with physical monomolecular deactivation paths Forexample singlet excited carbonyl groups contained in a polyethylene chain canundergo the Norrish type I reaction resulting in a free radical couple [seeEq (1-17)]

CH2 CH2 CH2 C CH2 CH2 CH2 CH2 CH2 C CH2 CH2

1-17

O O

More details of chemical deactivation processes are provided in Chapter 7


17Absorption and emission of polarized light

171Absorption

The absorption of linearly polarized light is characterized by the fact that onlythose chromophores with a component of the absorption transition moment lo-cated in the same direction as the electric (polarization) vector of the incidentlight can be excited No light will be absorbed if the direction of the transitionmoment is perpendicular to the electric vector of the incident light This di-chroic behavior is exhibited by anisotropic organic materials in the solid statesuch as single crystals of certain substances in which the transition moments ofall molecules are fixed in a parallel orientation In the case of linear polymersit is possible to generate some degree of optical anisotropy in highly viscous orrigid samples by aligning the macromolecules in a specific direction Variousmethods have been employed to achieve orientation such as mechanical align-ment Langmuir-Blodgett (LB) film deposition liquid-crystalline self-organiza-tion and alignment on specific substrates As a typical example Fig 19 showsabsorption spectra recorded from an LB film placed on the surface of a fused si-lica substrate and consisting of 100 monolayers of DPOPP (see Chart 16) [23]

Electron microscopy revealed that the LB film had a liquid-crystalline-likestructure This means that many polymer chains were oriented parallel to thesubstrate plane and exhibited a preferential orientation of their backbones alongthe dipping direction Absorption spectra recorded with the incident light polar-ized either parallel or perpendicular to the dipping direction show a maximumat 330 nm (376 eV) in both cases but A|| and A the absorbances parallel and


Fig 19 Absorption spectra of an LBfilm consisting of 100 monolayers ofDPOPP recorded with linearlypolarized incident light (|| and parallel and perpendicular to thedipping direction respectively)Adapted from Cimrova et al [23] withpermission from Wiley-VCH

perpendicular to the dipping direction respectively differ by a factor of aboutfive the in-plane order parameter S= (A|| ndash A)(A|| + A) being 067

It might be noted that in principle it is possible to create anisotropy upon ir-radiating an ensemble of randomly oriented photochromic chromophores withlinearly polarized light since photons are only absorbed by chromophores withtransition moments parallel to the electric vector of the incident light This ap-plies eg to thin films of poly(vinyl cinnamate) (see Chart 17) and its deriva-tives Exposure to linearly polarized light induces the preferential orientation ofliquid-crystal molecules in contact with the film surface [24] The photoalign-ment is likely to be caused by the trans-cis isomerization of the cinnamoylgroups a separate process to cross-linking through [2+2] addition which is amajor photoreaction of this polymer

The creation of anisotropy is treated in some detail in Section 44 which dealswith the trans-cis isomerization of azobenzene compounds

172Absorption by chiral molecules

A chiral molecule is one that is not superimposable on its mirror image It con-tains one or more elements of asymmetry which can be for example carbonatoms bearing four different substituent groups In principle chiral moleculescan exist in either of two mirror-image forms which are not identical and arecalled enantiomers Chiral molecules have the property of rotating the plane of po-larization of traversing linearly polarized monochromatic light a phenomenoncalled optical activity Linearly polarized light can be viewed as the result of thesuperposition of opposite circularly polarized light waves of equal amplitudeand phase The two circularly polarized components traverse a medium contain-ing chiral molecules with different velocities Thereby the wave remains plane-po-

17 Absorption and emission of polarized light 23

Chart 16 Chemical structure of poly(25-di-isopentyloxy-p-phenylene) DPOPP

Chart 17 Chemical structure of poly(vinyl cinnamate)

larized but its plane of polarization is rotated through a certain angle the opticalrotation OR In other words optical activity stems from the fact that nr and nl therefractive indices for the two circularly polarized components of linearly polarizedlight are different a phenomenon referred to as circular birefringence

Optically active compounds are commonly characterized by their specific rota-tion [] measured in solution [see Eq (1-18)]

13 100cd deg cm3 dm1 g1 1-18

where c is the concentration in units of g100 cm3 and d is the path length of thelight in dm [] depends on the wavelength of the light and the temperatureActually [] is proportional to the difference in the refractive indices nr and nl[] nrndashnl Since nr and nl have different dependences on [] also dependson A plot of [] vs yields the optical rotary dispersion (ORD) curve of the sub-stance In many cases ORD curves exhibit at wavelengths of light absorption asine-wave form which is referred to as the Cotton effect (see Fig 110) [25] The in-version point of the S-shaped curve (c) in Fig 110 corresponds to max the wave-length of the absorption maximum at which nr is equal to nl

In addition to their optical activity chiral molecules are characterized by theproperty of absorbing the two components of incident linearly polarized lightie left- and right-circularly polarized light to different extents This phenome-non called circular dichroism CD can be quantified by the difference in molarextinction coefficients l r CD is characterized by the fact that a linearlypolarized light wave passing through an optically active medium is transformedinto an elliptically polarized light wave With the aid of commercially availableinstruments the actual absorbance A of each circularly polarized light compo-


Fig 110 Schematic depiction of opticalrotary dispersion (ORD) curves for positiveand negative rotation (a) and (b) respec-tively for wavelength regions without

absorption The S-shaped curve (c) is typicalof the Cotton effect reflecting lightabsorption Adapted from Perkampus [25]with permission from Wiley-VCH

nent is measured yielding the difference Al Ar The latter is related to the el-lipticity given either in degrees (deg) or radians (rad) according to Eqs (1-19) and (1-20) respectively

2303Al Ar1804 deg 1-19

23034Al Ar rad 1-20

Commonly for the sake of comparison the molar ellipticity [] = 100 cd inunits of deg cm2 dmolndash1 is recorded where c is the concentration in mol Lndash1

and d is the optical path length If in the case of polymers such as proteinsthe molar concentration is related to the molar mass of the residue ie to therepeating (base) unit the mean residue weight ellipticity []MRW is obtained

In recent years circular dichroism spectroscopy has been widely applied ininvestigations concerning the molecular structure of chiral polymers It is apowerful tool for revealing the secondary structures of biological macromole-cules for instance of polypeptides proteins and nucleic acids in solution An


Fig 111 Circular dichroism spectra of poly(L-lysine) in its-helical -sheet and random coil conformations Adaptedfrom Greenfield et al [26] with permission from the AmericanChemical Society

important feature is the possibility of monitoring conformational alterations ofoptically active macromolecules by CD measurements Typical data are pre-sented in Fig 111 which shows CD spectra of poly(L-lysine) in three differentconformations [26] Poly(L-lysine) adopts three different conformations depend-ing on the pH and temperature random coil at pH 70 -helix at pH 108 and-sheet at pH 111 (after heating to 52 C and cooling to room temperature oncemore) These conformational transitions are due to changes in the long-rangeorder of the amide chromophores For detailed information on circular dichro-ism of chiral polymers the reader is referred to relevant publications [27ndash30]

173Emission

Provided that the transition moment does not change direction during the lifetimeof an excited state fluorescent light is polarized parallel to the incident light Forlinearly polarized incident light this implies that the direction of the electric vec-tor of both the incident and the emitted light is the same Therefore in the case oforiented polymers fluorescence can only be generated with linearly polarized lightif the components of the absorption transition moments of the chromophores arealigned parallel to the electric vector of the incident light If the alignment of themacromolecules is not perfect the emitted light is not perfectly polarized This iscommonly characterized by the degree of polarization P defined by Eq (1-21)

P I II I

1-21

Here I|| and I are the intensities of the fluorescence polarized parallel and per-pendicular to the electric vector of the incident light Usually set-ups with thegeometry shown in Fig 112 are employed for fluorescence measurements The


Fig 112 Geometry of experimentalset-ups employed in fluorescencedepolarization measurements

sample is excited with light incident along the x-axis and the fluorescence ismonitored along the y-axis M denotes the transition dipole moment

As a typical example Fig 113 shows fluorescence spectra recorded from anLB film of DPOPP (for the absorption spectra see Fig 19) The exciting lightwas polarized parallel to the dipping direction

In accordance with the conclusion derived from the absorption spectra theemission spectra also reveal the partially ordered structure of the film As in thecase of absorption I|| and I the fluorescence intensities parallel and perpendic-ular to the dipping direction respectively differ appreciably in this case by afactor of three to four Much higher dichroic ratios have been found with otheroriented systems eg with highly aligned films consisting of blends of poly-ethylene with 1 wt MEH-PPV (see Chart 18) [31 32] The films fabricated bytensile drawing over a hot pin at 110ndash120 C proved to be highly anisotropic (di-chroic ratio gt 60) with the preferred direction parallel to the draw axis

In principle oriented polymeric systems capable of generating linearly polar-ized light have the potential to be used as backlights for conventional liquid-crystal displays (LCDs) a subject reviewed by Grell and Bradley [33] In thisconnection systems generating circularly polarized (CP) light also became at-tractive CP light can be utilized for backlighting LCDs either directly with theaid of appropriate systems or after transformation into linearly polarized lightwith the aid of a suitable 4 plate [33] CP light has been generated for exam-ple with a highly ordered polythiophene bearing chiral pendant groups


Chart 18 Chemical structure of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] MEH-PPV

Fig 113 Fluorescence spectra of a DPOPP filmprepared by the LB technique I|| and I fluores-cence intensities parallel and perpendicular tothe dipping direction Exciting lightexc = 320 nm polarized parallel to the dippingdirection Adapted from Cimrova et al [23] withpermission from Wiley-VCH

poly34-di[(S)-2-methylbutoxy]thiophene (see Chart 112) [34] In this casehowever the dissymmetry factor ge was low ge is defined as 2(IL ndashIR)(IL + IR)and |ge| is equal to two for pure single-handed circularly polarized light IL andIR denote the left- and right-handed emissions respectively Circularly polarizedlight is produced quite efficiently when a conventional luminophore is em-bedded within a chiral nematic matrix consisting of a mixture of compounds Aand B (see Chart 19) [35] When this system was exposed to unpolarized lightof = 370 nm the dissymmetry factor ge approached ndash2 in the 400ndash420 nmwavelength range

Another aspect also considered in Subsection 18332 concerns fundamentaltime-resolved fluorescence studies Here the emphasis is placed on fluores-cence depolarization measurements which are very helpful in following rota-tional and segmental motions and for studying the flexibility of macromole-cules If the polymer under investigation does not contain intrinsically fluores-cent probes (eg certain amino acid moieties in proteins) then the macromole-cules have to be labeled with fluorescent markers Information concerning therate of rotation or segmental motion then becomes available provided that theemission rate is on a similar time scale Only when this condition is met canthe rate of depolarization be measured If the emission rate is much fasterthere is no depolarization whereas if it is much slower the depolarization willbe total

Commonly the emission anisotropy r(t) is determined as a function of timer(t) is defined by Eq (1-22)

rt It ItIt 2It 1-22

By irradiating a sample with a short pulse of linearly polarized light and separa-tely recording I|| and I as a function of time t after the pulse the sum S(t) =I|| + 2I and the difference D(t) = I|| ndash I may be obtained The application of anappropriate correlation function to r(t) = D(t)S(t) yields the relaxation time In


Chart 19 Chemical structures of compounds A and Bforming a chiral nematic matrix and of an oligomericluminophore

general the time dependence of r(t) is rather complex ie the decay of r(t) doesnot follow a single exponential decay function Theories have been developed toanalyze the experimentally observed decay functions However it is beyond thescope of this book to deal with the relevant theoretical work which has beenthoroughly reviewed elsewhere as part of the overall subject of fluorescence de-polarization [36 37] In simple cases r(t) decays according to a single exponen-tial decay law Provided that this applies to the rotational motion of macromole-cules the rotational relaxation time r can be evaluated by assuming sphericallyshaped macromolecules For a rotating spherical body r(t) is expressed byEq (1-23)

rt 25exp6Drt 1-23

The rotational diffusion constant Dr is given by Eq (1-24) the Einstein law

Dr 1r kTV 1-24

Here V is the volume of the sphere and is the viscosity of the solventAs can be seen in Table 14 the r values of proteins such as bovine serum al-

bumin and trypsin in aqueous solution lie in the ns range and become largerwith increasing molar mass The proteins were labeled with fluorescent markerssuch as 1-dimethylamino-5-sulfonyl-naphthalene groups (see Chart 110) [38]

Segmental motions and molecular flexibility have been studied for variouspolymers such as polystyrene and the Y-shaped immunoglobulins IgA and IgGRelaxation times in the range of 10ndash100 ns were found In these studies the


Table 14 Rotational correlation times r of proteins inaqueous solution at 25 C determined by time-resolvedfluorescence depolarization measurements [37]

Protein Molar mass (g molndash1) r (ns)

Apomyoglobin 17000 83Trypsin 25000 129Chymotrypsin 25000 151-Lactoglobulin 36000 203Apoperoxidase 40000 252Serum albumin 66000 417

Chart 110 Chemical structure of the 1-dimethylamino-5-sulfonyl-naphthalene group

polymers were labeled with small amounts of appropriate fluorescent markerssuch as anthracene in the case of PSt [39]

Again it is a prerequisite for such measurements that the fluorescence decaysat a rate similar to that of the motion under investigation Measurable rotationalrelaxation times are in the range 1 ns to 1 s corresponding to the rotation ofspecies with molar masses up to 106 g molndash1 in aqueous solution

18Applications

181Absorption spectroscopy

1811 UVVis spectroscopyThere are numerous applications reliant upon the ultraviolet and visible (UVVis) wavelength range For example absorption spectroscopy is applied to ana-lyze and identify polymers and copolymers containing chromophores that ab-sorb in this wavelength range such as aromatic or carbonyl groups In this con-text the investigation of photochemical reactions for instance of reactions oc-curring in degradation processes is noteworthy Moreover absorption measure-ments allow the monitoring of alterations in the tertiary structure ofmacromolecular systems for instance in the case of the denaturation of bio-macromolecules especially proteins and nucleic acids Figure 114 demonstratesthe increase in the optical absorption observed upon heating an aqueous solu-


Fig 114 Thermal denaturation of lysozyme in aqueoussolution Differential absorption vs temperature [lysozyme]10 g Lndash1 pH 145 [KCl] 02 m Adapted from Nicolai et al[40] with permission from John Wiley amp Sons Inc

tion of lysozyme a globular protein that acts as an enzyme in the cleavage ofcertain polysaccharides [40] The absorption change reflects the unfolding of thepolypeptide chains due to the destruction of intramolecular interactions such ashydrogen bonds (see Scheme 13)

The thermal denaturation of other superstructures such as those of collagenand deoxyribonucleic acid (DNA) may also be monitored by following the in-crease in the optical absorption Collagen is the most abundant protein in con-nective tissues and constitutes a major part of the matrix of bones In its nativestate it adopts a three-stranded helical structure Dissociation of the threechains at temperatures above 40 C is accompanied by an increase in optical ab-sorption DNA the carrier of genetic information and an essential constituentof the nuclei of biological cells contains the bases adenine guanine cytosineand thymine and hence absorbs UV light The intensity of its absorption spec-trum (max = 260 nm) is reduced by about 30 when single strands combine toform the double-stranded helix Conversely the optical absorption increasesupon denaturation [41] This is illustrated in Fig 115

Generally changes in optical absorption related to molecular alterations notinvolving chemical bond breakage are denoted by the terms hypochromicity (alsohypochromy) and hyperchromicity (also hyperchromy) depending on whether theoptical absorption decreases or increases respectively As regards nucleic acidsin solution hypochromicity applies to a decrease in optical absorbance whensingle-stranded nucleic acids combine to form double-stranded helices The hy-pochromic effect is not restricted to nucleic acids proteins and other polymersbut has also been observed with aggregates of dyes and clusters of aromaticcompounds In interpreting this effect it has been assumed that the electronclouds of chromophores brought into close proximity are strongly interactingThe resulting alteration in the electron density causes changes in the absorptionspectrum The hypochromicity phenomenon and relevant theories are discussedin detail in a recent monograph [42]

18 Applications 31

N H O C N H O C

Scheme 13 Destruction of hydrogen bonds

Fig 115 Thermal denaturation of DNA (E coli)Relative absorbance at 260 nm vs temperature atvarious concentrations of KCl (given in the graphin units of mol Lndash1) Adapted from Marmur et al[41] with permission from Elsevier

1812 Circular dichroism spectroscopyCircular dichroism (CD) spectroscopy is a form of absorption spectroscopy basedon measuring the difference in the absorbances of right- and left-circularly polar-ized light by a substance (see Section 172) Regarding polypeptides proteins andnucleic acids it is a powerful tool for analyzing secondary and tertiary structuresand for monitoring conformational changes In the case of proteins it allows thediscrimination of different structural types such as -helix parallel and antiparal-lel -pleated sheets and -turns and moreover allows estimation of the relativecontents of these structures Details are given in review articles [43ndash45]

Since appropriate instruments have become commercially available CD spec-troscopy has developed into a routine method for the characterization of thechirality of newly synthesized polymers As a typical example the rather highchiro-optical activity of the ladder-type poly(p-phenylene) of the structure shownin Chart 111 was revealed CD spectroscopically molar ellipticity [] = 22106 rad cm2 molndash1 (at max = 461 nm) corresponding to an anisotropy factor ofg == 0003 [46]

The following three examples serve to demonstrate the general importance ofCD spectroscopy (1) Consider first the case of optically active polythiophene de-rivatives They belong to the class of polymers of which the optical activity isbased on the enantioselective induction of main-chain chirality by the presenceof enantiomerically pure side groups In the case of PDMBT (Chart 112) CDspectroscopy permits the detection of a pronounced thermochromic effectWhen dichloromethane solutions that do not exhibit chiro-optical activity relatedto the transition at = 438 nm at 20 C are cooled to ndash30 C the onset ofabsorption is significantly red-shifted Moreover a CD spectrum exhibiting astrong bisignate Cotton effect (see Fig 116) is recorded The chiro-optical activ-ity which is observed for n-decanol solutions even at room temperature (g = = 002) is ascribed to highly ordered packing of the polythiophene chains inchiral aggregates [34]

(2) In the case of thin films of PMBET (see Chart 113) another optically ac-tive polythiophene derivative CD spectroscopy reveals stereomutation of themain chain As can be seen in Fig 117 a CD spectrum that is the mirror im-


Chart 111 Chemical structure of a ladder-typepoly(p-phenylene)

Chart 112 Chemical structure of poly34-di[(S)-2-methylbutoxy]thiophene PDMBT

age of the original spectrum is recorded when PMBET is rapidly cooled fromthe disordered melt to the crystalline state Apparently by rapid cooling of themelt a metastable chiral associated form of the polymer that exhibits the mir-ror-image main-chain chirality is frozen-in [47]

(3) A final example demonstrating the usefulness of CD spectroscopy con-cerns the detection of light-induced switching of the helical sense in polyisocya-nates bearing chiral pendant groups [48] Polyisocyanates (see Chart 114) existas stiff helices comprising equal populations of dynamically interconvertingright- and left-handed helical segments The relative population of these seg-ments is extraordinarily sensitive to chiral perturbations This is demonstratedby the CD spectra shown in Fig 118 They were recorded from polyisocyanatePICS (see Chart 114) that had been irradiated with circularly polarized light(CPL) of opposite handedness Initially the pendant groups consist of a racemicmixture of the two enantiomers and a CD spectrum is not observed Absorption

18 Applications 33

Fig 116 Normalized absorptionspectrum (dashed line)and CD spectrum (solid line) ofPDMBT recorded in dichloro-methane solution at -30 C Dottedline first derivative of theabsorption spectrum Adapted fromLangeveld-Voss et al [34] withpermission the American ChemicalSociety

Chart 113 Chemical structure of poly(3-2-[(S)-2-methylbutoxy]ethylthiophene) PMBET

Scheme 14 Isomerization of the pendant groups of PICS

of light induces isomerization at the C-C double bond (see Scheme 14) Thusirradiation with circularly polarized light which is absorbed by the two enantio-mers to different extents results in an optically active partially resolved mixtureand the CD spectra shown in Fig 118 are observed Remarkably an enantio-meric excess of just a few percent ie close to the racemic state converts thepolymer into one having a disproportionate excess of one helical sense In otherwords chiral amplification takes place since the minor enantiomeric grouptakes on the helical sense of the major enantiomeric group

Interestingly the helical sense of the polymer may be reversibly switched byalternating irradiation with (+)- or (ndash)-CPL or returned to the racemic state byirradiation with unpolarized light


Fig 117 CD spectra of PMBET recorded at room temperaturefrom thin films spin-coated onto glass plates after fast (a)and slow (b) cooling from 200 C to 20 C Adapted from Bou-man et al [47] with permission from Wiley-VCH

Chart 114 Chemical structure of polyisocyanates General structure left PICS right

1813 IR spectroscopyInfrared (IR) spectroscopy has become a very powerful chemical-analytical toolin the analysis and identification of polymers It also plays a prominent role intests related to chemical alterations generated by extrinsic forces and serves forexample in the monitoring of polymer degradation The wavelength regime ofimportance ranges from about 25 to 50 m (4000 to 200 cmndash1) This corre-sponds to the energies required to excite vibrations of atoms in molecules Pre-cisely speaking the full spectrum of infrared radiation covers the wavelengthrange from 075 to 103 m ie besides the aforementioned mid-IR region thereis the near-IR region (075 to 25 m) and the far-IR region (50 to 103 m)

IR light is absorbed when the oscillating dipole moment corresponding to amolecular vibration interacts with the oscillating vector of the IR beam The ab-sorption spectra recorded with the aid of IR spectrometers consist of bands at-tributable to different kinds of vibrations of atom groups in a molecule espe-cially valence and deformation (bending) vibrations as can be seen in Fig 119

Figure 120 presents a typical example of the application of IR spectroscopyHere the UV radiation-induced chemical modification of a polyester containingin-chain cinnamoyl groups (see Chart 115) is illustrated [49]

As can be seen in Fig 120 the FTIR spectrum of the unirradiated polymerfeatures absorption bands at 1630 1725 and 1761 cmndash1 which may be assigned

18 Applications 35

Fig 118 CD spectra of polyisocyanate PICS irradiated withcircularly polarized light (CPL) of opposite handedness atgt 305 nm The spectra were recorded in dichloromethanetetrahydrofuran (1 1) solution Adapted from Li et al [48]with permission from the American Chemical Society

Fig 119 Notation of group vibrations

to the stretching vibrations of vinylene double bonds and conjugated and non-conjugated carbonyl bonds respectively Upon irradiation the intensities of thevinylene and the conjugated carbonyl bands decrease whereas the band due tothe non-conjugated carbonyl groups intensifies with increasing absorbed doseThis behavior may be explained in terms of simultaneously occurring trans-cisisomerizations and [2+ 2] cycloadditions (dimerizations) The band at 1630 cmndash1

decreases since the extinction coefficient of cis C=C bonds is lower than that oftrans C=C bonds The growth in the intensity of the band at 1761 cmndash1 indicatesthe occurrence of dimerizations

Modern commercial IR spectrometers operating with the aid of a Michelsoninterferometer produce interferograms which upon mathematical decoding bymeans of the Fourier transformation deliver absorption spectra commonly re-ferred to as Fourier-transform infrared (FTIR) spectra [50] Comprehensive col-lections of IR spectra of polymers monomers and additives are available [51]Moreover the readerrsquos attention is directed to several books [52ndash58]


Chart 115 Chemical structure of the polyester referred to in Fig 120

Fig 120 FTIR spectra of a Cn-polyester recordedbefore and after irradiation with UV light (260ndash380 nm) to different absorbed doses Adaptedfrom Chae et al [49] with permission fromElsevier

182Luminescence

Many problems in the physics and chemistry of polymers have been investi-gated by means of fluorescence techniques Within the scope of this book it ismerely possible to point out the high versatility of these techniques rather thanto discuss the innumerable publications Among the features of luminescencethat account for the variety of its applications is the fact that emission spectracan be recorded at extremely low chromophore concentrations Thus a polymermay be labeled with such a small amount of luminophore that the labeling doesnot perturb the properties of the system As regards linear polymers in solutionit is possible to derive information on the conformational state and the behaviorof the macromolecules This concerns such topics as the interpenetration ofpolymer chains the microheterogeneity of polymer solutions conformationaltransitions of polymer chains and the structures of polymer associates Relevantwork has been reviewed by Morawetz [59] Here only one typical example is de-scribed which concerns the kinetics of HCl transfer from aromatic amino moi-eties to much more basic aliphatic amino groups attached to discrete macromol-ecules in this case poly(methyl methacrylate)s (see Scheme 15)

18 Applications 37

Scheme 15 HCl transfer from aromatic to aliphatic amino groups

The release of HCl from the aminostyrene groups increases the fluorescenceintensity since protonation prevents light emission Thus the rate of HCl trans-fer between the different macromolecules can be measured in a stopped-flowexperiment It was found that the rate constant of the reaction decreased withincreasing chain length of the interacting polymers [60] This result may be in-terpreted in terms of the excluded volume effect flexible polymer chains ingood solvent media strongly resist mutual interpenetration a phenomenon thatbecomes more pronounced with increasing chain length

Another quite different kind of luminescence application pertains to the gen-eration of polarized light with the aid of aligned systems Here the concept ofpolarizing excitonic energy transfer EET comes to prominence Thus in appro-priate systems randomly oriented sensitizer molecules harvest the incomingunpolarized light by isotropic absorption and subsequently transfer the energyto a uniaxially oriented polymer The latter emits light with a high degree of lin-ear polarization According to this concept all incident light can be funnelledinto the same polarization The incorporation of the polarizing EET process intocolored liquid-crystal displays (LCDs) would imply that dichroic polarizers areno longer required for the generation of polarized backlights in conventionalLCDs A system functioning in this way consists of a ternary blend of high mo-lar mass (4106 g molndash1) polyethylene 2 wt of a derivative of PPE and 2 wtof the sensitizer DMC (see Chart 116) [61] Blend films prepared by solution-casting from xylene are uniaxially drawn at 120 C to a draw ratio of about 80

183Time-resolved spectroscopy

1831 General aspectsWith the advent of powerful lasers capable of generating short light pulses a newera of research commenced [62ndash64] Notably the new light sources permit themeasurement of lifetimes of excited states and the detection of short-lived inter-mediates such as free radicals and ions The concomitant development of sophis-ticated detection methods has also brought about continuous progress during the


Chart 116 Chemical structures of a poly(25-dialkoxy-p-phenylene ethynylene) PPE and 7-diethylamino-4-methyl-coumarin DMC

last decades in the fields of polymer physics and chemistry [9 65ndash68] While re-searchers were initially fascinated by studying processes on the microsecond(1 s= 10ndash6 s) and nanosecond (1 ns= 10ndash9 s) time scale more recent researchhas concentrated on the picosecond (1 ps= 10ndash12 s) and femtosecond (1 fs= 10ndash

15 s) time region In this way a wealth of information has become available thatallows the identification of extremely short-lived intermediates and elucidatesthe mechanisms of many photophysical and photochemical processes The aimhere is not to review work on the technical development of pulsed lasers andon the invention of highly sensitive detection methods In a more general way in-formation is given on the wide-ranging potential of time-resolved measurementsand their benefits in the fields of polymer photophysics and photochemistry

Time-resolved measurements were initiated both by physicists who wereprincipally interested in photophysical processes that left the chemical struc-tures of the molecules intact and by chemists who were mainly interested inthe chemical alterations of the irradiated molecules but also in the associatedphotophysical steps The parallel development of these two lines of research isreflected in the terminology For example the term flash photolysis as used bychemists applies to time-resolved measurements of physical property changescaused by chemical processes induced by the absorption of a light flash (pulse)Flash photolysis serves to identify short-lived intermediates generated by bondbreakage such as free radicals and radical ions Moreover it allows the determi-nation of rate constants of reactions of intermediates Therefore this method isappropriate for elucidating reaction mechanisms

1832 Experimental techniquesFor pico- and femtosecond studies time-resolved measurements require power-ful pulsed laser systems operated in conjunction with effective detection tech-niques Relevant commercially available laser systems are based on Ti sapphireoscillators tunable between 720 and 930 nm (optimum laser power around800 nm) For nanosecond work Nd3+ YAG (neodymium-doped yttrium-alumi-num-garnet) (1064 nm) and ruby (6943 nm) laser systems are commonly em-ployed For many applications light pulses of lower wavelength are producedwith the aid of appropriate nonlinear crystals through second third or fourthharmonic generation For example short pulses of = 532 355 and 266 nm aregenerated in this way by means of Nd3+ YAG systems Moreover systems based

18 Applications 39

Fig 121 Schematic depiction of a set-upfor time-resolved optical absorptionmeasurements

on mode-locked dye lasers have occasionally been employed for ultrafast mea-surements in the fs and ps time domain [12]

Principally the pump and probe technique depicted in Fig 121 is applied intime-resolved transient absorption experiments A pump beam directed ontothe sample generates excited species or reactive intermediates such as free radi-cals The formation and decay of these species can be monitored with the aid ofan analyzing (probe) light beam that passes through the sample perpendicularto the direction of the pump beam In principle a set-up of this kind is alsosuitable for recording luminescence if it is operated without the probe beam


Fig 122 Schematic depiction of a set-up for time-resolvedoptical absorption measurements in the femtosecond timedomain SHG second harmonic generation crystal PDphotodiode OMA optical multichannel analyzer Adaptedfrom Lanzani et al [68] with permission from Wiley-VCH

A typical set-up employed for time-resolved measurements in the femtose-cond time domain is presented in Fig 122 [68] Here a Ti sapphire system op-erated in conjunction with a LiB3O5 crystal functioning as a frequency doublerprovides the pump pulse (= 390 nm repetition rate 1 kHz) The pulse intensity(excitation density) can be varied between 03 and 12 mJ cmndash2 For the genera-tion of the analyzing white light a fraction of the pump pulse is split off andfocused through a thin sapphire plate The resulting supercontinuum which ex-tends from 450 to 1100 nm is passed through the sample prior to hitting thedetector Through mechanical operation of the delay line transient absorptionspectra are recorded at various times after the pump pulse by averaging over100 to 1000 laser pulses

Modern detection systems are based on the charge-coupled device (CCD) tech-nique which is not indicated in the schematic of Fig 122

Prior to the advent of powerful lasers high-speed flash techniques were em-ployed as light sources in time-resolved studies Research was focused mainlyon luminescence studies aimed at determining fluorescence and phosphores-cence lifetimes In this connection the development and successful applicationof sophisticated methods such as the single-photon time-correlation methodand high-speed photography methods (streak camera) are worthy of note De-tailed technical information on these topics is available in a book by Rabek [69]The physical principles of lifetime determinations have been described by Birks[70]

1833 Applications of time-resolved techniques

18331 Optical absorptionOptical absorption measurements are much more difficult to perform thanemission measurements This applies for instance to the detection of specieshaving a low extinction coefficient at the relevant wavelengths The surroundingmolecules should be transparent which is important in the case of solutionsMoreover it has to be taken into account that invariably one has to measure anabsorbance difference and not an absolute quantity as in the case of lumines-cence In principle molecules that have been promoted to an excited state ofsufficiently long lifetime can absorb photons Provided that the absorption coef-ficients are large enough the absorption spectrum can permit identification ofthe excited state and from its decay the lifetime of the excited state is obtainedIn the relevant literature this kind of absorption is frequently denoted by theacronyms PIA or PA referring to photoinduced absorption In many cases ex-cited triplet states are relatively long-lived and can easily be detected by light ab-sorption measurements As a typical result Fig 123 shows the T-T absorptionspectrum ie the spectrum of excited triplet states of the polymer PPVK (seeChart 117) generated by irradiation in benzene solution at room temperaturewith a 15 ns pulse of 347 nm light The triplet lifetime amounts to several mi-croseconds in this case [71]

18 Applications 41

Commonly excited singlet states have very short lifetimes and can only be de-tected by means of femtosecond absorption spectroscopy A typical case is illus-trated in Fig 124 which shows the differential transmission spectrum ofMEH-DSB (see Chart 118)

The differential transmission is defined as TT = (TndashT0)T0 where T and T0

are the transmissions in the presence and the absence of the pump beam re-spectively It may be recalled that T = (II0) = endashd where I0 and I denote the lightintensities before and after the sample and d are the absorption coefficientand the sample thickness respectively The absorbance A is equal to d In thesmall signal limit commonly 10ndash5 to 10ndash3 ie (TT) 1 TT is proportionalto the change in the absorption coefficient (TT)ndashd Negative valuesof TT correspond to photoinduced absorption (PIA) Thus in Fig 124 theband between 600 and 1100 nm with a peak at about 900 nm reflects the ab-sorption of singlet intrachain excitons [72] Positive values of TT correspondto bleaching or stimulated emission SE Thus in Fig 124 the band between450 and 500 nm is assigned to bleaching due to depopulation of ground-stateelectrons and the band at around 535 nm coinciding with the photolumines-cence (PL) spectrum is ascribed to SE [72] The spectral features shown by thesolid line in Fig 124 are similar to those reported for many poly(arylene viny-lene)s The phenomenon of stimulated emission is dealt with in more detail inSection 622 Also typical of poly(arylene vinylene)s Fig 125 presents differen-tial transmission kinetic traces recorded at 800 nm at varying pulse intensitiesfor a thin film of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene]MEH-PPV The absorption decays on the ps time scale and the decay dynamicsdepends on the excitation density The higher the pulse intensity the faster is


Fig 123 Triplet-triplet absorption spectrumof poly(phenyl vinyl ketone) in benzenesolution at room temperature Recorded atthe end of a 15 ns pulse of 347 nm light

Chart 117 Chemical structure of poly(phenyl vinyl ketone)

18 Applications 43

Chart 118 Chemical structure of a phenylene vinylene oligomer

Fig 124 Femtosecond spectroscopy atexc = 400 nm pulse length 150 fs pulseenergy 1 mJ pulse repetition rate 1 kHzDifferential transmission spectrum of a thinfilm of MEH-DSB (solid line) recorded at theend of the pulse Also shown ground-state

absorption coefficient (dashed line) andphotoluminescence spectrum PL (dottedline) Adapted from Maniloff et al [72] withpermission from the American PhysicalSociety

Fig 125 Femtosecond spectroscopyDifferential transmission traces recorded atrec = 850 nm from thin films of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] MEH-PPV irradiated as indicatedin the legend of Fig 124 at varying photon

fluences from upper to lower curves101013 311014 and 931014 cmndash2respectively Adapted from Maniloff et al[72] with permission from the AmericanPhysical Society

the decay Since the decay dynamics of the PIA band at around 800 nm and ofthe SE band at 535 nm are correlated it is concluded that both bands arise fromthe same species namely intrachain excitons The intensity-dependent decay dy-namics may be interpreted in terms of exciton-exciton annihilation a processinvolving interaction of nearby excitons and resulting in non-radiative relaxationto the ground state [72]

18332 LuminescenceDuring the past decades time-resolved fluorescence measurements have helpedto address many problems in the polymer field A typical example concerns thedetermination of the rate of rotational and segmental motions of macromole-cules in solutions as dealt with in Section 173 Moreover time-resolved fluores-cence measurements permit the investigation of energy migration and excimerformation in linear polymers Down-chain energy migration in a linear polymerbearing overwhelmingly naphthalene plus a few anthracene pendant groupswas evidenced by a decrease in the naphthalene fluorescence and a concomitantincrease in anthracene fluorescence [17] Similarly the decay of the monomeremission was found to be correlated with the build-up of the excimer fluores-cence in the case of polystyrene in dilute solution in dichloromethane [73] Thisis illustrated in Fig 126

The remainder of this section focuses on the phenomenon of spectral or gainnarrowing which has been discovered in more recent fluorescence studies Ascan be seen in Fig 127 the shape of the spectrum of light emitted fromBuEH-PPV (see Chart 119) changes drastically when the intensity of the excit-ing light pulse is increased beyond a threshold value The broad emission spec-trum extending over a wavelength range of about 200 nm recorded at low inci-dent light intensity is transformed into a narrow band with 10 nm at highlight intensity [74]

The phenomenon of spectral narrowing is attributed to a cooperative effect inlight emission the so-called amplified spontaneous emission effect which involvesthe coherent coupling of a large number of emitting sites in a polymer matrix


Fig 126 Fluorescence spectra ofpolystyrene in oxygen-free CH2Cl2solution (1 g Lndash1) I Monomeremission recorded at the end of a10 ns flash (exc = 257 nm) IIExcimer emission recorded 45 nsafter the flash Adapted fromBeavan et al [73] with permissionfrom John Wiley amp Sons Inc

Spectral narrowing has been observed for thin polymer films (200ndash300 nmthick) on planar glass substrates The films act as wave guides since the refrac-tive index of the polymer is larger than that of the surrounding air or the glasssubstrate Immediately after absorption of a light pulse some photons are spon-taneously emitted from certain excited sites These photons are coupled into theguided-wave mode and stimulate radiative deactivation processes of other ex-cited sites upon propagation through the film a process denoted as amplifiedspontaneous emission The phenomenon of spectral narrowing is explained bythe fact that the emission of photons with the highest net gain coefficient is fa-vored [75]

References 45

Fig 127 Spectral narrowing in the caseof BuEH-PPV Emission spectrarecorded at different excitation pulseenergies Pulse duration 10 nsexc = 532 nm Film thickness 210 nm[74] Adapted from Lemmer et al [75]with permission from Wiley-VCH

Chart 119 Chemical structure of poly[2-butyl-5-(2-ethylhexyl)-14-phenylene vinylene] BuEH-PPV

References

1 J D Coyle Introduction to Organic Photo-chemistry Wiley Chichester (1986)

2 HH Jaffe M Orchin Theory and Appli-cations of Ultraviolet Spectroscopy WileyNew York (1962)

3 G M Barrow Introduction to MolecularSpectroscopy McGraw-Hill KogakushaTokyo (1962)

4 HG O Becker (ed) Einfuumlhrung in diePhotochemie Thieme Stuttgart (1983)

5 J Kopecky Organic Photochemistry A Vi-sual Approach VCH Weinheim (1992)

6 M Pope C E Swenberg Electronic Pro-cesses in Organic Crystals and Polymers2nd Edition Oxford University PressNew York (1999)

7 W Schnabel Polymer Degradation Princi-ples and Practical Applications HanserMuumlnchen (1981)

8 H-K Shim J-I Jin Light-Emitting Char-acteristics of Conjugated Polymers in K-SLee (ed) Polymers for Photonics Applica-tions I Springer Berlin Adv Polym Sci158 (2002) 193


9 NS Sariciftci (ed) Primary Photoexcita-tions in Conjugated Polymers MolecularExciton versus Semiconductor Band ModelWorld Scientific Singapore (1997)

10 J Cornil D A dos Santos D BeljonneZ Shuai J-L Bredas Gas Phase to SolidState Evolution of the Electronic and Opti-cal Properties of Conjugated Chains ATheoretical Investigation in G Hadziioan-nou PF van Hutten (eds) Semicon-ducting Polymers Wiley-VCH Weinheim(2000) p 235

11 K Pichler D Halliday DC BradleyPL Burn R H Friend A B Holmes JPhys Cond Matter 5 (1993) 7155

12 PA Lane SV Frolov Z V VardenySpectroscopy of Photoexcitations in Conju-gated Polymers in G Hadziioannou PFvan Hutten (eds) Semiconducting Poly-mers Wiley-VCH Weinheim (2000)p 189

13 SE Webber Chem Rev 90 (1990) 146914 IB Berlman Energy Transfer Parameters

of Aromatic Compounds Academic PressNew York (1973)

15 R F Cozzens R B Fox J Chem Phys50 (1969) 1532

16 C David M Lempereur G GeuskensEur Polym J 8 (1972) 417

17 J W Longworths MD Battista Photo-chem Photobiol 11 (1970) 207

18 J E Guillet Polymer Photophysics andPhotochemistry Cambridge UniversityPress Cambridge UK (1985)

19 J S Aspler CE Hoyle J E GuilletMacromolecules 11 (1978) 925

20 MS Choi T Yamazaki I Yamazaki TAida Angew Chem Int Ed 43 (2004)150

21 MR Benites ET Johnson S WeghornL Yu PD Rao J R Diers S I Yang CKirmaier D J Bocian D Holten J SLindsey J Mater Chem 12 (2002) 65

22 MS Choi T Aida T Yamazaki I Ya-mazaki T Aida Angew Chem Int Ed40 (2001) 3194

23 V Cimrova M Remmers D Neher GWegner Adv Mater 8 (1996) 146

24 K Ichimura Y Akita H Akiyama KKudo Y Hayashi Macromolecules 30(1997) 903

25 H-H Perkampus Encyclopedia of Spec-troscopy VCH Weinheim (1995)

26 NJ Greenfield G D Fasman ComputedCircular Dichroism Spectra for the Evalua-tion of Protein Conformation Biochemis-try 8 (1969) 4108

27 A Rodger B Norden Circular Dichroismand Linear Dichroism Oxford UniversityPress Oxford (1997)

28 G D Fasman (ed) Circular Dichroismand the Conformational Analysis of Biomo-lecules Plenum Press New York (1996)

29 K Nakanishi N Berova R W Woody(eds) Circular Dichroism Principles andApplications VCH Publishers Weinheim(1994)

30 R W Woody Circular Dichroism of Pep-tides in E Gross J Meienhofer (eds)The Peptides Analysis Synthesis BiologyAcademic Press New York (1985) Vol 7p 14

31 TW Hagler K Pakbaz J Moulton FWudl P Smith A J Heeger PolymCommun 32 (1991) 339

32 TW Hagler K Pakbaz K F Voss A JHeeger Polym Commun Phys Rev B44 (1991) 8652

33 M Grell DD C Bradley Adv Mater 11(1999) 895

34 BMW Langeveld-Voss RA J JanssenMPT Christiaans SC J MeskersHP JM Dekkers E W Meijer J AmChem Soc 118 (1996) 4908

35 SH Chen D Katsis A W Schmid J CMastrangelo T Tsutsui N T BlantonNature 397 (1999) 506

36 EA Anufrieva Yu Ya Gotlib Investiga-tion of Polymers in Solution by PolarizedLuminescence Adv Polym Sci 40Springer Berlin (1981) p 1

37 K P Ghiggino A Roberts D PhillipsTime-Resolved Fluorescence Techniques inPolymer and Biopolymer Studies AdvPolym Sci 40 Springer Berlin (1981)p 69

38 P Wahl CR Acad Sci 263 (1966)1525

39 See literature cited in [37]40 DF Nicolai GB Benedek Biopolymers

15 (1976) 242141 J Marmur P Doty J Mol Biol 5 (1962)

10942 NL Veksin Photonics of Biopolymers

Springer Berlin (2002)

References 47

43 R W Woody Circular Dichroism Meth-ods Enzymol 246 (1995) 34

44 W C Johnson Jr Methods Enzymol 210(1992) 426

45 W C Johnson Jr Proteins 7 (1990) 20546 R Fiesel J Huber U Scherf Angew

Chem 108 (1996) 223347 MM Bouman E W Meijer Adv Mater

7 (1995) 38548 J Li G B Schuster K-S Cheon MM

Green J V Selinger J Am Chem Soc122 (2000) 2603

49 B Chae SW Lee M Ree S B Kim Vi-brational Spectrosc 29 (2002) 69

50 W Kloumlpffer Introduction to Polymer Spec-troscopy Springer Berlin (1984)

51 DO Hummel Atlas of Polymer and Plas-tics Analysis 3rd Edition Wiley-VCHWeinheim (2005)

52 A Elliott Infrared Spectra and Structureof Organic Long-Chain Polymers ArnoldLondon (1969)

53 M Claybourn Infrared Reflectance Spec-troscopy of Polymers Analysis of Films Sur-faces and Interfaces Adhesion SocietyBlacksburg VA (1998)

54 R A Meyers (ed) Encyclopedia of Analyt-ical Chemistry Application Theory and In-strumentation Wiley Chichester (2000)

55 J M Chalmers P R Griffiths (eds)Handbook of Vibrational Spectroscopy Wi-ley Chichester (2002)

56 HW Siesler Y Ozaki S Kawata HMHeise Near-Infrared Spectroscopy Wiley-VCH Weinheim (2002)

57 J Workman Jr Handbook of OrganicCompounds NIR IR Raman and UV-VisSpectra Featuring Polymers and Surfac-tants Academic Press San Diego (2000)

58 HM Mantsch D Chapman InfraredSpectroscopy of Biomolecules Wiley NewYork (1996)

59 H Morawetz J Polym Sci Part APolym Chem 37 (1999) 1725

60 Y Wang H Morawetz Macromolecules23 (1990) 1753

61 A Montali C Bastiaansen P Smith CWeder Nature 392 (1998) 261

62 R R Alfano Semiconductors Probed byUltrafast Laser Spectroscopy AcademicPress New York (1984)

63 J L Martin A Mignus G A MourouA H Zewail (eds) Ultrafast Phenomena

Springer Series in Chemical PhysicsVol 55 Springer Berlin (1992)

64 G Porter Flash Photolysis into the Femto-second ndash A Race against Time in J ManzL Woumlste (eds) Femtosecond ChemistryWiley-VCH Weinheim (1995)

65 FC DeSchryver S De Feyter GSchweitzer (eds) Femtochemistry Wiley-VCH Weinheim (2001)

66 DW McBranch MB Sinclair UltrafastPhoto-Induced Absorption in Nondegener-ate Ground State Conjugated PolymersSignatures of Excited States in [9] p 587

67 J-Y Bigot T Barisien Excited-State Dy-namics of Conjugated Polymers and Oligo-mers in FC DeSchryver S De FeyterG Schweitzer (eds) Femtochemistry Wi-ley-VCH Weinheim (2001)

68 G Lanzani S De Silvestre G CerulloS Stagira M Nisoli W Graupner GLeising U Scherf K Muumlllen Photophys-ics of Methyl-Substituted Poly(para-Phenyl-ene)-Type Ladder Polymers in G Hadziio-annou PF van Hutten (eds) Semicon-ducting Polymers Wiley-VCH Weinheim(2000) p 235

69 J F Rabek Experimental Methods inPhotochemistry and Photophysics WileyChichester (1982)

70 J B Birks Photophysics of Aromatic Mole-cules Wiley-Interscience London (1970)p 94

71 W Schnabel J Kiwi Photodegradationin HHG Jellinek (ed) Aspects of Deg-radation and Stabilization of PolymersElsevier Scientific Publ Amsterdam(1978) p 195

72 ES Maniloff V I Klimov DWMcBranch Phys Rev B 56 (1997) 1876

73 SW Beavan JS Hargreaves D Phil-lips Photoluminescence in PolymerScience Adv Photochem 11 (1978) 207

74 F Hide MA Diaz-Garcia B J SchartzMR Anderson P Qining A J HeegerScience 273 (1996) 1833

75 U Lemmer A Haugeneder C Kallin-ger J Feldmann Lasing in ConjugatedPolymers in G Hadziioannou PF vanHutten (eds) Semiconducting PolymersWiley-VCH Weinheim (2000) p 309

21Introductory remarks

A photoconductive solid material is characterized by the fact that an electric cur-rent flows through it under the influence of an external electric field when it ab-sorbs UV or visible light There are two essential requirements for photoconduc-tivity (1) the absorbed photons must induce the formation of charge carriersand (2) the charge carriers must be mobile ie they must be able to move inde-pendently under the influence of an external electric field Photoconductivitywas first detected in inorganic materials for example in crystals of alkali metalhalides containing color centers (trapped electrons in anion vacancies) or in ma-terials possessing atomic disorder such as amorphous silicon or selenium Asregards organic materials dye crystals and more recently also various polymer-ic systems have been found to exhibit photoconductivity Two groups of photo-conducting polymeric systems may be distinguished (a) solid solutions of activecompounds of low molar mass in inert polymeric matrices also denoted as mo-lecularly doped polymers and (b) polymers possessing active centers in themain chain or in pendant groups Examples belonging to group (a) are polycar-bonate and polystyrene molecularly doped with derivatives of triphenylaminehydrazone pyrazoline or certain dyes (see Table 21) Molecularly doped poly-mers are widely used as transport layers in the photoreceptor assemblies ofphotocopying machines

Typical examples of photoconductive polymers (group (b)) are listed in Ta-ble 22 Concerning the field of conducting polymers including photoconduct-ing polymers the reader is referred to various books and reviews [1ndash21]

49

2Photoconductivity

22Photogeneration of charge carriers

221General aspects

Regarding inorganic semiconductors the photogeneration of charge carriers hasbeen explained in terms of the so-called band model according to which thenuclei of atoms are situated at fixed sites in a lattice [22] Since the charges ofthe nuclei are largely compensated by their inner-shell electrons an averageconstant potential is attributed to the outer-shell electrons denoted as valenceelectrons The energy levels of the valence electrons differ only slightly and aretherefore considered as being located in the so-called valence band (seeFig 21)

At T = 0 the absolute zero temperature all valence electrons reside in the va-lence band at higher temperatures some electrons are promoted to the so-called conduction band The probability of an electron being in a quantum stateof energy E is given by Eq (2-1)

2 Photoconductivity50

Table 21 Typical dyes applied as dopants in photoconducting polymeric systems

Chemical structure Denotation

Perylene dye

Azo dye

Quinone dye

Squaraine dye

M CdZnTiO etc Phthalocyanine dye

f E EF exp13E EF1 exp13E EF

2-1

Here f(E ndash EF) is the Fermi distribution function is equal to (kT)ndash1 where kis the Boltzmann constant T is the absolute temperature and EF is the Fermienergy

The Fermi level of inorganic semiconductors lies between the valence bandand the conduction band in contrast to metals for which the Fermi level lieswithin the valence band According to this model the phenomenon of dark con-ductivity is feasible Photoconductivity implies that upon irradiation electrons

22 Photogeneration of charge carriers 51

Table 22 Chemical structures of typical photoconducting polymers

Chemical structure Acronym Denotation

PVC Poly(N-vinyl carbazole)

PAC trans-Polyacetylene

PT Polythiophene

PFO Poly(dialkyl fluorene)

PPV Poly(p-phenylene vinylene)

PPP Poly(p-phenylene)

m-LPPP Methyl-substituted ladder-typepoly(p-phenylene)

R1 and R2 alkyl or aryl groups Polysilylene

PANI Polyaniline

are promoted from the valence band to the conduction band Thus the totalelectrical conductivity is composed of two terms representing the dark con-ductivity d and the photoconductivity p

d p 2-2

Band-to-band transitions of electrons require photon energies exceeding the en-ergy of the band gap Since the energy states of the conduction band are not lo-calized ie not attributable to specific atomic nuclei electrons transferred to theconduction band lose their local binding and become mobile Regarding poly-meric systems this aspect is at variance with recent experimental and theoreti-cal work which overwhelmingly led to the conclusion that in such systems lo-calized states are involved both in the photogeneration of charges and in thecarrier transport and that the theoretical model developed for inorganic semi-conductors is not applicable for polymeric systems At present the generationof charge carriers is explained in terms of the exciton concept and a generally ac-cepted carrier transport mechanism presumes charge hopping among discretesites as will be described in the following subsections

222The exciton model

The exciton model is based on the fact that in organic photoconductors thelight-induced transition of an electron to an excited state causes a pronouncedpolarization of the chromophoric group Because of the relatively high stabilityof this state it is considered to be an entity of special nature This entity calledan exciton is an excited state of quasi-particle character located above the va-lence band It resembles a hydrogen-like system with a certain binding energy


Fig 21 Energy levels of a semiconductor Also shown energylevel of an exciton state as generated upon photonabsorption

which can besides other non-radiative or radiative deactivation routes also giverise to the formation of a geminate electronhole pair Under certain conditionsthe latter can dissociate and thus give rise to the generation of free ie indepen-dent charge carriers

exciton 13he h e 2-3

It is generally accepted that the dissociation of electronhole pairs is induced orat least strongly assisted by an external electric field Whether electronholepair dissociation generally also occurs intrinsically ie in the absence of an ex-ternal electric field has not yet been fully established In certain cases such asin m-LPPP [23] or in PPV [24] this process has been evidenced However inthese and similar cases electronhole pair dissociation is likely to be due to thepresence of impurities such as molecular oxygen andor structural defects inthe macromolecular system such as conformational kinks or chain twists thatfunction as dissociation sites The existence of these sites and the capability ofexcitons to approach them are presumably prerequisites for dissociation In thisconnection it is notable that excitons are conjectured to diffuse over certain dis-tances It has been suggested that charge generation ie the formation of freecharge carriers occurs preferentially at specially structured sites on the surfaceof the sample

In view of the highly variable nature of photoconducting materials differenttypes of exciton states have been postulated For instance an exciton state witha radius of the order of 100 Aring a so-called Wannier exciton is assumed to beformed in amorphous silicon in which the wave function spreads over the elec-tronic orbitals of many Si atoms In contrast in conjugated polymers such aspoly(phenyl vinylene) or polysilanes (see Table 22) the formation of less ex-tended so-called Frenkel excitons with radii of the order of 10 Aring is assumed Inthis case the polymer system is considered to be an ensemble of short molecu-lar segments that are characterized by localized wave functions and discrete en-ergy levels and an exciton generated by the absorption of a photon exists withinthe intra-chain delocalization length For systems permitting the formation ofcharge-transfer (CT) states the existence of charge-transfer or quasi-Wannier ex-citons having radii exceeding those of Frenkel excitons is postulated This ap-plies for example to poly(methyl phenyl silylene) [25] In this case the absorp-tion of photons in main-chain segments generates Frenkel excitons which areconverted to CT excitons through intramolecular interaction with pendant phe-nyl groups (see Scheme 21)

Moreover CT excitons are thought to be formed by intermolecular interactionin certain polymeric systems containing small molecules A typical example ispoly(N-vinyl carbazole) doped with trinitrofluorenone (TNF) a system whichplayed a major role in early photoconductive studies on polymeric systems (seeChart 21)

As regards the nature of the so-called dissociation sites referred to above it maybe noted that generally any kind of disorder-induced kink may play an activating


role in the dissociation of electronhole pairs In the case of trans-polyacetylenewhich has been examined quite extensively so-called neutral solitons (seeChart 22) resulting from incomplete cis-trans isomerization are postulated to func-tion as dissociation sites Neutral solitons are characterized by a free spin and aretherefore detectable by electron-spin resonance (ESR) measurements [26]

223Chemical nature of charge carriers

In the earlier literature charge carriers generated in polymers are frequently de-noted as polarons and bipolarons and it is assumed that these charged speciesare formed instantaneously upon optical excitation [27] The fundamental andoften quite controversial debate on the nature of the primary photoexcitations


Scheme 21 Generation of charge-transfer excitons in poly(methyl phenyl silylene) [25]

Chart 21 Chemical structures of poly(N -vinyl carbazole) and trinitrofluorenone

Chart 22 Chemical structures of solitons formed in trans-polyacetylene

in -conjugated polymers has attracted much attention in the scientific commu-nity and has resulted in a series of articles being compiled in a book edited bySariciftci [9] This book is wholeheartedly recommended for further readingThe currently accepted notion that optical absorption generates primarily neu-tral excitations (excitons) rather than charged species was adopted in Sec-tion 222 The earlier model is based to some extent on the assignment of tran-sient optical absorption bands at around 06 and 16 eV recorded with PPV-typepolymers to bipolarons However this assignment was contradicted by unam-biguous experimental evidence for an attribution of these transient absorptionbands to singly-charged ions [28] The definition of the term polaron which cansometimes be rather elusive in older work has been subject to alterations andmany authors now denote the products of the dissociation of electronhole pairsas negative and positive polarons However by doing so the difficulty of pre-cisely describing the chemical nature of the charge carriers is merely circum-vented As a matter of fact the release of an electron should lead to a radicalcation and the capture of an electron to a radical anion Actually relatively littlework has hitherto been dedicated to clarifying the nature of photogeneratedcharge carriers Time-resolved spectroscopy has helped to evidence the existenceof radical cations acting as charge carriers in certain polymeric systems In thiscase radical cations were generated by hole injection from an indium tin oxide(ITO) electrode by applying an external electric field to polysulfone systems con-taining tris(stilbene) amine derivatives [29] Moreover the formation of radicalcations in poly(methyl phenyl silylene) with 13CC110ndash3 was evidenced bymeans of transient optical absorption measurements (absorption bands ataround 375 and 460 nm formed upon irradiation with 20 ns laser pulses= 347 nm) [25] In the case of m-LPPP irradiated with 380 nm laser pulses atransient optical absorption band at around 691 nm (191 eV) attributed to posi-tive polarons was detected (see below) [23] Obviously quite different charge car-riers will be produced depending on the chemical nature of the polymer For ex-ample in the case of trans-polyacetylene the dissociation of electronhole pairsat neutral solitons is considered to give rise to positively and negatively chargedsolitons (see Chart 22) [30]

224Kinetics of charge carrier generation

The research concerning the mechanism and kinetics of the photogeneration ofcharge carriers has focused on conjugated polymers since these are of great im-portance for applications in light-emitting diodes and organic photovoltaic cells(see Sections 621 and 63) Typical work performed with m-LPPP (see Table 22)revealed that charge carriers are generated within a few hundred femtosecondsin a very small yield in the absence of an external electric field [23] The poly-mer was irradiated with 180 fs pulses of 380 nm light at 77 K Transmission dif-ference spectra plotted as TT exhibited besides the emission and absorptionbands of excitons an absorption band at 19 eV (650 nm) attributable to individ-


ual positive polarons (holes) This band was formed within the duration of thepulse When an external electric field was applied the yield of charge carrierswas significantly increased As can be seen from the kinetic traces shown inFig 22 the formation of the polaron absorption corresponds to the decay of theexciton emission thus demonstrating that excitons dissociate into charge car-riers

Upon applying a field modulation technique it was possible to record directlyfield-induced changes in the TT spectra Therefore the kinetic traces inFig 22 reflect the time dependence of the field-induced differential transmis-sion (TT)FM which is the difference between TT recorded in the presenceand absence of the electric field (TT)FM = (TT)F ndash (TT)F = 0


Fig 22 Dissociation of excitons into chargecarriers in m-LPPP under the influence of anexternal electric field (13 V) Kinetic traceson different time scales demonstratingchanges in the field-induced differentialtransmission (TT)FM at 191 eV (hole

absorption) and 253 eV (exciton emission)following irradiation of a 100 nm thickpolymer film at 77 K with 180 fs pulses of380 nm light Trace (a) also shows the pulseprofile (dashed line) Adapted from Lanzaniet al [23] with permission from Wiley-VCH

225Quantum yield of charge carrier generation

It has been pointed out above that the deactivation of excitons may result in theformation of geminate electronhole pairs that can eventually form free chargecarriers This process proceeds with strong competition from charge recombina-tion and can be affected by an external electric field According to the Onsagertheory [31] the probability Pr of recombination can be estimated with the aid ofEq (2-4)

Pr exp rc

r

exp eFr

2kT131 cos

2-4

Here e is the elementary charge F is the electric field strength k is the Boltz-mann constant T is the temperature and is the angle between the vectorconnecting the charges and the direction of the electric field

The Onsager theory considers two potentials determining the fate of an elec-tronhole pair the Coulomb potential e2r (= dielectric constant) and the ther-mal energy kT Pairs having a radial distance r larger than rc will escape recom-bination At the critical radial distance rc the thermal energy is equal to theCoulomb potential [see Eq (2-5)]

kT e2

rc2-5

According to Eq (2-4) the recombination probability decreases with increasingfield strength ie the escape probability Pe = 1ndash Pr increases Therefore thequantum yield for charge carrier generation 13cc should increase with increasingfield strength Figure 23 shows a double logarithmic plot of the dependence of13cc on the electric field strength measured at T = 295 K for three polysilylenes[32]

The quantum yield increases dramatically by about three orders of magnitudein the cases of the polysilylenes PBMSi and PMPSi having aromatic substitu-ents whereas the fully aliphatic polysilane PDHeSi is quite ineffective in chargecarrier production presumably because CT excitons cannot be formed in thiscase Interestingly 13cc is markedly higher for the biphenyl-substituted polysi-lane than for the phenyl-substituted one which might be due to a larger initialelectronhole distance in the former case The curves in Fig 23 were obtainedwith the aid of Eq (2-6) [33] which is based on calculations by Mozumder [34]

13cc 13cc0

4r2f rFTgrdr 2-6

Here 13cc0 denotes the primary quantum yield f(r F T) is the dissociationprobability of pairs at radial distance r and g(r) is the initial spatial distribution


of electronhole pairs Satisfactory data fits were obtained by applying a Gaus-sian distribution function for electronhole pair distances [see Eq (2-7)]

gr 323 exp r2

2

2-7

Here is a material parameterRegarding the curves in Fig 23 data fitting was performed with 13cc0 = 085

and = 16 nm in the case of PBMSi and 13cc0 = 045 and = 13 nm in the caseof PMPSi These data are in accordance with the assumption that 13cc0 in-creases with increasing initial electronhole radial distance r0 since statistically is a measure of r013cc values are most accurately determined by the xerographic (electrophoto-

graphic) discharge method which is based on the determination of the light-in-duced change in the surface potential U= QC generated by a corona processQ and C denote the surface charge density and the capacitance per unit area re-spectively U is recorded at a given sampling frequency and the dischargequantum yield is obtained with the aid of Eq (2-8)

13cc 1efI

Q

t

tt0

CefI

Ut

tt0

0

edfI

Ut

tt0

2-8

with the following denotations dielectric constant (dimensionless) vacuumdielectric constant 0 = 88510ndash14 A s Vndash1 cmndash1 elementary charge e= 1602210ndash19 A s sample thickness d [cm] light intensity I [photons cmndash2 sndash1] surfacepotential U [V] and fraction of absorbed light f Figure 24 shows a schematicdepiction of a typical experimental set-up which includes a rotating metal disk


Fig 23 Quantum yield for charge carriergeneration as a function of the electric fieldstrength determined at 295 K for three poly-silylenes poly(biphenyl methyl silylene)

PBMSi poly(methyl phenyl silylene) PMPSiand poly(dihexyl silylene) PDHeSi Adaptedfrom Eckhardt [32] with permission from theauthor

carrying the sample Upon rotation (600ndash2400 rpm) the sample passes a contin-uous light beam and a condenser plate for determination of the change in thesurface potential

A typical result obtained upon irradiation of poly(methyl phenyl silylene) atexc = 337 nm is shown in Fig 25 [32]


Fig 24 Schematic illustration of a set-up used to determine13cc by means of the xerographic discharge method Adaptedfrom Eckhardt [32] with permission from the author

Fig 25 Light-induced decrease in the surface potentialrecorded for poly(methyl phenyl silylene) at exc = 337 nmt0 = onset of irradiation Adapted from Eckhardt [32] withpermission from the author

23Transport of charge carriers

The transport of charge carriers through a solid is characterized by the drift mo-bility which is defined as the hole or electron velocity per unit electric fieldstrength frequently given in units of cm2 Vndash1 sndash1 can be obtained with theaid of Eq (2-9) by measuring the transit time tr which is the time required forcharge carriers to pass a sample of thickness d when an external electric field ofstrength F is applied

dtrF

2-9

Commonly the so-called time-of-flight (TOF) method is applied to determine Figure 26 shows a schematic depiction of a typical set-up

A sandwich-type sample consisting of a semi-transparent ITO electrode apolymer layer and a metal (usually aluminum) electrode (see Fig 27a) is irra-diated with a short laser flash through the ITO electrode During the light flashwhich is totally absorbed by a very thin sheet at the surface of the polymer layercharge carriers are generated and start to drift towards the metal electrode un-der the influence of an external electric field The photocurrent is recorded as afunction of time after the flash Notably the transport of both sorts of chargecarriers cannot be recorded simultaneously In the case of a negatively polarizedmetal electrode hole migration can be observed while electron migration canbe followed with a positively polarized metal electrode For mobility measure-ments in thin samples or materials inappropriate for photochemical charge car-


Fig 26 Schematic illustration of a typical time-of-flight (TOF)set-up used for the determination of the mobility

rier generation (low absorption coefficient low quantum yield 13cc) a sandwich-type arrangement consisting of goldsiliconpolymergold layers (see Fig 27b)is used [35] Here after passing through the lower gold layer the light is totallyabsorbed by the silicon substrate thus generating charges that are injected intothe polymer layer

Usually only one sort of charge carrier is capable of migrating through thepolymer film In the cases of carbon-catenated -conjugated and silicon-cate-nated -conjugated polymers the photoconductivity is due to hole conductionOn the other hand electrical conductivity due to electron conduction has beenobserved with low molar mass compounds such as tris(8-oxyquinolato)alumi-num Alq3 dispersed in polymethacrylates bearing special pendant groups (seeChart 23 and also Table 63 in Section 6212)

Figure 28 shows a typical result obtained for conjugated polymers [36] Herecharge carriers are generated in a poly(methyl phenyl silylene) sample by a15 ns flash of 347 nm light The photocurrent is formed during the flash and afraction decays very rapidly until a plateau is reached In the subsequent phasethe current decreases slowly The initial phase after the flash is characterized bythe rapid formation of charge carriers and the rapid recombination of a fractionof them The plateau corresponds to the migration of the holes which drift atdifferent velocities through the sample and the end of the plateau correspondsto the time at which the fastest holes arrive at the metal electrode

23 Transport of charge carriers 61

Fig 27 Sandwich-type assembliesapplied in time-of-flight determinations ofcharge carrier mobility (a) carriergeneration in the polymer layer (b) carriergeneration in the silicon substrate

Chart 23 Chemical structure of tris(8-oxyquinolato)-aluminum Alq3

From Table 23 which lists typical values it can be seen that the hole mo-bility in conjugated polymers is lower than that in organic crystals and amor-phous silicon but much larger than that in undoped poly(N-vinyl carbazole)Therefore conjugated polymers have potential for applications in conductingopto-electronic and photonic devices In principle this also applies to liquid-crystal systems that can exhibit enhanced molecular order due to their self-orga-nizing ability as has been pointed out in a progress report [42]

The fact that liquid crystallinity enhances carrier transport as compared tonon-ordered systems was convincingly demonstrated in the case of poly(99-dioctylfluorene) A relatively high hole mobility of 910ndash3 cm2 Vndash1 sndash1 was ob-tained when the polymer was examined as a uniformly aligned nematic glassThis value is significantly larger than the = 410ndash4 cm2 Vndash1 sndash1 measured foran isotropic film of the same polymer [43] Although significant progress hasbeen made in developing materials with improved charge carrier mobilities itseems that future applications will require materials possessing much furtherimproved transport properties Apparently interchain interactions and morpho-logical complexities strongly control charge carrier transport in bulk polymericsystems Taking this into account recent work on hole transport has led to quitehigh mobility values For example high mobilities were measured for very thinfilms (70ndash100 nm) of poly(3-hexylthiophene) P3HT having a regioregularity of96 [40] (Regioregularity denotes the percentage of stereoregular head-to-tail at-


Table 23 Hole mobilities at T= 295 K and F105 V cmndash1

Polymer (cm2 Vndash1 sndash1) References

Crystals of low molar mass organic compounds 10ndash1ndash100 [5 28]Amorphous silicon 10ndash1 [5]m-LPPP 10ndash3 [37]Poly(99-dioctylfluorene) 10ndash4 [38]Poly(methyl phenyl silylene) 10ndash4 [32]Poly(p-phenylene vinylene) 10ndash5 [39]Polythiophene 10ndash5 [40]Poly(N-vinyl carbazole) 10ndash7ndash10ndash6 [41]

Fig 28 Time-of-flight experiment performedwith poly(methyl phenyl silylene) Photocurrenttrace recorded with a positively biased ITOelectrode at F= 25107 V mndash1 d = 2 mexc = 347 nm flash duration 20 ns Adaptedfrom Eckhardt et al [36] with permission fromTaylor amp Francis Ltd

tachments of thiophene rings bearing hexyl groups in the 3-position) The filmsconsisted of large amounts of microcrystalline domains embedded in an amor-phous matrix During film processing the macromolecules arranged by self-orga-nization into a lamellar structure composed of two-dimensional conjugated sheetsFor a lamellae orientation parallel to the substrate hole mobility values as high as01 cm2 Vndash1 sndash1 were found In this context work with isolated linear polymerchains (molecular wires) is also noteworthy [44] It revealed that the hole transportmobility along isolated polymer chains can exceed 01 cm2 Vndash1 sndash1 as can be seen inTable 24 Here values were obtained from a pulse radiolysis study on dilute poly-mer solutions Holes were generated by charge transfer from benzene radical ca-tions to the polymer By means of a time-resolved microwave conductivity methodit was shown that the conductivity of the solution increased significantly after theholes were produced indicating that the mobility of holes in the polymer chainsis considerably higher than the mobility of the initially formed benzene radicalcations

Interestingly electron transport has been observed with a diene compound ofthe structure shown in Chart 24


Table 24 Hole mobility in linear polymers in dilute solution in benzene [44]

Chemical structure Acronym (cm2 Vndash1 sndash1)

DEH-PF 074

MEH-PPV 043

m-LPPP 016

P3HT 002

PAPS6 023

For this compound which forms a smectic C phase at room temperature anelectron mobility of 1510ndash5 cm2 Vndash1 sndash1 was reported By virtue of its reactivegroups this diene compound can be photopolymerized to form a polymeric net-work [45]

24Mechanism of charge carrier transport in amorphous polymers

At present a hopping mechanism is generally accepted for the transport ofcharge carriers through amorphous polymeric media under the influence of anexternal electric field [23 46] After separation of electronhole pairs the inde-pendent charge carriers are temporarily trapped at certain sites The latter havethe quality of potential wells formed by single molecules or segments of poly-mer chains Assisted by an external electric field the carriers are removed fromthese sites by thermal activation and move until recaptured by other sites Withregard to this model Gill has formulated an empirical relationship [Eq (2-10)]for the dependence of the mobility on electric field strength and temperature[47]

FT 0 exp Ea0 F12

kTeff

2-10

Here Ea0 is the average activation energy = (e30)12 is the Poole-Frenkelfactor and Teff is an effective temperature where Teff

ndash1 = Tndash1 ndashT0ndash1 T0 is the tem-

perature at which Arrhenius plots of with varying F intersect and 0 =(T = T0)

More recently a relationship for the dependence of on F and T was derivedby Baumlssler [21 28] on the basis of the so-called disorder concept The latter takesinto account that carrier hopping in amorphous polymers is determined by theenergy state of the transport sites and by the geometrical localization of thesites The values of the energy states of the sites vary within a certain distribu-tion the so-called density of states (DOS) distribution which is referred to as di-agonal disorder The width of this distribution is characterized by a parameter Regarding the geometrical localization of the sites it is taken into accountthat they are randomly distributed within the three-dimensional system whichis referred to as off-diagonal disorder The width of this distribution is character-ized by the geometrical disorder parameter The two distributions can be il-


Chart 24 Chemical structure of a diene compound amenable to electron transport [45]

lustrated as follows Diagonal disorder transport sites are traps of varyingdepths off-diagonal disorder the trajectories of carriers do not follow lines par-allel to the field direction but show significant deviations therefrom especiallyat low electric field strengths as is demonstrated in Fig 29

In conclusion charge transport in amorphous polymers occurs by way of car-rier hopping within a positionally random and energetically disordered systemof localized states [48] The dependence of the carrier mobility on diagonal andoff-diagonal disorder is taken into account by Eq (2-11)

FT 0 exp 42

9

exp C2 2F12

132-11

Here kT with being the width of the Gaussian distribution of energystates C is an empirical constant and 0 is a material constant

According to Eq (2-11) ln is proportional to F12 and 1T2 Regarding thefield strength dependence of typical results obtained with poly(methyl phenylsilylene) are presented in Fig 210 [32]

Note that the square-root dependence does not hold for the entire field regimewhich is in accordance with findings for other polymers [28] Note also that Eq (2-11) predicts that the field dependence changes sign if gtkT and that the phe-nomenologically defined Gill temperature T0 is related to the disorder parameter of the system T0 =k For example T0 is equal to 387 K for = 3 and = 01 eV[28] The applicability of the model described above was scrutinized by Baumlssler [28]and is still being examined as indicated by recent publications [49ndash51] It has beenpointed out for instance that in the case of m-LPPP the dependence of on elec-tric field strength and temperature resembles that of molecular crystals exceptthat is two orders of magnitude lower a behavior at variance with the presentversion of the disorder model Attempts to modify the disorder model have tosome extent been focussed on the interaction of charge carriers with the surround-ing matrix ie on the so-called polaronic effect The latter implies that a localized

24 Mechanism of charge carrier transport in amorphous polymers 65

Fig 29 Schematic depiction of a carrier trajectory in apolymeric matrix reflecting the geometrical (off-diagonal) disorder The electric field acts along the DndashAdirection jump rate Adapted from Baumlssler et al [21]with permission from Wiley-VCH

carrier is strongly coupled either to local polarization or to vibrations andor rota-tions of the molecule at which it resides Since the coupling is induced by thecharge carrier itself the process is referred to as self-trapping and gives rise tothe denotation of charge carriers as polarons When a polaron moves it carriesalong the associated structural deformation As regards the hopping model po-laronic effects can be taken into account by considering that the activation energyfor the mobility in a random hopping system is composed of two components apolaronic component Ea

(p) and a disorder component Ea(d) [see Eq (2-12)]

Ea Epa Ed

a 2-12

Therefore the dependence of the charge carrier mobility on electric fieldstrength and temperature can be described by Eq (2-13)

FT 0 exp Ep

2kT 42

9

exp C 2 2

F12

132-13

Here Ep denotes the polaron binding energy

25Doping

It is possible to make inert polymers photoconductive and to improve the photo-conduction performance of conducting polymers by doping ie by the additionof appropriate low molar mass substances to the polymers Relevant work hasbeen reviewed by Mylnikov [3] Early studies with inert polymers such as poly-


Fig 210 Electric field dependence ofthe mobility of holes in poly-(methyl phenyl silylene) at varioustemperatures (1) 295 K (2) 312 K(3) 325 K (4) 355 K (5) 385 KAdapted from Eckhardt [32] withpermission from the author

carbonate polystyrene and poly(vinyl chloride) revealed that the hole mobili-ty and 13cc the quantum yield of charge carrier generation were increasedwhen electron-donating compounds such as those presented in Chart 25 wereincorporated as dopants Actually large amounts of dopants have to be appliedto accomplish significant variations in 13cc and

Figure 211 depicts the increase in 13cc with increasing triphenylamine con-tent in commercial bisphenol A polycarbonate (see Chart 26) [52] and Fig 212shows a plot of log vs 1T It can be seen that the hole mobility may be variedover several orders of magnitude by changing the TPA concentration [53] Hereirradiations were performed at wavelengths of exc = 300 and 337 nm respec-

25 Doping 67

Chart 25 Chemical structures of electron-donatingcompounds triphenylamine (TPA) isopropylcarbazole (IPC)and phenylcarbazole (PhC)

Fig 211 Doping of an inert polymer bisphenol Apolycarbonate with triphenylamine (TPA) The quantum yieldof charge carrier formation 13cc as a function of the TPAcontent exc = 300 nm Adapted from Borsenberger et al [52]with permission from the American Institute of Physics

tively at which the polycarbonate is transparent and the light is absorbed solelyby TPA

As regards photoconducting polymers typical work has been carried out withpoly(N-vinylcarbazole) PVK and polysilylenes The first commercial photocon-ductor was based on a 1 1 charge-transfer (CT) complex between PVK and trini-trofluorenone (TNF) [11] Similar photoconductor properties were found with a1 1 CT complex of TNF with poly[bis(2-naphthoxy)phosphazene] (see Chart 27)which is an insulator if dopant-free [54]

Results obtained with poly(methyl phenyl silylene) are presented in Table 25which demonstrate that at low concentration (3 mol) electron-accepting do-pants having zero dipole moment are capable of increasing both and 13cc Theincrease in 13cc is more pronounced the higher the value of the electron affinity


Chart 26 Chemical structure of bisphenol Apolycarbonate poly(oxycarbonyloxy-14-pheny-lene-isopropylidene-14-phenylene)

Fig 212 Doping of an inert polymerbisphenol A polycarbonate with triphenyl-amine (TPA) Temperature dependence ofthe hole mobility Plot of log vs 1T forvarious TPA contents denoted as weight

fraction x exc = 337 nm F= 7105 V cmndash1 denotes the activation energy Adaptedfrom Pfister [53] with permission from theAmerican Physical Society

EA Polar dopants also cause an increase in the quantum yield but the holemobility is concomitantly decreased [55]

Fullerene C60 is quite an effective dopant It is an excellent electron acceptorcapable of accepting up to six electrons Photoinduced electron transfer fromconducting polymers such as poly(3-octylthiophene) P3OT and poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] MEH-PPV to fullerene C60 occurs ona timescale of less than 1 ps A C60 content of a few percent is sufficient to en-hance 13cc in the ps time domain by more than an order of magnitude [56]

26Photoconductive polymers produced by thermal or high-energy radiation treatment

Certain polymers become photoconductive upon exposure to heat or high-en-ergy radiation an aspect that has been reviewed by Mylnikov [3] For examplepolyacrylonitrile (maximum sensitivity at = 420 nm) or polypyrrole (maximumsensitivity at = 500ndash600 nm) exhibit photoconductivity after heat treatmentwhich is thought to be due to the formation of conjugated double bonds High-

26 Photoconductive polymers produced by thermal or high-energy radiation treatment 69

Chart 27 Chemical structure of poly[bis(2-naphthox-y)phosphazene] P2NP

Table 25 The photoconduction performance of poly(methylphenyl silylene) containing electron-acceptor-type dopants[55]

Additive(3 mol)

EA a)

(eV)Dipole moment(Debye)

b)

(cm2 Vndash1 sndash1)cc

c)

None 22810ndash4 1910ndash2 d)

o-DNB g) 00 60 50210ndash5 2310ndash2 d)

m-DNB 03 38 14210ndash4 2310ndash2 d)

p-DNB 07 00 31010ndash4 3410ndash2 d)

Tetracene 10 00 30610ndash4 9610ndash2 e)

Chloranil 13 00 41210ndash4 12510ndash2 e)

TCNQ f) 17 00 57110ndash4 10010ndash2 e)

a) Electron affinityb) Hole mobilityc) Quantum yield of charge carrier formationd) exc =355 nme) exc =339 nmf) TCNQ tetracyanoquinoneg) DNB dinitrobenzene

energy electron irradiation on the other hand renders polyethylene photocon-ductive with maximum sensitivity in the near-infrared region This phenome-non was postulated as being due to radiation-generated donor- and acceptor-typetraps

27Photoconductive polymers produced by plasma polymerization or glow discharge

Various polymeric materials prepared by plasma polymerization or glow dis-charge become conductive when exposed to UV light This applies for exampleto a polymer obtained by plasma polymerization of styrene The polymer wasexamined as a thin sheet coated with gold layers on both sides [57] Also thinpolymer layers deposited by glow discharge of tetramethylsilane tetramethylger-manium or tetramethyltin on conducting substrates were found to be photocon-ductive in the wavelength region 200ndash350 nm [58]


References

1 D Mort D Pai (eds) Photoconductivityand Related Phenomena Elsevier Amster-dam (1976)

2 D Mort N Pfister (eds) Electronic Prop-erties of Polymers Wiley-InterscienceNew York (1982)

3 V Mylnikov Photoconducting PolymersAdv Polym Sci 115 (1994) 1

4 D Haarer Photoconductive Polymers AComparison with Inorganic Materials AdvSolid State Phys 30 (1990) 157

5 D Haarer Angew Makromol Chem183 (1990) 197

6 TA Skotheim (ed) Handbook of Con-ducting Polymers Marcel Dekker NewYork (1986)

7 TA Skotheim R L Elsenbaumer J RReynolds (eds) Handbook of ConductingPolymers 2nd Edition Marcel DekkerNew York (1997)

8 G Zerbi Organic Materials for PhotonicsElsevier Science Amsterdam (1993)


10 K Y Law Chem Rev 93 (1993) 44911 PM Borsenberger D S Weiss Organic

Photoreceptors for Xerography Marcel Dek-ker New York (1998)

12 PM Borsenberger D S Weiss OrganicPhotoreceptors for Imaging Systems MarcelDekker New York (1993)

13 NV Joshi Photoconductivity MarcelDekker New York (1990)

14 HS Nalwa (ed) Handbook of OrganicConductive Molecules and Polymers Vol 3Wiley New York (1997)

15 HS Nalwa (ed) Handbook of AdvancedElectronic and Photonic Materials and De-vices Academic Press San Diego (2001)

16 G Hadziioannou P F van Hutten(eds) Semiconducting Polymers Wiley-VCH Weinheim (2000)

17 M Pope C E Swenberg Electronic Pro-cesses in Organic Crystals and Polymers2nd ed University Press Oxford (1999)

18 D Fichou (ed) Handbook of Oligo- andPolythiophenes Wiley-VCH Weinheim(1998)

19 A Pron P Rannou Processible Conjugat-ed Polymers From Organic Semiconductorsto Organic Metals and SuperconductorsProg Polym Sci 27 (2002) 135

20 H Kies Conjugated Conducting PolymersSpringer Berlin (1992)

21 H Baumlssler Phys Stat Sol B 175 (1993)15

References 71

22 G von Buumlnau T Wolff PhotochemieGrundlagen Methoden AnwendungenVCH Weinheim (1987)

23 G Lanzani S de Sylvestre G CerulloS Stagira M Nisoli W Graupner GLeising U Scherf K Muumlllen Photo-physics of Methyl-Substituted Poly(para-phenylene)-Type Ladder Polymers in [16]p 235


25 S Nespurek V Herden W Schnabel AEckhardt Czechoslovak J Phys 48(1998) 477

26 J Knoester M Mostovoy Disorder andSolitons in trans-Polyacetylene in [16]p 63

27 R H Friend DDC Bradley P DTownsend J Phys D Appl Phys 20(1987) 1367

28 H Baumlssler Charge Transport in RandomOrganic Semiconductors in [16] p 365

29 M Redecker H Baumlssler HH HoumlrholdJ Phys Chem 101 (1997) 7398

30 M Loumlgdlund W R Salaneck ElectronicStructure of Surfaces and Interfaces in Con-jugated Polymers in [16] p 115

31 L Onsager Phys Rev 54 (1938) 55432 A Eckhardt PhD Thesis Technical

University Berlin (1995)33 V Cimrova I Kminek S Nespurek W

Schnabel Synth Metals 64 (1994) 27134 A Mozumder J Chem Phys 60 (1974)

430035 B J Chen C S Lee S T Lee P Webb

YC Chan W Gambling H Tian WHZhu Jpn Appl Phys 39 (2000) 1190

36 A Eckhardt V Herden S Nespurek WSchnabel Phil Mag B 71 (1995) 239

37 D Hertel U Scherf H Baumlssler AdvMat 10 (1998) 1119

38 M Redecker DD C Bradley M Inbase-karan EP Woo Appl Phys Lett 73(1998) 1565

39 E Lebedev T Dittrich V Petrova-KochS Karg W Bruumltting Appl Phys Lett 71(1997) 2686

40 H Sirringhaus P J Brown R HFriend MM Nielsen K Bechgaard

BMW Langeveld-Voss A I SpieringR A J Janssen E W Meijer D M deLeeuw Nature 401 (1999) 685

41 E Muumlller-Horsche D Haarer H ScherPhys Rev B 35 (1987) 1273

42 M OrsquoNeill S M Kelly Adv Mater 15(2003) 1135


44 FC Grozema LDA Siebbeles JMWarman S Seki S Tagawa U ScherfAdv Mater 14 (2002) 228

45 P Vlachos S M Kelly B Mansoor MOrsquoNeill Chem Commun (2002) 874

46 M Abkowitz H Baumlssler M Stolka PhilMag B 63 (1991) 201

47 W D Gill J Appl Phys 43 (1972) 503348 V I Arkhipov P Heremans EV Eme-

lianova G J Andriaenssens H BaumlsslerAppl Phys Lett 82 (2003) 3245

49 S Nespurek Macromol Symp 104(1996) 285

50 V I Arkhipov J Reynaert Y D Jin PHeremans EV Emelianova G J An-driaenssens H Baumlssler Synth Met 138(2003) 209

51 V I Arkhipov P Heremans EV Eme-lianova G J Andriaenssens H BaumlsslerChem Phys 288 (2003) 51

52 P Borsenberger G Contois A Ateya JAppl Phys 50 (1979) 914

53 G Pfister Phys Rev B 16 (1977) 367654 PG Di Marco G M Gleria S Lora

Thin Solid Films 135 (1986) 15755 A Eckhardt V Herden W Schnabel

Photoconductivity in Polysilylenes Dopingwith Electron Acceptors in N Auner JWeis (eds) Organosilicon Chemistry IIIWiley-VCH Weinheim (1997) p 617

56 B Kraabel CH Lee D McBranch DMoses NS Sariciftci A J HeegerChem Phys Lett 213 (1993) 389

57 S Morita M Shen J Polym Sci PhysEd 15 (1977) 981

58 N Inagaki M Mitsuuchi Polym SciLett Ed 22 (1978) 301


Electro-optic (EO) phenomena are related to the interaction of an electric fieldwith an optical process The classical electro-optic effects the Pockels and theKerr effect discovered in 1893 and 1875 with quartz and carbon disulfide re-spectively refer to the induction of birefringence in certain materials under theinfluence of an external electric field Application of an electric field to the sam-ple causes a change in the refractive index In the case of the Pockels effect nis linearly proportional to E the strength of the applied electric field [see Eq (3-1)] Hence it is also called the linear electro-optic effect In contrast n is pro-portional to E2 in the case of the Kerr effect [see Eq (3-2)]

Linear electro-optical effect Pockels effect n rE 3-1

Quadratic electro-optical effect Kerr effect n qE2 3-2

r (m Vndash1) and q (m Vndash2) are the Pockels and the Kerr constants respectively Eis the electric field strength (V mndash1) and (m) is the wavelength of the light

Pockels cells containing an appropriate crystal such as potassium dihydrogenphosphate and Kerr cells containing an appropriate liquid eg nitrobenzeneare used as light shutters (in conjunction with polarizers) and intensity modula-tors of linearly polarized laser light beams Actually the technical importance ofEO effects is increasing because of various applications in optical communica-tion devices particularly concerning EO modulators that are used in fiber-opticcommunication links In the search for novel EO materials organic compoundsand particularly polymeric systems have also been explored While polymers arecheap and easily processable many of them are inferior to inorganic crystals be-cause of their low thermal stabilities Therefore the application potential ofpolymeric systems is limited Nevertheless a large volume of research has beendevoted to the use of polymers in photonic devices based on EO effects Somehighlights regarding the achievements in this field are reported in this chapter

It should be emphasized that the Kerr effect refers to a quadratic ie a non-linear dependence of the refractive index on the strength of the externally ap-

73

3Electro-optic and nonlinear optical phenomena

plied electric field In this respect the Kerr effect is the first nonlinear opticalphenomenon that has gained both fundamental and practical importance Theinterest in nonlinear phenomena flourished after the construction of the firstruby laser in 1960 by TH Maiman [1] and the observation of second harmonicgeneration (SHG) ie frequency doubling of laser light in 1961 [2] Since thenthe field of nonlinear optics has developed very rapidly as demonstrated by aplethora of articles and books To a large extent these also cover research on or-ganic materials including polymers [3ndash14]

32Fundamentals

321Electric field dependence of polarization and dipole moment

Electric field-induced changes in refractive index can be explained with the aidof the following model under the influence of the electric field the charge dis-tribution in the molecules is perturbed and the molecules are polarized The di-pole moment pi induced by an electric field along the molecular axis can be ex-pressed by an expansion [see Eq (3-3)] [15]

pi 0

j

ijEj

jk

ijkEjEk

jkl

ijklEjEkEl 3-3

Here 0 denotes the permanent dipole moment The coefficients are tensorstermed as linear polarizability ij and first and second molecular hyperpolariz-abilities ijk and ijkl respectively The indices refer to the tensor elements ex-pressed in the frame of the molecule using Cartesian coordinates Ej Ek and El

denote the applied electric field strength components Commonly the responsetime ranges from picoseconds to femtoseconds Therefore if an alternating elec-tric field with a frequency of less than 1012 Hz is applied the direction of thepolarization alternates with the oscillations of the applied field

The polarization induced at the molecular level can cause a polarization inthe bulk of the sample and lead to macroscopically detectable property changesfor instance in the refractive index The macroscopic polarization PI induced bythe electric field can be expressed by the expansion given by Eq (3-4)

PI P0

J

1IJ EJ

JK

2IJKEJEK

JKL

3IJKLEJEKEL 3-4

Here P0 is the permanent polarization and (2) and (3) denote the second- andthird-order nonlinear optical three-dimensional susceptibility tensors The in-dices attached to the tensors refer to the tensor elements and the indices as-sociated with the E values refer to the components of the electric field strengthhere expressed in the laboratory frame

3 Electro-optic and nonlinear optical phenomena74

In the case of weak applied fields the higher terms in Eq (3-4) can be ne-glected and if the sample is not permanently polarized Eq (3-4) reduces toEq (3-5)

Plinear 1E 3-5

If the medium is isotropic (1) is a scalar ie the relationship between E andPlinear is independent of the direction of the field vector E and the polarizationis parallel to E Many polymers possess amorphous structures and their opticalproperties are isotropic However electro-optic polymeric systems containing po-lar moieties can be made anisotropic by orienting these moieties for exampleby electric field-induced or optical alignment In this case the polarization isnot necessarily parallel to the direction of E and its component in one directionis related to the field components in all three directions

PX 11EX 12EY 13EZ

PY 21EX 22EY 23EZ PI

J

IJEJ 3-6

PZ 31EX 32EY 33EZ

Note that the indices X Y and Z expressed in upper-case letters represent thecoordinates of the macroscopic laboratory frame As indicated in Fig 31 lower-case letters are used to denote the coordinates of the molecular frame

The susceptibility of an anisotropic medium is represented by a tensor Ten-sors are composed of 3a+1 elements where a is the number of interacting vec-tors and a+1 denotes the rank With a = 1 (1) is a second-rank tensor with32 = 9 elements which can be expressed by the matrix given in Eq (3-7)

1 11 12 1321 22 2331 32 33

3-7

Polarization can be induced in matter not only by an externally applied electricfield but also by the electric field of a passing light beam This kind of interac-tion does not lead to a loss of intensity of the beam in contrast to absorptionwhich reduces the intensity The overall situation taking into account both

32 Fundamentals 75

Fig 31 The macroscopic laboratory frame (X Y Z) and themolecular frame (x y z) Adapted from Kippelen et al [15] withpermission from Springer

kinds of interaction ie polarization and absorption can be described on the ba-sis of complex and frequency-dependent entities consisting of a real and animaginary part This concerns the dielectric constant the optical susceptibilityand the refractive index For example the complex refractive index n [seeEqs (3-8) and (3-9)] is given by the sum of the real part n and the imaginarypart ik the latter corresponding to light absorption [15]

n n ik 3-8

2kc

3-9

Here (cmndash1) is the linear absorption coefficient (sndash1) is the frequency ofthe optical field and c (cm sndash1) is the speed of light

When a high-intensity laser beam impinges on material its electromagneticfield induces electrical polarization that gives rise to a variety of nonlinear opti-cal properties because in this case the higher terms in Eq (3-4) are not negligi-ble The determination of the coefficients (2) and (3) that serve to characterizethe nonlinear properties is complicated by the fact that they are composed ofmany elements With a being equal to two and three (2) and (3) are composedof 3a+1 = 27 and 81 elements respectively Fortunately these tensors possesssymmetry properties that can be invoked to reduce the number of independentelements for instance when the optical frequencies involved in the nonlinearinteraction are far away from resonance (absorption) [15]

In the case of second harmonic generation for example the second-order sus-ceptibility tensor elements are symmetrical in their last two indices Therefore thenumber of independent tensor elements is reduced from 27 to 18 Moreover thetensor elements

2IJK can be expressed in contracted form

2IJ The index I takes

the value 1 2 or 3 corresponding to the three Cartesian coordinates and the indexL varies from 1 to 6 The values of L refer to the six different combinations of theindices J and K according to the following convention [15]

L 1 2 3 4 5 6

JK 11 22 33 23 or 32 13 or 31 12 or 21

Therefore (2) can be expressed by the matrix given by Eq (3-10)

2 211

212

213

214

215

216

221

222

223

224

225

226

231

232

233

234

235

236

3-10

For poled polymers that belong to the mm symmetry group some of the ten-sor elements vanish and the (2) tensor reduces to Eq (3-11) [15]


2 0 0 0 0

215 0

0 0 0 215 0 0

231

232

233 0 0 0

3-11

When Kleinman symmetry 2ijk

2ikj

2jkl

2jik

2kij

2kji

is valid [16]

215 is equal to

231 Therefore only two independent tensor elements namely

231 and

233 remain Methods that are commonly applied to determine macro-

scopic susceptibilities are based on geometrical arrangements permitting theusage of these simplifications Regarding the relationship between the macro-scopic susceptibilities and the molecular hyperpolarizabilities equations havebeen derived for the practically very important case of rigid polar moieties con-taining polymeric systems that have been or are subject to an alignment process[15] It is beyond the scope of this book to treat this subject in detail A typical re-sult concerning the relation of (2) to is given by Eqs (3-12) and (3-13) [17] Inthis case it was assumed that the macroscopic susceptibility of a given volumeis the sum of all corresponding molecular contributions in this volume and thateach molecular component is mapped onto the corresponding macroscopic vector

2ZZZ NFzzz cos3

3-12

2XXZ

2YYZ

2XZY

2YZY

2ZXX

2ZYY 1

2NFzzz cos sin2

3-13

Here N is the number of hyperpolarizable groups per unit volume (numberdensity) F is a factor correcting for local field effects and is the angle be-tween the permanent dipole 0 of the molecule (z direction) and the directionof the poling field (Z direction) The brackets indicate an averaging over all mo-lecular orientations weighted by an orientational distribution function

The importance of the hyperpolarizability and susceptibility values relates tothe fact that provided these values are sufficiently large a material exposed to ahigh-intensity laser beam exhibits nonlinear optical (NLO) properties Remark-ably the optical properties of the material are altered by the light itselfalthough neither physical nor chemical alterations remain after the light isswitched off The quality of nonlinear optical effects is crucially determined bysymmetry parameters With respect to the electric field dependence of the vectorP given by Eq (3-4) second- and third-order NLO processes may be discrimi-nated depending on whether (2) or (3) determines the process The discrimi-nation between second- and third-order effects stems from the fact that second-order NLO processes are forbidden in centrosymmetric materials a restrictionthat does not hold for third-order NLO processes In the case of centrosym-metric materials (2) is equal to zero and the nonlinear dependence of the vec-tor P is solely determined by (3) Consequently third-order NLO processes canoccur with all materials whereas second-order optical nonlinearity requiresnon-centrosymmetric materials

32 Fundamentals 77

The significances of the susceptibilities (1) (2) and (3) are related to specificphenomena (1) relates to optical refraction and absorption Common effects re-lated to (2) are frequency doubling (second harmonic generation SHG) andthe linear electro-optic effect (Pockels effect) Typical effects connected with (3)

are frequency tripling (third harmonic generation THG) sum and differencefrequency mixing two-photon absorption and degenerate four-wave mixing

322Electric field dependence of the index of refraction

Regarding light frequencies in the non-resonant regime electro-optic (EO) activityrelates to the control of the index of refraction of a material by application of anexternal electric field Either DC or AC (ranging from 1 Hz to more than100 GHz) voltages are applied The index of refraction n corresponds to the speedof light c in the material (n = c0c with c0 being the speed of light in vacuo) There-fore the electro-optic activity relates to a voltage-controlled phase shift of the lightThe change in the refractive index of a non-centrosymmetric material in a modu-lating electric field E can be represented by the expansion given by Eq (3-14) [18]

nIJ 12

n3IJrIJKEK 1

2n3

IJpIJKKE2K 3-14

Provided that higher terms are negligible Eq (3-14) reduces to Eq (3-15) whichrelates to the Pockels effect

nIJ 12

n3IJrIJKEK 3-15

The susceptibility tensor 2IJK is related to the Pockels tensor rIJK [Eq (3-16)] [19]

2IJK 1

2n4

I rIJK 3-16

2IJK is invariant under permutation of the first two indices Therefore a con-

densed notation resulting in only two indices L and K can be used The firstindex L represents the combination IJ and may have the value 1= XX 2 = YY3 = ZZ 4 = YZ 5 = ZX or 6 = XY= YX and the second index K may have the val-ue 1 = X 2= Y or 3 = Z [17]

Technical applications based on the Pockels effect require systems that are non-centrosymmetric on a macroscopic level This relates particularly to polymeric sys-tems containing physically admixed or chemically incorporated components withpermanent dipoles In such cases macroscopic second-order nonlinearity can beaccomplished by poling ie by aligning the permanent dipole moments of thecomponents with the aid of an external electric field that is applied at tempera-tures in the vicinity of the polymerrsquos glass transition temperature Tg The orderthus obtained is frozen-in by cooling to a low temperature TTg The refractive


index of the uniformly poled polymer is uniaxial with a long axis ne in the polingdirection (direction 3) and a short axis n0 perpendicular to the poling direction (di-rections 1 and 2) If a modulating electric field is applied in the poling directionthe two Pockels coefficients r33 and r31 can be discriminated They are described byEqs (3-17) and (3-18) in relation to the susceptibilities

2333 and

2311 and are re-

lated to the hyperpolarizability through Eqs (3-12) and (3-13)

n 12

n3er33Emod

2333Emod

ne NF cos3 Emod

ne3-17

n 12

n30r13Emod

2113Emod

n0

NF12

cos sin2

Emod

n03-18

Here N is the number density of hyperpolarizable groups is the angle be-tween the permanent dipole 0 of the molecule (z direction) and the directionof the poling field (Z direction) and F is a local field factor Commonlycos3 is larger than 05 cos sin2

Therefore the most efficient EO mod-

ulation is achieved if r33 is used rather than r13 [17]

33Characterization techniques

331Second-order phenomena

3311 Determination of the hyperpolarizability

Commonly two methods are employed to determine the hyperpolarizability (1)electric field-induced second harmonic generation EFISH and (2) hyper-Raleighscattering HRS HRS is applicable to both nonpolar and polar molecules as wellas ions but EFISH applies only to polar non-ionic molecules While in the EFISHmethod only the component of parallel to the dipole moment is measured HRSyields several of the tensor components In the case of EFISH one measures I2the intensity of light at frequency 2 emitted from a solution of the sample that issubmitted to an external electric field E0 and simultaneously irradiated with laserlight of frequency Provided that the external electric field is applied along the Z-axis in the laboratory frame and the laser light is polarized along the same axis themacroscopic polarization P(2) induced in the solution by the electric field of theincident laser wave E is given by Eq (3-19)

PZ2 3ZZZZE0EZEZ 3-19

Here 3ZZZZ is the macroscopic third-order susceptibility which is related to thefirst and second molecular hyperpolarizabilities and by Eq (3-20) [20]

33 Characterization techniques 79

3ZZZZ NF2F2

F0 z

5kT

3-20

N is the number density of chromophoric groups F2 F and F0 are local fieldfactors at frequencies 2 and zero is the ground-state dipole momentand z is the vectorial component of along the ground-state dipole momenttaken to be oriented along the z-axis in the molecular frameworkz zxx zyy zzz In the case of -conjugated chromophores is negligi-bly small in comparison with z5kT Therefore according to Eq (3-20) theproduct z is directly available from

3ZZZZ obtained by measuring the intensity

I2 of the second harmonic generated by sample solutions I2 is proportionalto

3ZZZZ [see Eq (3-21)]

I2 3ZZZZI2

E20 3-21

Commonly the evaluation of the susceptibility 3ZZZZ is related to a reference

standard A detailed description of both experimental techniques and data evalu-ation is given in the article by Singer et al [20]

In contrast to the EFISH method the hyperpolarizability can be measureddirectly by means of the HRS method developed by Clays and Persoons [21 22]This method involves measuring the intensity of the incoherently scattered fre-quency-doubled light from isotropic solutions As shown in Fig 32 an infraredlaser beam is focused on the center of a cell containing a solution of the NLO-active compound


Fig 32 Schematic depiction of a set-up for measuringsecond-order hyperpolarizability by means of the hyper-Rayleigh scattering method

The intensity of the scattered light I2 is proportional to the square of theintensity of the incident light I as given by Eq (3-22)

I2 g N1 2IJKsolvent

N2 2

IJKsolute

I2 3-22

Here g is a set-up dependent factor N1 and N2 are the number densities of solventand solute molecules respectively and 2

IJK

is the mean value of the square of

hyperpolarizability tensor components in the laboratory framework [23] It mustbe noted that the HRS process is extremely inefficient Typically the number ofscattered photons is 10ndash14 times the number of incident photons [20] In principlea low output intensity would be expected for an isotropic solution where the fieldsemitted from the individual NLO molecules interfere destructively That a measur-able amount of incoherently scattered harmonic light can be generated may be ra-tionalized by assuming that fluctuations in orientation can produce regions ofalignment [22] The rather low intensity of the scattered light requires the applica-tion of powerful lasers such as an Nd-YAG system producing 1064 nm lightpulses in conjunction with a sampling technique involving more than 100 pulses

3312 Determination of the susceptibility (2)

Several techniques have been developed for determining the second-order suscep-tibility (2) [24] Of practical importance are methods that may be employed foraligned polymeric systems containing polar moieties [4 8] Methods makinguse of the Pockels or linear electro-optic (EO) effect are based on the measurementof the variation in the refractive index of thin polymer films induced by an externalelectric field In this way values of the electro-optic coefficients r33 and r13 are ob-tained which are related to the corresponding (2) values through Eq (316)

A quite direct method for measuring (2) is based on second harmonic gen-eration SHG Figure 33 depicts a typical set-up used to determine the SHGcoefficients d31 and d33 defined as d =(2)2 by way of SHG measurements

A polarized laser beam of frequency passes through the polymer sampleand an IR-blocking filter The SHG signal is selected by means of an interfer-ence filter operating at the frequency 2 and is measured using a photomulti-plier tube connected to a boxcar integrator The intensity I2 is proportional tothe square of the SHG coefficient d and to the square of the intensity of thefundamental laser beam [see Eqs (3-23) and (3-24)] [8]

I2 Kd2I2 3-23

K 512t4T2t2

0p2 sin2 An2

n22

3-24

Here A is the area of the laser beam is the incident angle t0 t and T2

are transmission factors p is a projection factor () is an angular factor re-


lated to the sample thickness the fundamental wavelength and the refractionangles and n and n2 are the refractive indices of the sample at and 2The coefficient d of the polymer is obtained by comparing the I2 value withthat measured for a standard reference sample commonly Y-cut quartz withd11 = 049 pm Vndash1 at = 1064 m

332Third-order phenomena

Several measuring techniques giving evidence of third-order nonlinear behaviorare listed in Table 31 [26 27]

It is difficult to compare the third-order susceptibilities of systems examinedusing different measuring techniques Since they are based on fundamentallydifferent origins they do not yield identical (3) values Different nonlinearmechanisms contribute in a specific manner to (3) and values measured forthe same material by different techniques may differ by several orders of mag-nitude This applies for instance to the case of the combined resonant andnon-resonant interaction of light with matter A full expression of (3) reflectsnon-resonant and resonant contributions [see Eq (3-25)]

3 3nonresonant

3resonant 3-25

Resonance occurs at wavelengths around that of the absorption band Moreoverthe strong frequency (wavelength) dependence of (3) and the influence of repe-tition frequency and pulse duration of the laser on (3) have to be taken into ac-count It is beyond the scope of this book to describe the various measuring


Fig 33 Schematic depiction of a set-up for measuring secondharmonic generation (SHG) BS beam splitter PDphotodiode PMT photomultiplier tube Adapted fromJerphagnon et al [25] with permission from the AmericanInstitute of Physics

techniques However some of the most widely used methods are briefly consid-ered below with the additional aim of providing some insight into the fascinat-ing field of third-order nonlinear effects

3321 Third harmonic generationThe term third harmonic generation THG refers to the generation of a lightbeam that consists of photons having three times the energy of the photons ofthe input beam THG can be easily detected and is therefore widely employedin the third-order nonlinear characterization of newly developed materials [28]THG is a four-photon process in which three incident photons with angularfrequency create a photon with frequency 3 The off-resonant THG processcan be represented by a transition between virtual excited states as shown bythe dashed lines in Fig 34

In the case of THG the third-order susceptibility corresponds to a nonlinearpolarization component which oscillates at the third harmonic frequency of theincident laser beam Regarding the simplified case of an isotropic solution onlythe element

3XXXX3 of the third-order susceptibility tensor creates

a polarization at 3 which is parallel to the incident electrical field E as-sumed to be parallel to the X-axis [see Eq (3-26)]

P3 143XXXX3E3

3-26

For THG measurements pulsed laser systems operating at infrared wavelengths(typically 1064 1850 1907 or 2100 nm) are used Most commonly 3XXXX is ob-tained by relating the third-harmonic signal of the sample to that measured si-


Table 31 Measuring techniques for third-order susceptibilities

Method Acronym Denotation of process

Third harmonic generation THG (3) (3)Z-scan (3) (ndash ndash)Two-photon absorption TPA (3) (ndash ndash)Degenerate four-wave mixing DFWM (3) (ndash ndash)Electric field-induced second harmonic generation EFISH (3) (ndash2 0)Optical Kerr gate OKG (3) (ndashndash)

Fig 34 Energy level diagram illustrating third harmonic generationArrows denote photon energies horizontal solid lines represent energystates of the medium and dashed lines represent virtual excited states

multaneously with a fused silica plate serving as a reference The incident beamis focused on the sample in a vacuum chamber and a water filter removes thefundamental frequency from the output beam which is further attenuated sothat it lies within the linear range of the photomultiplier

3322 Self-focusingdefocusingThin polymer sheets allowing unhindered passage of a low-intensity light beamof a given non-resonant wavelength can act as lenses if a high-intensity beam ispassed through them This is a consequence of the intensity dependence of therefractive index n [see Eq (3-27)]

n n0 n2I 3-27

Here n0 denotes the linear refractive index (at low intensity I) and n2 is thenonlinear refractive index which can be measured by means of a Z-scan experi-ment [29 30] A typical set-up is schematically depicted in Fig 35 a

The incoming beam is split into two equal parts one part is guided to the de-tector D1 while the other is passed through the sample and an aperture priorto reaching the detector D2 Provided that the sample is nonlinearly active thephenomena outlined below will be observed if the sample is moved through thefocused laser beam along the optical axis Thus the transmission through theaperture is reduced if the sample is moved to the left of the original focus z0 be-cause the beam is defocused On the other hand if the sample is placed to the


Fig 35 (a) Schematic depiction of the Z-scan experimentBS beam splitter (b) Typical Z-scan curves for n2 gt 0 andn2 lt 0 Adapted from Gubler et al [30] with permission fromSpringer

right of z0 the beam is focused on the aperture and the transmission through it isincreased This applies in the case of n2 gt 0 The opposite behavior is observed ifn2 lt 0 Both cases are shown schematically in Fig 35b in which the signal ratioD2D1 is plotted against the distance z The nonlinear refractive index n2 can beobtained from the z-scan in the following way Tpv the difference in the transmit-tance between peak and valley is proportional to the phase distortion 130 accord-ing to the empirical relationship Tpv = k 130 where k is a constant determined bythe lay-out of the apparatus With 130 = (2)n2I0L one obtains Eq (3-28) [29]

n2 Tpv

2kI0L3-28

Here I0 and L denote the light intensity and the thickness of the sample re-spectively The third-order susceptibility (3) can then be obtained by usingEq (3-29) [26]

n2 122

cn03 3-29

This applies when esu units are used for both n2 and (3) It is interesting tonote that the set-up shown in Fig 35 a can also be used to determine the two-photon absorption coefficient 2 In this case the Z-scan experiment is per-formed without the aperture

3323 Two-photon absorption (TPA)The simultaneous absorption of two photons of equal energy can occur if a la-ser beam (ps or fs pulses) is focused within a material [31 32] The process de-picted schematically in Fig 36 is related to the excitation of a molecule to anenergy level h1 = 2 h2 by the simultaneous absorption of two photons of en-ergy h2 (=2)

Two-photon absorption is possible provided that both photons are spatiallyand temporally coincident It occurs with a probability proportional to thesquare of the light intensity

TPA can be measured by the transmission method or by the Z-scan techniqueMoreover two-photon fluorescence can serve to measure TPA absorption cross-sections provided that a fluorescent excited state is reached by TPA In nonlinear


Fig 36 Energy level diagram depictingsingle-photon and two-photonabsorptions

transmission experiments the transmission of the sample Tr is measured as afunction of the input intensity I0 At high incident intensities TPA is proportionalto I2

0 and there is a linear relationship between 1Tr and I0 [see Eq (3-30)]

1Tr

I0

I 1 2I0L 3-30

Here L is the sample thickness and 2 is the absorption coefficient for the puretwo-photon absorption process

3324 Degenerate four-wave mixing (DFWM) and optical phase conjugationDegenerate four-wave mixing (DFWM) is frequently employed to measure (3)

values and response times of polymeric systems The DFWM technique is basedon the interaction between three spatially distinguishable light beams of equalfrequency The interaction results in the generation of a fourth beam of thesame frequency Figure 37 shows the commonly used backward-wave geome-try with three incident beams spatially overlapping in the sample

The pump beams 1 and 3 are counterpropagating The signal beam 4 isemitted in the direction opposite to the probe beam 2 Its intensity depends on(3) and on the intensities of beams 1 2 and 3 according to Eq (3-31) [27]

I4 2

4c2n2 32L2I1I3I2 3-31

Here c n and L denote the velocity of light in vacuo the refractive index of thesample and the pathlength in the medium respectively Equation (3-31) holdsin the case of there being no linear or nonlinear light absorption The retrace-ment of the probe beam is characteristic of the phenomenon of optical phaseconjugation OPC [33] This refers to the property of materials to act as mirrorsand to reflect an incident light beam exactly in phase with its former phase Un-like a conventional mirror whereby rays are redirected according to the ordinarylaw of reflection a phase conjugate mirror also called a phase conjugator retro-reflects all incoming rays back to their origin Figure 38 illustrates the differ-ence between a conventional and a phase conjugate mirror

At a conventional mirror only the wave vector component normal to the surfacechanges sign while the tangential components remain unchanged The propaga-tion direction of the reflected ray depends on the angle between the surface normal


Fig 37 Degenerating four-wave mixing withcounterpropagating pump beams 1 and 3BS beam stopper

and the incident ray A phase conjugate mirror on the other hand changes the signof the complex wave vector so that the reflected ray is antiparallel to the incidentray Phase conjugation by degenerate four-wave mixing may result in reflectivitiesR = I4I2 exceeding 100 For example using picosecond pulses R = 25 has beenfound for poly(methyl methacrylate) doped with 510ndash4 mol Lndash1 rhodamine 6G[34] For detailed information concerning the DFWM technique and additionaltechniques not dealt with here the reader is referred to the literature [26 27]

34Nonlinear optical materials

341General aspects

Second-order NLO materials Originally second-order nonlinear optics was devel-oped with the aid of inorganic crystals such as lithium niobate LiNbO3 and po-tassium dihydrogen phosphate KH2PO4 (KDP) The nonlinear optical behaviorof these crystals is due to light-induced displacement of the ions in the latticeCertain organic substances having a non-centrosymmetric structure and con-taining delocalized -electrons behave similarly They undergo very fast light-in-duced intramolecular perturbations of their charge distributions In otherwords irradiation with light at non-resonant wavelengths causes an almost in-stantaneous shift in the -electron density over the molecule which accountsfor the large and fast polarization 2-Methyl-4-nitroaniline MNA and 4-di-methylamino-4-nitrostilbene DANS are typical organic compounds exhibitingsecond-order NLO activity (see Chart 31)

These compounds are so-called charge-transfer molecules having the generalstructure shown in Chart 32

Here an electron-donating and an electron-accepting moiety are connected byan extended -electron system In such compounds the electron displacementoccurs on a subpicosecond time scale and can be much more pronounced thanin inorganic crystals Polymeric organic systems are of practical importance

34 Nonlinear optical materials 87

Fig 38 The reflection of a ray of light off an ordinary mirrorand off a phase conjugate mirror

They consist either of polymers containing admixed AD compounds or of poly-mers with AD moieties chemically incorporated into the main chain or in pen-dant groups As pointed out above for an organic material to undergo a signifi-cant change in its dipole moment upon exposure to an intense light beam itneeds to have a non-centrosymmetric molecular structure This requirementalso pertains to the macroscopic level In other words both a large hyperpolariz-ability of the molecular constituents and a large macroscopic susceptibility (2)

are required Macroscopic non-centrosymmetry can be attained by aligning theassemblies so that the individual tensor components of add constructively

Third-order NLO materials Unlike for second-order NLO activities there areno molecular symmetry restrictions for the third-order nonlinear response ofmaterials In principle all materials including air are capable of exhibitingthird-order NLO activity Generally for most centrosymmetric compounds thehyperpolarizability is very small This does not apply however for organic -conjugated compounds It is the almost instantaneous shift in -electron densityover the whole molecule or extended parts of it that occurs upon irradiationwhich accounts for the large susceptibilities (3) of conjugated compounds Asregards the field of macromolecules -conjugated polymers such as polyacetyl-enes or polydiacetylenes (see Chart 33) exhibit pronounced third-order NLO ac-tivities (3) values of non-conjugated polymers such as poly(methyl methacry-late) are several orders of magnitude lower than those of conjugated polymers


Chart 31 Chemical structures of 2-methyl-4-nitroanilineMNA and 4-dimethylamino-4-nitrostilbene DANS

ACCEPTOR mdashndash[-conjugated system]mdashndash DONORChart 32 General structure of charge-transfer molecules (AD molecules)

Chart 33 Polymers exhibiting third-order NLO activitiesR R1 and R2 denote aliphatic or aromatic groups

Interestingly -conjugated polymers such as polysilanes (see Chart 33) also ex-hibit remarkably large third-order susceptibilities (3)

342Second-order NLO materials

3421 Guest-host systems and NLO polymersFundamentally there are two categories of second-order NLO polymeric sys-tems commonly also referred to as electro-optically active polymeric systems [435] (1) guest-host systems consisting of rigid solutions of small AD com-pounds in polymeric matrices and (2) systems consisting of polymers withAD moieties incorporated into either the main chain or side groups [36] Inthe latter case the rigidity of the polymeric matrix can be improved by chemicalcrosslinking General structures of such polymers are depicted in Fig 39

In this context research concerning non-centrosymmetric structures with su-pramolecular helical organization is interesting In the case of thermally stable(up to 400 C) polyesters containing -conjugated donor-acceptor segments (seeChart 34) the hyperpolarizability values turned out to be much larger thanthose of the respective monomeric chromophores

At a chiral unit content of 50 the second harmonic generation (SHG) effi-ciency of the polymer (at = 532 nm) is 48 times that of the monomer and isequal to 20710ndash30 esu This enhancement may be rationalized in terms of thedirectional orientation of dipole segments in the polymer as a consequence ofthe chiral organization of the polymer chains [37]

Typical low molar mass AD compounds and polymers containing AD moi-eties are listed in Table 32 [38] and Table 33 [39 40] In this context it is no-ticeable that electro-optically active compounds have been tabulated [7]


Fig 39 Schematic depiction of the structures of polymeric matrices containing AD moieties


Chart 34 Chemical structures of an electro-optically activepolyester and a chemically related monomer

Table 32 Characteristics of electro-optically activechromophores determined in chloroform solutionAdapted from Swalen and Moylan [38]

Denotation Structure maxa)

(nm) b)

(Debye)0

c)

(10ndash30 esu)

I 438 67 813

II 494 80 952

III 602 71 259

IV 698 104 359

V 680 83 479

a) Wavelength of maximumb) Dipole momentc) Off-resonance hyperpolarizability

At present various compounds are commercially available [41] Typical exam-ples are given in Table 34

Second-order NLO polymers have potential for technical applications (see Sec-tion 35 below) for example in electro-optic modulation and switching or fre-quency doubling A large body of compounds has hitherto been explored andat present relevant research is mainly focused on optimizing secondary proper-ties such as thermal stability adhesion thermal expansion etc


Table 33 Characteristics of electro-optically active poled polymer filmsAdapted from Bertram et al [39] and Lipscomb et al [40]

Chemical structure Acronym faca) TPol

b)

(C) c)

(m)r33

d)

(pm Vndash1)

Ber-1 100 155 42

3RDCYXY 15 mol 140 13 30

GT-P3 62 wt 180 1541 12

ROI-4 17 mol 215 13 16

a) Fraction of active compoundb) Poling temperaturec) Wavelengthd) Component of the Pockels coefficient tensor directed parallel to the applied elec-

tric field

3422 Orientation techniquesPractical applications demand optimum alignment of the AD moieties in thesample in a non-centrosymmetric fashion To this end the most commonapproach involves electric field-induced alignment of glassy ie amorphouspolymer films a process commonly referred to as poling Thereby a net orienta-tion of the molecular dipole moments along a polar axis of the macroscopicsample is attained Poling is carried out at a temperature close to the glass tran-sition temperature of the polymer matrix at which the molecules are relativelymobile Electric field-induced alignment can be achieved either by sandwichingthe polymer samples between electrodes which is referred to as electrode pol-ing or by corona poling Figure 310 shows a schematic diagram of a coronapoling set-up with wire-to-plane configuration

A corona discharge is induced upon application of an electric potential of sev-eral kV across the electrodes Ionized molecules from the air are forced by theelectric field to move to the surface of the sample The deposited ions induceimage charges on the earthed electrode Thereby a static electric field of about


Table 34 Commercially available NLO polymers [41]

Denotation Chemical Structure

Poly[4-(22-dicyanovinyl)-N-bis(hydroxyethyl)aniline-alt-(isophoronediisocyanate)]urethane

Poly4-(22-dicyanovinyl)-N-bis(hydroxyethyl)aniline-alt-[44-methylenebis(phenyl)isocyanate]urethane

Poly[4-(22-dicyanovinyl)-N-bis(hydroxyethyl)aniline-alt-p-phenylenediacrylate]

Poly[1-methoxy-4-(0-disperse red 1)-25-bis(2-methoxyethyl)-benzene]

Poly[1-methoxy-4-(0-disperse red 1)-25-phenylenevinylene]

106 V cmndash1 is generated across the sample which induces alignment of theNLO moieties with respect to the direction of the electric field Poled samplesare represented by Cv symmetry Alternative alignment methods are based onthe Langmuir-Blodgett (LB) and self-assembly techniques both of which are dif-ficult to perform

In the case of polymer systems containing photochromic chromophores egazo groups alignment can be achieved upon exposure to light instead of a staticelectric field This method is referred to as optical poling (see also Section 55)With such systems optimum results have been obtained by applying a com-bined electro-optical poling method As can be seen in Table 33 Pockels coeffi-cients exceeding 10 pm Vndash1 have been measured for appropriate polymers poledby the combined electro-optical method More detailed information concerningthe various alignment techniques can be obtained from review articles [4 8 4344]

343Third-order NLO materials

Table 35 presents a selection of (3) values of various conjugated polymers de-termined by THG measurements while Table 36 shows (3) values of somefull-ladder and semi-ladder polymers determined by means of the DFWM tech-nique

It must be noted that the (3) values reported in the literature vary over broadranges Therefore the values listed here reflect only the general behavior of sev-eral classes of compounds It can be seen in Table 35 that trans-polyacetylenes(PAs) and polydiacetylenes (PDAs) exhibit the largest third-order NLO suscept-ibilities The (3) value of cis-PA (not shown) is more than an order of magni-tude smaller than that of trans-PA Derivatives of poly-p-phenylene poly(phenyl-ene vinylene) and polythiophene also exhibit NLO activity but to a much lesserextent than PAs and PDAs As pointed out above polysilanes also possess quitelarge (3) values This is explained by the -conjugation of the silicon chainwhich implies a pronounced delocalization of -electrons A very large (3) value


Fig 310 Schematic diagramshowing a corona poling set-upwith wire-to-plane configurationThe tungsten wire is placedabove and parallel to the sampleAdapted from Eich et al [42] withpermission from the OpticalSociety of America


Table 35 Third-order susceptibilities (3) obtained by third harmonic generationmeasurements Adapted from Kajzar [28] and Nalwa [45]

Polymer Acronym c) (3)(esu) a) (nm) Remarks

trans-PA 5610ndash9 1907 Isotropic film

trans-PA 2710ndash8 1907 Oriented film

PDA-C4UC4 2910ndash10 1907 Oriented film

PDA-CH 1010ndash10 1907

PPV 1410ndash10 1450 Isotropic film

PBT 2910ndash11 1907 Spun film

PTV 3210ndash11 1850

PTT 210ndash11 1907 Isotropic film

PDES 3010ndash9 b) 620H


Table 35 (continued)


PDHS 110ndash11 1064

PVT 310ndash14 1907

a) Fundamental wavelengthb) Determined by the DFWM methodc) Abbreviations trans-PA trans-polyacetylene PDA-C4UC4 poly[57-dodecadiyne-

112-diol-bis(n-butoxycarbonyl methylurethane)] PDA-CH poly[16-di-(N-carba-zoyl)-24-hexadiyne] PPV poly(p-phenylene vinylene) PBT poly(3-butylthio-phene) PTV poly(25-thienylene vinylene) PTT poly(thieno-32-bithiophene)PDES poly(diethynylsilane) PDHS poly(di-n-hexylsilane) PVT poly(vinyl-toluene)

Table 36 Third-order susceptibilities (3) obtained by theDFWM method Adapted from Wijekoon et al [46]

Polymer Acronym (3) (esu) a) (nm)

PBT 1010ndash10 602

PBO 1010ndash10 602

LARC-TPI 2010ndash12 602

BBL 1510ndash11 1064

BBB 5510ndash12 1064

a) Fundamental wavelength

(310ndash9 esu) has been found for poly(diethynylsilane) PDES In this case a re-sponse time of 135 fs was measured [47] Compared with those of conjugatedpolymers the (3) values of non-conjugated polymers are very low For example(3) values of 40 and 3410ndash14 esu have been measured for poly(methyl meth-acrylate) and poly(vinyltoluene) respectively As regards the polymers listed inTable 36 it is notable that some of them for instance BBL and BBB are solu-ble and film-forming in spite of their quasi-two-dimensional structures Forpractical applications materials with large (3) values low optical losses andultrafast response times tresp are desired Ideal targets set for device applicationsare (3) 10ndash7 esu 102 cmndash1 and tresp1 ps Therefore appropriate materialsshould possess a figure of merit (3) of 10ndash9 esu cm Although most polymer-ic materials exhibit much lower (3) values various promising devices havebeen proposed and fabricated [45] For detailed information concerning third-or-der NLO properties of polymers and other compounds the reader is referred tothe literature [28 45 46]

35Applications of NLO polymers

The application potential of the effects dealt with in this chapter covers a broadfield extending from specific electro-optical devices to the all-optical computerFor many applications polymeric materials have proven appropriate and equiva-lent to inorganic materials This section is focused on two aspects the electro-optical (EO) or Pockels effect and two-photon absorption which have beenexploited extensively Technical developments relating to polymeric modulatorsoperating on the basis of the Pockels effect have reached the stage of commer-cialization [5]

351Applications relating to telecommunications

With the advent of optical fibers in telecommunications in the late 1970s practi-cal applications for nonlinear optical devices operating on the basis of the EOeffect became a serious goal Besides inorganic materials which were used ex-clusively in the early days more recently polymeric electro-optic materials havealso found use in a variety of device configurations They can function as tun-able Bragg wavelength filters ultra-high bandwidth signal modulators for tele-communications fast modulators for optical 3D sensing electrical-to-optical sig-nal transducers switches at nodes in optical networks and controllers of thephase of radiofrequency optical signals etc [5] Typical configurations theMach-Zehnder (MZ) interferometer and the birefringent modulator are depictedschematically in Fig 311

In the case of the MZ interferometer (Fig 311 a) application of an electricfield to one arm results in a phase retardation relative to the signal traversing


the second arm and in destructive interference at the output The phase retarda-tion of light traversing the material of optical path length L under the in-fluence of an electric field E is proportional to n the change in the index of re-fraction [see Eq (3-32)]

2nL

n3ErL

3-32

As a consequence of the voltage-controlled destructive interference the appliedelectrical signal is transduced onto the optical beam as an amplitude modula-tion The birefringent modulator depicted in Fig 311 b functions as an electri-cal-to-optical signal transducer Here both TM and TE optical modes traverse

35 Applications of NLO polymers 97

Fig 311 Electro-optic device configurations (a) Mach-Zehnder interferometer (b) birefringent modulator TM andTE denote transverse magnetic and transverse electricpolarization respectively

the EO material The application of an electric field produces a voltage-depen-dent birefringence which is turned into amplitude modulation with the aid of apolarizer positioned at the output of the device

The drive voltage VD required to achieve full-wave modulation is inverselyproportional to the EO coefficient of the material Since drive voltages of the or-der of 1 V or less are required for lossless communication links materials withlarge EO coefficients are desirable VD depends on the device configuration Forexample VD for the birefringent modulator exceeds that for the MZ-type modu-lator by a factor of 15 [5] It should be noted that the change in the refractiveindex (n = 05 n3rE) is rather small For example if n3 = 5 r = 510ndash12 m Vndash1and E = 106 V mndash1 n is equal to 12510ndash5

Very successful efforts in employing polymeric materials as modulators havebeen made with the guesthost systems shown in Table 37 The guest com-pounds are characterized by the cyanofuran moiety A thermally rather stablehost matrix denoted as APC is a copolymer poly[bisphenol A carbonate-co-44-(335-trimethylcyclohexylidene)diphenol] The systems shown in Table 37 areemployed in commercially available modulators the relevant industrial compa-nies are cited in Daltonrsquos review article [5] These polymeric systems are


Table 37 Characteristics of electro-optically activechromophores in a PMMA matrix Adapted from Dalton [5]

Denotation Chemical Structure a) (Debye) r b) (pm Vndash1)

FTC 1219 50

CLD 1342 70

GLD 1388 105

a) Dipole moment obtained by quantum mechanicalcalculation

b) Pockels coefficient at a number density of about151020 molecules cmndash3 measured at =13 m

superior to lithium niobate with respect to various important properties as canbe seen in Table 38

Pockels coefficients measured at the technologically important wavelengths13 and 155 m are higher than in the case of lithium niobate Moreover thedifference in the dielectric constants is important = 28 (LiNbO3) and = 25ndash4(EO polymer) The lower value corresponds to a decreased device power con-sumption and an enhanced speed of operation

352Applications relating to optical data storage

Potential applications of polymeric materials with large (3) values concernphotonic devices in various fields such as optical fiber communication opticalcomputing imaging dynamic holography optical switching and optical datastorage Two-photon absorption a third-order nonlinear effect (see Section3323) has gained importance for optical data storage [48] Two-photon absorp-tion is possible provided that both photons are spatially and temporally coinci-dent As this requirement has to be fulfilled optical sectioning can be accom-plished ie absorption events can be directed to selected layers In other wordsinformation can be recorded in previously defined layers of a film and therebythree-dimensional bit optical data storage within the volume of a recording me-dium is possible Photochemical free radical polymerization (see Section 102)can be employed to achieve optical data storage at a density as high as04 Tb cmndash3 with a bit spacing of 1 m and a layer spacing of 3 m [49 50] Forthis technique a recording medium consisting of a monomer solution contain-ing a photoinitiator is typically used Since the initiation is restricted to two-photon absorption the polymerization is confined to the region of the focusspot To prevent distortion of the recorded planes through shrinkage or flow ge-lation of the system by UV pre-irradiation is carried out Polymerization at therecorded bit changes the refractive index The pattern of recorded bits can thus


Table 38 Comparison of lithium niobate and polymeric EOmaterials Adapted from Dalton [5]

Property LiNbO3 EO Polymer

Pockels coefficient r (pm Vndash1) at = 13 m 31 gt 70Dielectric constant 28 25ndash4Refractive index n 22 16ndash17Figure of merit (n3r) 12 gt 100Optical loss (dB cmndash1) at = 13 m 02 02ndash11Maximum optical power (mW) 250 250Bandwidth length product a) f L (GHz cm) 10 gt 100

a) f Bandwidth in a device of Mach-Zehnder configurationL Interaction length of light with the modulating electricfield

be read by producing a phaseintensity map by means of differential interfer-ence contrast microscopy [51]

353Additional applications

Additional potential applications based on other nonlinear phenomena such assecond harmonic generation (frequency doubling of laser light) phase conjuga-tion and optical bistability may be envisaged Phase conjugation (see Sec-tion 3324) allows the distortionless transmission of images because upon re-tracement the beam reflected from a phase conjugator corrects every distortionof the probe beam Optical bistability is the basis for the transphasor the opticaltransistor a device switching light with light without the aid of an electrical cur-rent This can be achieved by focusing two laser beams a strong constant beamand a weak variable probe beam onto the front face of a Fabry-Perot interferom-eter containing a substance having a nonlinear refractive index Since the latterdepends on the light intensity constructive interference sets in at a certain in-tensity of the probe beam and the transmittance increases to a high level asshown in Fig 312 The term bistability refers to the existence of two quasi-stable levels

Another potential application relates to optical limiters ie materials that canbe used for the protection of eyes and sensors from intense light pulses andgenerally for devices that are required to have a high transmittance at low in-tensities and a low transmittance at high intensities [52 53] Appropriate sub-stances contain chromophores that exhibit nonlinear light absorption termedreverse saturable absorption Such chromophores become more strongly absorb-ing as the incident light intensity is increased The nonlinear response may beexhibited when chromophores absorb weakly in the ground state and stronglyin the excited state Optical limiting may also be due to two-photon (or moregenerally multi-photon) absorption (see Section 3323)


Fig 312 The transmittance behaviorof a transphasor (optical transistor)Plot of the transmitted intensity as afunction of the incident intensity

References 101

References


2 PA Franken LE Hill CW Peters GWeinreich Phys Rev Lett 7 (1961) 118

3 SK Yesodha CKS Pillai N TsutsumiStable Polymeric Materials for NonlinearOptics A Review Based on AzobenzeneSystems Prog Polym Sci 29 (2004) 45

4 F Kajzar K-S Lee AK-Y Jen Polymer-ic Materials and their Orientation Tech-niques for Second-Order Nonlinear OpticsAdv Polym Sci 161 (2003) 1

5 L Dalton Nonlinear Optical PolymericMaterials From Chromophore Design toCommercial Applications Adv Polym Sci158 (2002) 1

6 Z Sekkat W Knoll (eds) PhotoreactiveOrganic Thin Films Academic PressAmsterdam (2002)

7 MG Kuzyk CW Dirk (eds) Character-ization Techniques and Tabulations for Or-ganic Nonlinear Optical Materials MarcelDekker New York (1998)

8 J I Chen S Marturunkakul L Li S TTripathy Second-Order Nonlinear OpticalMaterials in TA Skotheim R L Elsen-baumer J R Reynolds (eds) Handbookof Conducting Polymers 2nd Edition Mar-cel Dekker New York (1998) p 727

9 S Bauer-Gogonea R Gerhard-Multhaupt Nonlinear Optical Electrets inR Gerhard-Multhaupt (ed) Electrets 3rd

Edition Vol 2 Laplacian Press MorganHill CA (1999) p 260

10 HS Nalwa S Miyata (eds) NonlinearOptics of Organic Molecules and PolymersCRC Press Boca Raton FL USA (1997)

11 DM Burland R D Miller C A WalshSecond-Order Nonlinearity in Poled Poly-mer Systems Chem Rev 94 (1994) 31

12 NP Prasad D J Williams Introductionto Nonlinear Optical Effects in Moleculesand Polymers Wiley New York (1991)

13 BS Wherrett in C Flytzanis J L Ou-dar (eds) Nonlinear Optics Materials andDevices Springer Berlin (1986)

14 M Canva G I Stegeman QuadraticParametric Interactions in Organic Wave-guides Adv Polym Sci 158 (2002) 87

15 B Kippelen N Peyghambarian Photore-fractive Polymers and their Applications

Springer Berlin Adv Polym Sci 161(2003) 87

16 DA Kleinmann Phys Rev 126 (1962)1977

17 G R Moumlhlmann C P J M van derVorst R A Huijts CT J WreesmannProc SPIE 971 (1988) 252

18 E Cavicchi J Kumar S Tripathy Non-linear Optical Spectroscopy of Polymers inH Baumlssler (ed) Optical Techniques toCharacterize Polymer Systems ElsevierAmsterdam (1989) p 325

19 CP J M van der Vorst D J PickenElectric Field Poling of Nonlinear OpticalSide Chain Polymers in VP Shibaev(ed) Polymers as Electrooptical and Photo-optical Active Media Springer Berlin(1996)

20 K D Singer SF Hubbard A SchoberLM Hayden K Johnson Second Har-monic Generation in [7] p 311

21 K Clays A Persoons Phys Rev Lett 66(1991) 2980 Rev Sci Instrum 63 (1992)3285

22 K Clays A Persoons L De Mayer Mod-ern Linear Optics Part 3 Adv ChemPhys Wiley New York (1993)

23 J A Delaire E Ishov K NakataniPhotoassisted Poling and Photoswitching ofNLO Properties of Spiropyrans and otherPhotochromic Molecules in Polymers andCrystals in Z Sekkat W Knoll (eds)Photoreactive Organic Thin Films Aca-demic Press Amsterdam (2002)

24 T Watanabe HS Nalwa S MiyataMeasurement Techniques for Refractive In-dex and Second-Order Optical Nonlineari-ties Chapter 3 in [10]

25 J Jerphagnon SK Kurtz J Appl Phys41 (1970) 1667

26 HS Nalwa Measurement Techniques forThird-Order Optical Nonlinearities Chap-ter 10 in [10]

27 J L Bredas C Adant P Tackx A Per-soons Third-Order Optical Response inOrganic Materials Theoretical and Experi-mental Aspects Chem Rev 94 (1994)243

28 F Kajzar Third Harmonic Generation in[7]


29 EW Van Stryland M Sheik-Bahae Z-Scan Chapter 8 in [7]

30 U Gubler C Bosshard Molecular Designfor Third-Order Optics Adv Polym Sci158 (2002) 125

31 T-C Lin S-J Chung K-S Kim XWang G S He J Swiatkiewicz HEPudavar P N Prasal Organics and Poly-mers with High Two-Photon Activities andtheir Applications Springer Berlin AdvPolym Sci 161 (2003) 157

32 S Kershaw Two-Photon AbsorptionChapter 7 in [7]

33 M Gower D Proch (eds) Optical PhaseConjugation Springer Berlin (1994)

34 K Abe M Amano T Omatsu OpticsExpress 12 (2004) 1243

35 HS Nalwa T Watanabe S Miyata Or-ganic Materials for Second-Order NonlinearOptics Chapter 4 in [10]

36 N Pereda J Extebarria CL Focia JOrtega C Artal MR Ros J C SeranoJ Appl Phys 87 (2000) 217

37 B Philip K Sreekumar J Polym SciPart A Polym Chem 40 (2002) 2868

38 J D Swalen CR Moylan Linear OpticalProperties Chapter 4 in [7]

39 R P Bertram E Soergel H Blank NBenter K Buse R Hagen SG Kostro-mine J Appl Phys 94 (2003) 6208

40 G F Lipscomb J I Thackara R LytelElectro-Optic Effect in [7]

41 Aldrich ChemFiles 4 (2004) 442 M Eich H Looser D Yoon R Twieg

G Bjorklund J Baumert J Opt SocAm B 6 (1989) 1590

43 F Kajzar J M Nunzi Molecular Orienta-tion Techniques in F Kajzar R Reinisch(eds) Beam Shaping Control with Non-

linear Optics Plenum Press New York(1998) p 101

44 S Bauer Appl Phys Rev 80 (1996)5531

45 HS Nalwa Organic Materials for Third-Order Nonlinear Optics Chapter 11 in[10]

46 W MK P Wijekoon PN Prasad Non-linear Optical Properties of Polymers inJ E Mark (ed) Physical Properties ofPolymers Handbook AIP Press Wood-bury NY (1995) Chapter 38

47 K S Wong S G Han ZV Vardeny JShinar Y Pang I Maghsoodi T J Bar-ton S Grigoras B Parbhoo Appl PhysLett 58 (1991) 1695

48 P Boffi D Piccinin MC Ubaldi (eds)Infrared Holography for Optical Communi-cations Techniques Materials and DevicesTopics in Applied Physics 86 SpringerBerlin (2003)

49 BH Cumpton S P Ananthavel S Bar-low D Dyer J E Ehrlich LL ErskineA A Heikal SM Kuebler IY S LeeD McCord-Maughon J Qin H RoumlckelM Rumi XL Wu S R Marder JWPerry Nature 398 (1999) 51

50 HB Sun S Matsuo H Misawa ApplPhys Lett 74 (1999) 786

51 D Day M Gu A Smallridge Review ofOptical Data Storage in [48] p 1

52 J W Perry Organic and Metal-ContainingReverse Saturable Absorbers for OpticalLimiters Chapter 13 in [10]

53 EW Van Stryland D J Hagan T XiaA A Said Application of Nonlinear Opticsto Passive Optical Limiting Chapter 14 in[10]

41The photorefractive effect

The photorefractive (PR) effect refers to the spatial modulation of the index ofrefraction in an electro-optically active material that is non-uniformly irradiatedNotably the refractive index of an electro-optically active material is electric fielddependent The PR effect is based on the light-induced generation and subse-quent migration of charge carriers and therefore is strongly connected to thephenomena of photogeneration and conduction of charge carriers in polymericsystems dealt with in Chapter 2 The PR effect was first observed in inorganicmaterials such as LiNbO3 BaTiO3 InP Fe and GaAs [1ndash9] and later also in or-ganic materials Work related to polymers has been reviewed [10ndash12] Materialsexhibiting the PR effect should be capable of forming charge carriers ie pairsof positively and negatively charged ions in a sufficiently high quantum yieldupon exposure to light and these charge carriers should migrate with a suffi-ciently high mobility A prerequisite for the occurrence of the PR effect is sepa-ration of the charges which is commonly accomplished if only one type ofcharge carrier is mobile and the material contains traps where the migratingcarriers are captured A non-uniform irradiation of polymeric materials can beaccomplished by placing foils in the interference region of two coherent lightwaves In this way a fringe pattern of brighter and darker regions ie ofstrongly and weakly or not at all irradiated regions is produced Notably thecharge separation due to the exclusive migration of charge carriers of the samesign from the irradiated to the non-irradiated regions results in the build-up ofa space-charge field ie of an internal electric field between the irradiated andunirradiated regions which allows the linear electro-optic effect (Pockels effectsee Section 31) to become operative In other words the formation of thespace-charge field gives rise to a change in the refractive index and in this waya refractive index fringe pattern is generated The magnitude of the refractiveindex modulation n frequently also referred to as the dynamic range dependson the space-charge field strength ESC according to Eq (4-1)

n n3reESC

24-1

103

4Photorefractivity

Here re is the electro-optic (or Pockels) coefficient for a given geometry and nis the refractive index

Commonly holes are the mobile charge carriers in photorefractive polymersSince the migration of holes by diffusion is a rather slow process a drift is en-forced by the application of an external electric field The latter not only pro-motes hole migration but also provides essential assistance during the photo-

4 Photorefractivity104

Fig 41 The photorefractive effect One-dimensional illustration of the chargegeneration by non-uniform irradiation of apolymer film and the subsequent generationof a refractive index grating through

transport and trapping of the mobile holesAdapted from Valley and Klein [13] andMoerner and Silence [12] with permissionfrom the American Chemical Society

Fig 42 Schematic depiction of the experimental geometryemployed for writing a refractive index grating in a PRpolymer Adapted from Moerner and Silence [12] withpermission from the American Chemical Society

generation process (see Section 22) Significantly there is a phase shift betweenthe irradiation pattern and the refractive index pattern as can be seen inFig 41 which illustrates the mechanism of grating formation

A schematic depiction of the formation of a grating in a polymer film locatedin an external electric field is shown in Fig 42

The grating is written by beams 1 and 2 which enter the film at angles of in-cidence 1 and 2 with respect to the sample normal The grating is written at awave vector KG at an angle with respect to the external electric field E0 Thespatial periodicity G of the grating is given by Eq (4-2)

G 0

2n sin131 22 4-2

Here n is the refractive index and 0 is the wavelength of the light in vacuo

42Photorefractive formulations

An organic photorefractive system has to contain different functional groupsproviding for the generation transport and trapping of charge carriers More-over a plasticizing function is required for certain formulations Apart from thelatter these requirements may in principle be met by fully functionalized poly-mers ie by polymers containing in their main chain and side chains the var-ious requisite functional groups However since this approach is rather difficultto implement research activities have concentrated mostly on the so-calledhostguest approach which is based on formulations consisting of a host poly-mer and various low molar mass guest compounds Typical polymers and lowmolar mass compounds used for formulations exhibiting a photorefractive effectare shown in Chart 41 and Chart 42 respectively

The system PMMA-PNA DEHTNF is a typical photorefractive formulationwith PMMA-PNA acting as the host polymer and DEH (30 wt) and TNF(01 wt) as charge-transporting agent and charge-generating sensitizer respec-tively In order to ensure bulk transport of the photogenerated holes by the hop-ping mechanism the concentration of the transporting agent has to be ratherhigh Typical examples of fully functionalized polymers are also presented inChart 41 (polymers VI [14] and VII [15]) In the case of polymer VI photoexcita-tion of the chromophores MHB+Brndash at = 647 nm induces electron transfer fromthe aromatic amino groups (Am) according to reaction (b) in Scheme 41 In thisway trapped electrons MHBBrndash and mobile radical cations Am+ are formedThe hole transport according to reaction (c) is a multiple successive electron-hop-ping process from neutral Am groups to neighboring radical cations

Polymer VII belongs to a group of conjugated polymers containing porphyrinor phthalocyanine complexes synthesized by Lu et al [16] Here the polymerbackbone consists of phenylene vinylene moieties which facilitate hole trans-

42 Photorefractive formulations 105

port through intramolecular migration and interchain hopping Charge carriersare formed as a result of the selective absorption of near-infrared light (eg He-Ne laser light = 6328 nm) by the porphyrin or phthalocyanine complexes andtrapping might occur at the side groups


Chart 41 Polymers employed in photorefractive formulations

43Orientational photorefractivity

During the development of new photorefractive materials the employment ofchromophoric compounds with a permanent dipole moment turned out to leadto unexpectedly high n values provided that the glass transition temperatureof the formulation was close to ambient temperature such that the chromo-phores were mobile and could become oriented under the influence of an elec-tric field a process referred to as poling Poling-induced orientation of the chro-mophoric molecules leads to macroscopic electro-optical properties and espe-cially to birefringence Notably the total effective electric field in a photorefrac-tivity experiment results from a superposition of the internal space-charge fieldand the externally applied electric field Consequently the spatial refractive in-dex modulation is controlled not only by the space-charge field but also by astrong contribution from the orientational birefringence a fact referred to bythe term orientational photorefractivity Notably in this case the refractive indexchange has a quadratic dependence on the total electric field which is a super-position of the internal space-charge field and the externally applied field andto a rough approximation the dependence of the dynamic range n on the fieldstrength E is given by Eq (4-3)

n pE2 pV2

d2 4-3

43 Orientational photorefractivity 107

Chart 42 Low molar mass compounds employed in photorefractive formulations

MHBBr h MHBBr aMHBBr Am MHBBr Am b

Am Am Am Am etc cScheme 41 Generation and transport of charge carriers in polymer VI

Here p is a material parameter V is the applied voltage and d is the samplethickness

DMNPAA and DHADC-MPN (see Chart 42) are typical optically anisotropiccompounds with permanent dipole moments which can be oriented in an elec-tric field at room temperature in formulations plasticized with ECZ and there-fore have low Tg values Typical values reported in the literature arep = 86 cm2 Vndash2 for the system DMNPAA PVK ECZ TNF and p = 333 cm2 Vndash2 forthe system DHADC-MPNPVK ECZTNFDM [10]

44Characterization of PR materials

Commonly the PR properties of materials are characterized and tested by two-beam coupling and four-wave mixing experiments Two-beam coupling (2BC) re-fers to the energy exchange between the two interfering laser beams employedto write the grating During the formation of the grating the two writing beamsdiffract from the forming grating ie each writing beam is partially diffractedin the direction of the other beam by the forming grating In a 2BC experimentthe change in the transmitted intensity of either of the write beams is recordedas the other write beam is switched on and the grating is formed This can beseen in Fig 43 which shows beam intensity as a function of time as recordedin two experiments in which the intensities of the two writing beams (beforethe sample) were kept equal [14]


Fig 43 Two-beam coupling experimentsyielding evidence for the occurrence of thePR effect in a film consisting of polymer VI(MHB+Brndash) The intensity of beam 1 wasmonitored as beam 2 was switched on att= 0 and switched off at t= 90 s and the

intensity of beam 2 was monitored as beam1 was switched on at t= 0 and switched offat t = 90 s = 647 nm E = 26 V mndash1 andd= 194 m I0 (1)= I0 (2)= 78 mW cmndash2Adapted from Vannikov et al [14] withpermission from Elsevier

In the first experiment in which beam 2 was switched on and off and beam1 was monitored the intensity of the latter decreased Conversely when beam 1was switched on and off and beam 2 was monitored the intensity of the latterincreased The occurrence of such asymmetric energy transfer unambiguouslyconfirms the PR nature of the optical encoding and allows a distinction to bemade between a grating based on the PR effect and other types of gratings

From plots of the type shown in Fig 43 the beam coupling ratio 0 as de-fined by Eq (4-4) can be determined

0 ILsat

IL04-4

Here I(L)sat and I(L)0 denote the intensity at saturation and at time t = 0 respec-tively of the writing beam under consideration measured after passage throughthe sample The beam coupling gain coefficient is given by Eq (4-5)

1L13ln0 ln 1 0 4-5

Here is the ratio of the intensities of the two beams before the sample and Lis the optical path length given by Eq (4-6)

L dcos

4-6

Here d is the sample thickness and is the angle of incidence of the beamwith respect to the sample normal

The total refraction index modulation n is given by Eq (4-7)

n

44-7

Typical results obtained with polymer VI at I1(0) = 720 mW cmndash2 = 22E = 8 V mndash1 = 647 nm and d = 74 m are = 313 cmndash1 n= 1610ndash3= sin2(L2) = 21 and = 4 s (grating build-up or response time) [14]

The four-wave mixing technique serves to measure the diffraction efficiency

during the writing process as a function of time and as a function of thestrength of the external electric field Figure 44 shows a schematic representa-tion of a typical set-up employed in four-wave mixing experiments

Notably a reading beam is used in addition to the two writing beams Com-monly the reading beam is of the same wavelength as the two writing beamsbut of a much lower intensity and it is counterpropagating one of the writingbeams is defined according to Eq (4-8) as the ratio of the intensities of thediffracted beam Id and of the incoming reading beam I0

44 Characterization of PR materials 109

Id

I04-8

Usually the electric field is applied to the sample by sandwiching the polymerbetween two transparent electrodes such as ITO (indium tin oxide)-coated glassslides The diffraction efficiency can be obtained from Kogelnikrsquos coupled-wave theory for thick holograms with the aid of Eq (4-9) [17]

sin2 fgdn

4-9

Here fg is a geometrical factor dependent on the polarization of the beams andthe experimental geometry and is the wavelength of the light of the readingbeam

45Applications

Photorefractive polymeric systems can be used to record in real-time and witha high storage density optically encoded information with low-power lasers suchas semiconductor diode lasers They are appropriate for recording hologramsThe storage of a large number of holograms at a single spot in the storage me-dium (multiplexing see Section 123) is possible Therefore there is a significantapplication potential Actually applications concerning dynamic holographic in-terferometry holographic storage and real-time processing have been demon-strated and future technical applications seem likely [18ndash22] With respect tocommercial applications it is noteworthy that the PR effect is reversible ie


Fig 44 Schematic depiction of a set-upfor a four-wave mixing experiment asemployed to measure diffraction efficiencyas a function of the strength of an externalelectric field Reading beam counterpropa-gating with writing beam (1) Diffractedbeam counterpropagating with writingbeam (2) Adapted from Kippelen et al[11] with permission from the InternationalSociety for Optical Engineering

previously recorded holograms can be erased by irradiation with a spatially uni-form light beam Moreover holograms can be overwritten

There is a long list of technical requirements for holographic materials suchas optical quality near-IR sensitivity large refractive index modulation short re-sponse time self-processing inertness and long shelf-life non-destructive read-out and low cost Successful technical applications depend on the availability ofmaterials that fulfil all or most of these requirements Interesting proposalshave been made to overcome still existing technical problems such as that con-cerning destructive readout To retrieve information from holograms with goodfidelity the reading and writing beams have to be of the same wavelengthHowever since the material is photosensitive at the relevant wavelength thereadout process partially erases the stored information According to Kippelenet al this problem can be overcome with the aid of a photorefractive systemcontaining a substituted diphenylacetylene (compound VII in Chart 42) that issensitive to two-photon absorption [23] In a system of the composition FTCNPVKBBPECZ (25 55 10 10 wt) charge carriers are generated exclusively bytwo-photon processes and holographic recording is achieved with high-intensitywriting beams (= 650 nm 025 mW each) For readout a low-intensity beam(= 650 nm 025 W) which does not affect the photorefractive system is suffi-cient

The requirements of high near-IR sensitivity and short response time arelargely fulfilled by applying a pre-irradiation method denoted as time-gated holo-graphic imaging [24] Pre-irradiation provides for charge carriers before the writ-ing starts and thus affords a significant reduction in response time Accordingto Mechner et al [24] pre-irradiation at = 633 nm prior to holographic record-ing at = 830 nm improved the response time by a factor of 40 (30 ms) in in-vestigations with a formulation containing TPD-PPV (polymer VIII in Chart 42)(see Table 41)

Note that holograms can also be generated in polymeric media by other meth-ods for instance by photopolymerization of appropriate monomers contained inspecial formulations (see Section 117)

45 Applications 111

Table 41 Composition of a photorefractive material suitablefor holographic recording by means of time-gated holographicimaging [24]

Components Content (wt) Function

Polymer VIII (TPD-PPV) 56 Conductive host matrix1 1 Mixture of 25-dimethyl-(4-p-nitrophenyl-azo)-anisole and 3-methoxy-(4-p-nitrophenylazo)-anisole

30 Electro-optical material

Diphenyl phthalate 13 Plasticizer[66]-Phenyl-C61-butyric acid methyl ester 1 Sensitizer


References

1 FS Chen J Appl Phys 38 (1967) 34182 P Guumlnter Holography Coherent Light

Amplification and Optical Phase Conjuga-tion with Photorefractive Materials PhysRep 93 (1982) 199

3 T J Hall R Jaura LM Conners PDFoote The Photorefractive Effect ndash A Re-view Prog Quant Electron 10 (1985)77

4 J Feinberg Photorefractive Nonlinear Op-tics Phys Today 41 (1988) 46

5 P Guumlnter J-P Huignard PhotorefractiveMaterials and Their Applications I and IIin Topics in Applied Physics 61Springer Berlin (1988)

6 MP Petrov SL Stepanov AV Kho-menko Photorefractive Crystals in Coher-ent Optical Systems Springer Berlin(1991)


8 P Yeh Introduction to PhotorefractiveNonlinear Optics Wiley New York (1993)

9 DD Nolte (ed) Photorefractive Effectsand Materials Kluwer Academic PublBoston (1995)

10 B Kippelen Overview of PhotorefractivePolymers for Holographic Data Storage inJ Coufal D Psaltis G T Sincerbox(eds) Holographic Data StorageSpringer Berlin Series in OpticalSciences 76 (2000) 159

11 B Kippelen N Peyghambarian CurrentStatus and Future of Photorefractive Poly-mers for Photonic Applications Crit RevOpt Sci Technol CR 68 (1997) 343

12 W E Moerner SM Silence PolymericPhotorefractive Materials Chem Rev 94(1994) 127

13 G C Valley M B Klein Opt Eng 22(1983) 704

14 A V Vannikov AD Grishina L Ya Per-eshivko T V Krivenko VV SavelyevL I Kostenko R W Rychwalski JPhotochem Photobiol A Chem 150(2002) 187

15 L Lu J Polym Sci Part A PolymChem 39 (2001) 2557

16 LQ Wang M Wang L Lu Adv Mater12 (2000) 974

17 H Kogelnik Bell Syst Tech J 48 (1969)2909

18 R Bittner K Meerholz G Steckman DPsaltis Appl Phys Lett 81 (2002) 211

19 C Poga PM Lundquist V Lee R MShelby R J Twieg DM Burland ApplPhys Lett 69 (1996) 1047

20 PM Lundquist R Wortmann C Gelet-neky R J Twieg M Jurich VY LeeCR Moylan D M Burland Science 274(1996) 1182

21 BL Volodin Sandalphon K MeerholzB Kippelen N Kukhtarev N Peygham-barian Opt Eng 34 (1995) 2213

22 BL Volodin B Kippelen K MeerholzB Jaridi N Peyghambarian Nature 383(1996) 58

23 B Kippelen P-A Blanche A Schuumllz-gen C Fuentes-Hernandez G Ramos-Ortiz J F Wang N PeyghambarianSR Marder A Leclercq D BeljonneJ-L Bredas Adv Funct Mater 12 (2002)615

24 E Mechner F Gallego-Gomez H Till-mann H-H Houmlrhold J C HummelenK Meerholz Nature 418 (2002) 959


There are substances that are transformed from form A into form B having adifferent absorption spectrum upon the absorption of light of wavelength 1

and that return to the initial state A either thermally or by the absorption oflight of wavelength 2 (see Scheme 51)

Substances capable of undergoing color changes in this way are denoted asphotochromic and the corresponding phenomenon is termed photochromism Ascan be seen from Table 51 in which typical photochromic systems are pre-sented photochromism can be based on various chemical processes

trans-cis (EZ) Isomerization occurs in azobenzene compounds (example (a))and also in the cases of azines stilbenes and certain biological receptors in liv-ing systems Pericyclic reactions (electrocyclizations) occur in the cases of spiro-pyrans and spirooxazines (examples (b) and (c)) and also with diarylethenes (ex-ample (d)) and fulgides (example (e)) Heterolytic bond cleavage resulting inionic dissociation occurs in the case of triphenylmethanes (example (f)) Con-cise information on organic photochromism including details of the variousfamilies of photochromic compounds and the chemical processes involved inphotochromic transformations is given in an IUPAC Technical Report [1]Moreover this subject has been dealt with in various review articles and booksthat emphasize its importance and potential for applications in the fields of mo-lecular switches and information storage [2ndash9] With respect to the present bookvarious publications focusing on polymers have to be pointed out [10ndash21]

The transformations presented in Table 51 are always accompanied bychanges in physical properties Besides the color changes there are alsochanges in dipole moment and in the geometrical structure at the molecularlevel Regarding bulk properties there are changes in the refractive indexwhich give rise to photo-induced birefringence and dichroism

113

5Photochromism

Scheme 51 Photochromic transformation of molecules

5 Photochromism114

Table 51 Typical photochromic processes

trans-cis Isomerization(a) Azobenzene

Pericyclic reactions(b) Spiropyrans

(c) Spirooxazines

(d) Diarylethenes

(e) Fulgides and fulgimides(X = O) (X = NR)

Heterolytic bond cleavage(f) Triarylmethanes

With respect to polymeric systems containing photochromic groups specialaspects have to be addressed For instance in linear macromolecules not onlythe chromophoric moieties but also neighboring units of the polymer chain orsurrounding molecules may be affected upon the absorption of photons by thechromophoric groups Conformational changes in linear polymers in solutioninduced in this way may lead to a change in viscosity or even to phase separa-tion For instance in liquid-crystalline polymeric systems phase transitions canbe generated In the case of rigid polymer matrices photomechanical effects areinduced ie photoisomerization causes shrinkage or expansion Interestinglystable relief surface gratings can be generated in polymer foils containingphotochromic moieties Notably the photostimulated conformational change inpolymers may result in an enormous amplification effect ie the absorption ofa single photon affects not only one moiety but also several neighboring onesor even the whole macromolecule

Potential applications of photochromic transformations relate to the reversiblecontrol of the properties of appropriate materials In this connection polymersoffer the advantage of easy fabrication and therefore a plethora of studies hasbeen devoted to polymers containing photochromic groups or to polymers withadmixed photochromic compounds Apparently among the various photochrom-ic polymeric systems dealt with in the literature those containing azobenzenegroups [19 20] have attracted the main interest although it seems that othersparticularly those containing diarylethenes [5] and furyl fulgides [6] deserve spe-cial attention because of their excellent performance Light-induced colorationdiscoloration cycles could be repeated more than 104 times with certain diaryl-ethenes thus proving their extraordinary resistance to fatigue [5] Thermal irre-versibility and fatigue resistance are prerequisites for applications related to datastorage and switching of photonic devices [21] which are considered in Chap-ter 12 of this book

52Conformational changes in linear polymers

521Solutions

Photochromic transformations may induce conformational changes in linearmacromolecules containing appropriate chromophoric groups Commonly thetransformation of these groups is accompanied by a change in polarity Thischange is most pronounced if the transformation generates electrically chargedgroups eg in the cases of triphenylmethane or spiropyran groups Howeverazobenzene groups also undergo a drastic change in polarity The change in thegeometry of the azobenzene group from the planar (trans or E-form) to the non-planar (cis or Z-form) leads to a decrease in the distance between the para car-bon atoms of the benzene rings from 99 to 55 Aring and to an increase in the di-

52 Conformational changes in linear polymers 115

pole moment from 05 to 55 D Regarding linear polymers containing pendantphotochromic groups the change in polarity affects not only the intermolecularinteraction between the chromophore and surrounding solvent molecules butalso the intramolecular interaction between pendant groups As a consequencerandom coil macromolecules undergo conformational alterations leading to ex-pansion or shrinkage For example a copolymer with pendant azobenzenegroups consisting of styrene and 4ndash6 mol 4-(methylacryloylamino)azobenzeneMAB (see Chart 51) precipitates in dilute cyclohexane solution at temperaturesabove the critical miscibility temperature upon irradiation with UV light Thisphenomenon is explained in terms of cis-azobenzene groups having in contrastto trans-azobenzene groups the capability of interacting rather strongly with sty-rene moieties Therefore immediately after trans-cis isomerization cis-azoben-zene groups interact preferentially with neighboring styrene moieties thuscausing a contraction of the coil Interactions of the cis-azobenzene groups withstyrene moieties of other macromolecules result in aggregation a process thatultimately leads to precipitation [22 23] This is illustrated schematically inFig 51

In solution coil expansion and contraction is readily reflected by changes inviscosity and in the intensity of scattered light As can be seen in Fig 52 theoptical absorption at 620 nm and the reduced viscosity specc increase simulta-neously when a poly(NN-dimethylacrylamide) sample containing 91 mol pen-dant triphenylmethane leucohydroxide groups is irradiated in dilute methanolsolution with UV light (gt 270 nm) In the dark the reduced viscosity returnsto the initial value The development of a green color in conjunction with theincrease in the viscosity indicates the formation of triphenylmethyl cations Ob-viously the polymer coils become expanded due to electrostatic repulsion of io-nized pendant groups formed according to Scheme 52 [24]

In the case of an azobenzene-modified poly(arylether ketone amide) (seeChart 52) a pronounced volume contraction due to photo-induced trans-cis iso-merization of the azobenzene groups was evidenced by means of size-exclusionchromatography (SEC) [25] When irradiated in dilute NN-diethylacetamide so-lution this polymer underwent a reduction in its hydrodynamic radius by a fac-tor of 27 corresponding to a contraction of the hydrodynamic volume by a fac-tor of about 20 This pronounced shrinkage effect is believed to be due to alarge number of conformationally restricted backbone segments because othermore flexible polyamides and polyurea polymers exhibit much weaker contrac-tion effects

5 Photochromism116

Chart 51 Chemical structures of co-monomer moieties styrene (left) and4-(methylacryloylamino)azobenzene(right)

The dynamics of conformational changes can be measured by following thechange in the light-scattering intensity Relevant studies relate to a polyamidecontaining in-chain azobenzene groups (see Chart 53) that was brought intothe compact form through trans-cis isomerization by continuous UV irradiationin NN-dimethylacetamide solution and subsequently exposed to a 20 ns flash of532 nm light On recording the changes in the optical absorption and in thelight-scattering intensity both at = 514 nm as a function of time it turned outthat the cis-trans isomerization was completed within the 20 ns flash and thatthe polymer chains unfolded on the ms time scale Obviously after isomeriza-tion the polymer chains maintain the initial compact conformation and thestrain energy built-up in this way causes coil expansion [26] The whole processis shown schematically in Scheme 53

The possibility of photo-inducing geometrical alteration in polymers in solu-tion has attracted special interest with regard to various polypeptides (seeChart 54)

Besides unordered random coil structures polypeptides are capable of assum-ing stable geometrically ordered structures namely -helix and -structures Asshown in Fig 53 these structures can be conveniently discriminated by record-ing circular dichroism (CD) spectra [14]


Fig 51 Coil contraction and precipitation of polystyrenebearing pendant azobenzene groups

5 Photochromism118

Fig 52 Coil expansion of poly(NN-dimethy-lacrylamide) containing pendant triphenyl-methane leucohydroxide (91 mol) inmethanol upon exposure to UV light

(gt 270 nm) (a) Optical absorption at= 620 nm (b) reduced viscosity specc(spec = (solutionsolvent)ndash1) Adapted fromIrie [11] with permission from Springer

Scheme 52 Photogeneration of triphenylmethyl cations inpoly(NN-dimethylacrylamide) containing pendanttriphenylmethane leucohydroxide groups

Chart 52 Chemical structure of an azobenzene-modified poly(arylether ketone amide)

Light-induced transformations from one structure to another have been stud-ied with many modified polypeptides [13 14] bearing pendant photochromicgroups such as azobenzene or spiropyran groups Typical examples are themodified poly(L-glutamic acids) PGA-1 and PGA-2 presented in Chart 55

The spiropyran-modified poly(L-glutamic acid) PGA-2 undergoes a coilhelixtransition upon exposure to visible light in hexafluoro-2-propanol solution Inthe dark the polypeptide containing 30ndash80 mol chromophore units in theopen charged form adopts a random coil conformation Irradiation causes iso-merization in the side chains as indicated by complete bleaching of the coloredsolution (see Scheme 54) The formation of the colorless and uncharged spiro-pyran form induces spiralization of the polypeptide chain The coilhelix tran-sition can be followed with the aid of CD spectra as shown in Fig 54


Chart 53 Chemical structure of a polyamide containing in-chain azobenzene groups

Scheme 53 Conformational change of a polyamidecontaining in-chain azobenzene groups due to cis-transisomerization

Chart 54 Chemical structures of poly(L-lysine) and poly(L-glutamic acid)

The coilhelix transition proceeds rapidly within seconds whereas the backreaction requires several hours for full conversion Notably in this case thephotochromic behavior of the spiropyran groups is opposite to that observed inother solvents (see example (b) in Table 5-1) The reverse photochromism is dueto the high polarity of hexafluoro-2-propanol which stabilizes the charged mero-cyanine form better than the uncharged spiropyran form

5 Photochromism120

Chart 55 Chemical structures of modified poly(L-glutamic acids)

Fig 53 Standard circular dichroism (CD)spectra of common polypeptide structures(1) -helix (2) -structure and (3) randomcoil Adapted from Pieroni et al [14] withpermission from Elsevier


Scheme 54 Isomerization of the spiropyran-modified poly(L-glutamic acid) PGA-2

Fig 54 Coilhelix transition of poly(glutamic acid) PGA-2containing 80 mol spiropyran units in the side chains CDspectra recorded in hexafluoro-2-propanol solution in the dark(1) and after exposure to sunlight (2) Adapted from Pieroniet al [14] with permission from Elsevier

522Membranes

As an extension of the work described in the previous section one goal was thedevelopment of artificial membranes the physical properties of which such aspermeability electrical conductivity and membrane potential could be con-trolled in response to light Typically in the case of membranes consisting ofpoly(L-glutamic acid) bearing azo groups in the side chains the water contentincreases upon light exposure Concomitantly the dissociation of acid groups isaccelerated and augmented and the potential across the membrane and thecross-membrane conductance are enhanced [15] Typical results are presented inFig 55

Moreover a low molar mass spiropyran compound entrapped in a membraneconsisting of plasticized poly(vinyl chloride) rendered the latter photoresponsiveA membrane potential change of more than 100 mV was induced by irradiationwith light [27] For further details and additional references the reader is re-ferred to the relevant reviews [11 28]

5 Photochromism122

Fig 55 Photoresponsive behavior of membranes of anazo-modified poly(L-glutamic acid) containing 12ndash14 molazobenzene groups at 60 C (a) Membrane potential(b) conductance and (c) absorbance at 350 nm Adaptedfrom Kinoshita [15] with permission from Elsevier

53Photocontrol of enzymatic activity

Photochromic groups covalently attached to enzymes are in certain cases cap-able of affecting the tertiary protein structure upon light-induced isomerizationAs a consequence the biocatalytic activity of the enzymes can be switched onand off [29] For example the catalytic activity of papain is inhibited when 4-carboxy-trans-azobenzene groups covalently linked to the lysine moieties of theenzyme undergo trans-cis isomerization (see Scheme 55) At a loading of fiveunits per enzyme molecule 80 of the catalytic activity is retained

The inactivity of enzyme molecules bearing cis-azobenzene groups is ex-plained by their incapability of binding to the reaction substrate Similarly thebinding of -d-manopyranose to concanavalin A is photocontrollable providedthat the enzyme is modified by the attachment of thiophenefulgide or nitro-spiropyran However the general applicability of this method has to be subjectto scrutiny because the photoswitching behavior is quite sensitive to the level ofloading Low loadings may result in a low switching efficiency and high load-ings often deactivate the biomaterials in both isomeric forms

54Photoinduced anisotropy (PIA)

Exposure of polymer films bearing azobenzene groups to linearly polarized laserlight induces optical dichroism and birefringence This is due to the fact thatduring exposure a major fraction of the chromophores becomes oriented per-pendicular to the polarization direction of the light Photons of linearly polar-ized light are preferentially absorbed by molecules with a transition momentparallel to the polarization plane of the light The absorbed photons inducetrans-cis isomerizations in conjunction with rotational diffusion The relaxationof the cis molecules results in trans molecules with a new orientation distribu-tion ie the fraction of trans molecules with a transition moment parallel to thepolarization plane of the incident light becomes smaller Continuous repetitionof this cycle steadily reduces this fraction and makes the system more transpar-ent to the incident light as the trans molecules can no longer be excited

54 Photoinduced anisotropy (PIA) 123

Scheme 55 Photoisomerization of azobenzene groupscovalently linked to the lysine moieties of papain

To sum up during the irradiation azobenzene groups with transition mo-ments that are not initially perpendicular to the polarization direction of the la-ser light undergo a series of trans-cis-trans isomerization cycles accompanied bya change in orientation until they finally line up in directions approximatelyperpendicular to the polarization direction of the laser light (see Fig 56)

In this way an orientation distribution with an excess of azobenzene groupsoriented in the direction perpendicular to the polarization plane of the laserlight is attained The resulting birefringence can be detected with the aid of an-other laser beam that is not absorbed by the photochromic compound Notablythe anisotropy can be erased if the sample is irradiated with circularly polarizedlaser light or is heated to a temperature in excess of the glass transition tem-perature This behavior is demonstrated for a typical case in Fig 57 Here itcan be seen that the birefringence (monitored at 633 nm) of a 400ndash500 nm thickfilm of pMNAP polymer (see Chart 56) is built up upon irradiation with a lin-early polarized laser beam (= 488 nm) [30] The birefringence relaxes down to acertain level when the writing beam is turned off and is completely eliminatedupon turning on a circularly polarized light beam (= 488 nm)

Photo-induced anisotropy (PIA) is quantitatively described by Eqs (5-1) and(5-2) by n in terms of the induced birefringence and by the parameter S interms of light absorption behavior

n n n 5-1

5 Photochromism124

Fig 56 Schematic illustration ofthe generation of anisotropy uponirradiation of a film containingphotochromic entities with linearlypolarized light

Fig 57 Generation of birefringence uponirradiation of pMNAP polymer with linearlypolarized light (= 488 nm) A light turned onB light turned off C circularly polarized lightturned on Adapted from Meng et al [30] withpermission from John Wiley amp Sons Inc

S A AA 2A 5-2

Here A|| and A and n|| and n denote the absorbances and the refractive in-dices at orientations parallel and perpendicular to the polarization plane of theexciting probe light respectively

In recent years optical dichroism and birefringence based on photo-inducedtrans-cis-trans isomerization of azobenzene groups has been observed with pre-oriented liquid-crystalline polymers [31-35] at temperatures above the glass tran-sition temperature and also with various amorphous polymers at temperatureswell below the glass transition temperature In the case of a polyimide (seeChart 57) a quasi-permanent orientation can be induced [36ndash38] Here the azo-benzene groups are rather rigidly attached to the backbone and photoisomeriza-tion occurs at room temperature ie 325 C below the glass transition tempera-ture Tg = 350 C This behavior is in accordance with the fact that the isomeriza-tion quantum yields of azobenzene compounds are very similar in solution andin polymer matrices 13(trans cis)01 and 13(cis trans) 05


Chart 56 Chemical structure ofpMNAP polymer used for the photo-generation of birefringence(see Fig 57)

Chart 57 Chemical structure of a polyimide bearing pendant azobenzene groups

Because of the importance of the PIA phenomenon for applications in opticaldata storage systems a large variety of homopolymers and copolymers has beenstudied and the reader is referred to the literature cited in a relevant review arti-cle [39] In this connection it is also worthwhile to cite work performed with cy-clic siloxane oligomers bearing pendant photochromic groups Compounds ofthis family possessing relatively high glass transition temperatures and capableof forming cholesteric liquid-crystalline phases have been examined as potentialoptical recording materials [40]

55Photoalignment of liquid-crystal systems

It has been shown in Section 54 that linearly polarized laser light induces achange in the orientation of azobenzene groups contained in polymers Interest-ingly this change in orientation can be greatly amplified if the azobenzenegroups are contained in liquid-crystalline polymers This phenomenon whichhas been the subject of extensive investigations [16 41ndash44] is described here insome detail for the case of a methacrylate-based copolymer consisting mainly ofnon-photosensitive mesogenic side groups and a small fraction of azobenzene-containing side groups (see Chart 58) [45]

Initially this copolymer is an isotropic (polydomain) liquid-crystalline polymerwith a glass transition temperature of Tg = 45 C and a clearing temperature(transition from nematic to isotropic phase) of TN-I = 112 C Irradiation with lin-early polarized light at = 366 nm (28 mW cmndash2) and T = 106 C ie just belowTN-I induces anisotropy By repetitive trans-cis-trans isomerization the opticalaxis of the azobenzene groups becomes aligned perpendicular to the electricvector of the incident light In this way a cooperative motion of the neighboringphotoinactive mesogenic groups is triggered Thus the entire assembly of me-sogenic side groups becomes aligned in one direction and forms a monodomain

5 Photochromism126

Chart 58 Chemical structures of the components of a liquid-crystalline copolymer exhibiting amplified photoalignment(see Fig 58)

nematic phase This was evidenced by measuring the transmittance of an irra-diated (exc = 633 nm) copolymer film placed between a pair of crossed polarizersat various rotation angles As can be seen in Fig 58 the transmittance hasmaxima at 45 135 225 and 315 and minima at 0 90 180 and 270

Materials such as the LC copolymer considered here possess an applicationpotential for image storage This is demonstrated in Fig 59 which shows (a)the transmittance response of the copolymer during alternating irradiation withpolarized and unpolarized light and (b) a one-year-old stored image which wasgenerated by irradiation of a copolymer film through a standard photo mask[45]

The field of liquid-crystalline polymers is still growing and a significant num-ber of the relevant papers deal with subjects related to photochemical andphotophysical problems as has been documented in several reviews [46ndash48]The progress in research is demonstrated here by referring to an interesting de-velopment concerning the photochromic amplification effect based on the sur-face-assisted alignment of liquid-crystalline compounds in cells possessing so-called command surfaces [16 41ndash43] The latter consist of silica glass plates orpolymer films bearing attached photochromic groups at an area density of aboutone unit per nm2 The light-induced isomerization of the photochromic moi-eties triggers reversible alignment alterations of the low molar mass liquid-crys-talline compounds contained in the cell Chemical structures of appropriatecompounds forming nematic crystalline phases are shown in Chart 59

It should be noted that the intermolecular interaction between surface azo-benzene units and liquid-crystal molecules is strongly determined by theirchemical nature an aspect that has been thoroughly investigated [43] but is notelaborated here It is estimated that the amplification involves up to 104 liquid-

55 Photoalignment of liquid-crystal systems 127

Fig 58 Alignment of liquid-crystalcopolymer MACB-CNB6 upon30 min of exposure to polarized lightat = 366 nm (28 mW cmndash2) at106 C (a) Transmittance of probelight (633 nm) through a 2 m thickcopolymer film placed betweencrossed polarizers as a function ofthe rotation angle (b) Experimentalset-up Adapted from Wu et al [45]with permission from Elsevier

crystalline molecules per elementary isomerization process The response timeof the cells is determined by relax the relaxation time of the nematic phase Val-ues of relax typically range from 50 to 300 ms [43] and so are several orders ofmagnitude longer than isomerization times which are of the order of picose-conds Figure 510 schematically depicts for the case of azobenzene chromo-phores as the active entities at the surface how irradiation with unpolarizedlight induces an alignment change from the homeotropic to the planar homoge-neous state

Notably this kind of alignment change can also be accomplished by applyingan electric field On the other hand alignment changes between planar homo-

5 Photochromism128

Fig 59 (a) Transmittance responseof copolymer MACB-CNB6 duringirradiation with polarized light (A toB) and unpolarized light (C to D) at106 C (b) One-year-old imagestored in the liquid-crystalcopolymer The film was coveredwith a photo mask during irradiationwith polarized light at = 366 nm(28 mW cmndash2) and 106 C Adaptedfrom Wu et al [45] with permissionfrom Elsevier

Chart 59 Compounds forming nematic liquid-crystallinephases appropriate for photoalignment [43]

geneous states not realizable with the aid of an electric field can be achievedby employing linearly polarized light An alignment change induced by an azi-muthal in-plane reorientation of the photochromic groups is depicted schemati-cally in Fig 511

It has been reported that cells fabricated with azobenzene-modified surfacesand operating on the basis of alternate irradiation with UV and visible light be-come inactive after about 2000 cycles which is thought to be due to side reac-tions occurring with a quantum yield of about 10ndash4 [43]


Fig 510 Light-induced surface-assisted alignment change ina liquid-crystal cell Schematic depiction of the out-of-planechange from the homeotropic state to the planar homoge-neous state upon exposure to unpolarized UV light Adaptedfrom Ichimura [43] with permission from Springer

Fig 511 Light-induced surface-assisted alignment change ina liquid-crystal cell Schematic depiction of the in-planechange between homogeneous planar states under theinfluence of linearly polarized light Adapted from Ichimura[43] with permission from Springer

56Photomechanical effects

561Bulk materials

The idea of transforming light into mechanical energy has fascinated many re-searchers In the early studies reviewed by Irie [11] contractionexpansion be-havior in conjunction with isomerization of photochromic entities either ad-mixed to or chemically incorporated into polymer films was found Howeverthe dimensional changes were only marginal amounting to 1 or less and onscrutiny turned out in many cases to be due to the local increase in tempera-ture arising from non-radiative transitions rather than to isomerization of thechromophores

Large real effects on the other hand were observed with hydrogels A typicalresult is presented in Fig 512 which shows how a polyacrylamide gel contain-ing 19 mol triphenylmethane leucocyanide swells upon irradiation with UVlight at 25 C [49] The swelling is correlated to a 18-fold increase in the relativeweight

It can also be seen in Fig 512 that in the dark the gel slowly attains the ini-tial weight More recently rigid films (501005 mm) of polyurethanendashacrylateblock copolymers containing nitrospiropyrans and nitro-bis-spiropyrans havebeen irradiated with 325 nm light at 20 C in 5 min lightdark cycles [50] Thefilms expanded during irradiation and shrank in the dark with a response timeof a few seconds in each case The highest photomechanical responses were ob-served at a high acrylate content (72) which rendered the system least elastic

The possibility of converting light into mechanical energy has been impres-sively demonstrated with cross-linked liquid-crystalline polymeric systems con-taining azobenzene groups that were prepared by polymerizing previouslyaligned mixtures of acrylate 1-AC and diacrylate 2-AC (see Chart 510) [51]

Figure 513 shows how a film prepared from an 8020 mol mixture of 1-ACand 2-AC bends upwards towards the incident light (= 360 nm) It becomes flat

5 Photochromism130

Fig 512 Photomechanical effects UV-light-stimulated dilatation of a polyacrylamide gelcontaining pendant triphenylmethane leucocyanidegroups (19 mol) at 25 C Adapted from Irieet al [49] with permission from the AmericanChemical Society

again upon irradiation at = 450 nm These processes are completed within90 s The anisotropic bending phenomenon caused by trans-cis isomerizationmay be explained in terms of a volume contraction The latter is limited to athin surface layer of the 10 m thick film in which the incident light is totallyabsorbed Since the film mobility requires segment relaxation the bending phe-nomenon can be observed with rigid films at T gt Tg in this case at T = 90 C orat room temperature with films swollen in a good solvent such as toluene

The phenomenon of light-induced dimensional alterations in polymer films hasbeen exploited for the generation of regular surface structures in azobenzene-con-taining polymers The technique employed is based on the fact that azobenzenegroups undergo reorientation due to repeated trans-cis-trans isomerization upon

56 Photomechanical effects 131

Chart 510 Monomers used to prepare cross-linked polymericsystems exhibiting photomechanical effects

Fig 513 Photomechanical effects Schematicillustration of UV-light-induced bending of across-linked liquid-crystalline polymer filmcontaining azobenzene groups Light isabsorbed at the upper surface layer of the filmand causes anisotropic contraction Adaptedfrom Ikeda et al [51] with permission fromWiley-VCH

irradiation with polarized light (see Section 54) and that the target film is inhomo-geneously irradiated The reorientation results in a driving force that initiatesmass transport from irradiated to unirradiated areas The experimental set-uporiginally used to generate large surface gratings is shown in Fig 514 a [52 53]

The gratings are optically inscribed onto the films with a single beam of anargon ion laser (488 nm irradiation power between 1 and 100 mW) split by amirror and reflected coincidently onto the film surface which is fixed perpen-dicular to the mirror The diffraction efficiency is monitored with the aid of aHe-Ne laser beam (1 mW = 633 nm) Changing the incident angle of the writ-ing beam allows the intensity profile spacing on the sample and thereby thegrating spacing to be changed Under such conditions irradiation of the poly-mer films for a few seconds at an intensity between 5 and 200 mW cmndash2 pro-duces reversible volume birefringence gratings with low diffraction efficiency If

5 Photochromism132

Fig 514 Photomechanical effectsGeneration of surface relief gratings inpoly(4-(2-acryloyloxy)ethylamino-4-nitroazo-benzene) by light-induced mass transport(a) Experimental set-up (b) Sinusoidal

surface relief profiles examined with the aidof an atomic force microscope Adaptedfrom Rochon et al [53] with permission fromthe American Physical Society

the film is exposed for a longer period (up to a few minutes) an irreversibleprocess creates an overlapping and highly efficient surface grating Thus thereis an initial rapid growth corresponding to the production of the reversible vol-ume birefringence grating and a slower process which irreversibly creates sur-face gratings observable by atomic force microscopy (AFM) with efficiencies ofup to 50 Figure 514 b shows a typical grating generated in this case at thesurface of a film of a polymer having the structure depicted in Chart 511

Surface gratings have been generated in various azobenzene-modified poly-mers epoxy polymers polyacrylates polyesters conjugated polymers poly(4-phenylazophenol) and cellulose [54ndash56]

562Monolayers

Monolayers of a polypeptide consisting of two -helical poly(L-glutamate)slinked by an azobenzene moiety (see Chart 512) become bent in the main


Chart 511 Chemical structure of poly(4-(2-acry-loyloxy)ethylamino-4-nitroazobenzene)

Chart 512 Chemical structure of a poly(L-glutamate) with in-chain azobenzene groups

Chart 513 Chemical structure of a hairy-rod-type poly(gluta-mate) bearing pendant azobenzene groups

chain to an angle of about 140 upon light-induced trans-cis isomerization As aresult the area of the monolayer shrinks [57]

Photomechanical effects in monolayers have also been observed in othercases for example with so-called hairy-rod type poly(glutamate)s (see Chart 513)[58]

57Light-induced activation of second-order NLO properties

Apart from the aforementioned property alterations photochromicity is fre-quently also connected with changes in nonlinear optical (NLO) properties Thisis due to the fact that the two molecular species in a photochromic couple com-monly exhibit different molecular NLO properties Relevant studies have beenperformed with thin polymer films For example if spiropyran is transformedto merocyanine the first hyperpolarizability increases considerably The sec-ond harmonic generation (SHG) increases by a factor of ten when a previouslyelectric field-poled PMMA film doped with a spiropyran (see Chart 514) is irra-diated at = 355 nm [59] Subsequent irradiation at = 514 nm at which mero-cyanine absorbs strongly induces the reverse reaction resulting in a drop of theSHG signal to almost zero Figure 515 shows how the SHG signal changes inresponse to alternating irradiation with UV and visible light

Clearly the SHG signal decreases with increasing number of cycles indicat-ing that in the absence of an external electric field the chromophores becomeincreasingly disorientated ie the NLO activity of the system is deactivated Ana-logous behavior has been observed with a PMMA film doped with a furyl ful-gide (see Chart 515) In this case the ring-opening and -closure reactions needless free volume Therefore the matrix is less disturbed and the SHG signal de-creases more slowly with increasing number of cycles

Interestingly the disorientation-induced distortion of the matrix can beavoided if the photoswitching is performed under an external electric field Thiswas demonstrated in the case of the polyimide of the structure shown inChart 516 [60]

Here the SHG signal decays under irradiation due to trans-cis isomerizationand recovers almost completely in the dark after the light is switched off Theinfluence of the external electric field is thought to allow a compensation of thephoto-induced distortion through photo-assisted poling

5 Photochromism134

Chart 514 Chemical structure of 6-nitro-133-trimethylspiro[2H-1-benzopyran-22-indoline] [59]

57 Light-induced activation of second-order NLO properties 135

Chart 515 Chemical structure of furyl fulgide FF-1

Fig 515 Light-induced generation ofsecond-order NLO properties in an electricfield-poled PMMA film doped with 25 wtof a spiropyran (see Chart 514) Alternatingirradiation at = 355 nm and =514 nm

Upper part Second harmonic generation(SHG) Lower part Optical absorption of themerocyanine isomer at =532 nm Adaptedfrom Atassi et al [59] with permission fromthe American Chemical Society

Chart 516 Chemical structure of a polyimide with pendant azobenzene groups

58Applications

581Plastic photochromic eyewear

Besides classical inorganic glasses there are certain optical plastics that are em-ployed in the transparency and eyewear industry For instance thermoset resinsbased on allyl diglycol carbonate poly(methyl methacrylate) derivatives and bis-phenol A polycarbonates have been used to produce commercial plastic non-photochromic and photochromic lenses As far as has been disclosed by themanufacturers indolinospironaphthoxazines INSO and pyridobenzoxazines

5 Photochromism136

Fig 516 UV activation and thermal bleach profiles at 10 C20 C and 30 C of a commercial photochromic lens based onindolinospironaphthoxazine Adapted from Crano et al [61]with permission from Springer

Chart 517 Chemical structures of compounds that render plastic lenses photochromic

QISO (see Chart 517) have received much attention among the compoundscapable of rendering plastic lenses photochromic

The photochromic compounds are incorporated at a concentration of 01ndash03either by admixing or by chemical bonding In the latter case modified compoundswith appended polymerizable functionalities are employed Photochromic lensesoperate on the basis of UV activation and thermal bleaching as shown in Fig 516

As with most photochromic lenses the performance of plastic photochromiclenses is temperature-dependent In addition to variable light attenuation photo-chromic lenses offer protection against UV light Photochromic plastics coated ontoclassical glass lenses provide abrasionscratch resistance and highly functional anti-reflectivity For further details the reader is referred to a review article [61]

582Data storage

The availability of two states associated with the common photochromic processis a promising basis for erasable optical data storage systems as outlined in areview article by Irie [62] Besides sufficiently high quantum yields and rapid re-sponses for both the forward and the reverse reaction important requirementsfor device application include a high storage capacity a long archival lifetimeand good intrinsic fatigue characteristics and cyclability ie the number oftimes the interconversion can be made without significant performance lossObviously a development of the recorded image should not be necessary

Photochromic compound families that have been considered for employmentin data storage systems include for example fulgides and diarylethenes Com-pounds that have been examined for instance are the furyl fulgide FF-1 (seeChart 515) [63] and the diarylethene shown in Scheme 56 When dispersed ina polystyrene film the latter system exhibited a strong fatigue resistance in atest using a low-power readout laser (633 nm 20 nW) The initial optical densityof 05 remained unchanged during more than 105 readout cycles [5 64]

In this connection the importance of fatigue resistance should be pointedout If form A of a chromophoric couple AB undergoes a side reaction with aquantum yield 13side = 0001 and B converts to A without loss 63 of the initialmolecules of A will be decomposed after 1000 cycles Thus 13side should be lessthan 00001 if the system is expected to endure more than 104 cycles [65]

58 Applications 137

Scheme 56 Photoisomerization of 3-(1-octyl-2-methyl-3-indolyl)-4-(235-trimethyl-1-thienyl)maleic anhydride

The search for materials appropriate for data storage has also been extendedto liquid-crystalline copolymers containing photochromic moieties and inten-sive studies have been focused on copolymers containing pendant azobenzenegroups because of the possibility of generating anisotropy Indeed alignment al-terations induced in such copolymers by exposure to linearly polarized light canbe permanently frozen-in and stored Since long durability is a prime require-ment for information storage materials with a high glass transition tempera-ture (higher than 100 C) seemed to be most appropriate [66] However in thecase of a liquid-crystalline polyester (P6a12 see Chart 518) containing azoben-zene side groups holographically recorded gratings endured at room tempera-ture over a period of several years and up to 104 write-record-erase cycles couldbe accomplished [67 68] Notably erasure is achieved by heating this polyesterto approximately 80 C This temperature is much higher than the glass transi-tion temperature of about 30 C and corresponds to the clearing temperature atwhich the liquid-crystalline domains form the mesophase melt

Similarly good long-term optical storage properties at room temperature havebeen reported for a liquid-crystalline copolymer composed of the moietiesshown in Chart 519 with phase transitions at 487 C (Tg) 832 C (SC) and969 C (SA) [69]

5 Photochromism138

Chart 518 Chemical structure of a polyester with pendant azobenzene groups

Chart 519 Chemical structures of the constituents of acopolymer with good optical storage properties

Large induced birefringences [see Eq (5-1)] up to n = 036 at 780 nm are ob-tained with liquid-crystalline copolymers containing the methyl methacrylate co-monomer presented in Chart 520 [70 71]

Since such copolymers possess besides a high storage capacity a high storagecyclability and moreover withstand temperatures up to 120 C they are utilizedby Bayer Material Science for high-tech storage systems The holography-relatedapplication potential of these materials includes forgery-proof storage systemsID cards for access control to high security areas etc [72]

Regarding the heat resistance of potential storage materials work on oligo-peptides (see Chart 521) is also noteworthy Holograms written in DNO films(write = 488 nm read = 633 nm) remained stable at room temperature for up toone year and were not erased upon exposure to 80 C for one month [73]

References 139

Chart 520 Chemical structure of a base unit of copolymersused for forgery-proof storage systems

Chart 521 Chemical structure of oligopeptides with good optical storage properties

References

1 H Bouas-Laurent H Duumlrr OrganicPhotochromism Pure Appl Chem 73(2001) 639

2 J C Crano R J Guglielmetti (eds) Or-ganic Photochromic and ThermochromicCompounds Vol 1 Photochromic FamiliesPlenum Press New York (1999)

3 G H Brown (ed) Photochromism Tech-niques in Chemistry III Wiley-Inter-science New York (1971)

4 H Duumlrr H Bouas-Laurent (eds) Photo-chromism Molecules and Systems ElsevierAmsterdam (1990)

5 M Irie Chem Rev 100 (2000) 16856 Y Yokoyama Chem Rev 100 (2000)

17177 G Berkovic V Krongauz V Weiss

Chem Rev 100 (2000) 17418 S Kawata Y Kawata Chem Rev 100

(2000) 17779 N Tamai H Miyasaka Chem Rev 100

(2000) 187510 CB McArdle (ed) Applied Photochromic

Polymer Systems Blackie Glasgow(1992)

11 M Irie Adv Polym Sci 94 (1990) 27

5 Photochromism140

12 O Nuyken C Scherer A Baindl A RBrenner U Dahn R Gaumlrtner S Kiser-Roumlhrich R Kollefrath P Matusche BVoit Prog Polym Sci 22 (1997) 93

13 F Ciardelli O Pieroni PhotoswitchablePolypeptides in [21]

14 O Pieroni A Fissi G Popova ProgPolym Sci 23 (1998) 81

15 T Kinoshita Prog Polym Sci 20 (1995)527

16 K Ichimura Chem Rev 100 (2000)1847

17 N Hampp Chem Rev 100 (2000) 175518 J A Delaire K Nakatani Chem Rev

100 (2000) 181719 S Xie A Natansohn P Rochon Chem

Mater 5 (1993) 40320 G S Kumar G Neckers Chem Rev 89

(1989) 191521 BL Feringa (ed) Molecular Switches

Wiley-VCH Weinheim (2001)22 M Irie H Tanaka Macromolecules 16

(1983) 21023 M Irie W Schnabel Light-Induced Con-

formational Changes in Macromolecules inSolution as Detected by Flash Photolysis inConjunction with Light Scattering Measure-ments in B Sedlacek (ed) Physical Op-tics of Dynamic Phenomena and Processesin Macromolecular Systems de GruyterBerlin (1985) p 287

24 M Irie M Hosoda Makromol ChemRapid Commun 6 (1985) 533

25 MS Beattie C Jackson G D JaycoxPolymer 39 (1998) 2597

26 M Irie W Schnabel Macromolecules 14(1983) 1246

27 J Anzai T Osa Tetrahedron 50 (1994)4039

28 O Pieroni F Ciardelli Trends in PolymSci 3 (1995) 282

29 I Willner Acc Chem Res 30 (1997)347

30 X Meng A Natansohn P Rochon JPolym Sci Polym Phys 34 (1996)1461

31 M Eich J H Wendorff B Reck HRingsdorf Makromol Chem RapidCommun 8 (1987) 59

32 M Eich J H Wendorff MakromolChem Rapid Commun 8 (1987) 467

33 NCR Holme L Nikolova PS Rama-nujam S Hvilsted Appl Phys Lett 70(1997) 1518

34 H Ringsdorf C Urban W Knoll MSawodny Makromol Chem 193 (1992)1235

35 FT Niesel J Rubner J Springer Mak-romol Chem Chem Phys 196 (1995)4103

36 Z Seccat P Pretre A Knoesen WVolksen VY Lee RD Miller J WoodW Knoll J Opt Soc Am B 15 (1998)401

37 Z Seccat J Wood W Knoll W VolksenR D Miller A Knoesen J Opt SocAm B 14 (1997) 829

38 Z Seccat J Wood EF Aust W KnollW Volksen R D Miller J Opt SocAm B 13 (1996) 1713

39 J A Delaire K Nakatani Chem Rev100 (2000) 1817

40 FH Kreuzer Ch Braumluchle A Miller APetri Cyclic Liquid-Crystalline Siloxanes asOptical Recording Materials in [48]

41 K Ichimura Y Suzuki T Hosoki KAoki Langmuir 4 (1988) 1214

42 T Ikeda S Horiuchi DB Karanjit SKrihara S Tazuke Macromolecules 23(1990) 36 and 42

43 K Ichimura Photoregulation of Liquid-Crystal Alignment by Photochromic Mole-cules and Polymeric Thin Films in [48]

44 (a) V P Shibaev S G Kostromin S AIvanov Comb-Shaped Polymers with Meso-genic Side Groups as Electro- and Photoop-tical Active Media in [48] (b) VP Shi-baev A Bobrovsky N Boiko ProgPolym Sci 28 (2003) 729

45 Y Wu A Kanazawa T Shiono T IkedaQ Zhang Polymer 40 (1999) 4787

46 D Creed Photochemistry and Photophysicsof Liquid-Crystalline Polymers in V Rama-murthy K S Schanze (eds) Molecularand Supramolecular Organic and Inorgan-ic Photochemistry Vol 2 Marcel DekkerNew York (1998)

47 CB McArdle (ed) Side-Chain Liquid-Crystal Polymers Blackie Glasgow (1989)

48 V P Shibaev (ed) Polymers as Electroopti-cal and Photooptical Active MediaSpringer Berlin (1996)

49 M Irie D Kungwatchakun Macromole-cules 19 (1986) 2476

50 EA Gonzalez-de los Santos J Lozano-Gonzalez A F Johnson J Appl PolymSci 71 (1999) 267

References 141

51 T Ikeda M Nakano Y Yu O TsutsumiA Kanazawa Adv Mater 15 (2003) 201

52 DY Kim S K Tripathy L Li J KumarAppl Phys Lett 66 (1995) 1166

53 P Rochon E Batalla A NatansohnAppl Phys Lett 66 (1995) 136

54 T Fukuda K Sumaru T Kimura HMatsuda J Photochem Photobiol AChem 145 (2002) 35

55 S Yang L Li A L Cholly J KumarSK Tripathy J Macromol Sci PureAppl Chem A 38 (2001) 1345

56 NK Viswanathan S BalasubramanianJ Kumar SK Tripathy J MacromolSci Pure Appl Chem A 38 (2001)1445

57 M Higuchi N Minoura T KinoshitaColloid Polym Sci 273 (1995) 1022

58 H Menzel Macromol Chem Phys 195(1994) 3747

59 Y Atassi J A Delaire K Nakatani JPhys Chem 99 (1995) 16320

60 Z Sekkat P Pretre A Knoesen WVolksen VY Lee RD Miller J WoodW Knoll J Opt Soc Am B 15 (1998)401

61 J C Crano WS Kwak CN WelchSpiroxazines and Their Use in Photo-chromic Lenses in [10]

62 M Irie High-Density Optical Memory andUltrafine Photofabrication Springer Se-ries in Optical Sciences 84 (2002) 137

63 J Whittall Fulgides and Fulgimides ndash aPromising Class of Photochromes for Appli-cation in [10]

64 T Tsujioka F Tatezono T Harada KKuroki M Irie Jpn J Appl Phys 33(1994) 5788

65 M Irie K Uchida Bull Chem SocJpn 71 (1998) 985

66 R Natansohn P Rochon C Barret AHay Chem Mater 7 (1995) 1612

67 NCR Holme S Hvilsted PS Rama-nujam Appl Optics 35 (1996) 4622

68 NCR Holme S Hvilsted PS Rama-nujam Opt Lett 21 (1996) 1902

69 Y Tian J Xie C Wang Y Zhao H FeiPolymer 40 (1999) 3835

70 BL Lachut SA Maier HA AtwaterMJ A de Dood A Polman R HagenS Kostromine Adv Mater 16 (2004)1746

71 R P Bertram N Benter D Apitz ESoergel K Buse R Hagen SG Kostro-mine Phys Rev E 70 (2004) 041802-1

72 Forgery-Proof Information Storage Genu-ine Security Bayer Scientific MagazineResearch 16 (2004)

73 R H Berg S Hvilsted P S Ramanu-jam Nature 383 (1996) 506

61Electrophotography ndash Xerography

According to Schaffertrsquos definition [1] electrophotography concerns the formationof images by the combined interaction of light and electricity and xerography is aform of electrophotography that involves the development of electrostatic chargepatterns created on the surfaces of photoconducting insulators The term xerogra-phy originates from the Greek words xeros (dry) and graphein (to write) which to-gether mean dry writing The xerographic process invented by Carlson in 1938 [2] isthe basis for copying documents with the aid of copying machines The impor-tance of xerography in our daily lives is unquestionable in view of the ubiquitousemployment of copying machines At present virtually all copiers use xerographyWith the advent of semiconductor lasers and light-emitting diodes xerography isalso widely applied in desktop printing [3ndash8] The principle of the xerographic pro-cess is outlined briefly in the following and depicted schematically in Fig 61

The essential part of a copying machine is the photoreceptor which nowadaysconsists mostly of organic material In order to make a copy of a document thephotoreceptor surface is first positively or negatively corona charged and subse-quently exposed to the light reflected from the document The resulting patternof exposed and unexposed areas at the photoreceptor corresponds to areas wherethe corona charges were neutralized or remained unaltered respectively Electro-statically charged toner particles brought into contact with the exposed photorecep-tor adhere exclusively to those areas that still carry charges To complete the copy-ing process the toner particles are transferred to a sheet of paper which is pressedonto the photoreceptor and then fixed (fused) by a thermal (infrared) treatment

Modern copying machines employ dual-layer photoreceptors (see Fig 62) Inthis way charge generation and charge transport are separated The charge genera-tion layer (CGL 05ndash50 m) is optimized for the spectral response and the quan-tum yield of charge carrier formation and the charge transport layer (CTL 15ndash30 m) is optimized for the drift mobility of the charge carriers and for wear re-sistance

Dual-layer systems have the advantages of high sensitivity long process life-time and a reduction in the hysteresis of latent image formation The transportlayer requires the displacement of either electrons or holes Since most trans-

143

6Technical developments related to photophysical processesin polymers

port layers are formulated to transport holes dual-layer receptors are usuallynegatively charged

Numerous compounds have been tested and applied commercially as charge-generation and charge-transport materials as can best be seen from the bookby Borsenberger and Weiss [4]

6 Technical developments related to photophysical processes in polymers144

Fig 61 Schematic depiction of the xerographic process for apositively corona-charged single-layer photoreceptor

Fig 62 Schematic depiction of the light-induced dischargeprocess for a negatively corona-charged dual-layer photo-receptor CGL and CTL denote the charge generation layerand the charge transport layer respectively

The first all-organic photoreceptor was a single-layer device consisting of a1 1 molar mixture an electron-donor polymer poly(N-vinyl carbazole) and anelectron acceptor TNF (see Chart 21) A very effective dual-layer system desig-nated by the acronym TiO(F4-Pc) TTA contains a dispersion of tetrafluorotita-nylphthalocyanine in poly(vinyl butyral) in the charge-generation layer and amixture of tris(p-tolylamine) and polycarbonate in the charge-transport layerHighly sensitive charge-generation systems appropriate for visible and also fornear-infrared light were obtained upon doping polymers with pigment particlesof dyes In this case the CG layers consist of a light-sensitive crystalline phasedispersed in the polymeric matrix Besides phthalocyanines pigments employedcomprise azo compounds squaraines and polycyclic aromatic compounds (thechemical structures of which are shown in Table 21) Improved sensitivitieshave sometimes been achieved with pigment mixtures As a typical exampleFig 63 presents results obtained with a dual-layer system [8 9] Here the CGlayer consisted of a dispersion of the triphenylamine triazo pigment AZO-3 (seeChart 61) in poly(vinyl butyral) in a 4 10 weight ratio while the CT layer con-sisted of a mixture of bisphenol A polycarbonate and the triarylamine derivativeMAPS (see Chart 61) in a 10 9 weight ratio

Note that the value of the quantum yield of charge carrier formation is veryhigh about 045 at F= 3105 V cmndash1 and remains practically constant over theinvestigated wavelength range from 470 to 790 nm Interestingly the quantumyield found for the single-layer system was about one order of magnitude lowerThe very high quantum yield is interpreted in terms of exciton dissociation atthe interface between the two layers and injection of practically all of the holesinto the charge-transport layer

61 Electrophotography ndash Xerography 145

Fig 63 Charge generation in a dual-layerphotoreceptor system The quantum yield ofcharge generation as a function of the wave-length of the incident light at

F = 3105 V cmndash1 () and F= 08105 V cmndash1

() See text for system characterizationAdapted from Williams [8] with permissionfrom John Wiley amp Sons Inc

Regarding the charge-transport layers materials for hole and electron trans-port have to be discriminated A large number of hole-transport materials con-tain arylamine moieties Moreover polysilylenes are well-suited for hole trans-port A key requirement for dual-layer systems is a high efficiency of charge in-jection from the generation layer into the transport layer Moreover it is impor-tant that the charge transport is not impeded by trapping and that the transittime is short compared to the time between exposure and development Formost applications a hole mobility between 10ndash6 and 10ndash5 cm2 Vndash1 sndash1 is suffi-cient

The requirements for electron-transport materials cannot be fulfilled easilyFor instance an appropriate compound should be weakly polar and have a lowreduction potential ie a high electron affinity Actually the electron affinityshould be higher than that of molecular oxygen which is always present Forthis reason and because of some additional difficulties electron-transport layershave not yet been used in commercial applications [4]

62Polymeric light sources

One of the most fascinating developments in recent times concerns the genera-tion of light with the aid of polymers This development is characterized by twoinventions which are described in the following subsections the polymericlight-emitting diode and the polymer laser


Chart 61 Chemical structures of the triphenylamine triazopigment AZO-3 and the triarylamine derivative MAPS

621Light-emitting diodes

6211 General aspectsPolymeric light-emitting diodes operate on the basis of electroluminescence ieluminescence generated by the application of high electric fields to thin poly-mer layers Devices based on the electroluminescence of organic materials com-monly denoted as organic light-emitting diodes OLEDs are used for examplefor mini-displays in wrist watches and chip cards for flexible screens and foremitting wall paper In contrast to liquid-crystal displays (LCDs) OLED displayscan be seen from all viewing angles OLED devices can be extremely thin flex-ible and of low weight Moreover production costs and energy consumptionare low Consequently the potential for making large-area multicolor displaysfrom easily processable polymers has initiated a large number of research pro-

62 Polymeric light sources 147

Table 61 Poly(p-phenylene vinylene)s used in light-emitting diodes [11 12 20]

Polymer Acronym EL Maximum (nm)

PPV 540

PMPPV 560

MEH-PPV 590

PMCYH-PV 590

PDFPV 600

PPFPV 520

jects in the area of polymer light-emitting diodes as has been documented byseveral reviews [10ndash23]

The phenomenon of polymer-based electroluminescence was first demon-strated in the case of poly(p-phenylene vinylene) PPV ( energy gap25 eV) [24] and was later also observed with many PPV derivatives and otherfully -conjugated polymers Typical representatives are shown in Tables 61 and62 Table 61 relates to PPV and some of its derivatives whereas Table 62 listsother classes of polymers that have been employed in LED work


Table 62 Polymers employed in light-emitting diodes [10a]

Polymer class Structure of typical polymer Characteristics

Polythiophenesp-Type (hole-transporting) polymers Alkylgroups provide for solubility in organic sol-vents Emission tunable from UV to IRthrough varying the substituent

Poly-p-phenylenesp-Type polymers of rather high thermal sta-bility mostly used in the form of polymerscontaining oligo-p-phenylene sequencesEmit light in the blue wavelength range

Polyfluorenesp-Type polymers of improved thermal andphotostability (relative to PPV) Emit lightprimarily in the blue wavelength range

R typically hexyl octyl ethylhexyl

Cyano polymersPolymers eg PPV derivatives containingelectron-withdrawing cyano groups The lat-ter provide for electron transport thus com-plementing the hole-transport property

Pyridine-containingpolymers

Highly luminescent polymers soluble in or-ganic solvents High electron affinity affordsimproved electron transportQuaternization of nitrogen allows manipula-tion of the emission wavelength

Oxadiazole-containingpolymers

Oxadiazole groups provide for efficient elec-tron transport Insertion of these groupsinto p-type polymers facilitates bipolar car-rier transport

In this connection the reader is referred to a rather comprehensive reviewdealing with the various classes of polymers tested for LED application [10 a]and to a list of appropriate commercially available materials [25]

As can be seen from Fig 64 a an OLED consists in the simplest case of apolymer film placed between two electrodes one of them being light-transpar-ent such as indium tin oxide (ITO) and the other being a metal of low workfunction eg barium calcium or aluminum

Holes and electrons are injected from the ITO electrode (anode) and the me-tal electrode (cathode) respectively The energy level diagram under forwardbias is shown in Fig 65 More sophisticated OLEDs possess multilayer struc-tures as shown in Fig 64 b


Fig 64 (a) Structure of a single-layer polymer LED(b) Structure of a multilayer polymer LED

Fig 65 Energy level diagram of a single-layer polymer LEDunder forward bias The z-direction is parallel to the currentdirection and hence perpendicular to the layer Adapted fromGraupner [13] with permission from the Center forPhotochemical Sciences Bowling Green

As can be seen from the typical luminancendashvoltage characteristic presented inFig 66 light generation requires a minimum voltage the turn-on voltage atwhich light emission commences

The luminance increases drastically on further increasing the voltage immedi-ately beyond the onset and later approaches saturation The curve in Fig 66 refersto a 240 1 blend of the polymers denoted as MEH-PPV and PCzDBT20 (seeChart 62) [26] In this case red light with a maximum at about 680 nm is emittedHere the turn-on voltage is quite low (lt 2 V) and the external quantum yield israther high 13ext = 0038 13ext represents the number of photons penetrating thedevice surface to the outside generated per injected electron The availability ofhighly efficient OLEDs emitting light of the primary colors ndash red green and bluendash is important for the realization of full color display applications

6212 MechanismThe injection of charges from the electrodes into the bulk organic material isdetermined by various parameters Since holes are injected into the highest oc-cupied molecular orbital (HOMO) and electrons into the lowest unoccupied mo-lecular orbital (LUMO) matching of energy levels is required This is demon-


Fig 66 Luminancendashvoltage characteristic for the polymerblend PCzDBT20MEH-PPV (1240) Adapted from Niu et al[26] with permission from Wiley-VCH

Chart 62 Polymers contained in the blend referred to in Fig 66

strated for a two-layer OLED of the structure shown in Chart 63 by the energylevel diagram presented in Fig 67 [12]

This diagram illustrates the equivalence of the valence band with the ioniza-tion potential (IP) and the HOMO as well as that of the conduction band withthe electron affinity (EA) and the LUMO Notably electron and hole injectionare controlled by the energy barrier between the contact and the organic materi-al In the absence of surface states and a depletion region due to impurity dop-ing the energy barriers are given by Eqs (6-1) and (6-2)

Eh IP 13anode for holes 6-1

Eel 13cathode EA for electrons 6-2

Here 13anode and 13cathode denote the work functions of the contact materialsDepending on the magnitude of E the current flow through an OLED can beeither space-charge limited (SCL) ie transport-limited or injection-limited Pre-requisites for SCL are that the injection barrier is rather low and that one of thecontacts supplies more charge carries per unit time than can be transportedthrough the organic material layer Commonly injection-limited conduction isdescribed by Fowler-Nordheim (FN) tunneling into the transport band or by Ri-chardson-Schottky (RS) thermionic emission [27 28] The FN model ignores im-age-charge effects and assumes tunneling of electrons from the contact throughthe barrier into a continuum of states The RS model assumes that electronscapable of ejection from the contact have acquired sufficiently high thermal en-ergies to cross the potential maximum resulting from the superposition of theexternal and the image-charge potentials These models were developed forband-type materials However it turned out that they are inadequate for describ-


ITO anodehole-transporting layer (HTL)emitting layer (EML)metal cathode

Chart 63 Structure of a two-layer OLED

Fig 67 Energy level diagram fora two-layer polymer LEDshowing the ITO anode thehole-transporting layer HTL theemitting and electron-transporting layer EML and themetal cathode EV denotes thevacuum potential

ing the currentndashvoltage dependence measured for disordered organic materials[29] In organic materials the charge carriers are not very mobile because theyare localized and the transport involves localized discrete hopping steps withina distribution of energy states For charge carrier injection of electrons from ametal contact into such organic hopping systems a Monte Carlo simulationyielded excellent agreement with the experimentally observed dependence of theinjection current on electric field strength and temperature [30 31] It is basedon the concept of temperature and field-assisted injection from the Fermi levelof an electrode into the manifold of hopping states Under the influence of theapplied electric field the injected oppositely charged carriers migrate throughthe system towards the electrodes and a portion of them eventually combine toform excited electron-hole singlet states so-called singlet excitons The latter un-dergo radiative decay to only a small extent that is to say electroluminescencequantum yields in terms of emitted photons per injected electron are relativelylow and amount to only a few per cent even in the best cases Competing pro-cesses are operative such as singlet-triplet crossing singlet-exciton quenchingetc Figure 68 shows typical photoluminescence and electroluminescence spec-tra recorded for PPV and two PPV derivatives


Fig 68 Photoluminescence (a)and electroluminescence spec-tra (b) of PPV PMCYH-PV andPPFPV Adapted from Shim etal [11] with permission fromSpringer

Obviously in these cases the maxima of both types of emission spectra arealmost the same indicating that the emission originates from the same speciesIn both cases the peak position is red-shifted when strongly electron-donatinggroups are attached to the conjugated backbone of the polymer Therefore it ispossible to tune the color of the electroluminescent emission by varying thechemical nature of the substituent A blue color can be obtained by wideningthe gap through shortening the conjugation length and lowering the elec-tron density in the conjugated backbone In the case of PPFPV the emissionmaximum lies in the greenish-blue region Here the strong electron-withdraw-ing influence of the perfluorobiphenyl group lowers the electron density in the


Table 63 Hole and electron transport materials employed in polymer LEDs [10a]

Chemical structure Acronym

Hole transport materials

TPD

PPV

PVK

PMPS

Electron transport materials

PBD

Alq3

PMA-PBD

polymer chain and thus causes a shift of the maximum from 540 nm (PPV) toabout 520 nm

Notably the major steps in the electroluminescence mechanism are injectiontransport and recombination of charge carriers Good carrier transport and effi-cient recombination in the same material are antagonists because the combina-tion probability is low if the charge carriers swiftly migrate to the electrodeswithout interaction with their oppositely charged counterparts A solution tothis dilemma was found with devices consisting of several layers In manycases a layer allowing swift hole transport and blocking of the passage of elec-trons has been combined with a layer permitting only electron transport andserving as an emitting layer Table 63 presents typical hole and electron trans-port materials [10 a]

6213 Polarized light from OLEDsProvided that the macromolecules in a thin film employed as an emitting layerin a LED device are well oriented the emitted light is largely polarized [31] Re-garding conjugated polymers this phenomenon has attracted broad interest be-cause low-cost techniques for chain alignment in such polymers are availablePolarized electroluminescence is useful for certain applications for instance forthe background illumination of liquid-crystal displays (LCDs) [20 32] The firstLED device emitting polarized light was realized with the stretch-oriented poly-thiophene PTOPT (see Chart 64) [33]

The methods commonly used for chain alignment in polymer films havebeen reviewed [34] They comprise the Langmuir-Blodgett technique rubbing ofthe film surface mechanical stretching of the film and orientation on pre-aligned substrates As an example electroluminescence spectra of the orientedsubstituted poly(p-phenylene) presented in Chart 65 are shown in Fig 69 a [35]

The device prepared by the Langmuir-Blodgett (LB) technique had the struc-ture shown in Chart 66


Chart 64 Chemical structure of poly[3-(4-octylphenyl)-22-bithiophene] PTOPT

Chart 65 Chemical structure of an orientedsubstituted poly(p-phenylene) [35]

As demonstrated schematically in Fig 69 b the rigid rod-like macromoleculesare oriented parallel to the substrate plane and their backbones exhibit a prefer-ential orientation along the dipping direction employed during LB processing

From the emission spectra recorded with the polarization of the light paralleland perpendicular to the dipping direction the polarization ratio can be esti-mated to be somewhat greater than three

6214 White-light OLEDsIn many cases OLED devices have been developed that contain polymers ashole-transport media and low molar mass organic or inorganic compounds asemitting materials This pertains for instance to certain white-light-emittingLEDs two of them being exemplified here The first case refers to a device con-taining CdSe nanoparticles in the emitting layer These particles are embeddedin a polymer namely PPV A device having the multilayer structure shown inChart 67 produces almost white light under a forward bias of 35ndash50 V [36]

The second case refers to a device containing a platinum compound such asFPt-1 or FPt-2 in the emitting layer (see Chart 68)

A device having the multilayer structure shown in Chart 69 emits white lightwith 13ext = 19 at a brightness of 100 cd mndash2 (J = 2 mA cmndash2) The white lightresults from the simultaneous monomer (blue) and excimer (green to red)emission of the Pt compound [37]


Fig 69 (a) Electroluminescence spectra ofthe oriented substituted poly(p-phenylene)SPPP The emission spectra were recordedwith the polarization direction parallel andperpendicular to the dipping direction

employed during preparation by the LBtechnique (b) Schematic depiction of rigidrod-like macromolecules oriented parallel tothe substrate plane Adapted from Cimrovaet al [35] with permission from Wiley-VCH

ITO anode100 monolayers SPPPAl cathodeChart 66 Device used for recording the electroluminescence spectra depicted in Fig 69a

622Lasers

6221 General aspectsThe term laser is an acronym (light amplification by stimulated emission of ra-diation) that denotes a technical device operating on the basis of the stimulatedemission of light A laser emits monochromatic spatially coherent and stronglypolarized light The essential parts of a laser device are an active material and aresonator ie an optical feedback (see Fig 610)

In classical laser systems such as Ti sapphire-based systems or semiconduc-tor laser diodes the active materials are inorganic compounds In recent yearssuitable organic active materials have been introduced [38ndash41] These organicmaterials may be divided into two classes hostguest systems consisting of ahost material doped with organic dye molecules and systems consisting of con-jugated polymers Typical dyes used in hostguest systems are rhodamines cou-marins and pyrromethenes and these are dissolved in polymeric hosts such aspoly(methyl methacrylate) or methacrylate-containing copolymers In some


ITO anodePEI(CdSe-PPV)Al cathodeChart 67 Device used to produce almost white light PEIpoly(ethylene imine) ndash(CH2ndashCH2ndashNH)nndash

Chart 68 Chemical structures of Pt-containing compounds used to produce white light

ITO anodePEDOTPSS(FPt2-CBP)BCPLiFAl cathodeChart 69 Device used to produce white light PEDOTpoly(34-ethylenedioxythiophene) PSS poly(styrene sulfonicacid) CBP 44-di(N-carbazolyl)-biphenyl (see Chart 610)BCP bathocuproine (29-dimethyl-47-diphenyl-1-10-phenan-throline)

Fig 610 Schematic illustration of an opticallypumped laser device Adapted fromKranzelbinder et al [38] with permission fromthe Institute of Physics Publishing Bristol UK


Chart 610 Chemical structures of 44-di(N-carbazolyl)-biphenyl CBP and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-134-oxadiazole PBD

Table 64 Conjugated polymers used as laser materials

Polymer a) Chemical structure Resonator Excitationconditions

Ithresholdb)

(J cmndash2)Ref

DOO-PPV Microring = 532 nm= 100 ps

01 [43]

BEH-PPV Microring = 555 nm= 100 fs 25 [44]

BuEH-PPV Microcavity = 435 nm= 10 ns

45 [45]

m-LPPPFlexibledistributedfeedback

= 400 nm= 150 ps

37 [46]

PDOPT Microcavity = 530 nm= 90 fs

012 [47]

a) Acronyms used in this column DOO-PPV poly(25-dioctyloxy-p-phenylene viny-lene) BEH-PPV poly[25-di-(2-ethylhexyloxy)-p-phenylene vinylene] BuEH-PPVpoly[2-butyl-5-(2-ethylhexyl)-p-phenylene vinylene] m-LPPP ladder-type poly(p-phenylene) bearing methyl groups PDOPT poly[3-(25-dioctylphenyl)thiophene]

b) Threshold pulse intensity for lasing

cases low molar mass materials have been employed as host materials such asCBP or PBD (see Chart 610)

In systems of the type PBDpoly(p-phenylene vinylene) derivative the hostmaterial PBD absorbs the pump light and transfers the excitation energy tothe polymer here the emitting guest [42] Appropriate conjugated polymers cit-ed in the literature are presented in Table 64

It seems that m-LPPP a ladder-type poly(p-phenylene) is one of the mostpromising materials for laser application It is soluble in nonpolar organic sol-vents thus enabling the facile preparation of thin layers on substrates that maypossess structured uneven surfaces

6222 Lasing mechanismAt present polymer lasers are operated by optical pumping ie through the ab-sorption of light by the active material A four-level energy scheme similar tothat used for organic laser dyes serves to explain the lasing mechanism in thecase of conjugated polymers As can be seen in Fig 611 the absorption of aphoton corresponds to a transition from the lowest vibronic level of the groundstate S0 to a higher-lying vibronic level of the singlet state S1

Rapid (non-radiative) internal conversion leads to the lowest vibronic excita-tion level of the S1 manifold Subsequent transition from this level to one of thevibronic excitation levels of the S0 manifold is radiative and corresponds toeither spontaneous or stimulated emission SE In terms of a simple modelstimulated emission is generated through the interaction of the excited mole-cules with other photons of equal energy This process can only become impor-tant with respect to other competitive processes such as spontaneous emissionwhen the concentration of excited states is very high ie when the populationof the upper state exceeds that of the lower state a situation denoted by theterm population inversion In other words the Boltzmann equilibrium of statesmust be disturbed Notably the lasing transition relates to energy levels that arenot directly involved in the optical pumping process The laser potential of anactive material is characterized by Eq (6-3)


Fig 611 Energy scheme illustrating stimulated emission in conjugated polymers

Iout Iin expNexcL 6-3

Here Iin and Iout denote the intensities of the incoming and outgoing beam re-spectively is the cross-section for stimulated emission Nexc is the concentra-tion of excited S1 states and L is the path length of the light in the sample Theterm gnet = Nexc represents the net gain coefficient of the material

As pointed out above the transition from spontaneous to stimulated emissionrequires population inversion In other words SE becomes significant whenNexc exceeds a critical value Nexc(crit) which characterizes the lasing thresholdExperimentalists frequently denote the threshold in terms of the energy ormore exactly the intensity Ithreshold of the excitation light pulse Figure 612shows a schematic depiction of the dependence of the laser output on the inten-sity of the excitation light pulse

Typical Ithreshold values are given in Table 64 In films of conjugated poly-mers Nexc(crit) is about 1018 cmndash3 if a resonator is not operative Significantlythe employment of appropriate feedback structures lowers the threshold by sev-eral orders of magnitude

6223 Optical resonator structuresAs has been pointed out above a laser basically consists of an active materialand a resonator The latter enables the build-up of certain resonant modes andessentially determines the lasing characteristics In most conventional devicesthe optical feedback is provided by an external cavity with two end mirrorsforming the resonator With the advent of polymers as active materials variousnew feedback structures were invented Initially a microcavity resonator deviceof the type shown schematically in Fig 613 a was employed [48]

This device consisted of a PPV layer placed between a highly reflective distrib-uted Bragg reflector DBR and a vacuum-deposited silver layer functioning as thesecond mirror The emission characteristics at different intensities of the pumpinglight are shown in Fig 613 b At low intensity the emission consisted of three dif-ferent modes whereas at high intensity it was concentrated into the mode of thehighest gain Moreover the directionality of the emitted light was enhanced by in-creasing the intensity of the exciting light Both effects were taken as evidence for


Fig 612 Schematic depiction of thedependence of the intensity of the lightemitted from a laser device on theintensity of the exciting light

the occurrence of lasing During the ensuing development resonators in theshape of microspheres microrings and flat microdisks were designed As an ex-ample Fig 614a shows a schematic depiction of a cylindrical microring laser de-vice with an outer diameter of D= 11 m and a lateral length of about 100 m con-sisting of a thin DOO-PPV film coated onto an optical fiber

When the device was excited with 532 nm light pulses (= 100 ps) at an intensitybelow the lasing threshold (100 pJpulse) the spectrum shown in Fig 614 b ex-tending over about 100 nm was emitted Dramatic changes occurred when the in-tensity of the excitation light pulse exceeded the lasing threshold the emissionspectrum collapsed into several dominant microcavity modes [43]

Another device the flexible distributed Bragg reflector laser with an activelayer structure supporting second-order feedback makes full use of the advanta-geous properties of polymers namely flexibility large-area fabrication and low-cost processing [41 42] As can be seen in Fig 615 the device consists of aone-dimensionally periodically structured flexible substrate coated with an m-LPPP layer which acts as a planar wave guide The substrate possesses a peri-odic height modulation with a period of = 300 nm

The surface of the polymer layer exhibits a height modulation with the same per-iod but a smaller amplitude (lt 10 nm) It should be pointed out that the polymerlayer in the device considered here functions as a distributed Bragg reflector and the


Fig 613 The microcavity a vertical cavitylasing device (a) Schematic depiction of thedevice consisting of a distributed Braggreflector a PPV layer and a silver layer(b) Spectra emitted at two different pump

laser energies Eexc = 005 Jpulse (dashedline) and Eexc = 11 Jpulse (solid line)Pulse duration 200ndash300 ps Adapted fromTessler et al [48] with permission fromMcMillan Publishers Ltd

resonant modes for laser oscillation in this strongly frequency-selective feedbackdevice correspond to the wavelength satisfying the Bragg condition [see Eq (6-4)]

m 2n 6-4Here m is the order of diffraction n is the refractive index and is the gratingperiod (height modulation period) Optical feedback is accomplished by way ofthe second-order diffraction mode (m= 2) which is fed into the counter-propa-gating wave The first-order light (m= 1) is coupled out from the waveguide andpropagates perpendicular to the film Provided that the energy of the excitinglight pulses (pulse duration 150 fs 400 nm spot size diameter 200 m) ex-ceeds the threshold value Ethreshold = 15 nJ highly polarized laser light(= 488 nm) is emitted perpendicular to the film plane An improvement overthis method of mode selection was achieved with the aid of two-dimensionallynano-patterned substrates [49] The device depicted schematically in Fig 616emits a monomode beam perpendicular to its surface


Fig 614 Microring laser device (a)and spectra emitted at excitation lightintensities below (b) and above (c) thethreshold intensity Active materialDOO-PPV coated onto an optical fiberAdapted from Frolov et al [43] withpermission from the American Instituteof Physics

Fig 615 Schematic illustration of a one-dimensionally patterned flexible distributed Braggreflector laser device Active layer 400 nm m-LPPPSubstrate 125 m thick poly(ethylene terephthalate)film covered with acrylic coating Adapted fromKallinger et al [46] with permission from Wiley-VCH

Compared to the one-dimensionally structured device the lasing threshold is30 lower and the divergence of the emission is drastically reduced In accor-dance with the 2D laser operation the emitted light is not polarized in this case

6224 Prospects for electrically pumped polymer lasersAt present an electrically driven polymer laser has yet to be realized [39] Never-theless low-cost polymer laser diodes could be an attractive alternative to thewidely used inorganic laser diodes In principle an electrically pumped polymerlaser could be realized with the aid of an appropriate feedback structure pro-vided that the excitation density Nexc(crit) ie the concentration of excitons ex-ceeded the lasing threshold (see Section 6222) From research concerning opti-cally pumped polymer lasers it is known that Nexc(crit) is about 1018 cmndash3 Thisvalue corresponds to a critical current density of 105 to 106 A cmndash2 [50] How-ever the highest current densities hitherto obtained are about 103 A cmndash2 ieseveral orders of magnitude below the required value Therefore besides thesearch for appropriate device structures and appropriate highly conducting ma-terials strategies aiming at an electrically pumped polymer laser are also con-cerned with achieving much higher exciton concentrations An approach in thisdirection may lie in the application of sharp-edge shaped electrodes with the po-tential of generating locally very high electric fields enabling the formation oflocally very high charge carrier concentrations through field-induced emission

63Polymers in photovoltaic devices

Photovoltaic (PV) cells generate electric power when irradiated with sunlight orartificial light Classical PV cells based on inorganic semiconducting materials


Fig 616 Schematic illustration of a flexiblepolymer laser device consisting of anm-LPPP layer spin-coated onto a two-dimensionally structured flexible poly(ethy-lene terephthalate) substrate The laser light

is emitted perpendicular to the substrateAdapted from Riechel et al [49] withpermission from the American Institute ofPhysics

such as silicon GaAs CdTe or CuInSe2 consist of layers doped with smallamounts of additives that provide n-type (electron) or p-type (hole) conductivity[51ndash59] A ldquobuilt-inrdquo electric field exists across the junction between the two layerswhich sweeps electrons from the n to the p side and holes from the p to the n sideFigure 617 shows the essential features of a (sandwich-structured) p-n homojunc-tion silicon solar cell

The absorption of photons having energies greater than the band gap energypromotes electrons from the valence to the conduction band thus generatinghole-electron pairs The latter rapidly dissociate into free carriers that move in-dependently of each other As these approach the junction they come under theinfluence of the internal electric field which actually prevents recombinationAt present most of the industrially produced photovoltaic cells consist of mono-crystalline or polycrystalline and to some extent of amorphous silicon (a-Si) Dif-ferent types of junctions may be distinguished homojunctions are p-n junctionsformed by adjacent p- and n-doped regions in the same semiconductor of bandgap Ug whereas heterojunctions are formed between two chemically differentsemiconductors with different band gaps Moreover there are p-i-n junctionswhich are formed by interposing an intrinsic undoped layer between p and nlayers of the same semiconductor

Certain organic materials also possess semiconductor properties and can beemployed in PV cells a fact that has recently been attracting growing interestsince the advent of novel polymeric materials [22 60ndash66] Table 65 lists sometypical polymers used in solar cells

Criteria commonly used to characterize PV cells comprise Jsc the short-circuitcurrent density Voc the open-circuit voltage 13cc the quantum efficiency for

63 Polymers in photovoltaic devices 163

Fig 617 Schematic depiction of a p-n homojunctioncrystalline silicon solar cell Typical dimensions of commercialwafers 10 cm10 cm03 mm Adapted from Archer [67]with permission from the World Scientific PublishingCompany


Table 65 Chemical structures of semiconducting polymersused in organic solar cell devices [60ndash66]


MDMO-PPV Poly[2-methoxy-5-(37-dimethyl-octyloxy)-14-phenylene vinylene]

MEH-PPV Poly[2-methoxy-5-(2-ethyl-hexyl-oxy)-14-p henylene vinylene]

MEH-CN-PPV Poly[2-methoxy-5-(2-ethyl-hexyl-oxy)-14-phenylene (1-cyano)vinyl-ene]

CN-PPV Poly[25-di-n-hexyloxy-14-phenyl-ene (1-cyano)vinylene]

P3HT Poly(3-hexylthiophene)

POPT Poly[3-(4-octylphenyl)thiophene]

PEOPT Poly3-[4-(147-trioxaoctyl)-phenyl]thiophene

PEDOT Poly(34-ethylenedioxy thiophene)

PDTI Thiophene-isothianaphthenecopolymer

PTPTB Benzothiadiazole-pyrrolecopolymer

charge carrier generation ie the number of electrons formed per absorbedphoton ffill the fill factor and mp the maximum power conversion efficiencyffill and mp are defined by Eqs (6-5) and (6-6) respectively [67]

ffill impVmpIscVoc 6-5

mp impVmpDr ffilliscVocDr 6-6

Here imp and Vmp denote the current and the voltage at maximum power andDr (W cmndash2) is the incident solar irradiance

Compared with inorganic PV cells organic PV cells resemble the heterojunc-tion type apart from the fact that organic materials do not support the forma-tion of a space-charge region at the junction Figure 618 shows a schematic de-piction of a cell simply formed by the superposition of two layers of semicon-ducting organic materials with different electron affinities and ionization poten-tials One layer functions as the electron donor (p-type conductor) and the otheras the electron acceptor (n-type conductor) In this case the absorption of aphoton is confined to a molecule or to a region of a polymer chain where anexcited state is created This localized excited state is frequently termed an exci-ton (see Section 222) It refers to an electron-hole pair in semiconductor termi-nology Charge separation at the interphase requires that the difference in ener-gies of the hole states and the electron states exceeds the binding energy of theelectron-hole pairs This amounts to about 100 meV and is much larger thanthe input energy required for charge separation in inorganic semiconductorsThe efficiency of charge separation is critically determined by the exciton diffu-sion range since after its generation the exciton must reach the junction in or-der to dissociate into two free charge carriers Actually the exciton diffusionrange is at most a few nanometers and therefore a portion of the excitons gen-erated in the bulk of the layer do not dissociate In the course of efforts to over-come this flaw of flat-junction organic solar cells new architectures consistingof phase-separated polymer blends were devised [68ndash70] Figure 619 shows thestructure of such a system and the charge transfer from an exciton at a donoracceptor heterojunction These blend systems consist of interpenetrating bicon-tinuous networks of donor and acceptor phases with domain sizes of 5ndash50 nmand provide donoracceptor heterojunctions distributed throughout the layerthickness In this case the mean distance that the excitons have to travel toreach the interface is within the diffusion range and therefore efficiencies for


Fig 618 Schematic depictionof a flat-heterojunction organicsolar cell

the conversion of incident photons to electric current of over 50 have beenachieved Such systems can be formed for example from blends of donor andacceptor polymers such as MEH-PPV and CN-PPV [68 69] or from compositesof conducting polymers with buckminsterfullerenes such as MEH-PPV+ C60 orpoly(3-hexylthiophene) (P3HT) + C60 [70ndash74] In the latter cases the preparationof appropriate composites is facilitated by using fullerene derivatives with im-proved solubility such as PCBM the structure of which is presented inChart 611 [65 75]

In typical experiments thin (100 nm) films of polymer blends were depositedby spin coating from a solution of the two polymers Alternatively two thin filmsof a hole-accepting and an electron-accepting polymer that had been deposited onITO or metal substrates were laminated together in a controlled annealing pro-


Fig 619 Schematic diagram depicting charge transfer froman exciton at a donoracceptor heterojunction in a compositeof two conducting polymers

Chart 611 Chemical structure of 1-(3-methoxycarbonyl)-propyl-1-phenyl-[66]C61 PCBM

cess In the latter case a 20ndash30 nm deep interpenetration between the two layerswas revealed by atomic force microscopy [76] Performance characteristics of someof these organic PV cells and those of silicon cells are shown in Table 66

Obviously the performance of organic cells having bicontinuous networkstructures with quantum efficiencies of about 50 and power conversion effi-ciencies of about 5 remains far inferior to that of silicon cells but is highlyimproved as compared to that of flat-junction organic cells which have bothquantum efficiencies and power conversion efficiencies of less than 01

In conclusion for various reasons certain organic materials and especiallypolymers are attractive for use in photovoltaics There is the prospect of inex-pensive production of large-area solar cells at ambient temperature since high-throughput manufacture using simple procedures such as spin-casting or spraydeposition and reel-to-reel handling is feasible It is possible to produce verythin flexible devices which may be integrated into appliances or building mate-rials Moreover it seems that new markets will become accessible with the aidof polymer-based photovoltaic elements This concerns daily life consumergoods such as toys chip cards intelligent textiles and electronic equipmentwith low energy consumption

64Polymer optical waveguides

641General aspects

With the advent of semiconductor lasers a new technique of information trans-mission based on optical fibers was developed [77] Instead of propagating dataelectronically by the transport of electrons through coaxial copper cables the

64 Polymer optical waveguides 167

Table 66 Performance characteristics of solar cells

Material system Jsca)

(mA cmndash2)Voc

b)

(V)ffill

c) mpd)

()cc

e) Ref

P3HTPCMB (1 08) 95 063 068 51 [70a]P3HTPCMB (1 1) 106 061 067 44 [70c]MDMO-PPVPCBM 525 082 061 25 050 (470 nm) [70d]POPTMEH-CN-PPV ca 1 ca 1 032 19 029 [76]Amorphous silicon 194 089 074 127 090 [61]Monocrystalline silicon 424 071 083 247 gt090 [61]

a) Short-circuit current densityb) Open-circuit voltagec) Fill factord) Maximum power conversion efficiencye) Quantum efficiency for charge carrier generation

new technique permits optical data transfer by laser light pulses guided throughbranching optical networks operated with the aid of optical fibers Optical fibersconsist of a highly transparent core and a surrounding cladding of refractive in-dices ncore and ncladding respectively Provided that ncore gt ncladding light enteringthe fiber at an angle ltmax is totally reflected at the cladding boundary and isthus transmitted through the fiber

At present copper conductors are still used in short-distance data communi-cation However they can no longer cope with the high bandwidth demands ofmodern communication systems Therefore copper wiring systems are going tobe replaced by high-bandwidth fiber-optic systems The size and weight of opti-cal fiber cables are significantly lower than those of coaxial copper wire cablesin which the single wires must be carefully isolated to prevent electromagneticinterference

642Optical fibers

6421 Polymer versus silica fibersInitially the new fiber-optic technique was based solely on inorganic glass fibersbut in recent years polymeric optical fibers have also become attractive and appearto be in great demand for the transmission and the processing of optical commu-nications compatible with the Internet [78ndash84] As compared with silica fiberspolymer fibers have a larger caliber are cheaper to prepare and easier to processHowever because of their greater light attenuation and their lower frequencybandwidth for signal transmission polymer fibers can only be employed in infor-mation networks over distances of several hundred meters Typical properties ofpolymer and inorganic glass optical fibers are compared in Table 67

Silica fibers are still unsurpassed as regards attenuation and bandwidth buttheir diameter has to be kept rather small to provide for the required cable flex-ibility Consequently skillful hands and high precision tools are required to con-nect silica fibers in a time-consuming process Polymer fibers have a much low-


Table 67 Typical properties of step-index optical fibers [85]

Property PMMA a) Polycarbonate Silica glass

Attenuation coefficient (dB kmndash1) b) 125 at 650 nm 1000 at 650 nm 02 at 1300 nmTransmission capacity Ctrans (MHz km) c) lt 10 lt 10 102 to 103

Numerical aperture 03 to 05 04 to 06 010 to 025Fiber diameter (mm) 025 to 10 025 to 10 910ndash3 to 12510ndash1

Maximum operating temperature (C) 85 85 ca 150

a) Poly(methyl methacrylate)b) = (10L) log (P0PL) P0 and PL input and output power L fiber lengthc) Ctrans product of bandwidth Wband and fiber length L Ctrans = WbandL

Wband044 L tndash1 t t2out t2

in12 tout and tin width (FWHM) of output andinput pulses

er modulus than inorganic glass fibers and can therefore be of a much largerdiameter without compromising their flexibility Since their numerical apertureis larger the acceptance angle ie the light gathering capacity is larger com-pared to that of glass fibers Due to the large core diameter and the high nu-merical aperture the installation of polymer optical fibers is facilitated and in-stallation costs are much lower than for silica glass fiber networks Hence poly-mer optical fibers are suitable for short-distance data communication systemsthat require a large number of connections [85] Generally polymer optical fibersystems are applicable in local area networks (LANs) fiber-to-the-home systemsfiber-optic sensors industrial environments automotive applications eg me-dia-oriented system transport (MOST) devices etc Actually data transmissionrates increase in parallel with the number of devices connected to a system andtransmission rates of 400 Mbit sndash1 or more are envisaged With already existingand commercially available polymer optical fibers of a sufficiently large band-width these requirements can be fulfilled Another interesting field of applica-tion relates to lighting and illumination In this context end or point-sourcelighting and side- or line-lighting devices are to be discriminated The formerare used for motorway signaling and the latter for night illumination of build-ings to give typical examples [85]

The introduction of polymer optical fibers may have an impact on the devel-opment of next-generation light sources for optical communication To date theemission wavelength of semiconductor lasers has been adapted to the absorp-tion characteristics of silica fibers Since polymer optical fibers may be used indifferent wavelength regions a change in an important boundary condition forlight source engineering is anticipated

6422 Compositions of polymer optical fibers (POFs)Polymer optical fibers have been prepared from various amorphous polymerssuch as polycarbonate poly(methyl methacrylate) polystyrene and diglycol dial-lylcarbonate resin [79 80] In these cases the light attenuation of the respectiveoptical fibers is due to absorption by higher harmonics of CndashH vibrations Sub-stitution of hydrogen by deuterium fluorine or chlorine results in a shift of theabsorption due to overtone vibrations to higher wavelengths and reduces the at-tenuation at key communication wavelengths as is apparent from Table 68


Table 68 Light attenuation (approximate values) caused byabsorption due to overtone vibrations at key communicationwavelengths in units of dB kmndash1 [79]

(nm) CndashCl CndashF CndashD CndashH

840 lt10ndash8 10ndash4 101 104

1310 10ndash5 100 103 105

1550 10ndash3 101 105 106

Actually commercial polymeric optical fibers made from a perfluorinatedpolymer (see Chart 612) exhibit an attenuation of 15 dB kmndash1 at = 1300 nmSingle-channel systems can be operated at a transmission rate of 25 Gbit sndash1

over a distance of 550 m at = 840 or 1310 nm [79 86] Besides the intrinsic fac-tors for optical propagation loss mentioned above namely absorption and Ray-leigh light scattering there are extrinsic factors such as dust interface asymme-try between core and cladding variation in core diameter etc that may also af-fect the light transmission

6423 Step-index and graded-index polymer optical fibersTable 67 presents the properties of large-core step-index polymer optical fibers SI-POFs They are characterized by a single refractive index which extends overthe entire core and changes abruptly at the corecladding interface SI-POFspossess a low bandwidth due to extensive pulse broadening An increased band-width is achieved with graded-index polymer optical fibers GI-POFs which pos-sess a refractive index profile over the core Refractive index profiles can be ob-tained by special techniques eg by polymerizing a mixture of two monomersdiffering in size and refractive index in rotating tubes or by photochemical par-tial bleaching of a dopant contained in a polymer [79]

643Polymer planar waveguides

Planar ie rectangular waveguide components are applied in many photonicdevices They can be easily manufactured at low cost Typical applications relateto computer backplanes combining electrical and optical cables [87] thermo-op-tical switches [88] optical splitters of multichannel high-density planar light-wave circuits [89] and polyimide-based electro-optical (EO) modulators [90]

644Polymer claddings

Polymers also play a role in the case of specialized optical equipment wherethe different parts are connected by silica fibers This applies for example toinstruments used for spectroscopic process analysis ie for real-time control ofchemical processes [91] To prevent physical damage the fibers are coated withpoly(vinyl chloride) or acrylate-based polymers Fibers coated with polyimidewithstand temperatures up to 350 C


Chart 612 Chemical structure of a perfluorinatedpolymer used to make optical fibers

References 171

References

1 R M Schaffert Electrophotography 2ndEdition Focal Press London (1975)

2 CF Carlson US Patent 2297691 (1942)3 K Y Law Chem Rev 93 (1993) 4494 PM Borsenberger D S Weiss Organic

Photoreceptors for Xerography DekkerNew York (1998)

5 PM Borsenberger D S Weiss OrganicPhotoreceptors for Imaging Systems Dek-ker New York (1993)

6 LB Schein Electrophotography and De-velopment Physics 2nd Edition LaplacianPress Morgan Hills CA USA (1996)

7 LB Schein Electrophotography and De-velopment Physics Springer Berlin(1992)

8 EM Williams The Physics and Technol-ogy of Xerographic Processes Wiley NewYork (1984)

9 M Umeda M Hashimoto J ApplPhys 72 (1992) 117

10 (a) L Akcelrud Electroluminescent Poly-mers Prog Polym Sci 28 (2003) 875(b) K Mullen U Scherf (eds) OrganicLight-Emitting Devices Synthesis Proper-ties and Applications Wiley New York(2006)

11 H-K Shim J-I Jin Light-Emitting Char-acteristics of Conjugated Polymers in K-SLee (ed) Polymers for Photonics Appli-cations I Springer Berlin Adv PolymSci 158 (2002) 193

12 T Bernius M Inbasekaran J OrsquoBrienW-S Wu Progress with Light-EmittingPolymers Adv Mater 12 (2000) 1737

13 W Graupner Science and Technology ofOrganic Light-Emitting Diodes The Spec-trum 15 (2002) 20

14 B Ruhstaller SA Carter S Barth HRiel W Riess J C Scott J Appl Phys89 (2001) 4575

15 DY Kim HN Cho CY Kim BlueLight Emitting Polymers Prog PolymSci 25 (2000) 1089

16 A Greiner Design and Synthesis of Poly-mers for Light-Emitting Diodes PolymAdv Technol 9 (1998) 371

17 J R Sheats YL Chang DB RoitmanA Socking Chemical Aspects of PolymericElectroluminescent Devices Acc ChemRes 32 (1999) 193

18 L J Rothberg A J Lovinger Status andProspects for Organic ElectroluminescenceJ Mater Res 11 (1996) 3174

19 A Kraft A Grimsdale A B HolmesElectroluminescent Conjugated Polymers ndashSeeing Polymers in a New Light AngewChem Int Ed 37 (1998) 402

20 R H Friend RW Gymer A B HolmesJ H Burroughes R N Marks C TalianiDD C Bradley DA dos Santos JLBredas M Loumlgdlund W R SalaneckElectroluminescence in Conjugated Poly-mers Nature 397 (1999) 121

21 A Bolognesi C Botta D FacchinettiM Jandke K Kreger P Strohriegl ARelini R Rolandi S Blumstengel Polar-ized Electroluminescence in Double-LayerLight-Emitting Diodes with PerpendicularlyOriented Polymers Adv Mater 13 (2001)1072

22 M Schwoerer HC Wolf Elektrolumines-zenz und Photovoltaik Chapter 11 in MSchwoerer HC Wolf Organische Mole-kulare Festkoumlrper Wiley-VCH Weinheim(2005)

23 S Miyata HS Nalwa (eds) OrganicElectroluminescent Materials and DevicesGordon amp Breach Amsterdam (1997)

24 J H Burroughes DD C Bradley ARBrown R N Marks K Mackay R HFriend PL Burns A B Holmes Nature347 (1990) 539

25 OLED Cross Reference by Material Func-tion HW Sands Corp httpwwwhwsandscomproductslistsoledcross_reference_material_function_oledhtm

26 Y-H Niu J Huang Y Cao Adv Mater15 (2003) 807

27 J Kalinowski Electronic Processes in Or-ganic Electroluminescence in S MiyataHS Nalwa (eds) Organic Electrolumi-nescent Materials and Devices Gordon ampBreach Amsterdam (1997) p 1

28 H Baumlssler Polym Adv Technol 9(1998) 402

29 S Barth U Wolf H Baumlssler P MuumlllerH Riel H Vestweber PF Seidler WRieszlig Phys Rev B 60 (1999) 8791

30 (a) U Wolf V I Arkhipov H BaumlsslerPhys Rev B 59 (1999) 7507 (b) V I Ar-


khipov U Wolf H Baumlssler Phys Rev B59 (1999) 7514

31 DD C Bradley RH Friend H Linden-berger S Roth Polymer 27 (1986) 1709

32 M Grell DD C Bradley M Inbasekar-an E R Woo Adv Mater 9 (1997) 798

33 P Dyreklev M Berggren O InganaumlsMR Andersson O Wennerstroumlm THjertberg Adv Mater 7 (1995) 43



36 M Gao B Richter S Kirstein Adv Ma-ter 9 (1997) 802

37 BW D Andrade J Brooks V Adamo-vich ME Thompson SR Forrest AdvMater 14 (2002) 1032

38 G Kranzelbinder G Leising OrganicSolid-State Lasers Rep Prog Phys 63(2000) 729

39 IDF Samuel G A Turnbull PolymerLasers Recent Advances Materials Today7 (2004) 28

40 U Lemmer A Haugeneder C Kallin-ger J Feldmann Lasing in ConjugatedPolymers in G Hadziioannou P vanHutton (eds) Semiconducting PolymersChemistry Physics and Engineering Wiley-VCH Weinheim (2000) p 309

41 U Lemmer C Kallinger J FeldmannPhys Blaumltter 56 (2000) 25

42 Z Bao Y M Chen R B Cai L Yu Mac-romolecules 26 (1993) 5228

43 SV Frolov A Fujii D Chinn ZV Var-deny K Yoshino R V Gregory ApplPhys Lett 72 (1998) 2811

44 Y Kawabe Ch Spielberg A SchuumllzgenMF Nabor B Kippelen EA Mash PAllemand M Kuwata-Gonokami K Ta-keda N Peyghambarian Appl PhysLett 72 (1998) 141

45 MD McGehee R Gupta S VeenstraEK Miller MA Diaz-Garcia A J Hee-ger Phys Rev B 58 (1998) 7035

46 C Kallinger M Hilmer A HaugenederM Perner W Spirkl U Lemmer JFeldmann U Scherf K Muumlllen AGombert V Wittwer Adv Mater 10(1998) 920

47 T Granlund M Theander M BerggrenM Andersson A Ruzeckas V Sund-

strom G Bjork M Granstrom O Inga-nas Chem Phys Lett 288 (1998) 879

48 N Tessler G J Denton R H FriendNature 382 (1996) 695

49 S Riechel C Kallinger U Lemmer JFeldmann A Gombert V Wittwer UScherf Appl Phys Lett 77 (2000) 2310

50 F Hide B J Schwartz MA Diaz-Gar-cia A J Heeger Chem Phys Lett 256(1996) 424

51 MD Archer R Hill (eds) CleanElectricity from Photovoltaics ImperialCollege Press London (2001)

52 R Messenger G Ventre PhotovoltaicSystems Engineering CRC Press Boca Ra-ton FL USA (1999)

53 J Perlin From Space to Earth The Storyof Solar Electricity Aatec PublicationsAnn Arbor MI USA (1999)

54 R H Bube Photovoltaic Materials Imper-ial College Press London (1998)

55 H-J Lewerenz H Jungblut Photovol-taik Springer Berlin (1995)

56 MA Green Silicon Solar Cells AdvancedPrinciples and Practice Centre for Photo-voltaic Devices and Systems Universityof New South Wales Sydney (1995)

57 SR Wenham MA Green ME WattApplied Photovoltaics Centre for Photo-voltaic Devices and Systems Universityof New South Wales Sydney (1995)

58 LD Partain (ed) Solar Cells and TheirApplications Wiley-Interscience NewYork (1995)

59 T Markvart (ed) Solar Electricity WileyChichester (1994)

60 (a) N S Sariciftci Plastic Photovoltaic De-vices Materials Today 7 (2004) 36 (b)C J Brabec V Dyakonov J Parisi NSSariciftci (eds) Organic PhotovoltaicsConcept and Realization Springer Berlin(2003)

61 J Nelson (a) Organic and Plastic SolarCells Chapter IIe-2 in T Markvart LCatantildeer (eds) Practical Handbook ofPhotovoltaics Fundamentals and Applica-tions Elsevier Oxford (2003) (b) Materi-als Today 5 (2002) 20

62 J JM Halls R H Friend Organic Photo-voltaic Devices in Ref [51] p 377

63 J-F Nierengarten G Hadziioannou NArmaroli Materials Today 4 (2001) 16

References 173

64 (a) C J Brabec Organic PhotovoltaicsTechnology and Markets Solar Energy Ma-ter Solar Cells 83 (2004) 273 (b) C JBrabec N S Sariciftci J Keppler Mate-rials Today 3 (2000) 5

65 A Dhanabalan J K J van Duren PA vanHal JL J van Dongen R A J JannssenAdv Funct Mater 11 (2001) 255

66 SE Shaheen D Vangeneugden R Kie-booms D Vanderzande T Fromherz FPadinger C J Brabec N S SariciftciSynth Met 121 (2001) 1583

67 MD Archer The Past and Present inRef [51] p 1

68 J JM Halls CA Walsh N C Green-ham EA Marseglia RH Friend S CMoratti A B Holmes Efficient Photo-diodes from Interpenetrating Networks Na-ture 376 (1995) 498

69 G Yu J Gao J C Hummelen F WudlA J Heeger Science 270 (1995) 1789

70 (a) H Hoppe NS Sariciftci Morphologyof PolymerFullerene Bulk HeterojunctionSolar Cells J Mater Chem 16 (2006) 45(b) M Al-Ibrahim H-K Roth U Zho-khavets G Gobsch S Sensfuss SolarEnergy Mater Solar Cells 85 (2005) 13(c) G Li V Shrotriya J Huang Y YadT Moriarty K Emery Y Yang NatureMater 4 (2005) 864 (d) SE ShaheenC J Brabec NS Sariciftci F PadingerT Fromherz J C Hummelen ApplPhys Lett 78 (2001) 841

71 I Riedel M Pientka V DyakonovCharge Carrier Photogeneration and Trans-port in Polymer-Fullerene Bulk-Heterojunc-tion Solar Cells Chapter 15 in W Bruumlt-ting (ed) Physics of Organic Semiconduc-tors Wiley-VCH Weinheim (2005)

72 N Armaroli E Barigefletti P CeroniJ-E Eckert J-F Nicoud J-F Nierengar-ten Chem Commun (2000) 599

73 J-E Eckert J J Nicoud J-F Nierengar-ten S-G Liu L Echegoyen F Barigel-letti N Armaroli L Ouali V KrasnikovG Hadziioannou J Am Chem Soc122 (2000) 7467

74 J-F Nierengarten J-E Eckert J J Ni-coud L Ouali V Krasnikov G Had-ziioannou Chem Commun (1999) 617

75 CJ Brabec V Dyakonov PhotoinducedCharge Transfer in Bulk HeterojunctionComposites in Ref [60b]

76 M Granstroumlm K Petritsch A C AriasA Lux MR Andersson RH FriendNature 395 (1998) 257

77 H Zanger Fiber Optics Communicationand Other Applications McMillan NewYork (1991)

78 HS Nalwa Polymer Optical FibersAmerican Scientific Publishers Steven-son Ranch CA USA (2004)

79 W Daum J Krauser P E Zamzow OZiemann POF ndash Polymer Optical Fibersfor Data Communication Springer Berlin(2002)

80 K Horie H Ushiki FM Winnik Mo-lecular Photonics Fundamentals and Prac-tical Aspects Kodansha-Wiley-VCHWeinheim (2000)

81 A Weinert Plastic Optical Fibers Princi-ples Components Installation MCD Ver-lag Erlangen (1999)

82 J Hecht City of Light The Story of FiberOptics Oxford University Press NewYork (1999)

83 T Kaino Polymers for Light Wave and In-tegrated Optics LA Hornak (ed) Dek-ker New York (1992)

84 M Kitazawa POF Data Book MCRTechno Research Tokyo (1993)

85 MA de Graaf Transmissive and EmissivePolymer Waveguides for Communicationand Illumination University Press Facili-ties Eindhoven The Netherlands (2002)

86 G-D Khoe H van den Boom I T Mon-roy High Capacity Transmission SystemsChapter 6 in [78]

87 J Moisel J Guttman H-P Huber OKrumpholz M Rode R BogenbergerK-P Kuhn Opt Eng 39 (2000) 673

88 N Keil HH Yao C Zawadski KLoumlsch K Satzke W Wischmann J VWirth J Schneider J Bauer M BauerElectron Lett 37 (2001) 89

89 J T Kim CG Choi J Micromech Mi-croeng 15 (2005) 1140

90 S Ermer Applications of Polyimides toPhotonic Devices in K Horie T Yamashi-ta (eds) Photosensitive Polyimides Funda-mentals and Applications TechnomicLancaster PA USA (1995)

91 J Andrews P Dallin Spectroscopy Eu-rope 15 (2003) 23

Part IILight-induced chemical processes in polymers


According to the Grotthus-Draper law chemical changes can only be producedin a system by absorbed radiation It has been pointed out in Chapter 1 thatlight absorption involves electronic transitions As regards organic moleculessuch transitions occur with a high probability if some of the constituent atomsare arranged in special bonding positions Such arrangements are termed chro-mophoric groups (Chapter 1 Table 11) They become resonant at certain light fre-quencies Resonance gives rise to absorption bands in the absorption spectrum(Chapter 1 Figs 14 and 15) The chemical activity of a chromophoric groupmay originate from two features (a) The bonding strength between adjacentatoms is strongly reduced when an electron is promoted to a higher levelTherefore a chemical bond can be cleaved if the atoms separate upon vibrationThis type of monomolecular bond cleavage is a very rapid process (ca 10-12 s)that cannot be prevented by any means after the absorption of a photon (b)The electronic excitation leads to a relatively stable state The lifetime of the ex-cited state is so long (occasionally approaching the ms range) that in the con-densed phase chromophoric groups have many encounters with the surround-ing molecules thus enabling bimolecular chemical interactions Thereby theoriginal chemical bond is relinquished and a new bond is formed This type ofbond cleavage can be prevented by energy quenching (see Chapter 1) ie throughenergy transfer from the excited chromophore to an additive functioning as anenergy acceptor The bond scission processes mentioned above are energeticallyfeasible since the photon energies associated with radiation of wavelengthsranging from 250 nm (496 eV) to 400 nm (31 eV) correspond to the bond dis-sociation energies of common covalent bonds ie about 35 eV for CndashH CndashCand CndashO bonds (in aliphatic compounds) Although these considerations applyto both small and large molecules there are certain aspects pertaining to poly-mers that merit special attention and these are dealt with in this chapter Thesubsequent sections are related overwhelmingly to phenomena associated withapplication aspects Cross-linking and main-chain scission for example playkey roles in lithographic applications and photo-oxidation reactions are ofprominent importance for the behavior of polymers in outdoor applications

177

7Photoreactions in synthetic polymers

It should be emphasized that a plethora of research papers and patents havebeen devoted to the field of photoreactions in synthetic polymers However onlya few important results are highlighted in this chapter For more detailed infor-mation the reader is referred to relevant books and reviews [1ndash28]

711Amplification effects

Photochemical reactions in polymers may result in amplification effects as be-comes obvious if we consider the example of the photochemical coupling of twomolecules In a system of linear chain macromolecules consisting of a largenumber of base units the formation of a given small number of cross-linksmay lead to an enormous property change This is so because each cross-linkconnects two chains with many base units which are all then affected Conse-quently the polymer may become insoluble in solvents if on average each mac-romolecule only contains one cross-link site On the other hand a propertychange is hardly detectable if the same number of cross-links is generated in asystem consisting of small molecules because in this case each cross-link in-volves only two small molecules and leaves the other molecules unaffected

712Multiplicity of photoproducts

The deactivation of identical electronically excited chromophores can result inthe cleavage of different chemical bonds This common phenomenon is demon-strated for two polymers polystyrene and poly(methyl methacrylate) inSchemes 71 and 72 Note that the bond cleavage probabilities are not equalie the quantum yields for the individual processes may differ by orders ofmagnitudes

As indicated in Schemes 71 and 72 several different free radicals are gener-ated upon exposure to light These radicals undergo various reactions eg hy-drogen abstraction reactions thereby generating new free radicals and couplingreactions In this way a variety of products is eventually formed as is demon-strated in Scheme 73 for the case of polystyrene

Notably this scheme does not cover all of the initially formed free radicals(see before Scheme 71) Therefore the number of photoproducts formed inthe case of polystyrene exceeds that shown in Scheme 73

Obviously photochemical methods based on the direct absorption of light bythe polymer can hardly be envisaged for chemical modifications of commercialpolymers Most practical applications especially those devoted to photolithogra-phy concern light-induced changes in the solubility of polymers as a conse-quence of intermolecular cross-linking or main-chain scission In these casesonly reactions causing changes in the average molar mass are important be-cause other photoreactions and the resulting products are ineffective with re-spect to the desired property change

7 Photoreactions in synthetic polymers178

71 Introductory remarks 179

Scheme 71 Primary reactions in the photolysis of polystyrene [9]

Scheme 72 Primary reactions in the photolysis of poly(methyl methacrylate) [14]

713Impurity chromophores

Commonly commercial polymers contain impurities originating from the poly-merization or from processing These impurities although mostly present intrace amounts only play an undesired role because they are capable of absorb-ing the near-UV portion (290ndash400 nm) of the solar radiation reaching the earthand therefore jeopardize or curtail the stability of the polymers in outdoor ap-plications hastening degradation According to the structures of their repeatingunits some of the practically important linear polymers such as polyethylenepolypropylene and poly(vinyl chloride) should be transparent to light ofgt 250 nm However commercial polymer formulations contain impurity chro-mophores (see Table 71) which absorb UV light Consequently these formula-tions are subject to severe degradation in the absence of stabilizers

Some of the chromophores shown in Table 71 are chemically incorporatedinto the polymers such as carbonyl groups or carbon-carbon double bondswhereas others are adventitiously dispersed such as polynuclear aromatic com-pounds and metal salts The latter are almost invariably present in many poly-mers Oxygen-polymer charge-transfer complexes have been postulated as addi-tional UV light-absorbing species Apart from the latter the impurity chromo-phores listed in Table 71 function as free radical generators as illustrated inScheme 74 Hydroperoxide groups the most common and important of chro-mophores yield highly reactive hydroxyl radicals Carbonyl groups can give riseto the formation of various kinds of free radicals as outlined in Section 714Moreover they may act as donors in energy-transfer processes which also ap-


Scheme 73 Reactions of a benzyl-type macroradical formedin the photolysis of polystyrene [9]

plies for polynuclear aromatic compounds Metal salts produce free radicals byelectron-transfer processes In the case of poly(vinyl chloride) allyl-type chlorineatoms are split off

Most of the radicals generated by photoreactions of impurity chromophorescan abstract hydrogen atoms from the surrounding polymer This applies espe-cially to hydroxyl and chlorine radicals

Dioxygen-polymer charge-transfer complexes are assumed to form hydroper-oxide groups [Eq (7-1)]


Table 71 Impurity chromophores commonly contained incommercial polyalkenes or poly(vinyl chloride)s

Structure of chromophore Denotation

Hydroperoxide group

Carbonyl group

-Unsaturated carbonyl group

Double bonds

Conjugated double bonds

Polynuclear aromatics (eg naphthalene an-thracene rubrene)

Metal ions

Charge-transfer complex

13RH O2CTh 13RH O

2 CT R OOH ROOH 7-1

714Photoreactions of carbonyl groups

The detrimental environmental degradation of unstabilized commercial poly-meric products consisting of polyethylene polypropylene poly(vinyl chloride)etc is frequently due to very small amounts of ketonic carbonyl groups Elec-tronically excited ketone groups can undergo different processes in particularthe so-called Norrish type I and Norrish type II reactions as illustrated inScheme 75 for the case of a copolymer of ethylene and carbon monoxide


Scheme 74 Generation of free radicals by photoreactions ofimpurity chromophores and ensuing hydrogen abstractionfrom the polymer

According to the Norrish type I reaction a carbon-carbon bond in a position to the carbonyl group is cleaved The resulting ketyl radical is very likely to re-lease carbon monoxide [Eq (7-2)]

R C R COO

7-2

The Norrish type II process refers to a CndashC bond cleavage initiated by the ab-straction of a hydrogen in a -position with respect to the carbonyl group

Note that Norrish-type reactions are not only of importance in relation to var-ious polymers containing ketonic impurities but they also play a dominant rolein the photolysis of all polymers containing carbonyl groups as constituent moi-eties such as polyacrylates polymethacrylates poly(vinyl acetate) polyestersand polyamides

72Cross-linking

The formation of intermolecular cross-links ie covalent bonds between differ-ent polymer chains causes an increase in the average molar mass and even-tually combines all of the macromolecules into a three-dimensional insolublenetwork Cross-linking can be accomplished in various ways Several methodsrely on reactions of electronically excited pendant groups on the polymerchains others on reactions of various kinds of reactive species in the groundstate that are photogenerated in polymeric systems Typical of the former reac-tion type are [2+2] cycloadditions that occur in the case of linear polymers bear-

72 Cross-linking 183

Scheme 75 Light-induced main-chain cleavage of poly-ethylene containing traces of carbonyl groups

ing pendant C=C bonds typical examples of the latter process are reactions ofnitrenes generated in polymeric systems containing azide groups [17]

Photo-cross-linking of thick polymer films is a difficult task because thepenetration depth is limited to thin layers if the light is strongly absorbed Ahigh absorptivity on the other hand is required for effective photo-cross-link-ing Therefore only the photo-cross-linking of thin films (1 m) is of practicalimportance This process has found widespread application in photolithography(see Section 91) The following subsections are largely devoted to systems thathave been employed for photolithographic applications although some systemsof as yet purely academic interest are also discussed

721Cross-linking by cycloaddition of C=C bonds

The reaction of an excited alkene molecule in its S1 or T1 state with an alkenemolecule in its ground state produces a cyclobutane derivative [Eq (7-3)]

7-3


Scheme 76 Light-induced cross-linking and trans cis isomerization of poly(vinyl cinnamate)

In this reaction which occurs in competition with isomerization two bondsare lost with the formation of two new bonds Since two electrons of eachalkene molecule are involved the reaction is called [2+2] or simply [2+2] cy-cloaddition As discovered by Minsk [29] linear polymers containing C=C bondsin pendant groups also undergo light-induced [2+2] cycloaddition reactions Thisleads to the formation of intermolecular cross-links as demonstrated here forthe classical case of poly(vinyl cinnamate) Exposure of the polymer to UV light(exp = 365 nm) results both in [2+2] cycloaddition and trans cis isomerization(Scheme 76)

Besides cinnamate compounds various other compounds containing C=Cbonds also undergo light-induced cycloaddition reactions (see Chart 71)

Scheme 77 shows as a typical example the photo-cross-linking of a co-poly-peptide [30]


Chart 71 Structures of moieties suitable for the cross-linkingof linear polymers through cycloaddition

Scheme 77 Photo-cross-linking of a co-polypeptide consistingof L-ornithine and -7-coumaryloxyacetyl-L-ornithine residues[30]

722Cross-linking by polymerization of reactive moieties in pendant groups

Photo-cross-linking of linear polymers can be achieved by light-induced poly-merization of reactive moieties in pendant groups located on different macro-molecules a process analogous to the polymerization of low molar mass com-pounds which is treated in Chapter 10 Provided that the pendant groups arecapable of approaching to within the reaction distance and their concentrationis high enough they undergo chain reactions which can propagate by way ofvarious mechanisms that are started with the aid of appropriate photoinitiatorsFrom the technical point of view free radical polymerizations of unsaturatedcarbon-carbon bonds are most important In principle cationic polymerizationsinvolving the ring opening of epoxides and glycidyl ethers (see Chart 72) arealso suitable

Although in contrast to free radical polymerizations cationic polymerizationsare unaffected by O2 their importance is somewhat limited by the scarcity ofappropriate macromolecules and suitable photoinitiators [3] However this doesnot apply to the photopolymerization of low molar mass epoxides (see Sec-tion 103) In this context applications of photo-cross-linked epoxides in variousfields such as stereolithography volume holography and surface coating arenotable [16]

A typical example involving the polymerization of unsaturated pendantgroups relates to the fixation of surface relief gratings that are optically in-scribed with the aid of a 488 nm laser beam (see Section 561) onto a film of acopolymer bearing pendant azobenzene groups (chemical structure shown inChart 73)

The generation of the relief gratings involves trans cis isomerization of thependant azobenzene groups and the subsequent fixation is achieved by cross-linking with UV light at 80 C ie by polymerization of the acrylic groups withthe aid of a photoinitiator (see Chart 74)


Chart 72 Structures of moieties suitable for cross-linking by photopolymerization

This process results in an improved thermal stability of the gratings [31] An-other example relates to the photo-cross-linking of a copolymer of the structureshown in Chart 75 [32]

Here the alkynyl side groups are polymerized to form a three-dimensionalnetwork when the copolymer is exposed to UV light (320ndash390 nm) in the pres-ence of 5 mol tungsten hexacarbonyl W(CO)6 (see also Subsection 102241)The polymerization is presumed to be initiated by the formation of a 2-alkynetungsten pentacarbonyl complex 2-RCCRW(CO)5


Chart 73 Co-monomers (1 1 molar ratio) contained in apolymer used to generate surface relief gratings

Chart 74 Chemical structure of 4-(methylthio)-2-morpholino-propiophenone used as a photoinitiator in the cross-linking ofthe copolymer of Chart 73

Chart 75 Chemical structure of a copolymer consisting ofpropargyl acrylate (345 left) and methyl methacrylate(655 right)

723Cross-linking by photogenerated reactive species

This mode of photo-cross-linking has attracted attention for applications in re-sist technology since it became apparent that the photodecomposition of organ-ic azides in polymeric systems leads to insolubility Azide groups can be chemi-cally attached to polymer chains as demonstrated here by two examples

Alternatively bisazides ie low molar mass compounds containing two azidegroups can be added to the polymer Several commercially used bisazides arepresented in Table 72 Many linear polymers can be photo-cross-linked with theaid of bisazides [17] Of note in this context is poly(cis-isoprene) which containssome cyclized structures (Chart 77) It has been frequently applied as a resistmaterial in photolithography applications

A water-soluble bisazide (see Chart 78) is applicable for the photo-cross-link-ing of water-processable polymeric systems containing polyacrylamide or poly(vi-nyl pyrrolidone)


Chart 76 Base units of polymers bear-ing pendant azide groups

Table 72 Bisazides of practical importance for the photo-cross-linking of linear polymers [17]

Denotation Chemical structure

26-Bis(4-azidobenzal)-4-methylcyclohexane

44-Diazidostilbene

44-Diazidobenzophenone

44-Diazidobenzalacetone

When an azide group decomposes after absorption of a photon an electricallyneutral very reactive intermediate called a nitrene is formed Immediately afterdecomposition the latter is in an electronically excited singlet state which candecay to the ground state the triplet nitrene [see Eqs (7-4) and (7-5)]

RN3 h 1RN N2 7-41RN 3RN 7-5

Both nitrene species are very reactive since the nitrogen possesses only six va-lence electrons Singlet nitrene can insert into CndashH bonds of the polymer andin the case of unsaturated polymers can add to C=C bonds both in single-stepprocesses (Scheme 78)

As shown in Scheme 79 triplet nitrene can abstract a hydrogen atom fromneighboring macromolecules thus forming an amino radical and a carbonmacroradical (reaction (a)) The two radicals have correlated spins and can


Chart 77 Cyclized structure in poly(cis-isoprene)

Chart 78 Chemical structure of a water-soluble bisazide

Scheme 78 Reactions of singlet nitrene with saturated and unsaturated polymers

therefore only couple after spin inversion (reaction (b)) The amino radical mayalso abstract a hydrogen atom from a different site to produce a primary amine(reaction (c)) Cross-links are formed by coupling reactions namely by the com-bination of macroradicals (reaction (d)) and if bisazides are employed after theconversion of both azide groups according to reaction (e) [17]

Free radical mechanisms also serve to explain the photo-cross-linking of var-ious polymers such as that of polyethylene accomplished with the aid of light-absorbing additives such as benzophenone quinone benzoin acetophenone ortheir derivatives When electronically excited by light absorption these additiveseither directly abstract hydrogen from the polymer or decompose into free radi-cals capable of abstracting hydrogen as shown in Schemes 710 and 711

Macroradicals P can form cross-links by combination reactions according toEq (7-6)

P P PP 7-6


Scheme 79 Cross-linking of polymers through the reaction of triplet nitrene

The occurrence of these reactions is restricted to the amorphous phase Thereforethe photo-cross-linking process has to be performed at temperatures exceeding thecrystalline melting point in the case of highly crystalline polymers such as poly-ethylene The cross-linking efficiency can be strongly enhanced by the additionof small amounts of multifunctional compounds such as triallyl cyanurate TAC(see Chart 79) or by the incorporation of special diene moieties into copolymerssuch as ethylene propylene diene copolymers (EPDM elastomers) [33]


Scheme 710 Generation of macroradicals by the reaction ofelectronically excited benzophenone and anthraquinone with apolymer PH

Scheme 711 Generation of free radicals by -cleavage inelectronically excited acetophenone and benzoin derivativesand subsequent formation of macroradicals P by hydrogenabstraction from macromolecules PH

The reaction mechanism in this case is shown in Scheme 712 It is based onthe fact that allyl-type hydrogens are readily abstracted by reactive radicals suchas ketyl species Side-chain macroradicals generated in this way combine toform intermolecular cross-links

724Cross-linking by cleavage of phenolic OH groups

Typical of this type of photo-cross-linking is the case of poly(4-hydroxystyrene)(see Chart 710) [34]

The deactivation of excited singlet phenolic groups proceeds by two mainroutes cleavage of the OndashH bonds and intersystem crossing to the triplet stateas shown in Scheme 713


Chart 79 Chemical structure of triallyl cyanurate

Scheme 712 Generation of pendant macroradicals acting asprecursors for the cross-linking of an EPDM elastomercontaining ethylidene norbornene moieties (other co-monomer moieties are not shown) Initiatorhydroxycyclohexyl phenyl ketone [33]

The phenoxyl radicals can couple to form cross-links (Scheme 714)If dioxygen is present additional phenoxyl radicals are formed by reaction ac-

cording to Eq (7-7) ie by the reaction of triplet excited phenolic groups with O2

7-7

Therefore the cross-linking quantum yield is significantly increased if the irra-diation is performed in the presence of dioxygen

73Simultaneous cross-linking and main-chain cleavage of linear polymers

As has been pointed out in Section 712 polymers commonly undergo differentkinds of bond ruptures simultaneously upon exposure to light ie bond cleav-age processes occur both in side chains and in the main chain of linear poly-mers Bond rupture in side chains results in the formation of lateral macroradi-

73 Simultaneous cross-linking and main-chain cleavage of linear polymers 193

Scheme 713 Primary steps in the photolysis of poly(4-hydroxystyrene)

Chart 710 Chemical structure of poly(4-hydroxystyrene)

cals which can give rise to the release of low molar mass compounds and canalso form inter- and intramolecular cross-links Therefore it is often the casethat main-chain scission and cross-linking occur simultaneously These pro-cesses cause changes in the molar mass distribution and in the average molarmass of the polymer which has been treated theoretically [35ndash37] The depen-dence of the weight-average molar mass Mw (g molndash1) of linear polymers under-going simultaneous main-chain cleavage and cross-linking on the absorbed doseDabs (photons gndash1) is given by Eq (7-8)

1MwD

1Mw0

13S2

213X

Dabs

NA7-8

where 13(S) and 13(X) denote the quantum yields for main-chain cleavage andcross-linking respectively and NA is Avogadrorsquos number Equation (7-8) holdsfor the case that the initial molar mass distribution is of the most probable typeand that main-chain ruptures and cross-links are randomly distributed alongthe polymer chains Cross-linking predominates if 13(S) lt 413(X) In this casethe reciprocal average molar mass decreases ie Mw increases with increasingabsorbed dose On the other hand main-chain cleavage predominates if13(S) gt 413(X) In this case the reciprocal average molar mass increases ie Mw

decreases with increasing absorbed dose In this context it should be noted thatpredominant main-chain cleavage causes a deterioration of important mechani-cal properties that are related to the molar mass of the polymer Several linearpolymers are characterized with respect to the predominance of cross-linking ormain-chain cleavage in Table 73

Interestingly polyacrylonitrile poly(methyl acrylate) and polystyrene behavedifferently in the rigid state and in dilute solution This may be explained interms of lateral macroradicals being generated upon the release of side groupsin a primary step The combination of these radicals competes with decomposi-tion through main-chain rupture In dilute solution where radical encountersare much less probable than in the rigid state main-chain rupture predomi-


Scheme 714 Coupling of phenoxyl radicals

nates This mechanism is illustrated for the case of polyacrylonitrile inScheme 715

When linear polymers undergo predominantly cross-linking a three-dimen-sional insoluble network is formed The absorbed dose at which the insolublenetwork begins to form is the gel dose Dgel It corresponds to an average of onecross-link per weight-average molecule [35] and a simple equation may be de-rived from Eq (7-8) for the relationship between Dgel and 13(X)

Dgel NA

13XMw07-9

Equation (7-9) holds in the absence of main-chain scission ie at 13(S) = 0 Inthis case the reciprocal molar mass approaches infinity at the gel dose ie1MwDgel 0

Quantum yields of photoproducts of selected polymers are presented in Ta-ble 74 It can be seen that both 13(S) and 13(X) are low (lt 01) The quantum


Scheme 715 Main-chain cleavage and cross-linking of polyacrylonitrile

Table 73 Predominant effects upon UV irradiation of polymers in the absence of oxygen [27]

Polymer Rigid state Dilute solution

Poly(methyl methacrylate) degradation degradationPoly(-methyl styrene) degradation degradationPoly(phenyl vinyl ketone) degradation degradationPolyacrylonitrile crosslinking degradationPoly(methyl acrylate) crosslinking degradationPolystyrene crosslinking degradation

yields of volatile products resulting from side-group degradation are also quitelow for most polymers apart from poly(methyl methacrylate)

74Photodegradation of selected polymers

It is not intended to present a comprehensive treatise on the photoreactions inpolymers in this book Actually many polymers exhibit analogous behaviorHowever this certainly does not apply to poly(vinyl chloride) or polysilanes andtherefore these two types of polymers are discussed to some extent in the fol-lowing subsections

741Poly(vinyl chloride)

Poly(vinyl chloride) PVC is one of the most widely used polymers CommercialPVC products commonly contain plasticizers (up to 40) such as phthalates ormellitates If exposed to UV or solar radiation for prolonged periods PVC productssuffer from a deterioration of their mechanical and electrical properties and areeventually discolored [11 19 21] Unsaturated moieties are believed to be the mostimportant initiator species with carbonyl groups as the next most important Thelatter can undergo Norrish-type reactions (see Section 714) Moreover excited car-bonyl groups can transfer energy to unsaturated moieties or abstract hydrogens Inaddition hydroperoxide and peroxide groups formed during autoxidation of thepolymer (see Section 75) can contribute to the initiation process [11]


Table 74 Photoproduct quantum yields of polymers in the rigid state deter-mined at room temperature in vacuo [27]

Polymer SX (S)102 (X)102 (nm) Volatile products

(102 )

Poly--methylstyrene 01ndash06 2537 -methylstyreneH2 (17102)

Poly(methyl methacrylate) 12ndash39 2537 CH3OH (48)HCOOCH3 (14) COH2 CO2

Poly(phenyl vinyl ketone) 60 313Poly(vinyl acetate) 14 66 47 2537 CH3COOH (10) CO2

(065) CO (069) CH4

(038)Poly(ethylene terephthalate) 27 016 006 313Poly(methyl acrylate) 10 019 019 2537 HCHO (2) CH3OH

(02) HCOOCH3 (08)Poly(p-methylstyrene) 052 2537 H2 (6) CH4 (004)

The discoloration is due to a dehydrochlorination process resulting in the for-mation of long conjugated polyene sequences in the polymer chain [Eq (7-10)]Polyenes can give rise to photo-cross-linking reactions

7-10

It is generally accepted that the elimination of HCl occurs by way of a free radi-cal chain reaction As shown in the lower part of Scheme 716 chlorine atomsfunction as propagating species Likely initiation mechanisms involving some ofthe impurity chromophores listed in Table 71 are presented in the upper partof Scheme 716

The solar light-induced dehydrochlorination of PVC plasticized with phtha-lates has been reported to be sensitized by the plasticizer [38 39] In markedcontrast more recent work has revealed a weak protective effect of phthalateswith respect to CndashCl bond cleavage and polyene formation Phthalates are likelyto quench electronically excited states of impurity chromophores [40]

74 Photodegradation of selected polymers 197

Scheme 716 Mechanism of the light-induced dehydrochlorination of poly(vinyl chloride)

742Polysilanes

Polysilanes (alternative denotations polysilylenes poly-catena-silicons) of thegeneral structure shown in Chart 711 exhibit an absorption band in a relativelylong-wavelength region ie between 300 and 400 nm reflecting the -conjuga-tion of electrons in the silicon chain

In addition to their other interesting properties polysilanes are photoconduc-tive [41] (see Chapter 2) and therefore are attractive with regard to practical ap-plications [42 43] However to the detriment of their technical applicabilitypolysilanes show a pronounced trend to suffer photodegradation Light absorp-tion induces main-chain scission and extrusion of silylene as depicted inScheme 717

The lifetime of the excited state giving rise to main-chain cleavage is shorterthan 100 ps [44]


Scheme 717 Main-chain degradation of polysilanes

Chart 711 Chemical structure of a base unit of polysilane

75Oxidation

Oxidation processes are initiated when polymers absorb visible or UV light inthe presence of air [7 12 24-26] In most cases these processes occur as chainreactions initiated by the light-induced generation of free radicals Since someof the reaction products are chromophoric groups capable of initiating new ki-netic chains themselves the oxidation becomes auto-accelerated during expo-sure As a consequence of autoxidation important mechanical properties ofpolymeric materials may suffer a sudden breakdown during continuous expo-sure to light This is demonstrated in Fig 71 which shows how the impactstrength of an ABS polymer drops drastically after a certain exposure time [45]

The schematic representation in Fig 72 shows how at first the oxygen uptakeincreases exponentially with increasing irradiation time ie the reaction rate isaccelerated After prolonged irradiation the autoacceleration is followed by anautoretardation stage due to a depletion in the O2 concentration in the interiorof the sample or to reaction products interfering with the propagation process

The behavior depicted in Fig 72 is observed with many polymers upon expo-sure to sunlight including with commercial polyalkenes such as polyethyleneand polypropylene In the latter cases impurity chromophores act as initiatorsof the autoxidation process (see Scheme 74 in Section 713) Important elemen-tary reactions determining the autoxidation process are described in the follow-ing Free radicals RX

formed during the initiation phase abstract hydrogenatoms from macromolecules PH thus forming macroradicals P [Eq (7-11)]

75 Oxidation 199

Fig 71 Photodegradation of an acrylonitrilebutadienestyrene (ABS) copolymer at 30 C Plot of the impact strengthvs the simulated natural exposure time (xenon-arc radiation055 W mndash2 at 340 nm) Adapted from Davis et al [45] withpermission from Elsevier

RX PH RXH P 7-11

The ensuing chain reaction which is propagated by the macroradicals produceshydroperoxide groups (see Scheme 718)

Hydroperoxide groups can be photolytically cleaved provided that the wave-length of the incident light is lower than about 300 nm [Eq (7-12)]

POOHh PO OH 7-12

The radicals generated in this way can initiate additional chain reactions (chainbranching) by abstracting hydrogens from neighboring macromolecules for in-stance by reaction according to Eq (7-13)

OH PH H2O P 7-13

The kinetic chains are terminated by radical coupling reactions (seeScheme 719)

The combination of peroxyl radicals (reaction (a) in Scheme 719) is assumedto proceed via a tetroxide P-O4-P a short-lived intermediate Various reaction


Fig 72 Autoxidation of polymers Schematic represen-tation of the oxygen uptake as a function of timeAdapted from Schnabel [24] with permission from CarlHanser

Scheme 718 Propagation of the chain reaction in the autoxidation process

P O2 POO

POO PH POOH P

POO POO a Products

POO P b POOP

P P c P P

Scheme 719 Termination reactions in the autoxidation process

paths that may be envisaged in the case of secondary peroxyl radicals are shownin Scheme 720 [46] Reaction (a) in Scheme 720 refers to the so-called Russelmechanism The extent to which each individual reaction occurs depends onthe chemical nature of the polymer as well as on other parameters particularlythe temperature The oxyl radicals formed by reaction (b) can abstract hydrogenin inter- andor intramolecular reactions Alternatively they can decompose withthe formation of carbonyl groups (see Scheme 721)

In conclusion the salient features of the light-induced oxidation of polymersare the formation of hydroperoxide peroxide and carbonyl groups the latter inthe form of both aldehyde and keto groups Moreover certain reactions such asreaction (d) in Scheme 720 and reaction (b) in Scheme 721 result in main-chain cleavage as far as the oxidation of linear macromolecules is concernedMain-chain cleavage leads to a deterioration in certain important mechanicalproperties Therefore the photo-oxidation of polymers is deleterious and shouldbe avoided in commercial polymers Appropriate stabilization measures are dis-cussed in Section 93

75 Oxidation 201

Scheme 720 Decay processes of secondary peroxyl radicals [46]

Scheme 721 Reactions of oxyl radicals

76Singlet oxygen reactions

The ground state of molecular oxygen (3O2) is a triplet state with two unpairedelectrons In addition to the reactions outlined in Section 75 3O2 can undergoenergy-transfer reactions with many compounds such as dyes and polynucleararomatics provided that the difference in the energy levels exceeds 94 kJ molndash1In these reactions the first excited state of molecular oxygen ie singlet oxygen(1O2

) is formed as is illustrated by the reaction of triplet excited carbonylgroups present in a polymer with 3O2 according to Eq (7-14)

7-14

1O2 is unreactive towards saturated hydrocarbons but reacts with unsaturated

substances with a rate constant of 103 to 104 L molndash1 sndash1 [47] This reaction re-sults in the insertion of hydroperoxide groups [Eq (7-15)]

7-15

In conclusion singlet oxygen plays a role in the photo-oxidative degradation ofpolymers containing olefinic unsaturations Polymers that do not contain thesegroups eg poly(vinyl chloride) poly(methyl methacrylate) polystyrene etc areunreactive [24]

77Rearrangements

Certain organic molecules are modified by a rearrangement of some of theirconstituent groups upon light absorption Typical processes that have gainedimportance in the polymer field are the photo-Fries rearrangement of aromaticesters amides and urethanes (see Scheme 722) and the o-nitrobenzyl ester re-arrangement (see Scheme 723) In the latter case nitronic acid forms as a long-lived intermediate Its decay in polymeric matrices is non-exponential (kineticmatrix effect) up to temperatures exceeding the glass transition temperaturerange [49]


Regarding linear polymers rearrangements can involve the main chain as inthe case of a polycarbonate (see Scheme 724) or pendant groups as in the caseof poly(4-acetoxy styrene) which is converted into poly(3-acetyl-4-hydroxy sty-

77 Rearrangements 203

Scheme 722 Photo-Fries rearrangement of a carbonate

Scheme 723 Mechanism of the o-nitrobenzyl ester photo-rearrangement [48 49]

rene) (see Scheme 725) or with polymers bearing o-nitrobenzyl ester pendantgroups (see Scheme 726)

Photo-rearrangements in polymers are important because they can lead topronounced property changes For example polymers containing o-nitrobenzylpendant groups become soluble in aqueous solution since benzyl ester groupsare converted into carboxyl groups Therefore such polymers are applicable aspositive-tone photoresists in lithographic processes [50 51] (see Section 91)


Scheme 724 Photo-rearrangement of a polycarbonate

Scheme 725 Photo-rearrangement of poly(4-acetoxy styrene)

Scheme 726 Photo-rearrangement of polymers bearing o-nitrobenzyl pendant groups

References 205

References

1 (a) J C Salamone (ed) Polymeric Materi-als Encyclopedia CRC Press Boca RatonFL USA (1996) (b) Abridgement of (a)J C Salamone (ed) Concise PolymericMaterials Encyclopedia CRC Press BocaRaton FL USA (1999)

2 G Scott Polymers and the EnvironmentRoyal Society of Chemistry Cambridge(1999)

3 H-J Timpe Polymer Photochemistry andPhoto-Crosslinking in R Arshady (ed)Desk Reference of Functional PolymersSynthesis and Applications AmericanChemical Society Washington DC(1997) p 273

4 S I Hong S Y Joo D W Kang Photo-sensitive Polymers in R Arshady (ed)Desk Reference of Functional PolymersSynthesis and Applications AmericanChemical Society Washington DC(1997) p 293

5 B Raringnby B Qu W Shi Photocrosslink-ing (Overview) in [1(a)] Vol 7 p 5155

6 J Paczkowski Photocrosslinkable Photopo-lymers (Effect of Cinnamate Group Struc-ture) in [1(a)] Vol 7 p 5142

7 J F Rabek Photodegradation of PolymersPhysical Characteristics and ApplicationsSpringer Berlin (1996)

8 R L Clough NC Billingham K T Gil-len (eds) Polymer Durability Stabiliza-tion and Lifetime Prediction AmericanChemical Society Washington DC Ad-vances in Chemistry Series 249 (1996)

9 W Schnabel I Reetz Polystyrene and De-rivatives Photolysis in [1(a)] Vol 9p 6786

10 V V Krongauz AD Trifunac Processesin Photoreactive Polymers Chapman ampHall New York (1995)

11 A L Andrady Ultraviolet Radiation andPolymers in J E Mark Physical Propertiesof Polymers Handbook AIP Press Wood-bury NY (1995) Chapter 40

12 G Scott (ed) Atmospheric Oxidation andAntioxidants Elsevier Amsterdam(1993)

13 NS Allen M Edge Fundamentals ofPolymer Degradation and StabilisationElsevier Applied Science London (1992)

14 Z Osawa Photoinduced Degradation ofPolymers in S H Hamid MB AminA G Maadhah (eds) Handbook of Poly-mer Degradation Dekker New York(1992)

15 H Boumlttcher J Bendig MA Fox GHopf H-J Timpe Technical Applicationsof Photochemistry Deutscher Verlag fuumlrGrundstoffindustrie Leipzig (1991)

16 V Strehmel Epoxies Structures Photoin-duced Cross-Linking Network Propertiesand Applications in HS Nalwa (ed)Handbook of Photochemistry and Photo-biology American Scientific PublishersStevenson Ranch CA USA (2003) Vol2 p 2

17 A Reiser Photoreactive Polymers TheScience and Technology of Resists WileyNew York (1989)

18 J Guillet Polymer Photophysics andPhotochemistry Cambridge UniversityPress Cambridge (1985)

19 C Decker Photodegradation of PVC inED Owen (ed) Degradation and Stabili-zation of PVC Elsevier Applied ScienceLondon (1984) p 81

20 S Tazuke Photocrosslinking of Polymersin NS Allen (ed) Developments in Poly-mer Photochemistry ndash 3 Applied ScienceLondon (1982) Chapter 2 p 53

21 ED Owen Photodegradation and Stabili-zation of PVC in NS Allen (ed) Devel-opments in Polymer Photochemistry ndash 3Applied Science London (1982) Chapter5 p 165

22 Z Ozawa Photodegradation and Stabili-zation of Polyurethanes in NS Allen(ed) Developments in Polymer Photochem-istry ndash 3 Applied Science London(1982) Chapter 6 p 209

23 W Schnabel Laser Flash Photolysis ofPolymers in N S Allen (ed) Develop-ments in Polymer Photochemistry ndash 3 Ap-plied Science London (1982) Chapter 7p 237

24 W Schnabel Polymer Degradation Princi-ples and Practical Applications HanserMuumlnchen (1981) Chapter 4

25 R Arnaud J Lemaire PhotocatalyticOxidation of Polypropylenes and Polyunde-canoamides in N S Allen (ed) Develop-


ments in Polymer Photochemistry ndash 2 Ap-plied Science London (1981) Chapter 4p 135

26 A Garton D J Carlsson DM WilesPhoto-oxidation Mechanisms in Commer-cial Polyolefins in NS Allen (ed) Devel-opments in Polymer Photochemistry ndash 1Applied Science London (1980) Chapter4 p 93

27 W Schnabel J Kiwi Photodegradationin HHG Jellinek (ed) Aspects of Deg-radation and Stabilization of PolymersElsevier Amsterdam (1979)

28 J F McKellar NS Allen Photochemistryof Man-Made Polymers Applied ScienceLondon (1979)

29 LM Minsk J G Smith W P Van Deu-sen J W Wright J Appl Polym Sci 11(1959) 302

30 K Ohkawa K Shoumura M YamadaA Nishida H Shirai H YamamotoMacromol Biosci 1 (2001) 149

31 H Takase A Natansohn P RochonPolymer 44 (2003) 7345

32 C Badaru ZY Wang Macromolecules36 (2000) 6959

33 B Raringnby Photoinitiated Modifications ofSynthetic Polymers Photocrosslinking andSurface Photografting in NS Allen MEdge I R Bellobono E Selli (eds) Cur-rent Trends in Polymer PhotochemistryHorwood New York (1995) Chapter 2p 23

34 K Nakabayashi R Schwalm W Schna-bel Angew Makromol Chem 195(1992) 191

35 A Charlesby Atomic Radiation and Poly-mers Pergamon Press Oxford (1960)Chapter 10

36 O Saito Statistical Theory of Crosslinkingin M Dole (ed) The Radiation Chemistryof Macromolecules Academic Press NewYork (1972) Chapter 11

37 CL Moad D J Windzor Prog PolymSci 23 (1998) 759

38 IS Biggin DL Gerrard G E Wil-liams J Vinyl Technol 4 (1982) 150

39 DL Gerrard HJ Bowley KP J Wil-liams IS Biggin J Vinyl Technol 8(1986) 43

40 A I Balabanovich S Denizligil WSchnabel J Vinyl Add Technol 3 (1997)42

41 R G Kepler J M Zeigler LA HarrahSR Kurtz Phys Rev B 35 (1987) 2818

42 R D Miller J Michl Chem Rev 89(1989) 1359

43 R D Miller Radiation Sensitivity of Solu-ble Polysilane Derivatives in J M ZeiglerFW G Fearon (eds) Silicon-Based Poly-mer Science A Comprehensive ResourceAmerican Chemical Society WashingtonDC (1990) Advances in Chemistry Se-ries 224 Chapter 24

44 Y Ohsako CM Phillips J M ZeiglerR M Hochstrasser J Phys Chem 93(1989) 4408

45 P Davis BE Tiganis L S Burn PolymDegrad Stab 84 (2004) 233

46 C von Sonntag The Chemical Basis ofRadiation Biology Taylor amp Francis Lon-don (1987) Chapter 4

47 H Bortolus S Dellonte G Beggiato WCorio Eur Polym J 13 (1977) 185

48 K H Wong H Schupp W SchnabelMacromolecules 22 (1989) 2176

49 G Feldmann A Winsauer J Pfleger WSchnabel Macromolecules 27 (1994)4393

50 H Barzynski D Saumlnger MakromolChem 93 (1981) 131

51 E Reichmanis R Gooden CW Wil-kins H Schonehorn J Polym SciPolym Chem Ed 21 (1983) 1075


Biopolymers play a key role in many light-triggered biological processes such as inphotomorphological processes in plants and in the photomovements of bacteriaMoreover biopolymers participate in energy transduction processes related tothe conversion of solar energy into chemical energy (photosynthesis) and to theconversion of chemical energy into light (bioluminescence) Apart from these ben-eficial effects light can also have a harmful effect on polymers and cause chemicaldamage resulting in a deactivation of their biological activity While the deleteriousaction is commonly restricted to UVB and UVC light ( 200ndash320 nm) ie to photonshaving energies high enough to cleave chemical bonds the regulatory action relatesto light of longer wavelengths ie UVA ( 320ndash400 nm) and visible light In thelatter case effective biopolymers contain chromophoric groups capable of absorbinglight in the 400ndash800 nm wavelength region This chapter which deals with bothmodes of action of light is organized according to the important biopolymer familiesof nucleic acids proteins lignins and polysaccharides (see Chart 81) However itshould be kept in mind that very often members of these families exist in closeproximity in biological objects and are sometimes even linked by chemical bonds

For relevant literature concerning the broad field of light-induced effects inbiopolymers and biological objects the reader is directed to several reviews andbooks [1ndash17]

The polymers presented in Chart 81 absorb UV light to quite different ex-tents Nucleic acids absorb more strongly than proteins This can be seen inFig 81 which shows absorption spectra of aqueous solutions of DNA and bo-vine serum albumin recorded at equal concentrations In contrast to the ratherstrongly absorbing nucleotide residues in DNA only a few of the amino acid re-sidues in proteins absorb light measurably in the UV region This pertainsmainly to the aromatic amino acids phenylalanine tyrosine and tryptophan (seeChart 82)

Lignins a major component of wood (15ndash30 wt) are phenolic polymersbased on three structural units the content of which depends on the type ofwood trans-p-coumaryl alcohol (I) trans-coniferyl alcohol (II) and trans-sinapylalcohol (III) (see Chart 83)

207

8Photoreactions in biopolymers

The optical absorption spectra of lignins extend into the visible wavelength re-gion and exhibit peaks at about 205 and 280 nm and shoulders at 230 and340 nm [18] Polysaccharides such as cellulose and amylose essentially do notabsorb light at gt 200 nm Very weak absorption bands observable in somecases in the region between 250 and 300 nm are due to intrinsic impuritiessuch as acetal groups or carboxyl groups replacing hydroxyl groups [17 19]

Special biopolymers containing covalently bound chromophoric groups absorbvisible light and act as photoreceptors They play a regulatory role in important

8 Photoreactions in biopolymers208

Chart 81 Biopolymer structures depicting(a) different nucleotides contained in humandeoxyribonucleic acid DNA (b) part of aprotein chain consisting of various aminoacid residues with R being H (glycine) CH3

(alanine) (CH2)4NH2 (lysine) CH2SH(cysteine) etc (c) the base unit of thecellulose chain representing the class ofpolysaccharides and (d) part of a lignin withtypical structural elements

biological processes Typical photoreceptors are proteins belonging to the carote-noid (rhodopsin) phytochrome and cryptochrome families In this context thechlorophyllic protein complexes are also of note They function as light-harvestingantenna pigments and auxiliary cofactors in the photosynthetic process and are


Fig 81 Optical absorption spectra of aqueous solutions of anucleic acid (calf thymus DNA) and a protein (bovine serumalbumin) both recorded at a concentration of 19710ndash2 g Lndash1Adapted from Harm [12] with permission from CambridgeUniversity Press

Chart 82 Chemical structures of aromatic amino acids


Chart 83 Substituted phenyl propanols that constitute the structural units of lignins

Table 81 Photoactive chromophores (pigments) of photoreceptor proteins [9 20ndash25]

Typical chromophore Photoreceptor class Typical functions

Carotenoids(a) Photoantennas in the photo-synthetic system of plants (b) Cat-alytic pigments in animal andbacterial rhodopsins

11-cis Retinal

Flavins(a) Photoantennas in enzymes(b) Cofactors for photolyaseblue-light photoreceptors

Flavin

Phytochromes

(a) Photoreceptors exerting mor-phogenic control in plants(b) Accessory antennas in thelight-harvesting complexes ofphotosynthetic systems

Phytochromobilin

PterinsPhotoantennas in the majority ofphotolyasecryptochrome blue-light photoreceptors

510-Methenyltetrahydrofolate(MTHF)

Xanthopsins YellowProteins

Sensory blue light receptorswater-soluble controlling the lifeof bacteria in saline lakes

4-Hydroxycinnamate

therefore of profound biological importance The chemical structures of typicalchromophoric groups contained in these proteins are presented in Table 81

In conclusion proteins play a range of roles in relation to the exposure of bio-logical objects to light of different wavelengths UV light acts harmfully since itcauses chemical changes leading to the deactivation of specifically acting pro-teins such as enzymes However light-induced chemical changes might alsotrigger the synthesis of special proteins As regards irradiation with visible lightit is most important that certain proteins serve as light-harvesting agents inphotosynthesis and as photoreceptors and photosensors in photomorphogenicprocesses in plants The various aspects are referred to briefly in the followingsections

82Direct light effects

8 21Photoreactions in deoxyribonucleic acids (DNA)

The energy-rich UV light portion of the terrestrial solar spectrum ( 280ndash400 nm) is harmful to most organisms and can even cause skin cancer in hu-mans (basal and squamous cell carcinoma melanoma) This is mainly due to

82 Direct light effects 211



NaphthodianthronesBlepharismins

Photosensors in ciliated protozo-ans exhibiting step-up photopho-bic and negative phototacticresponses

Stentorin

Chlorophylls Photoantennas in the light-harvest-ing complexes and electron donorsin the reaction center of the photo-synthetic system

Chlorophyll a

light-induced chemical modifications in DNA bases commonly termed UV-in-duced DNA lesions The absorption of light converts the bases into their excitedsinglet or triplet states from which chemical reactions can ensue The resultingbase modifications are accompanied by a change in the base-pairing propertieswhich in turn causes mutations [26ndash29] There are a number of feasible photo-lesions based on the cleavage of chemical bonds with the concurrent generationof free radicals Besides these dimeric photoproducts may be formed in greatabundance through a molecular rather than a free radical mechanism Notablypyrimidine bases are essentially involved in the generation of lesions of biologi-cal importance although both purine and pyrimidine residues are rather strongabsorbers in the far-UV region Actually the quantum yield of photodecomposi-tion differs significantly It amounts to about 10ndash4 for purines ie one or two or-ders of magnitude lower than that for pyrimidines [12]

8211 Dimeric photoproductsThe pyrimidine bases thymine (T) and cytosine (C) form dimers at sites withadjacent pyrimidine moieties so-called dipyrimidine sites in the DNA chainwhich have been well characterized with respect to chemical structure and mu-tagenic potential The dimerization presented in Scheme 81 is a [2+2] cy-cloaddition (see Section 73) involving the two C(5)=C(6) double bonds leadingto cyclobutane structures denoted by the symbol T lt gt T or generally Pyr lt gt Pyr

The dimerization can in principle lead to three isomers cis-syn trans-syn Iand trans-syn II but due to the constraints imposed by the DNA double strandthe cis-syn dimer shown in Scheme 81 is the major photoproduct [27]

Another type of dimeric lesions are pyrimidinendashpyrimidone (Pyr[6-4]Pyr) di-mers formed by a Paterno-Buumlchi-type reaction at dipyrimidine sites between theC(5)=C(6) double bond of the first pyrimidine and the C(4)=O carbonyl groupof the second base This kind of dimerization is demonstrated in Scheme 82for the case of adjacent thymine moieties


Scheme 81 Dimerization of adjacent thymine moieties in DNA by [2+2] cycloaddition

Analogous photoproducts may form between any types of adjacent pyrimi-dines T-T T-C C-T and C-C except that the (6-4) photoproduct does not format C-T sites Adeninendashthymine heterodimers (see Chart 84) have also been de-tected [29 30]

The UV-induced generation of cyclobutane dimers is greatly dependent ondouble-helix conformational factors In dormant spores of various bacillus spe-cies for example a group of small acid-soluble proteins specifically bind toDNA thereby enforcing a particular conformation that is unfavorable for theformation of harmful cyclobutane-type lesions As a consequence these dor-mant spores are much more resistant to UV radiation than the correspondinggrowing cells in which DNA strands reassume conformations favorable for theformation of cyclobutane-type lesions [31]

Notably photodimers of the cyclobutane type are cleaved by irradiation withfar-UV light (240 nm) with a quantum yield of almost unity by way of the so-called [2+2] cycloreversion reaction In living cells dimer lesions can be repairedby the nucleotide excision repair pathway which is based on the excision of asmall piece of DNA around the lesion Lesions not removed from the genomelead to cell death or mutagenesis


Chart 84 Structure of an adeninendashthyminephotodimer [29]

Scheme 82 Dimerization of adjacent thymine moieties inDNA by a Paterno-Buumlchi-type reaction

8212 Other DNA photoproductsAdditional photoproducts commonly generated via free radical mechanismshave been identified These include single-strand breaks cross-links betweenthe strands of the same double helix and between different DNA strands andadjacent protein molecules and the so-called photohydrates (see Chart 85)

822Photoreactions in proteins

Gross changes in proteins due to UV irradiation include disturbance of the naturalconformation aggregation and chain cleavage all of which lead to denaturationThe structural proteins keratin (wool) collagen elastin and fibroin (silk) undergolosses in mechanical strength and elasticity (wool tenders) and sometimes colorchanges (yellowing) These changes are due to chemical alterations

In order to assess possible photochemical events one has to take into accountthat proteins are heterogeneously composed linear polymers (see Chart 81)The amino acid residues are connected by amide (peptide) bonds ndashCOndashNHndashNature uses 20 amino acids to synthesize a great variety of proteins which arecharacterized by amino acid sequence size and three-dimensional structureMany proteins are intramolecularly cross-linked by disulfide links (RndashSndashSndashR)ie they consist of several covalently connected chains Alternatively two ormore protein chains can be linked by non-covalent forces Proteins consisting ofthe 20 natural amino acids absorb light at lt 320 nm The low-wavelength por-tion of the terrestrial solar spectrum extending to about 290 nm is mainly ab-sorbed by the aromatic amino acids (see Chart 82) Therefore the sunlight-in-duced photochemistry of proteins essentially relates to these moieties Atlt 290 nm light is also absorbed by the other amino acid residues whichgreatly increases the variety of possible bond ruptures In view of these facts itis clear that the photochemistry of proteins is extremely complex and thereforeonly certain aspects have been thoroughly investigated to date


Chart 85 Photohydrates of cytosine (a) and of thymine (b) [30]

(a) (b)

8221 Chemical alterations by UV lightTryptophan (Trp) tyrosine (Tyr) cystine (Cys) and phenylalanine (Phe) moietiesplay a determinant role regarding UV light-induced chemical alterations inmany proteins After the absorption of light by these moieties in most casesmainly by Trp and Tyr they undergo photoionization and participate in energy-and electron-transfer processes This not only holds for structural proteins suchas keratin and fibroin [11] but also for enzymes in aqueous media such as lyso-zyme trypsin papain ribonuclease A and insulin [7] The photoionization ofTrp andor Tyr residues is the major initial photochemical event which resultsin inactivation in the case of enzymes A typical mechanism pertaining to Trpresidues (see Scheme 83) commences with the absorption of a photon and thesubsequent release of an electron In aqueous media the latter is rapidly sol-vated By the release of a proton the tryptophan cation radical Trp+ is con-verted to the tryptophan radical Trp

In many proteins such as -lactalbumin which consists of 123 amino acidmoieties the electron released from a Trp moiety is attached by way of an intra-molecular process to a disulfide group of a cystine bridge in a position adjacentto the indole ring of the Trp moiety [32]

As shown in Scheme 84 the resulting disulfide anion radical dissociates intoa thiolate ion RndashSndash and a thiyl radical RndashS Proton transfer from the tryptophancation radical to the thiolate ion leads to the tryptophan radical Trp and thethiol RSH The final stage of the process is governed by radical coupling whichmay result in sulfenylation of the Trp moiety yielding TrpndashSndashR or in inter-molecular cross-linking ie in the formation of enzyme dimers or trimers

Disulfide bridges can also be ruptured by reaction with the triplet excited moi-eties 3Trp or 3Tyr the formation of which accompanies the electron release


Scheme 83 Photolysis of proteins Reactions involving tryptophan moieties [7]

In this process the triplet species undergo an electron transfer with cystinemoieties thus forming the disulfide radical anion (see Scheme 85)

Intermediates occurring in these mechanisms have been identified by ESRmeasurements and by flash photolysis studies using optical absorption detec-tion For example ESR measurements on wool keratins revealed the formationof sulfur-centered radicals of the structure RCH2S which in this case are as-sumed to result from a reaction of electronically excited tyrosine moieties withcystine residues [11] In many proteins cross-links are formed In the case ofkeratin and collagen the cross-links are of the tryptophan-histidine and dityro-sine types [11] Cross-links formed by the combination of RndashS or RndashSndashS radi-cals both intermolecularly and intramolecularly with incorrect sites are consid-ered to be an important source of photoaggregation effects [8] ESR measure-ments have also yielded evidence of CndashH and CndashN bond ruptures [8]

8222 Formation of stress proteinsUV light induces the formation (expression) of so-called stress proteins in mam-malian skin cells [34] Stress proteins (shock proteins) are also generated byother stress factors such as hyperthermia and comprise a heterogeneous groupof proteins with molar masses ranging from 104 to 11105 g molndash1 They func-tion as molecular chaperones by transiently binding to unfolded proteins aftersynthesis as well as to denatured proteins in stressed cells thus promoting theirrefolding and correct assembly In this way they protect proteins from misfold-ing and irreversible denaturation The molecular mechanism of the formationof stress proteins has not yet been elucidated although it is supposed that theirformation is triggered by oxidative damage


Scheme 84 Rupture of cystine bridges by the attachment ofelectrons stemming from the photoionization of tryptophan[32 33]

Scheme 85 Reaction of tryptophan triplets with cystine moieties

8223 Effects of visible light ndash photoreceptor actionPhotoreceptors ie proteins containing chromophores absorbing visible light (seeTable 81) play a key role in many light-triggered biological processes For instancein plants they regulate and participate in energy transduction processes during theconversion of solar energy into chemical energy (photosynthesis) and trigger andsupport photomorphological processes Moreover photoreceptors are responsiblefor the photomovements of certain bacteria and regulate the circadian rhythm ofhigher animals Circadian (circa= round about and dies= day) rhythms are oscilla-tions in the biochemical physiological and behavioral functions of organisms witha periodicity of approximately 24 hours Detailed information on this fascinatingfield is available from the cited literature [6 9 20 22 35ndash44] Upon light absorp-tion the chromophores of photoreceptors undergo molecular transformations thatresult in the formation of signaling states in the protein The regulatory action re-lates to UVA ( 320ndash400 nm) and visible light ( 400ndash800 nm) In most proteinac-eous photoreceptor systems such as cytochromes and phytochromes the chromo-phores are covalently linked to the protein [35] On the other hand chlorophyll moi-eties are specifically associated with intrinsic proteins of the photosynthetic mem-brane thus forming chlorophyll-protein (non-covalent) complexes

Depending on their chemical nature chromophores undergo different modesof light-induced molecular transformation As can be seen in Table 82 thetransformation modes include trans-cis isomerization charge transfer and en-ergy transfer

The chromophores act as photosensing-phototransducing devices because theyare not isolated but rather are embedded in and interacting with a molecular apo-protein framework The latter senses the light-induced molecular modifications inthe chromophores and in turn gives rise to the signaling state The intimate in-teraction between chromophore and protein determines the physiological andspectroscopic properties of the photoreceptors In recent years photobiological re-search has been largely focused on photoreceptors and has revealed some very in-teresting results This is illustrated here for the typical case of the family of phy-tochromes which are present in plants and certain bacteria [20 37ndash39] Certainphytochromes exert morphogenic control functions in higher and lower plants al-gae and mosses relating to for example blooming the opening of hooks ofshoots or the germination of seeds Other phytochromes function as accessorylight-harvesting antennae in conjunction with the photosynthetic systems of cer-tain algae Plant phytochromes consist of polypeptide chains of about 1100 amino


Table 82 Transformation modes of chromophores in photoreceptors

Transformation mode Chromophores

trans-cis Isomerization Retinals 4-hydroxy-cinnamate bilinsCharge transfer Flavins stentorins blepharisminsEnergy transfer Pterins flavins

acid moieties (molar mass 12ndash13105 g molndash1) and a single open-chain tetrapyr-role chromophore of the bilin family (see Table 81 and Scheme 86) which iscovalently bound via an S-cysteine linkage to the apoprotein The polypeptidechain is composed of two domains the globular N (amino) terminal domain bear-ing the chromophore and the regulatory C (carboxyl) terminal domain [39] Thetwo domains are connected by a flexible protease-sensitive hinge region contain-ing the Q (Quail) box Active phytochrome entities are dimers ie they consistof two polypeptide strands (see Fig 82)


Scheme 86 Mechanism of the PrPfr photocycle for phytochromobilin Adapted from [20]

Fig 82 Schematic illustration of the interdo-main signal transmission in a dimeric oatphytochrome Q Quail box PAS Per-Arnt-Sim motif Q and PAS constitute the regula-tory core region HD Histidine kinase-related domain PKS1 Phytochrome kinase

substrate 1 NDPK2 Nucleosidediphosphate kinase 2 PIF3 Phytochromeinteracting factor 3 Adapted from Bhoo etal [39] with permission from RoutledgeTay-lor amp Francis Group LLC

The photomorphogenic control functions are triggered by trans cis and cistrans double-bond isomerizations of the chromophore induced by red (r) and far-red (fr) light respectively The PrPfr photocycle is illustrated in Scheme 86

The Pr to Pfr isomerization induces a transformation from random to -helicalconformation in part of the N-terminal domain and thus triggers a series ofconformational changes in other structural peptide motifs especially in the C-terminal domain (see Fig 82) Here certain regulatory sites become exposedand thus capable of interacting with signal transducer proteins such as PIF3(phytochrome interacting factor 3) NDPK2 (nucleoside diphosphate kinase 2)etc In this way the enzymatic activity of these proteins is significantly in-creased Moreover the Q-box in the hinge region becomes uncovered thus per-mitting the phosphorylation of the serine moiety in position 598 of the chainThe phosphorylation at Ser-598 exerts an accelerating effect on the associationof PIF3 and NDPK2 and the phosphorylation of PKS1 (phytochrome kinasesubstrate 1) The latter is a protein that is complexed to the Pr state of the phy-tochrome and is released from the photoactivated Pfr state after phosphorylationto give downstream signals through a kinase cascade [39] Recall that a kinaseis an enzyme that catalyzes the phosphorylation of a substrate here a proteinIn conclusion the light-induced isomerization of carbon-carbon double bondsin the chromophore causes a series of conformational changes within the twodomains of the phytochrome These changes trigger the association of signaltransducer proteins with the phytochrome and allow phosphorylation and phos-phate transfer at various sites These are key steps initializing the downstreamof processes that eventually result in transcriptional regulation

8224 Repair of lesions with the aid of DNA photolyasesThe repair of dimer lesions induced with the aid of light of relatively long wave-length that is not absorbed by the dimer sites ( 300ndash400 nm) is based on photo-receptor action as dealt with in Section 8223 above It occurs if DNA photolyasesie structure-specific (not sequence-specific) enzymes are present in the systemduring the irradiation [6] Photolyases are proteins of 450-550 amino acids contain-ing two non-covalently bound chromophore cofactors (see Chart 86)

One of the cofactors is always flavin adenine dinucleotide FAD and the sec-ond one is either methenyltetrahydrofolate MTHF or 8-hydroxy-78-dides-methyl-5-deazariboflavin 8-HDF

The repair of lesions by photolyases is the basis of the so-called photoreactiva-tion of organisms A striking example is the resurrection of UV-killed Escheri-chia coli by subsequent exposure to a millisecond light flash which is demon-strated by the results shown in Fig 83

The reaction mechanism can be summarized as follows In a dark reactionthe enzyme binds to DNA and flips out the pyrimidine dimer from the doublehelix into the active cavity After the photochemical repair the reaction partnersare moved out of the cavity As shown in Scheme 87 MTHF (or alternatively 8-HDF) is converted into an excited state MTHF upon absorption of a photon



Chart 86 Cofactors of photolyases

Fig 83 Photoreactivation of UV-killed E coli cells Lower linecells irradiated with UV light and plated on a growth mediumUpper line UV-irradiated cells exposed to a 1 ms light flashbefore plating Adapted from Sancar [6] with permission fromthe American Chemical Society

Excited reduced flavin (FADH) formed by energy transfer from MTHFtransfers an electron to Pyr lt gtPyr the pyrimidine dimer In a subsequent con-certed reaction the latter is split into two pyrimidines and an electron is trans-ferred to the nascently formed FADH

823Photoreactions in cellulose

It was pointed out in Section 81 that polysaccharides do not absorb light atgt 200 nm Therefore photochemical alterations caused by light of longer wave-lengths are due to the action of impurity chromophores This also holds for cel-lulose which is a major component of plants Some plants such as jute flaxhemp and cotton contain up to 90 cellulose Neat cellulose forms gaseousproducts (CO CO2 and H2) upon exposure to UV light (= 2537 nm) ESRstudies have revealed the generation of H radicals and various carbon-centeredfree radicals The degree of crystallinity of the cellulose fibrils is reduced [17] IfO2 is present during the irradiation carbonyl carboxyl and peroxide groups areformed even at gt 340 nm Main-chain scission occurs and the brightness is re-duced [45] This is because irradiation at lt 360 nm leads to homolysis of thepreviously formed hydroperoxide groups (see Scheme 88)

The OH radicals resulting from this process are very reactive ie they ab-stract hydrogens from neighboring molecules and thus initiate further decom-position processes For detailed information concerning the photochemistry ofcellulose the reader is referred to the relevant literature [17 46]

824Photoreactions in lignins and wood

Wood contains 15ndash30 lignin an aromatic UV- and visible-light-absorbing poly-mer with a very complex structure (see Chart 81) and photochemical alterationsof wood are essentially determined by reactions initiated by bond breakages in the


Scheme 87 Reaction mechanism of the repair of pyrimidinedimer lesions in DNA with the aid of photolyases

RO OHh RO OH

Scheme 88 Generation of hydroxyl radicals during the photolysis of hydroperoxide groups

lignin component Due to a lack of systematic investigations little is known aboutthe complex mechanism of the photoreactions in lignins Scheme 89 illustratesbond-breakage processes suggested in the literature [16 47]

The formation of phenoxyl radicals has been revealed by ESR measurementsPhenoxyl radicals can be transformed into quinoid structures (see Scheme 810)which are thought to be responsible for the yellowing of the surfaces of woodproducts

Because of the capability of lignins to absorb near-UV and visible light evenindoor yellowing and darkening of wood surfaces due to slow photooxidationprocesses is unavoidable More detailed information concerning the photochem-istry of lignins and wood is available in relevant review articles [16 47]

83Photosensitized reactions

Various applications are based on the indirect action of light on polymers con-tained in biological objects Many biopolymers do not absorb visible light andabsorb UV light only to a limited extent Therefore sensitizers are used to ac-complish light-induced chemical alterations Sensitizers which are in an elec-tronically excited state after light absorption either react directly with substratepolymers or decompose into fragments capable of reacting with the polymers


Scheme 89 Photoreactions of lignins

Scheme 810 Formation of quinoid structures in lignins

Sensitizers can be employed for agricultural purposes as herbicides and insecti-cides or for medical purposes as antibacterial and antiviral agents Moreoversensitizer-based methods serve as tools for the analysis of the interaction facesof polymer complexes and the sequence-selective photocleavage of double-stranded DNA The ways in which photosensitized reactions are utilized are il-lustrated by the following typical examples The first case relates to the photo-chemotherapy of cancer cells in superficial solid tumors [48] The so-calledphotodynamic therapy PDT is based on the selective incorporation of a photosen-sitizer into tumor cells followed by exposure to light (commonly at = 600 nm)Cytotoxic products namely singlet oxygen 1O2

and superoxide radical anionsO

2 are generated upon irradiation and these are postulated to start a cascadeof biochemical processes that inactivate neoplastic cells The precise mechanismhas not yet been elucidated [49] However it has been established that chemicalalterations of the cytoskeleton trigger a sequence of reactions eventually causingcell apoptosis The cytoskeleton consists of a complex array of highly dynamicprotein structures that organize the cytoplasma of the cell The basic proteinac-eous constituents having molar masses ranging from 4104 to 7104 g molndash1are microtubules and globular or linear microfilaments (actins and keratins re-spectively) The cytoskeleton structure disorganizes and reorganizes continu-ously depending on the shape and state of division of the cells as well as onsignals received from the environment Assembly and disassembly of the cyto-skeletal elements are severely disturbed or inhibited by light-induced damageChart 87 presents the chemical structures of several PDT sensitizers Relevantresearch work has been reviewed [50]

The second example relates to photochemical cross-linking as a tool for study-ing metastable protein-nucleic acid and protein-protein assemblies [51ndash54] Pro-tein-protein and protein-nucleotide interactions are maintained by a multitudeof weak non-covalent interaction forces From an analytical perspective it isuseful to stabilize such complexes by trapping the interaction partners bymeans of a cross-linking technique so as to generate covalent bonds betweenthem The process of protein assembly can be time-resolved in a snapshot man-ner if the cross-linking period is significantly shorter than the lifetimes of inter-mediate stages reached during the complexing of two or more protein mole-cules ie during dimerization or oligomerization respectively The method dis-cussed here denoted by the acronym PICUP (photo-induced cross-linking ofunmodified proteins) in the case of the oligomerization of unmodified proteinsinvolves exposing the assemblies to a short high-power laser pulse therebygenerating a number of cross-links that is sufficient to stabilize the interactionpartners The aim of the subsequent analysis is then to define binding sites byidentifying the composition of the cross-linked domains of the partners Massspectrometry has been successfully applied for this purpose and it appears thatthe desired information can be obtained more quickly and with greater sensitiv-ity in this way than by NMR or X-ray crystallography [53] The information ob-tained can be used as a basis for three-dimensional molecular modeling of pro-tein oligonucleotide interfaces Commonly the cross-linking reaction is per-

83 Photosensitized reactions 223

formed with the aid of sensitizers that absorb light at wavelengths exceeding300 nm since photo-cross-linking by direct irradiation of the complexes withfar-UV light suffers from serious disadvantages such as low cross-linking yieldstrand breakage and oxidation

In studies of the dynamics of protein oligomerization in the context of inves-tigations exploring amyloidoses ie diseases including Alzheimerrsquos disease ruthe-nium(II) complexes are used [52 55] To this end tris(22-bipyridyl)dichloro-ruthenium(II) Ru(II)bpy3Cl2 (see Chart 88) and ammonium persulfate(NH4)2S2O8 are homogeneously dispersed in an aqueous protein solution


Chart 87 Sensitizers employed in the photochemotherapy ofcancer cells TPP meso-tetraphenylporphine TMPyP meso-tetra(4-N-methylpyridyl)porphine MB methylene blueTB toluidine blue ZnPc zinc(II) phthalocyanine TPPotetraphenylporphyrene


Chart 88 Structure of tris(22-bipyridyl)dichloro ruthenium(II) Ru(II)bpy3Cl2

Scheme 811 Photoreaction of Ru(II)bpy32+ complexes with persulfate ions [53]

Table 83 Nucleobases bearing photosensitizer groupscommonly used for nucleic acidprotein cross-linking studies[51 53]

Structure of nucleobase Denotation max

(nm)operation

(nm)

4-Thiouridine 330 gt 300

Azido-substitutednucleobases

280 gt 300

IodouridineIodocytidine

290300 gt300

Bromouridine 275 gt 300

Upon photoexcitation Ru(III) complexes and sulfate radicals are produced(see Scheme 811) Both resultant species Ru(III)bpy3

3+ and SO4ndash are potent

one-electron oxidants and can generate protein radicals by hydrogen abstractionfrom protein molecules The combination of the protein radicals leads to cross-links

If nucleic acidprotein complexes are to be explored photosensitive groupsare synthesized and incorporated into the nucleic acids Typical sensitizer-bear-ing nucleobases are shown in Table 83

A typical cross-linking reaction is presented in Scheme 812A third example concerns the sequence-selective photocleavage of double-

stranded DNA [14 56ndash58] The advantage of using photoreagents for this pur-pose is that they are inert in the dark and react only under irradiation with lightof an appropriate wavelength that is not absorbed by neat DNA Strand cleavagecan be accomplished by attack of either sugar or nucleobase moieties In the lat-ter case cleavage of DNA usually requires alkaline treatment after irradiation


Scheme 812 Cross-linking of a nucleic acid with a protein bythe reaction of a 5-iodouracil group with a tryptophan sidegroup

Scheme 813 Cleavage of a DNA strand following theabstraction of a hydrogen atom from a sugar moiety by anelectronically excited photoreagent X

On the other hand attack at a sugar moiety can lead to direct cleavage of theDNA strand In this case a common mechanism is based on hydrogen abstrac-tion (see Scheme 813) The resulting sugar radicals can decompose by a varietyof pathways to yield low molar mass products and DNA fragments


Scheme 814 Intra-chain hydrogen abstraction from the sugarmoiety in poly(uridylic acid) involving an uracil radical formedby addition of an OH radical

Chart 89 Structures of typical photochemical nucleases usedfor sequence-specific cleavage of DNA strands L LinkerR sequence-specific DNA-binding compound [56]

Although mechanistic details which are discussed in the relevant literature[14 59 60] cannot be dealt with here the following aspect should at least bepointed out an attack at the nucleobase might induce chemical alterations inthe sugar moiety that eventually result in strand breakage This applies for ex-ample to the intramolecular hydrogen abstraction suggested in the case ofpoly(uridylic acid) (see Scheme 814) [59]

The hydrogen abstraction process is in principle unselective since abstract-able hydrogens are present in all sugar moieties Strand ruptures originatingfrom attacks at the nucleobases are also intrinsically unselective However se-quence selectivity can be accomplished if the photoreagent binds to one or afew sequences of the DNA strand The focus of relevant research is on synthe-sizing conjugates composed of a photosensitizer group and a sequence-specificDNA-binding compound also denoted as photochemical nucleases [56] Appropri-ate photoactive groups (listed eg in [14]) include complexes of transition metalions such as Ru(II) Rh(III) and Co(II) polycyclic aromatic compounds such asanthraquinone and naphthalene diimide porphyrins and related compounds(chlorins sapphyrins) phthalocyanines and fullerenes (see Chart 89)


References

1 W M Horspool F Lenci (eds) CRCHandbook of Organic Photochemistry andPhotobiology 2nd Edition Boca RatonFlorida (2004)

2 W M Horspool P-S Song (eds) CRCHandbook of Organic Photochemistry andPhotobiology 1st Edition Boca RatonFlorida (1995)

3 H Morrison (ed) Bioorganic Photochem-istry Wiley New York (1990)

4 A R Young LO Bjorn J Moan WNultsch (eds) Environmental UV Photo-biology Plenum Press New York (1993)

5 HS Nalwa (ed) Handbook of Photo-chemistry and Photobiology American Sci-entific Publ Stevenson Ranch Califor-nia (2003)

6 A Sancar Structure and Function of DNAPhotolyase and Cryptochrome Blue-LightPhotoreceptors Chem Rev 103 (2003)2203

7 L I Grossweiner Photochemistry of Pro-teins A Review Curr Eye Res 3 (1984)137

8 K M Schaich Free Radical Initiation inProteins and Amino Acids by Ionizing andUltraviolet Radiation and Lipid Oxidationndash Part II Ultraviolet Radiation and Photo-

lysis CRC Crit Rev Food Sci Nutr 13(1980) 131

9 A Sancar Cryptochrome The SecondPhotoactive Pigment in the Eye and its Rolein Circadian Photoreception Ann RevBiochem 69 (2000) 31

10 NL Veksin Photonics of BiopolymersSpringer Berlin Heidelberg (2002)

11 G J Smith New Trends in Photobiology(Invited Review) Photodegradation ofKeratin and other Structural Proteins JPhotochem Photobiol B Biol 27 (1995)187

12 W Harm Biological Effects of UltravioletRadiation Cambridge University PressCambridge (1980)

13 CH Nicholls Photodegradation andPhotoyellowing of Wool in N S Allen(ed) Developments in Polymer Photochem-istry ndash 1 Appl Science Publ London(1980) Chapter 5 p 125

14 B Armitage Photocleavage of NucleicAcids Chem Rev 98 (1998) 1171

15 J Barber (ed) The Light Reactions Else-vier Amsterdam (1987)

16 DN S Hon N Shiraishi (eds) Woodand Cellulosic Chemistry 2nd EditionDekker New York (2001)

References 229

17 P J Baugh Photodegradation and Photo-oxidation of Cellulose in NS Allen (ed)Developments in Polymer Photochemistry ndash2 Appl Science Publ London (1981)Chapter 5 p 165

18 A Sakakibara Y Sano Chemistry of Lig-nin Chapter 4 in [16]

19 A Bos J Appl Polym Sci 16 (1972)2567

20 K Schaffner W Gaumlrtner Open-Chain Tet-rapyrroles in Light Sensor Proteins Phyto-chromes The Spectrum 12 (1999) 1

21 G EO Borgstahl D E Williams E DGetzoff Biochemistry 34 (1995) 6278

22 J Hendriks K J Hellingwerf PhotoactiveYellow Protein the Prototype XanthopsinChapter 123 in [1]

23 Y Muto T Matsuoka A Kida Y OkanoY Kirino FEBS Lett 508 (2001) 423

24 R Dai T Yamazaki I Yamazaki P SSong Biochim Biophys Acta 1231(1995) 58

25 Y Shichida T Yoshizawa PhotochemicalAspects of Rhodopsin Chapter 125 in [1]

26 MG Friedel DNA Damage and RepairPhotochemistry Chapter 141 in [1]

27 SY Wang (ed) Photochemistry andPhotobiology of Nucleic Acids AcademicPress New York (1976)

28 F Cadet P Vigny The Photochemistry ofNucleic Acids Vol 1 Chapter 1 in [3]

29 DL Mitchell D Karentz The Inductionand Repair of DNA Photodamage in theEnvironment p 345 in [4]

30 DL Mitchell DNA Damage and RepairChapter 140 in [1]

31 P Setlow Environ Mol Mutagen 38(2001) 97

32 A Vanhooren B Devreese K Vanhee JVan Beeumen I Hanssens Biochem 41(2002) 11035

33 DV Bent E Hayon J Am Chem Soc97 (1975) 2612

34 F Trautinger Stress Proteins in the Photo-biology of Mammalian Cells Vol 4 Chap-ter 5 in [5]

35 J Breton E Naberdryk Pigment and Pro-tein Organization in Reaction Center andAntenna Complexes Chapter 4 in [15]

36 H Zuber The Structure of Light-Harvest-ing Pigment Protein Complexes Chapter 5in [15]

37 K Schaffner SE Braslavski SE Holz-warth Protein Environment Photophysicsand Photochemistry of Prosthetic BiliproteinChromophores in H-J Schneider HDuumlrr (eds) Frontiers in SupramolecularOrganic Chemistry and PhotochemistryVCH Weinheim (1991) p 421

38 SE Braslavski W Gaumlrtner K SchaffnerPhytochrome Photoconversion Plant Celland Environment 6 (1997) 700

39 SH Bhoo P S Song Phytochrome Mo-lecular Properties Chapter 129 in [1]

40 G Checcuci A Sgarbossa F LenciPhotomovements of Microorganisms An In-troduction Chapter 120 in [1]

41 SC Tu Bacterial Bioluminescence Bio-chemistry Chapter 136 in [1]

42 V Tozzini V Pellegrini F BeltramGreen Fluorescent Proteins and Their Ap-plications to Cell Biology and BioelectronicsChapter 139 in [1]

43 NK Packham J Barber Structural andFunctional Comparison of Anoxygenic andOxygenic Organisms Chapter 1 in [15]

44 M Salomon Higher Plant PhototropinsPhotoreceptors not only for Phototropismin A Batschauer (ed) Photoreceptors andLight Signalling Comprehensive Seriesin Photochemistry and PhotobiologyVol 3 Royal Soc Chem Cambridge(2003) p 272

45 J Malesic J Kolar M Strlic D KocarD Fromageot J Lemaire O HaillandPolym Degrad Stab 89 (2005) 64

46 DN S Hon Weathering and Photochem-istry of Wood Chapter 11 in [16]

47 B George E Suttie A Merlin X De-glise Photodegradation and Photostabilisa-tion of Wood ndash the State of the Art PolymDegrad Stab 88 (2005) 268

48 T J Dougherty J G Levy Clinical Appli-cations of Photodynamic Therapy Chapter147 in [2]

49 BW Henderson S O Gollnick Mechan-istic Principles of Photodynamic TherapyChapter 145 in [2]

50 A Villanueva R Vidania J C StockertM Canete A Juarranz Photodynamic Ef-fects on Cultured Tumor Cells CytoskeletonAlterations and Cell Death MechanismsVol 4 Chapter 3 in [5]

51 K Meisenheimer T Koch Crit Rev Bio-chem Mol Biol 32 (1997) 101


52 G Bitan DB Teplow Acc Chem Res37 (2004) 357

53 H Steen ON Hensen Analysis of Pro-tein-Nucleic Acid Interaction by Photo-chemical Crosslinking Mass SpectromRev (2002) 163

54 B Bartholomew RT Tinker G A Kas-savetis EP Geiduschek Meth Enzy-mol 262 (1995) 476

55 DA Fancy I Kodadek Proc Natl AcadSci USA 96 (1999) 6020

56 A S Boutorine PB Arimondo Se-quence-Specific Cleavage of Double-Stranded DNA in MA Zenkova (ed)Artificial Nucleases Nucleic Acids andMolecular Biology Vol 13 Springer Ber-lin (2004) p 243

57 T Da Ros G Spalluto A S BoutorineR V Bensasson M Prato DNA-Photo-cleavage Agents Curr Pharm Design 7(2001) 1781

58 IE Kochevar DA Dunn Photosensi-tized Reactions of DNA Cleavage and Ad-dition Vol 1 Chapter 1 p 299 in [3]


60 W K Pogozelski DT Tullius OxidativeStrand Scission of Nucleic Acids RoutesInitiated by Hydrogen Abstraction from theSugar Moiety Chem Rev 98 (1998) 1089

91Polymers in photolithography


In modern-day technical terminology lithography denotes a technology used topattern the surfaces of solid substrates Lithography as invented by Alois Sene-felder in 1798 is a printing technique used by artists who draw (Greek gra-phein) directly onto a stone (Greek lithos) surface with greasy ink which adheresto the dry stone and attracts printing ink while the background absorbs waterand repels the printing ink The patterning of surfaces with the aid of light iscalled photolithography It serves to generate macrostructures in the millimeterrange and is applied for example in the fabrication of printed circuit boardsand printing plates In its currently most important version lithography heredenoted as microlithography refers to the generation of microstructures on topof semiconductor (mostly silicon) wafers Photomicrolithography has served asthe essential tool in the information and electronic revolution It is still unavoid-able in the mass production of computer chips containing fine-line featuresnow in the sub-75 nm range thus permitting an information density exceeding109 integrated circuits (IC) per cm2 This miniaturization technique is renderedpossible by polymers although they are not contained in the final productsStimulated by the demand for further progress in the miniaturization of de-vices outlined by the SIA International Roadmap [1] a large body of researchand development still focuses on the improvement of the classical microlitho-graphic techniques and the development of novel ones [2ndash4]

912Lithographic processes

The lithographic process that is widely used to generate microstructures espe-cially in the context of the fabrication of microdevices is shown schematicallyin Fig 91 It is based on the interaction of electromagnetic or particle radiationwith matter Since direct irradiation of the substrate (eg silicon wafers) does

231

9Technical developments related to photochemical processesin polymers

not result in the generation of microstructures of the required quality the tech-nically utilized processes are performed with wafers coated with a thin layer ofa radiation-sensitive material The required fine-line structures are generatedwithin this thin layer essentially in two steps irradiation through a stencil (herecalled the mask) and subsequent (commonly liquid) development The radiation-sensitive material is called the resist (material) because it has to be resistant toetching agents ie chemicals capable of reacting with the substrate Etching iscarried out after development ie after the removal of either the irradiated orthe unirradiated resist All of these steps are illustrated in Fig 91 which relatesto photolithography Most of the resists that have been employed to date arepolymer-based ie they consist wholly or partly of an amorphous polymer

As regards the manufacture of microdevices photolithography is the key tech-nology On the other hand charged particle beam lithography using electron orion beams (eg H+ He2+ Ar+) serves to fabricate photomasks In this case acomputer-stored pattern is directly converted into the resist layer by addressingthe writing particle beam

In applying the process depicted in Fig 91 the mask may either be placed di-rectly onto the wafer (contact printing) or may be positioned a short distance infront of the wafer (proximity printing) In either case the minimum feature sizeamounts to a couple of micrometers and thus does not satisfy todayrsquos industrialdemands However fine-line features down to the sub-micrometer range can beobtained with projection techniques as described in the next subsection

9 Technical developments related to photochemical processes in polymers232

Fig 91 Schematic illustration ofthe lithographic process

9121 Projection optical lithographyProjection optical lithography has been the mainstream technology in the semi-conductor industry for the last two decades [2] Figure 92 shows a schematic de-piction of an optical projection system consisting of a laser light source a maska projection lens and a resist-coated wafer The projection of the pattern of themask onto the resist layer provides a demagnification ratio of up to 4

Regarding a periodic fine structure assembly consisting of lines and spacesthe minimum line resolution of the pattern in terms of the minimum achiev-able feature size LWmin can be estimated with the aid of Eq (9-1)

LWmin k1

NA9-1

Actually LWmin is equal to p2 Here p denotes the pitch ie the distance madeup of a pair of lines and spaces is the wavelength of the exposure light and k1 isa system factor that depends on various parameters such as resist response pat-tern geometry in the mask etc NA is the numerical aperture given by Eq (9-2)

NA n sin 9-2

Here n is the refractive index and is the acceptance angle of the lens (seeFig 92) According to Eq (9-1) a decrease in LWmin can be accomplished by de-

91 Polymers in photolithography 233

Fig 92 Schematic illustration of an optical pro-jection system

creasing k1 or or by increasing NA In the past all three approaches havebeen implemented in following industryrsquos roadmap for the miniaturization ofelectronic devices [1] For instance a significant enhancement in resolution wasachieved by using excimer lasers operating at short wavelengths 248 nm (KrF)193 nm (ArF) and 157 nm (F2) as can be seen from Table 91 Sub-100 nm fea-tures can be generated with the aid of ArF and F2 lasers and sub-50 nm fea-tures with extreme ultraviolet (EUV) sources The numerical aperture may be in-creased with the aid of lenses with increased acceptance angle Most recently aquite radical approach to enhanced resolution has been introduced althoughnot yet applied in manufacturing namely liquid immersion lithography [5ndash7]This new technology is based on an increase in the refractive index n by repla-cing the ambient gas (air nitrogen) with a transparent liquid Using water withn= 14366 at = 193 nm and T = 215 C the numerical aperture NA is increasedby 44 at a given sin [2] The revolutionary development in miniaturizationbecomes evident if one considers that the storage capacity of dynamic randomaccess memory (DRAM) devices has been increased from less than 1 Megabit(1 Mb= 106 bit) to several Gigabit (1 Gb= 109 bit) This increase in storage capaci-ty has been accomplished by lowering LWmin from gt 1 m to less than 007 m

A different approach whereby the resolution may be improved by 50ndash100is based on the use of phase-shifting transmission masks The latter containopaque regions as conventional masks do but some of the apertures are cov-ered with a transparent phase-shifting material which reverses the phase of thelight passing through them The interaction of phase-shifted with non-phase-


Table 91 Correlation of radiation wavelength and minimumfeature size in dynamic random access memory (DRAM)devices

LWmin (m) Light source Wavelength (nm)

08 Hg discharge lamps 436 (g-line) 365 (i-line)05 Hg discharge lamps 436 365 250035 KrF excimer lasers 248025 KrF excimer lasers

ArF excimer lasers248193

018 ArF excimer lasers 1930090 F2 excimer lasers

ArF excimer lasersa)157193

0065 F2 excimer lasersArF excimer lasersa)

157193

0045 EUV sourcesb) 135 c)

a) Using hard resolution enhancement technology (RET)including the immersion technique and phase-shift masktechnology

b) Laser- and discharge-produced plasmas [8] and compactelectron-driven extreme ultraviolet (EUV) sources [9]

c) Si L-shell emission

shifted light brings about destructive interference at the resist plane This re-sults in sharply defined contrast lines because the resist is only sensitive to theintensity of the light and not to its sign [10]

9122 Maskless lithographyThe tools used for projection optical lithography as described in the previoussection include very expensive parts for instance the mask and the heavy (over1000 kg) reduction lens The projection of the image of the mask onto the sili-con wafer requires such a heavy reduction lens Moreover the design and fabri-cation of the features of the mask are associated with high costs and long de-lays The cost of the masks producing one chip can exceed $2 million Innova-tions that have stemmed from these difficulties concern the development ofmaskless optical techniques Actually non-optical techniques such as electron-beam and ion-beam lithography have existed for many years They are em-ployed in photo-mask production but are inappropriate for the large-scale pro-duction of chips Novel techniques relating to optical projection are based onprotocols differing from that described above in Section 9121 Zone-plate arraylithography ZPAL seems to play a prominent role among the novel techniques[3] In ZPAL an array of diffractive lenses focuses an array of spots onto thesurface of a photoresist-coated substrate This is accomplished by passing lightfrom a continuous-wave laser through a spatial filter and a collimating lens tocreate a clean uniform light beam The latter is incident on a spatial light mod-ulator which replaces the mask Under digital control it splits the beam intoindividually controllable beamlets Subsequently the beamlets are passedthrough a telescope such that each is normally incident upon one zone plate inthe array By simple diffraction the zone plate consisting of circular concentriczones focuses the light on a spot of the resist layer The zones in the platecause a phase shift of the transmitted light The radii of the zones are chosensuch that there is constructive interference at the focus Lines and spaces with adensity of 150 nm have been patterned with a ZPAL system operated at400 nm Sub-100 nm linewidths are expected to be realized with systems operat-ing at lower wavelengths At present continuous-wave lasers emitting at= 198 nm are commercially available [3]

Imprinting lithography is another maskless technique capable of generatingsub-100 nm patterns It is essentially a nanomolding process in which a trans-parent patterned template is pressed into a low-viscosity monomer layer dis-pensed onto the surface of a wafer Thereby the relief structure of the templateis filled After photopolymerization of the monomer with the aid of UV light(see Chapter 10) the template is separated leaving a solid polymer replica ofthe template on the surface of the wafer With the aid of subsequent etchingprocesses the pattern is fixed on the waferrsquos surface [4]


913Resists

A resist material suitable for computer chip fabrication has to fulfil various re-quirements the most important of which are the following The material mustbe suited for spin casting from solution into a thin and uniform film that ad-heres to various substrates such as metals semiconductors and insulators Itmust possess high radiation sensitivity and high resolution capability The as-pect ratio of radiation-generated fine-line features (height-to-width ratio of lines)is desired to be high but is limited by the risk of pattern collapse Moreoverthe resist material must withstand extremely harsh environments for examplehigh temperature strong acids and plasmas

On the aforementioned roadmap of progressive miniaturization major advancesin resolution have been achieved through the use of light of shorter wavelengthsNew resist materials with low absorptivities (optical density less than 04) at thesewavelengths had to be found because near-uniform exposure throughout the resistlayer needs to be maintained For example Novolak resists which function well at365 nm are too opaque at 248 nm and protected p-hydroxystyrene-based polymersthat operate well at 248 nm are too opaque at 193 nm at which acrylate- and cy-cloalkene-based polymers are used At 157 nm only transparent fluorocarbon-based polymers containing CndashF bonds appear to operate satisfactorily

Liquid development which is commonly applied in lithographic processes isbased on the radiation-induced alteration of the solubility of the irradiated resistareas (see Fig 91) Solubility is decreased by intermolecular cross-linking (nega-tive mode) or increased by main-chain degradation of the polymer (positivemode) Moreover radiation-induced chemical alterations of functional groupson the polymers can lead to a solubility change Very importantly radiation-in-duced conversion of additives controlling the solubility behavior of the polymercan also bring about the desired effect For example an additive that normallyfunctions as a dissolution inhibitor may accelerate the dissolution after exposureto light In the following subsections typical resist systems are presented With-in the frame of this book the aim is not to provide an exhaustive treatment ofthis subject More information can be obtained from relevant review articles [1-25] In this context one should note that details of the compositions of resistsystems and of the chemical nature of components are commonly withheld bythe manufacturers

9131 Classical polymeric resists ndash positive and negative resist systemsThe earliest photoresists used in integrated circuit manufacture consisted ofpolymers that were rendered insoluble by photo-cross-linking and thus operatedin the negative tone mode For instance partially cyclized poly(cis-isoprene) con-taining a bisazide as additive served for a long time as the ldquoworkhorserdquo resistmaterial in photolithography applications [15] This system has already been de-scribed in Section 723 Subsequently Novolak-based positively functioning sys-


tems (see Chart 91) were used as reliably performing ldquoworkhorserdquo resists formany years Typical commercial formulations consist of a phenol-formaldehyde-type polycondensate containing a high proportion of cresol moieties and a disso-lution inhibitor eg a diazonaphthoquinone DNQ commonly 2-DNQ The poly-mer remains soluble since polycondensation is halted before the system be-comes cross-linked It dissolves very slowly in aqueous base This dissolutionprocess may be greatly enhanced upon irradiation

As illustrated in Scheme 91 the photolysis of DNQ (quantum yield 015ndash030) induces the release of nitrogen (N2) which is followed by a Wolff rear-rangement to give an indene ketene In the presence of water this reacts toform the corresponding 3-indene carboxylic acid The latter accelerates the dis-solution of the exposed areas of the coating on top of the wafer [13 18]

Among the large family of classical resists polyimides are renowned for theirhigh temperature resistance (up to 500 C) and their excellent electrical insula-tion properties Therefore polyimides are appropriate materials for mask fabri-cation and can serve as passivation layers and interlayer dielectrics [20 21 25]To this end microstructures are generated from polyimide precursors for in-stance polyamic acid esters [26] The ester groups contain reactive functionseg carbon-carbon unsaturations The unsaturated moieties can undergo cyclo-additions or (in the presence of a photoinitiator) polymerization reactions uponexposure to UV light In this way the polyamic acid ester is cross-linked thusacting in the negative tone mode After removal of the unexposed material imi-dization of the cross-linked polyamic acid ester by thermal treatment results ininsoluble polyimide The overall process is illustrated in Scheme 92


Chart 91 Chemical structure ofNovolak resin

Scheme 91 Photolysis of 2-diazonaphthoquinone 2-DNQ [13]

A host of resist systems that undergo changes in their solubility due to chem-ical alterations upon exposure to deep UV light (240ndash280 nm) has been de-scribed in the literature [11 15 16] Tables 92 and 93 list some typical exam-ples and commercially available resists respectively They also show sensitivityvalues of the resists

By general convention the sensitivity S is related to the thickness d of theresist layer measured after exposure and development and is obtained from ex-posure characteristic curves as are illustrated in Fig 93 In the case of positive-ly functioning resists S D00

exp corresponds to the exposure light dose requiredto completely remove the irradiated polymer from the substrate ie the dose atwhich the normalized thickness of the resist layer is equal to zero dirrd0 = 0 Inthe case of negatively acting resists the sensitivity is reported as S D05

exp orsometimes as S D08

exp or S D09exp corresponding to dirr = 05 d0 dirr = 08 d0 or

dirr = 09 d0 respectively Dexp is the product of light intensity and irradiation


Scheme 92 Photo-cross-linking of polyamic acid esters andsubsequent thermal imidization R denotes a reactive groupeg ndashOndashCH2ndashCH=CH2

Fig 93 Schematic representation of exposure characteristiccurves for positive and negative resists Adapted fromSchlegel and Schnabel [27] with permission from Springer

time and is commonly given in units of mJ cmndash2 A higher sensitivity corre-sponding to a lower exposure dose implies a faster production rate

9132 Chemical amplification resistsPast efforts to improve the fabrication of microdevices have been closely connectedwith attempts to increase the resist sensitivity S In the case of the resists de-scribed in Section 9131 S is limited by the quantum yields which are much low-


Table 92 Sensitivities of deep UV positive-tone resists [15]

Polymer S (mJ cmndash2) a) (nm) b)

Poly(methyl methacrylate) 3300 240

Poly(methylisopropenyl ketone) 700 280

Poly(perfluorobutyl methacrylate) 480 240

Poly(methyl methacrylate-co-glycidyl methacrylate) 250 250

Poly(methyl methacrylate-co-indenone) 20 240

Poly(butane sulfone) 5 185

Diazoquinone-containing Novolak resins 90 248

a) Sensitivityb) Wavelength of incident light

er than unity typically 02ndash03 Quantum yields can rarely be increased In the bestcase S would be improved by a factor of three to five if the quantum yield couldbe increased to unity the maximum value Therefore the introduction into litho-graphy in the early 1980s of processes based on the concept of chemical amplifica-tion represented a truly significant advance [28] Chemical amplification meansthat a single photon initiates a cascade of chemical reactions This applies for in-stance to the photogeneration of a Broslashnsted (protonic) acid capable of catalyzingthe deprotection of functional groups attached to the backbone of linear polymerssuch as PBOCSt or PTBVB (see Chart 92)

The protonic acid is formed upon irradiation with UV light (eg at= 248 nm) when the polymers contain a small amount of an appropriate acidgenerator such as an iodonium or sulfonium salt (see Scheme 93)

Upon baking the exposed resist system at elevated temperatures (gt100 C)the photogenerated acid catalyzes the cleavage of CndashO bonds as illustrated inScheme 94 The deprotected polymer host is soluble in aqueous base develop-ers Typical turnover rates for one acid molecule are in the range of 800ndash1200cleavages Resists thus amplified may attain a photosensitivity of 1ndash5 mJ cmndash2

[14] thus significantly surpassing the sensitivity of non-amplified commercialresists (see Table 93)


Table 93 Sensitivities S of some commercial deep UV resists (250 nm) [15]

Resist S (mJ cmndash2) Tone

RD 2000NPoly(vinyl phenol) containing diazidodiphenyl sulfone a)

20 Negative

Kodak KTFRCyclized polyisoprene rubber containing azide

20 Negative

AZ-1350JNovolak resin containing diazonaphthoquinone

90 Positive

a)

Chart 92 Chemical structures of poly(t-butoxycarbonyl oxy-styrene) PBOCSt and poly(t-butyl-p-vinyl benzoate) PTBVB

Resist systems based on PBOCSt turned out to be very sensitive towards air-borne impurities These difficulties were overcome by employing another chem-ically amplified resist a random copolymer consisting of p-hydroxystyrene andt-butyl acrylate (see Chart 93)

This system denoted as Environmentally Stable Chemical Amplification PositivePhotoresist ESCAP has become the standard 248 nm resist in device manufac-ture by leading chip makers It is capable of printing features with a density of125 nm [29]

Photogenerated acids can also catalyze various other reactions eg the cross-linking of polymers containing epoxide groups (see Chart 94) or Claisen andpinacol rearrangements in polymers as shown in Scheme 95 Resist systemsoperating on the basis of these reactions have been proposed [12 13]

Besides the onium salts considered above various other organic compoundsare capable of acting as acid generators [27] Typical examples are presented inTable 94


Scheme 93 Proton generation by photolysis of diphenyliodo-nium and triphenylsulfonium salts For a detailed mechanismsee Section 103

Scheme 94 Acidolysis of PBOCSt a protected poly(p-hydroxystyrene)

Chart 93 Structure of a randomcopolymer forming the host polymerof ESCAP [29]

9133 Resists for ArF (193 nm) lithographyArF lithography employing ArF lasers emitting 193 nm light has been devel-oped with the aim of generating sub-100 nm features Since the industriallywidely used 248 nm resists containing aromatic (eg hydroxystyrene) moietiesare too opaque at 193 nm novel polymers of much lower absorptivity at thiswavelength are needed These polymers are required to withstand dry etching


Chart 94 Structures of polymers containing epoxide groupscapable of undergoing photoacid-catalyzed cross-linking [12]

Scheme 95 Acid-induced Claisen (a) and pinacol (b) rearrangements [12]


Table 94 Organic photoacid generators

Acid generators Acid

o-Nitrobenzyl sulfonates

Imino sulfonates

2-Aryl-46-bis(trichloromethyl)triazinesHCl

oo-DibromophenolsHBr

Table 95 Chemical amplification resists applicable in 193 nm lithography

Resist system Chemical structure of typical base units References

Random copolymers of norbornene methyl-cyclopentyl ester and norbornene hexafluoro-isopropanol

[29][30]

Random co- and terpolymers containingnorbornene derivatives and maleic anhydride

[31]

Alternating copolymers of vinyl ether andmaleic anhydride

[32]

Random co- and terpolymers containingacrylate or methacrylate moieties with pendantalicyclic groups

[32 33]

agents and to be base-soluble when chemical amplification based on the depro-tection of carboxylic or phenolic groups is the imaging mechanism of choice

Table 95 presents families of random copolymers with cycloaliphatic struc-tures in the main chain or in side groups that are appropriate for lithographicapplications Cycloaliphatic moieties such as adamantyl groups offer etch dur-ability while carboxylic acid groups which become available through amplifieddeprotection processes impart base solubility

The components of the copolymers are cycloaliphatic monomers (norbor-nene) and vinyl ether maleic anhydride acrylate and methacrylate In additionvinyl sulfonamides have been used as co-monomers in the synthesis of randomcopolymers capable of functioning as acid-amplified resists An example is pre-sented in Chart 95 A high sensitivity S = 2 mJ cmndash2 was measured for a co-polymer (Chart 95) with n= 04 and m= 06 (resist thickness d= 220 nm devel-oper aqueous tetramethylammonium hydroxide solution) Triphenylsulfoniumperfluoro-1-butane sulfonate served as acid generator [33]

Notably the liquid immersion technique (see Section 9121) in conjunctionwith high refractive index fluids can be applied to generate 32 nm structures(see Fig 94a) [7b]


Chart 95 Structure of poly[N-(1-adamantyl)vi-nylsulfonamide-co-(2-methyl)adamantyl methac-rylate] a random copolymer that absorbs lightonly weakly at = 193 nm [33]

Fig 94 (a) 32 nm line and space structures(X-SEM graphs) generated by means of193 nm immersion lithography and(b) 60 nm structures generated by means of

157 nm lithography Adapted from Mulkenset al [7b] and Hohle et al [39] respectivelywith permission from the author (a) andfrom Carl Hanser (b)

9134 Resists for F2 (157 nm) lithographyPhotoresists employed at 248 nm and 193 nm are too opaque at 157 nm thewavelength of light emitted by F2 lasers However sufficiently transparentfluorocarbon-based polymers containing non-absorbing CndashF bonds operate satis-factorily at 157 nm [30 34] Therefore new fluoropolymers also functioning asacid-amplified resists were synthesized Chart 96 shows the structures of copo-lymers containing 4-(2-hydroxy hexafluoro isopropyl) styrene units

At = 157 nm the fluorine-containing homopolymers and copolymers pre-sented in Chart 97 and in Scheme 96 have absorption coefficients rangingfrom 30 to 40 mndash1 [35ndash40] At a resist thickness lower than 100 nm theyturned out to be capable of imaging 40 nm lines with a 100 nm pitch [35]


Chart 96 Chemical structures of random copolymers usedfor 157 nm lithography (a) poly[4-(2-hydroxy hexafluoro iso-propyl) styrene-co-t-butyl acrylate] and (b) poly[4-(2-hydroxyhexafluoro isopropyl) styrene-co-t-butyl methacrylate] [35]

Chart 97 Chemical structures of monomer moieties ofhomopolymers and random copolymers capable of acting as157 nm resists [36ndash38]

Here the excellent performance of these polymers is demonstrated by the60 nm structures shown in Fig 94 b

Absorption coefficients of about 05 mndash1 allow imaging of 200 nm thickfilms At present however there are problems concerning pattern developmentMoreover difficulties regarding lenses and masks have to be resolved As yetCaF2 is the only feasible lens material since fused quartz is not transparent at157 nm However CaF2 is crystalline and therefore intrinsically birefringentConsequently lenses have to be made from elements with different crystal or-ientations

914The importance of photolithography for macro- micro- and nanofabrication

Photolithography is industrially employed also for the generation of macrostruc-tures of dimensions up to several millimeters Typical examples in this contextinclude the fabrication of printed circuit boards picture tubes and printingplates For details the reader is referred to the literature [21] Actually printingplates are mostly made from photopolymer systems functioning on the basis ofphotopolymerization of appropriate monomers This aspect is dealt with in Sec-tion 115 Currently photolithography continues to play a dominant role in thesemiconductor industry with regard to the production of microdevices How-ever with miniaturization being extended to nanofabrication methods using ex-treme ultraviolet (EUV) radiation (= 13 nm) and soft X-rays (synchrotron radia-tion) might become important in the future In addition to the fact that photo-lithography involves high capital and operational costs it is not applicable tononplanar substrates To overcome this disadvantage alternative methods havebeen developed At present soft lithography seems to be a promising new tech-nique for micro- and nanofabrication The soft lithographic process consists oftwo parts the fabrication of elastomeric elements (masters) ie stamps ormolds and the use of these masters to pattern features in geometries defined


Scheme 96 Acidolysis of polymers appropriate for 157 nm lithography [39 40]

by the mastersrsquo relief structure The formation of a master includes a photo-lithographic step ie the relief structure is generated by shining light througha printed mask onto the surface of a photoresist film After development thelatter is subsequently impressed in an elastomer [41ndash43]

Photomicrolithography also plays a major role in the field of micromachiningwhereby photofabrication provides a tool for making inexpensive high aspectratio microstructures having dimensions of several micrometers For exampleheight-to-width ratios as high as 18 1 at a resist thickness of up to several hun-dred m and minimum feature sizes down to 3 m can be realized with a neg-ative-tone resist containing epoxide groups (see Chart 94) Cross-linking of theirradiated resist is achieved through a photoacid-amplified mechanism [44 45]In this case irradiations can be performed at 365 nm Metallization of the poly-mer patterns (with steep edges more than 88) by galvanization or othermeans and subsequent removal of the polymer results in metal structureswhich opens up a plethora of applications Additional resist systems tested inrelation to this technique include the positive-tone system NovolakDNQ (seeSection 9131 and Fig 95) and negative-tone polyimides (see Section 9131)

Notably the patterning of thick layers commonly consisting of multiple coatsof spun-cast polymer necessitates a high transparency of the resist systemTherefore care has to be taken that the maximum exposure depth exceeds thethickness of the layer In special cases the initiatorsensitizer is photobleachedthus causing the penetration depth of the incident light to increase during expo-sure


Fig 95 High aspect ratio micro-structures (height 50 m spacing15 m) Resist system NovolakDNQ(see Section 9131) Adapted fromMaciossek et al [44] with permissionfrom Leuze

92Laser ablation of polymers

921General aspects

9211 Introductory remarksMaterial can be ejected when a laser beam or more generally speaking a highintensity light beam is directed onto a polymer sheet On the basis of this phe-nomenon commonly called laser ablation mechanical machining such as cut-ting and drilling of polymeric materials is possible Moreover microstructurescan be generated with laser beams of small diameter Since its discovery therehave been attempts to utilize laser ablation as a photolithographic tool [46 47]However because of several disadvantages such as contamination of the sur-rounding surfaces with debris carbonization and insufficient sensitivity it hasnot become a serious competitor to conventional photolithographic techniquesat least as far as the use of readily available polymers is concerned At presentthere is growing interest in exploiting laser ablation for various practical applica-tions such as laser desorption mass spectrometry or laser plasma thrusters forthe propulsion of small satellites Moreover basic research is still focused onthe mechanism of laser ablation The increasing importance of laser ablationhas been recognized by two renowned scientific journals which have publishedspecial issues devoted to various aspects of this interesting field [48 49] Mostpublished laser ablation work concerns the irradiation of polymers with femto-or nanosecond pulses provided by excimer lasers operating at wavelengths of157 193 248 308 and 351 nm In more recent work diode-pumped solid-stateNd YAG lasers generating 10 ns light pulses at the harmonic wavelengths 532355 and 266 nm (pulse energy several mJ) have also been applied especiallyfor the micromachining of plastics [50]

9212 Phenomenological aspectsThe ablation is quantified by means of the ablation rate ie the ablated depthper pulse Generally the ablation rate is insignificant at fluences below a thresh-old fluence Above this threshold the ablation rate increases dramatically This isdemonstrated in Fig 96 [51] for a commercial polyimide It can also be seen inFig 96 that the threshold fluence decreases with shortening wavelength

A sharp rise in the etch rate at the threshold is found only at the lowest laserwavelength (193 nm) At higher wavelengths the curves bend smoothly up-wards in an exponential fashion indicating that there is also ablation below thethreshold fluence point obtained by extrapolating the linear portion of the curveto zero ablation rate This was corroborated by a study on poly(methyl methacry-late) concerning the so-called incubation effect [52] The latter refers to the phe-nomenon of the polymer surface being etched less deeply by the initially ap-plied pulses than by subsequent pulses of the same fluence Actually material


is even ejected during the incubation period However it cannot be released be-cause of insufficient formation of gaseous products The latter are needed tobuild up a pressure sufficient for the ejection of large fragments Therefore theinitially etched pit is refilled Evidence for the ejection of fragments was ob-tained with the aid of acoustic signals detected in the 2ndash85 MHz range [53] InFig 97 it can be seen that the longitudinal 20 MHz signal increases drasticallyin the fluence range around the threshold deduced from ablation depth andtemperature measurements

92 Laser ablation of polymers 249

Fig 96 Laser ablation of polyimide(KaptonTM) at different wavelengths (givenin the graph) The ablation rate obtained bysingle-shot experiments as a function of the

fluence The changes in the film thicknesswere measured with the aid of a quartzcrystal microbalance Adapted from Kuumlper etal [51] with permission from Springer

Fig 97 Laser ablation of polyimide(KaptonTM) at = 193 nm The longitudinalacoustic signal (20 MHz) received by apiezoelectric transducer as a function of thefluence The arrow indicates the threshold

fluence obtained by recording the signalvoltage produced at a pyroelectrical crystal(LiTaO3) Adapted from Gorodetsky et al[53] with permission from the AmericanInstitute of Physics

The signal increase is interpreted as arising from the transfer of momentumof the ablated particles to the remaining substrate The particles acquire a ki-netic energy of Ekin = mv22 (v particle velocity of the order of 105 cm sndash1 mparticle mass) The force exerted by the ablated particles on the sample surfacegives rise to a pulse of acoustic energy which propagates through the sampleThe signal detected below the threshold is thought to be of thermoelastic andto some extent of photoelastic origin

9213 Molecular mechanismBoth photochemical and photothermal reactions contribute to the release of vol-atile fragments a process that leads to the breakage of a certain number ofchemical bonds in the polymer within a short period A versatile model that ad-dresses the fact that ablation always requires the application of a large numberof laser pulses and that rationalizes the dependence of the ablation rate on flu-ence wavelength pulse length and irradiation spot size has been proposed bySchmid et al [54] Accordingly the absorption of laser light leads to the elec-tronic excitation of chromophoric groups in the polymer The subsequent deacti-vation processes involve both direct bond breakage in the excited state and re-laxation ie internal conversion to a highly excited vibrational state of the elec-tronic ground state In the latter case the interaction with surrounding mole-cules can lead to thermal activation resulting in further bond breakage Thechemical alterations that accompany these reactions lead to modified chromo-phores with absorption cross-sections differing from those of the original onesIf the number of broken bonds exceeds a threshold value a thin layer of thepolymer is ablated and the ablated material forms a plume that expands three-dimensionally and continues to absorb laser radiation The ablation plume con-sists of gaseous organic products and particulate fragments and in the case ofbiological tissues also of water vapor and water droplets The expansion of theplume into the surrounding air is coupled with the generation of acoustic tran-sients that for high volumetric energy densities evolve into shock waves [55]In principle simultaneous multi-photon absorption may also be involved in la-ser ablation of neat polymers although it seems to be important only at thelarge pulse fluences attained with sub-ps pulses

922Dopant-enhanced ablation

Ablation can be significantly enhanced by the presence of dopants ie by addi-tives that strongly absorb laser light Dopant-enhanced ablation is important incases in which the laser light is only weakly absorbed by the polymer matrixTypical examples of such systems are poly(methyl methacrylate) containing acri-dine or tinuvin-328 (exc = 308 or 351 nm) [56 57] and nitrocellulose doped withstilbene-420 coumarin-120 or rhodamine 6G (exc = 337 nm) [58] In thesecases different mechanisms can become operative [57] Degradation of the poly-


mer matrix can be caused by thermal energy transferred from the dopant to thepolymer In other words most of the electronically excited dopant molecules de-activate through vibronic relaxation (internal conversion) to vibronically excitedground states from which energy is transferred to surrounding macromole-cules Alternatively the additive may be excited to higher electronic levels bymulti-photon absorption and subsequently decompose into various fragmentswhich leads to explosive decomposition of the polymer matrix

923Polymers designed for laser ablation

Novel photopolymers have been developed to overcome certain disadvantagessuch as debris contamination and insufficient sensitivity encountered in the ap-plication of laser ablation in lithographic techniques Of note in this context arenovel linear polymers containing photochemically active chromophores in themain chain [59] In relation to the 308 nm laser light generated by XeCl excimerlasers polymers containing triazene or cinnamylidene malonic acid groups werefound to be much more appropriate than a commercial polyimide (see Chart 98)

The TC and CM polymers decompose exothermically at well-defined posi-tions Thereby gaseous products are formed which carry away the larger frag-ments In the case of the triazene polymer (see Scheme 97) the fragmentationpattern has been analyzed with the aid of time-of-flight mass spectrometry

A comparison of characteristic ablation parameters (see Table 96) reveals thatthe polymer containing triazene groups possesses a lower threshold fluence anda higher etch rate than the other two polymers and is therefore most appropri-ate for technical processes based on laser ablation of polymers


Chart 98 Chemical structures of polymers appropriate for laser ablation at = 308 nm

924Film deposition and synthesis of organic compounds by laser ablation

Thin films with special chemical and physical properties can be deposited on asubstrate upon irradiating a target material located in a closed system in theneighborhood of the substrate with a laser beam [60 61] A schematic depictionof such a set-up with a targetndashsubstrate distance of 20 mm is shown in Fig 98Besides silicon wafers appropriate substrate materials include ZnSe KBr andquartz

In deposition studies with polyacrylonitrile it was found that the compositionof the deposited films could be controlled by varying the laser wavelength andthe fluence per pulse Films containing varying amounts of cyano side groupshave been generated in this way [63 64] Moreover poly(tetrafluoroethylene)and poly(methyl methacrylate) have been used as target materials for the de-position of thin films [65 66] Films possessing an Si-C network structure havebeen obtained by laser ablation of poly(dimethylsilane) or hexaphenyldisilane(see Chart 99) With blends of these two compounds films of increased hard-ness were obtained [67 68]


Scheme 97 Laser decomposition of the TC polymer [59]

Table 96 Ablation parameters of polymers [59]

TC Polymer CM Polymer Polyimide a)

linear (cmndash1) b) 100000 102000 95000Fthreshold (mJ cmndash2) c) 27 63 60D (nmpulse) d) 267 90 61

a) 125 m KaptonTMb) Linear absorption coefficientc) Threshold fluenced) Etch rate at F =100 mJ cmndash2

34910-Perylenetetracarboxylic dianhydride PTCDA has been used as a tar-get material for the generation of films consisting essentially of polyperi-naphthalene (see Chart 910) [62 69] Films annealed at 350 C immediatelyafter deposition possessed an electrical conductivity of 10ndash3 S cmndash1

Proteins such as collagen (see Chart 911) keratin and fibroin have also beensuccessfully employed as target polymers in the generation of films [70] Theprimary structure of the target protein is retained in the deposited film as wasinferred from IR spectroscopic analysis Interestingly relevant research led tothe application of lasers for medical purposes Nowadays excimer laser beamsare frequently employed by ophthalmologists for the purpose of keratectomy ie


Fig 98 Schematic representation of a set-up used for filmdeposition with the aid of laser ablation Adapted from Nishioet al [62] with permission from the Editorial Office of JPhotopol Sci Technol

Chart 99 Chemical structures of poly(dimethylsilane) left and hexaphenyldisilane right

Chart 910 Chemical structures of 34910-perylenetetra-carboxylic dianhydride left and polyperinaphthalene right

cornea reprofiling and sculpting As a matter of fact a large portion of the cor-nea consists of a collagen hydrogel

925Laser desorption mass spectrometry and matrix-assisted laser desorptionionization (MALDI)

Laser beam ablation in conjunction with mass spectrometry has contributedgreatly to the progress in polymer analysis made in recent years [71] Laser de-sorption mass spectrometry (LDMS) refers to the irradiation of a polymer surfacewith a high-power laser beam and the subsequent mass analysis of the ablatedspecies For this purpose the ablated species are ionized by irradiation with an-other laser beam or with an electron beam Typical LDMS work pertains to thecharacterization of polyamide-66 [72] and perfluorinated polyethers [73] and tothe detection of additives in polymers [73 74] A particular kind of LDMS calledmatrix-assisted laser desorptionionization (MALDI) has recently become quiteimportant [75ndash77] The development of the analysis of proteins by means ofMALDI has been recognized by the award of the Nobel prize for chemistry toK Tanaka in 2002 MALDI is characterized by specific sample preparation tech-niques and low fluences in order to create the analyte ions Fundamentally theanalyte is embedded within a solid matrix in a molecularly dispersed state byplacing a droplet of a solution containing analyte and matrix compound on asubstrate and subsequently vaporizing the solvent Alternatively a layered targetmay be formed by casting solutions of both analyte and matrix on a substrateThis target is then placed in the source of a mass spectrometer and the ablationof both matrix and analyte molecules is induced by irradiation with a laserbeam (usually at = 337 nm at which the matrix absorbs the laser light) Theablated neutral analyte molecules are cationized in the gas phase by reactionwith protons (eg analytes bearing amine functions) or metal cations (eg oxy-gen-containing analytes react with Na+ unsaturated hydrocarbons react withAg+) The resulting ions are extracted into the mass spectrometer for mass anal-ysis Most of the matrix materials used in polymer MALDI are aromatic organicacids that can readily supply protons such as 25-dihydroxybenzoic acid -cya-no-hydroxycinnamic acid ferulic acid indole acrylic acid or trans-retinoic acidIf metal cationization is required the source of the appropriate metal must be


Chart 911 Chemical structures of base units contained in collagen


Fig 99 MALDI mass spectra of high molarmass polystyrene samples with nominalmolar masses of 31105 (A) 60105 (B)and 93 105 (C) The peaks at lower mass-

to-charge ratios relate to multiply-chargedions Adapted from Schriemer et al [78] withpermission from the American ChemicalSociety

contained within the matrix The mechanisms of ionization in MALDI are notyet well understood In many cases cations are likely to form rather stable com-plexes with ablated analyte molecules in the gas phase

An outstanding quality of polymer MALDI is that it offers the possibility ofmeasuring molar masses Very accurate values can be obtained for oligomerswith molar masses up to several thousand g molndash1 but the determination ofmuch higher molar masses is difficult Nevertheless the successful analysis ofa polystyrene sample of molar mass 15106 g molndash1 has been claimed [78] Typ-ical MALDI mass spectra of high molar mass polystyrene samples are shown inFig 99

For more detailed information concerning this interesting field the reader isreferred to relevant literature reviews [79ndash82]

926Generation of periodic nanostructures in polymer surfaces

The possibility of generating periodic sub-100 nm line structures in polyimideby direct laser ablation was demonstrated as long ago as 1992 [83] Structureswith a period of 167 nm and line widths varying from 30 to 100 nm were pro-duced by 248 nm laser irradiation by means of an interferometric techniqueThe polyimide film was exposed to 500ndash800 laser shots at a pulse fluence rang-ing from 34 to 58 mJ cmndash2 Work of this kind is important because of possibleapplications in the fabrication of optical microdevices such as high-speedphotonic switches or gratings for coupling light into waveguides Actually grat-ing couplers can be easily produced by laser ablation at any position of the wave-guide which implies good prospects for employment in the industrial fabrica-tion of waveguides This aspect has been outlined in work concerning the gen-eration of periodic nanostructures in PDA-C4UC4 a polydiacetylene (for thechemical structure see Table 35) by UV laser pulses (248 nm 130 fs) [84 85]

927Laser plasma thrusters

A potential application of polymer laser ablation concerns the propulsion ofsmall satellites (1ndash10 kg) used in space science [86] Laser plasma thrustersLPTs operating with small powerful diode lasers emitting in the near-infraredwavelength range (930ndash980 nm) have been proposed Polymers intended toserve as fuel for a thruster are required to possess a large momentum couplingcoefficient Cm defined by Eq (9-3)

cm mvW

9-3

Here mv is the target momentum of the laser-ejected material and W is theenergy absorbed by the polymer per laser pulse The triazene polymer (TC poly-


mer) dealt with in Section 923 doped with carbon seems to be a promisingfuel candidate for application in LPTs for microsatellites This was concluded onthe basis of a high absorption coefficient at 930 nm a large Cm value a lowthreshold fluence and a high ablation rate [59]

93Stabilization of commercial polymers


No polymer is capable of withstanding prolonged exposure to solar radiationTherefore commercial polymers are stabilized with small amounts of additivesdenoted as light stabilizers Research and development concerning light stabi-lizers dates back to the time when polymers first became constructive materialsand industrial companies started to fabricate a plethora of plastic items Actu-ally the development of efficient light stabilizers has been a critical factor in re-lation to the growth of the plastics industry Mechanistic aspects regarding thephotodegradation of polymers are outlined in Chapter 7 where it is shown thatthe absorption of a photon by a chromophoric group generates an electronicallyexcited state and that the latter can undergo various deactivation modes Com-monly chemical deactivation results in the formation of free radicals which arereactive and attack intact molecules Extremely important in this context are re-actions involving molecular oxygen The aims of the strategies that are currentlyemployed to stabilize commercial polymers are to interfere with the absorptionof light with the deactivation of excited states and with the reactions of freeradicals Therefore stabilizers may be divided into three classes UV absorbersenergy quenchers and radical scavengers It should be noted however that a stabi-lizer molecule may protect a polymer by more than one mechanism Radicalscavengers are commonly denoted as chain terminators chain breakers or anti-oxidants

Screening is the most obvious and historically most familiar method of protec-tion Surface painting which serves as a means of protection for many materi-als is not applicable for most plastics because of incompatibility problemsHowever intrinsic screening is widely applied It is based on the addition of ef-fective light absorbers denoted as pigments ie hyperfinely dispersed com-pounds with extinction coefficients that significantly exceed those of the poly-mers Most prominent in this context is carbon black Other pigments and fil-lers of industrial importance include ZnO MgO CaCO3 BaSO4 and Fe2O3Light stabilizers for commercial polymers are required to be physically compati-ble with the polymers They should not readily be transformed into reactive spe-cies Moreover they should not alter the mechanical or other physical propertiesof the polymer before during or after exposure to light For instance theyshould be resistant to discoloration The different classes of light stabilizers are

93 Stabilization of commercial polymers 257

discussed in the following subsections For further reading several books andarticles concerned with polymer stabilization are recommended [87ndash109]

932UV absorbers

9321 Phenolic and non-phenolic UV absorbersUV absorbers (UVAs) are colorless compounds having high absorption coeffi-cients in the UV part of the terrestrial solar spectrum They transform the ab-sorbed radiation energy into harmless thermal energy by way of photophysicalprocesses involving the ground state and the excited state of the molecule Typi-cal UVAs are listed in Tables 97 and 98

Effective UVAs are required to have absorption maxima lying between 300and 380 nm preferably between 330 and 350 nm and an inherent photostabil-ity Various UVAs including derivatives of benzotriazoles 135-triazines andoxanilides fulfil these requirements and are therefore widely applied in coat-ings [87]


Table 97 Typical phenolic UV absorbers capable of formingan intramolecular hydrogen bond [87 107]


o-Hydroxybenzophenones

R1 H alkyl

R2 H alkyl phenyl

R3 H alkyl

R4 H butyl

2-(2-Hydroxyphenyl)benzo-triazoles

2-(2-Hydroxyphenyl)-135-triazines

Phenyl salicylates

9322 Mechanistic aspectsEfficient phenolic UVAs are characterized by a planar structure and a capacityto form intramolecular hydrogen bonds ie OmiddotmiddotmiddotHmiddotmiddotmiddotO or OmiddotmiddotmiddotHmiddotmiddotmiddotNbridges which allow intramolecular proton tunneling in the excited state Theprocess referred to in the literature as excited-state intramolecular proton transfer(ESIPT) is illustrated in Scheme 98

The formation of the tautomeric form S1 by proton tunneling proceeds with arate constant of about 1011 sndash1 The subsequent processes namely dissipation ofenergy by internal conversion (IC) to the ground state S0 of the tautomeric formand regeneration of the original ground state S0 by reverse proton transfer(RPT) are complete within 40 ps Mechanisms based on intramolecular H-tun-neling have been proposed for benzotriazoles and 135-triazines as well as for(non-phenolic) oxanilides (see Scheme 99)


Table 98 Typical non-phenolic UV absorbers [87]


Cyanoacrylates

Oxanilides

Scheme 98 Excited-state intramolecular proton transfer(ESIPT) in the case of 2-hydroxybenzophenone

A mechanism involving intramolecular charge separation after photoexcitationserves to explain the UVA properties of (non-phenolic) cyanoacrylates (seeScheme 910)

933Energy quenchers

Energy quenchers accept energy from excited chromophores tethered to poly-mers and thus prevent harmful chemical transformations Commonly the gen-erally undesired chemical deactivation of the excited chromophore throughbond rupture (eg via Norrish type I and II processes) or rearrangements (egvia the photo-Fries rearrangement) and energy transfer to the quencher arecompeting processes (see Scheme 911)

Therefore the photodegradation of polymers cannot be completely suppressedby energy quenchers Energy transfer from P to Q is possible if the energylevel of the excited state of the chromophore is higher than that of thequencher Excited quencher molecules are deactivated to the ground state byemission of light or dissipation of thermal energy (see Scheme 912)


Scheme 99 Excited-state intramolecular proton transfer (ESIPT) in the case of oxanilides

Scheme 910 Light-induced intramolecular charge separationin the excited state in cyanoacrylates

Scheme 911 Schematic illustration of the action of energy quenchers

The importance of quenchers derives mainly from their ability to interactwith excited carbonyl groups which are present in many thermoplastics espe-cially in polyalkenes Commercially available energy quenchers include com-plexes and chelates of transition metals such as those shown in Chart 912

It may be the case that energy quenchers also act as UVAs ie that they alsoprotect the polymer by light absorption


Scheme 912 Schematic illustration of the deactivation of excited quencher molecules

Chart 912 Chemical structures of typical nickel chelates usedas quenchers in polyalkenes [93]

934Chain terminators (radical scavengers)

Chain terminators interrupt the propagation of the oxidative chain reaction [re-actions (a) and (b) in Scheme 913 see also Scheme 718] and thus prevent dete-rioration of the mechanical properties of polymers

The chain propagation would be totally prevented if all macroradicals P gen-erated during the initiation stage were scavenged according to reaction (c)However reaction (a) proceeds at a relatively large rate even at ambient tem-perature and low O2 pressure Therefore in practically relevant situations theconcentration of P will be much lower than that of POO [99] Consequentlyan effective chain terminator is required to react rapidly with POO (reaction(d)) and the products of this reaction must be inert towards the polymer Hin-dered amines based on the 2266-tetramethylpiperidine (TMP) structure (seeChart 913) satisfactorily fulfil these requirements especially in the case of poly-alkenes In the literature they are referred to as hindered amine stabilizers(HASs) or frequently also as hindered amine light stabilizers (HALSs) The stabi-lizing power of a typical HAS is demonstrated by the results shown in Fig 910

Hindered amine stabilizers are transparent to visible and terrestrial UV light(300ndash400 nm) In polymeric matrices they are oxidized in a sacrificial reactionby way of a not yet fully understood mechanism to stable nitroxyl (aminoxyl)radicals gtNndashO A mechanism based on the reaction of HASs with alkyl hydro-peroxides and alkyl peroxyl radicals is presented in Scheme 914 [87]

A mechanism involving charge-transfer complexes formed by HAS polymerO2 and ROO ie [HASmiddotmiddotmiddotO2] [polymermiddotmiddotmiddotO2] [HASmiddotmiddotmiddotROO] has been pro-posed [111] It is considered to contribute in the early stages of the hinderedamine stabilization mechanism [87] The oxidation of TMP derivatives as illus-trated in Scheme 915 commences when the polymer is processed It continueslater when the polymer is exposed to light


Scheme 913 Schematic illustration of elementary reactionsoccurring in a polymeric matrix containing O2 and a radicalscavenger (chain terminator CT)


Chart 913 Chemical structures of typical commercial hindered amine stabilizers [109]

Fig 910 Photooxidation of a commercial polypropylene in theabsence and presence of a typical HAS (for chemicalstructure see Chart 913 uppermost) Adapted from Schnabel[110] with permission from Carl Hanser

The reaction of TMPO with alkyl radicals yields amino ethers as illustratedin Scheme 916

Amino ethers are capable of reacting with peroxyl radicals thereby regenerat-ing nitroxyl radicals This is considered to be the reason for the high stabilizerefficiency of many hindered amines (see Scheme 917)


Scheme 914 Schematic illustration of the oxidation ofhindered amine stabilizers by alkyl hydroperoxides and alkylperoxyl radicals [87]

Scheme 915 Oxidation of a 2266-tetramethylpiperidine(TMP) derivative to the corresponding nitroxyl radical ie thepiperidinoxyl radical TMPO

Scheme 916 Formation of amino ethers by the reaction of TMPO with alkyl radicals

Scheme 917 Regeneration of nitroxyl radicals by the reactionof amino ethers with alkyl peroxyl or acyl peroxyl radicals

Besides the beneficial role that nitroxyl radicals play in the stabilization ofpolyalkenes hydrogen abstraction according to Scheme 918 may have an ad-verse effect [87]

The macroradicals P generated in this process can initiate oxidative chain re-actions and thus reduce the stabilizing power of hindered amines

935Hydroperoxide decomposers

Besides hindered amines (see Section 934) there are compounds that are cap-able of functioning as long-term hydroperoxide decomposers These include al-kyl and aryl phosphites and organosulfur compounds such as dialkyl dithio-carbamates dithiophosphates and dithioalkyl propionates (see Chart 914)

These compounds are commonly used to stabilize thermoplastic polymersduring processing in the melt at temperatures up to 300 C Their contributionto the long-term stabilization of polymers at ambient temperatures is small butnot negligible Phosphite stabilizers destroy hydroperoxides stoichiometrically ina sacrificial process as shown in Scheme 919


Scheme 918 Reaction of nitroxyl radicals with polymers

Chart 914 Chemical structures of hydroperoxide decomposers [93 94]

Scheme 919 Reaction of phosphites with hydroperoxides

Scheme 920 Reaction of dialkyl dithiopropionates with hydroperoxides

Metal dialkyl dithiocarbamates are oxidized to sulfur acids which act as ioniccatalysts for the non-radical decomposition of hydroperoxides When the metalis nickel or another transition metal they also function as UVAs Dialkyl dithio-propionates are oxidized by hydroperoxides as shown in Scheme 920

936Stabilizer packages and synergism

Frequently different classes of light stabilizers are combined to optimize stabilizingefficiency [112] For example UVAs and HALSs used in combination often providebetter photostability than either class alone Light stabilizers are also used in com-bination with additives that protect the polymers against thermal degradation dur-ing processing such as hindered phenols and phosphates [113] Consequently var-ious bifunctional and trifunctional photostabilizers have been synthesized andsome have been selected for use in commercial applications (see Chart 915)


Chart 915 Chemical structures of typical bifunctional stabilizers [87]

In the context of multifunctionality carbon black a polycrystalline materialmerits special mention The surface layer of carbon black particles may containquinones phenols carboxy phenols lactones etc Therefore apart from being apowerful UV absorber and a quencher of excited states (such as those of carbo-nyl groups) carbon black acts as a scavenger of free radicals in chain-breakingreactions and as a hydroperoxide decomposer [114 115] In polyethylene carbonblack forms a complex with macroradicals [115]

937Sacrificial consumption and depletion of stabilizers

All polymer systems eventually undergo a loss in durability during long-termoutdoor application However the presence of stabilizers at concentrations be-tween 025 and 30 provides for longevity The ultimate outdoor lifetime ofpolymer articles such as coatings is determined by the sacrificial consumptionandor depletion of the stabilizers During outdoor application the concentra-tion of the active form of the stabilizer is continually reduced and eventuallyreaches a level below the critical protection value determining the ultimate life-time of polymer coatings The term sacrificial consumption refers to the chemicalalterations that stabilizer molecules undergo in protecting the polymer matrixStabilizer molecules are also consumed by direct or sensitized photolysis (egby the attack by free radicals) photooxidation reactions with atmospheric pollu-tants etc processes that are covered by the term depletion Stabilizer depletioncan also be caused by physical loss ie by migration of the stabilizer moleculesThis relates for example to coatings in which stabilizer molecules may migratefrom the clearcoat to the basecoat or plastic substrate These problems may bealleviated by the use of physically persistent stabilizers High molar massstabilizers (M gt 500 g molndash1) including oligomers with appropriate molecularstructures (M= 3500ndash5000 g molndash1) are sufficiently physically persistent and donot evaporate at the elevated temperatures of curing [87]

In the case of UV absorbers forming intramolecular hydrogen bonds the loss ofstabilizer efficiency may be due to the interruption of intramolecular hydrogenbonds and the formation of intermolecular hydrogen bonds with H-acceptors (car-bonyl groups) generated by photooxidation of the polymer matrix Thus the


Chart 916 Nitroso (a) and nitro compounds (b) andnitrogen-free compounds ((c) and (d)) formed during thephotolysis of hindered amines [117]

ESIPT mechanism (see Section 9322) can no longer be repeated Regarding hin-dered amine stabilizers depletion is caused by the reaction of acyl radicals stem-ming from Norrish reactions with nitroxyl radicals Nitroso and nitro compounds(see Chart 916) are formed when nitroxyl radicals are photolyzed [117]


References

1 Semiconductor Industry Association TheInternational Roadmap for SemiconductorsInternational SEMATECH Austin TX(2003) See also P M Zeitzoff SolidState Technol (2004) January p 30 andP Gargini R Doering Solid State Tech-nol (2004) January p 72

2 M Rothschild Projection Optical Litho-graphy materials today 8 (2005) 18

3 R Menon A Patel D Gil HI SmithMaskless Lithography materials today 8(2005) 26

4 (a) D J Resnick SV Sreenivasan CGWillson Step amp Flash Imprint Lithographymaterials today 8 (2005) 34 (b) SVSreenivasan CG Willson NE Schu-maker D J Resnick Proc SPIE 4688(2002) 903

5 M Switkes M Rothschild J Vac SciTechnol B 19 (2001) 2353

6 B J Lin Proc SPIE 4688 (2002) 117 (a) S Owa H Nagasaka J Microlith

Microfabr Microsyst 3 (2004) 97 (b) JMulkens P Graupner Proc 2nd Sympo-sium on Immersion LithographyBruges Belgium (2005)

8 U Stamm J Phys D Appl Phys 37(2004) 3244

9 A Egbert A Ostendorf BN ChichkovT Missala MC Schuumlrman K Gaumlbel GSchriever U Stamm Lambda High-lights 61 (2002) 6

10 MD Levenson Microlithogr World 6(1992) 6

11 H Ito Functional Polymers for Micro-lithography Nonamplified Imaging Sys-tems Chapter 23 in R Arshady (ed)Desk Reference of Functional PolymersSyntheses and Applications AmericanChemical Society Washington DC(1997)

12 H Ito Functional Polymers for Micro-lithography Amplified Imaging SystemsChapter 24 in R Arshady (ed) Desk

Reference of Functional Polymers Synthesesand Applications American Chemical So-ciety Washington DC (1997)

13 NP Hacker Photoresists and Their Devel-opment in V V Krongauz AD Trifunac(eds) Processes in Photoreactive PolymersChapman amp Hall New York (1995)p 368

14 M Madou Fundamentals of Microfabrica-tion CRC Press Boca Raton FL USA(1997)


16 CG Willson Organic Resist Materials ndashTheory and Chemistry in L F ThompsonCG Willson MJ Bowden (eds) Intro-duction to Microlithography AmericanChemical Society Washington DC 1sted (1983) p 87 2nd ed (1994) p 139

17 W S DeForest Photoresist Materials andProcesses McGraw-Hill New York (1975)

18 LE Bogan Novolak Resins for Micro-lithography in J C Salamone (ed) Con-cise Polymeric Materials Encyclopedia CRCPress Boca Raton FL USA (1999)

19 J N Helbert (ed) Handbook of VLSI Mi-crolithography Principles Tools Technologyand Application 2nd Edition Noyes PublPark Ridge NJ USA (2001)

20 J J Lai Polymer Resists for Integrated Cir-cuit (IC) Fabrication in J J Lai (ed)Polymers for Electronic Applications CRCPress Boca Raton FL USA (1989)

21 J H Bendig H-J Timpe Photostructur-ing in H Boumlttcher (ed) Technical Appli-cations of Photochemistry Deutscher Ver-lag fuumlr Grundstoffindustrie Leipzig(1991) p 172

22 P Rai-Choudhury Handbook of Micro-lithography Micromachining and Microfab-rication Vol 1 Microlithography Vol 2Micromachining and Microfabrication

References 269

SPIE Optical Engineering Press Belling-ham Washington (1997)

23 HJ Levinson W H Arnold OpticalLithography Chapter 1 in [22]

24 H Ito E Reichmanis A Nalamasu TUeno (eds) Micro- and Nanopatterning ofPolymers ACS Symposium Series 706American Chemical Society WashingtonDC (1998)

25 K Horie T Yamashita (eds) Photosensi-tive Polyimides Fundamentals and Applica-tions Technomic Lancaster PA USA(1995)

26 H Ahne R Rubner Applications of Poly-imides in Electronics Chapter 2 in [25]

27 L Schlegel W Schnabel Polymers in X-ray Electron-Beam and Ion-Beam Lithogra-phy in J P Fouassier J F Rabek (eds)Radiation Curing in Polymer Science andTechnology Vol 1 Elsevier AppliedScience London (1993)

28 H Ito G Willson Polym Eng Sci 23(1983) 1012

29 H Ito Chemical Amplification Resists In-ception Implementation in Device Manu-facture and New Developments J PolymSci Part A Polym Chem 41 (2003)3863

30 W Li PR Varanasi MC Lawson RWKwong K J Chen H Ito H TruongR D Allen M Yamamoto E KobayashiM Slezak Proc SPIE 5039 (2003) 61

31 J-C Jung M-H Jung G Lee H-HBaik Polymer 42 (2001) 161

32 HW Kim S J Choi SG Woo J TMoon J Photopolym Sci Technol 13(2000) 419

33 T Fukuhara Y Shibasaki S Ando SKishimura M Endo M Sasago MUeda Macromolecules 38 (2005) 3041

34 A K Bates M Rothschild T M Bloom-stein T H Fedynyshyn R R Kunz VLiberman M Switkes Review of Technol-ogy for 157 nm Lithography IBM J Resamp Dev 45 (2001) 605

35 TH Fedynshyn R R Kunz R F SintaM Sworin WA Mowers R B Good-man SP Doran Proc SPIE 4345(2001) 296

36 S Kodama T Kaneko Y Takebe S Oka-da Y Kawaguchi N Shida S IshikawaM Toriumi T Itani Proc SPIE 4690(2002) 76

37 T Yamazaki T Furukawa T Itani TIshikawa M Koh T Araki M ToriumiT Kodani H Aoyama T YamashitaProc SPIE 5039 (2003) 103

38 H Ito GM Wallraff P Brock N Fen-der H Truong G Brayta D C MillerMH Sherwood RD Allen Proc SPIE4345 (2001) 273

39 C Hohle M Sebald T Zell Metallober-flaumlche 57 (2003) 45

40 F Houlihan R Sakamuri A RomanoD Rentkiewicz R R Dammel W Con-ley D Miller M Sebald N StepanenkoK Markert U Mierau I Vermeir CHohle T Itani M Shigematsu E Kawa-guchi Proc SPIE 5376 (2004) 134

41 Y Xia G M Whitesides Soft Litho-graphy Angew Chem Int Ed 37 (1998)550

42 J A Rogers R Nuzzo Recent Progress inLithography materials today 8 (2005) 50

43 B Gates Nanofabrication with Molds andStamps materials today 8 (2005) 44

44 A Maciossek G Gruumltzner F ReutherGalvanotechnik 91 (2000) 1988

45 M Madou Surface MicromachiningChapter 5 in [14]

46 R Srinivasan V Mayne-Banton ApplPhys Lett 41 (1982) 576

47 Y Kawamura K Toyoda S NambaAppl Phys Lett 40 (1982) 374

48 H Masuhara NS Allen (eds) Photo-ablation of Materials J PhotochemPhotobiol A Chem Special Issue 145(2001) 3

49 S Georgiou F Hillenkamp (eds) LaserAblation of Molecular Substrates ChemRev Special Issue 103 (2003) 2

50 A Ostendorf K Koumlrber T Nether TTemme Lambda Physics Highlights 57(2000) 1

51 S Kuumlper J Branon K Brannon ApplPhys A 56 (1993) 43

52 R Srinivasan B J Braren K G Casey JAppl Phys 68 (1990) 1842

53 G Gorodetsky TG Kazyaka R L Mel-cher R Srinivasan Appl Phys Lett 46(1985) 828

54 H Schmid J Ihlemann B Wolff-RottkeK Luther J Troe Appl Phys A 83(1998) 5458


55 A Vogel V Venugopalan Mechanisms ofPulsed Laser Ablation of Biological TissuesChem Rev 103 (2003) 577

56 R Srinivasan B J Braren R W DreyfusL Hadel DE Seeger J Opt Soc AmB3 (1986) 785

57 R Srinivasan B J Braren Appl Phys A45 (1988) 289

57 CE Kosmidis CD Skordoulis ApplPhys A 56 (1993) 64

58 T Lippert C David J T Dickinson MHauser U Kogelschatz SC LangfordO Nuyken C Phipps J Robert A Wo-kaun J Photochem Photobiol AChem 145 (2001) 145

59 J Hobley S Nishio K Hatanaka H Fu-kumura Laser Transfer of Organic Mole-cules Chapter 5 of Vol 2 in HS Nalwa(ed) Handbook of Photochemistry andPhotobiology American Scientific Pub-lishers Stevenson Ranch CA USA(2003)

60 DB Chrisey A Piqueacute R A McGill J SHorwitz BR Ringeisen DM BubbPK Wu Laser Deposition of Polymer andBiomaterial Films Chem Rev 103 (2003)533

62 (a) S Nishio Y Tsujine A MatsuzakiH Sato T Yamabe J Photopol SciTechnol 13 (2000) 163 (b) S Nishio KTamura Y Tsujine T Fukao M Naka-no A Matsuzaki H Sato T Yamabe JPhotochem Photobiol A Chem 145(2001) 165

63 S Nishio T Chiba A Matsuzaki HSato Appl Surf Sci 106 (1996) 132

64 S Nishio T Chiba A Matsuzaki HSato J Appl Phys 79 (1996) 7198

65 G B Blanchet S I Shah Appl PhysLett 62 (1993) 1026

66 SG Hansen T E Robitaille Appl PhysLett 52 (1988) 81

67 M Suzuki Y Nakata H Nagai K GotoO Nishimura T Oku Mat Sci Eng A246 (1998) 36

68 M Suzuki M Yamaguchi L RamonatX Zeng J Photochem Photobiol AChem 145 (2001) 223

69 M Yudasaka Y Tasaka M Tanaka HKamo Y Ohki S Usamai S Yoshi-mura Appl Phys Lett 64 (1994) 3237

70 Y Tsuboi N Kimoto M Kabeshita AItaya J Photochem Photobiol A Chem145 (2001) 209

71 SD Hanton Chem Rev 101 (2001)527

72 A C Cefalas N Vassilopoulos E Saran-topoulou K Kollis C Skordoulis ApplPhys A Mater Sci Process 70 (2000) 21

73 MS de Vries HE Hunziker PolymPrepr 37 (1996) 316

74 Q Zhan R Zenobi S J Wright PR RLangridge-Smith Macromolecules 29(1996) 7865

75 K Tanaka H Waki Y Ido S Akita YYoshida T Yoshida Rapid CommunMass Spectrom 2 (1988) 151

76 M Karas F Hillenkamp Anal Chem60 (1988) 2299

77 U Bahr A Deppe M Karas F Hillen-kamp U Giessman Anal Chem 64(1992) 2866

78 DC Schriemer L Li Anal Chem 68(1996) 2721

79 HJ Raeder W Schrepp Acta Polym 49(1998) 272

80 MWF Nielsen Mass Spectrom Rev 18(1999) 309

81 R Zenobi R Knochenmuss Mass Spec-trom Rev 17 (1998) 337

82 H Pasch W Schrepp MALDI-TOF MassSpectrometry of Synthetic PolymersSpringer Heidelberg (2003)

83 HM Phillips DL Callahan R Sauer-brey G Szabo Z Bor Appl Phys A 54(1992) 158

84 G I Stegeman R Zanoni CT SeatonMater Res Soc Symp Proc 109 (1988)53

85 J-H Klein-Wiele MA Mader I BauerS Soria P Simon G Marowsky SynthMetals 127 (2002) 53

86 C Phipps J Luke Diode Laser-DrivenMicrothrusters A New Departure for Mi-cropropulsion AIAA Journal 401 Janu-ary (2002)

87 J Pospisil S Necircpurek Photostabilizationof Coatings Mechanisms and PerformanceProg Polym Sci 25 (2000) 1261

88 J Pospisil S Necircpurek Highlights in theInherent Chemical Activity of Polymer Sta-bilizers in HS Hamid (ed) Handbookof Polymer Degradation Dekker NewYork (2000) p 191

References 271

89 J Pospisil Aromatic and HeteroaromaticAmines in Polymer Degradation AdvPolym Sci 124 (1995) 87

90 J Pospisil Chain-Breaking Antioxidantsin Polymer Stabilization in G Scott(ed) Developments in Polymer Stabiliza-tion Applied Science Publishers Lon-don (1979) p 1

91 J Pospisil PP Klemchuk (eds) Oxida-tion Inhibition in Organic Materials VolsI and II CRC Press Boca Raton FLUSA (1990)

92 H Zweifel Plastics Additives Handbook5th Edition Hanser Muumlnchen (2001)

93 H Zweifel Stabilization of PolymericMaterials Springer Berlin (1998)


95 G Scott (ed) Atmospheric Oxidationand Antioxidants Elsevier London(1993)

96 G Scott Mechanisms of Polymer Degra-dation and Stabilisation Elsevier AppliedScience London (1990)

97 A Valet Stabilization of Paints Vin-centz Hannover (2000)

98 V Ya Shlyapintokh Photochemical Con-version and Stabilization of PolymersHanser Muumlnchen (1984)

99 Yu A Shlyapnikov SG KiryushkinA P Marin Antioxidative Stabilizationof Polymers Taylor amp Francis London(1996)

100 W W Y Lau P J Qing Polymeric Stabi-lizers and Antioxidants Chapter 4 in RArshady Desk Reference of FunctionalPolymers Syntheses and ApplicationsAmerican Chemical Society Washing-ton DC (1997)

101 N Grassie G Scott Polymer Degrada-tion and Stabilisation Cambridge Uni-versity Press Cambridge (1985)

102 R Gaumlchter H Muumlller (eds) Plastics Ad-ditives 3rd Edition Hanser Muumlnchen(1990)

103 J F Rabek Photostabilization of Poly-mers Elsevier Applied Science London(1990)

104 B Raringnby J F Rabek PhotodegradationPhotooxidation and Photostabilization ofPolymers Wiley London (1975)

105 S Al-Malaika A Golovoy CA Wilkie(eds) Specialty Polymer Additives Black-well Oxford (2001)

106 F Gugumus The Many-sided Effects ofStabilizer Mass on UV Stability of Poly-olefins Chapter 9 in [105]

107 R E Lee C Neri V Malatesta R MRiva M Angaroni A New Family ofBenzotriazoles How to Modulate Proper-ties within the Same Technology Chapter7 in [105]

108 C Decker Photostabilization of UV-Cured Coatings and Thermosets Chapter8 in [105]

109 J Sedlaacuter Hindered Amines as Photostabi-lizers Chapter 1 of Vol II in [91]

110 W Schnabel Polymer Degradation Prin-ciples and Practical Applications HanserMuumlnchen (1981)

111 F Gugumus Polym Degrad Stab 40(1993) 167

112 S Yachigo Synergistic Stabilization ofPolymers in S H Hamid MB AminA G Maadhah (eds) Handbook of Poly-mer Degradation Dekker New York(1992) p 305

113 J P Galbo Light Stabilizers (Overview)in JC Salamone (ed) Concise Polymer-ic Materials Encyclopedia CRC PressBoca Raton FL USA (1999) p 749

114 NS Allen J M Pena M Edge CMLiauw Polym Degrad Stab 67 (2000)563

115 J M Pena NS Allen M Edge CMLiauw I Roberts B Valange PolymDegrad Stab 70 (2000) 437

116 ECD Nunes A C Babetto JA MAgnelli Polim Cienc Tecnol AprilJune (1997) 66

117 DM Wiles J PT Jensen D J CarlsonPure Appl Chem 55 (1983) 165

Part IIILight-induced synthesis of polymers

101Introduction

While the previous chapters have demonstrated how light can affect the physi-cal behavior of polymers and chemically modify or degrade them this chaptershows how light can be used as a tool to make polymers In other words var-ious kinds of polymers can be synthesized by light-induced chemical processesa technique commonly denoted by the term photopolymerization In accordancewith the widely accepted terminology polymerization denotes a chain reaction(chain polymerization) and consequently photopolymerization refers to the syn-thesis of polymers by chain reactions that are initiated upon the absorption oflight by a polymerizable system Notably light serves only as an initiating toolIt does not interfere with the propagation and termination stages of the chainprocess Both radical and ionic chain polymerizations can be photoinitiated pro-vided that appropriate initiators and monomers are employed It is commonpractice to add small amounts of photoinitiators to formulations to be polymer-ized Photoinitiators are compounds that are thermally stable and capable of ab-sorbing light with relatively high absorption coefficients in the UV andor visi-ble wavelength ranges Industrially employed photopolymerization processesoverwhelmingly rely on the use of easily available UV light sources emitting inthe 300ndash400 nm wavelength range Actually many highly efficient UV photoini-tiators which are stable in the dark are commercially available The handlingof UV-sensitive systems is easy and does not require special precautions such assafety light conditions which are mandatory for the application of systems sen-sitive to visible light In many cases photoinitiation can replace other initiationtechniques including thermochemical or electrochemical initiation Photoinitia-tion parallels initiation by high-energy radiation such as -radiation or electronbeam radiation Initiation by high-energy radiation proceeds in the absence ofinitiators but is less specific than photoinitiation since high-energy radiation si-multaneously generates various kinds of free radicals of differing reactivity aswell as free ions

Both free radical and ionic polymerizations are restricted to certain types ofmonomers Many olefinic and acrylic monomers are readily polymerizable by afree radical mechanism whereas other compounds such as oxiranes (epoxides)

275

10Photopolymerization

and vinyl ethers are solely polymerizable by a cationic mechanism Photopoly-merizations can be readily performed at ambient or at an even much lower tem-perature Moreover solvent-free formulations can be used Therefore there areimportant technical applications for instance in the field of curing of coatingsand printing inks Technical aspects are described in Chapter 11

According to the large number of publications and patents concerned withphotopolymerization that continue to appear this field is still expanding Thisremarkable development has been documented in various books and reviews[1ndash40]

102Photoinitiation of free radical polymerizations

1021General remarks

The synthesis of macromolecules by the free radical chain polymerization oflow molar mass compounds denoted as monomers commences with the gen-eration of free radicals which is conveniently performed through photoreactionsof initiator molecules The subsequent processes ie propagation includingchain transfer and termination are thermal (dark) reactions which are not af-fected by light The simplified overall mechanism is described in Scheme 101

Two types of compounds are employed as photoinitiators of free radical poly-merizations which differ in their mode of action of generating reactive free rad-icals Type I initiators undergo a very rapid bond cleavage after absorption of aphoton On the other hand type II initiators form relatively long-lived excitedtriplet states capable of undergoing hydrogen-abstraction or electron-transfer re-actions with co-initiator molecules that are deliberately added to the monomer-containing system

1022Generation of reactive free radicals

10221 Unimolecular fragmentation of type I photoinitiatorsTypical type I photoinitiators are listed in Table 101 Most of them contain aro-matic carbonyl groups which act as chromophores Since the dissociation en-ergy of the CndashC bond adjacent to the benzoyl group is lower than the excitationenergy of the excited state these compounds undergo rapid bond cleavage re-sulting in the formation of a pair of radicals one of them being a benzoyl-typeradical (see Scheme 102)

Phosphinoyl radicals are much more reactive towards olefinic compoundsthan carbon-centered radicals For example the rate constants for the additionof diphenylphosphinoyl radicals (see Scheme 102) to vinyl monomers are of theorder 106 to 107 mndash1 sndash1 ie one or two orders of magnitude larger than those

10 Photopolymerization276

102 Photoinitiation of free radical polymerizations 277

Scheme 101 Reaction scheme illustrating the photoinitiatedfree radical polymerization of monomer M commonly acompound with a C=C bond

Scheme 102 Photofragmentations by -cleavage of benzoinmethyl ether and 246-trimethylbenzoyl diphenylphosphineoxide


Table 101 Chemical structures of typical type I free radical photoinitiators

Class Chemical structure

Benzoin and benzoin ethers

R methyl ethyl

ethyl isopropyl

n-butyl isobutyl

Benzil ketals R methyl

Acetophenones

Hydroxyalkylphenones

Phenylglyoxylates

S-Phenyl thiobenzoates

O-Acyl--oximo ketones

Morpholino-acetophenones

Acylphosphine oxides

Acylphosphonates

Halogenated compounds

for benzoyl or other carbon-centered radicals formed by the photolysis of ben-zoin or the other compounds listed in Table 101 [41 42]

In spite of the large number of available photoinitiators [4] the search fornew initiators is ongoing For example S-(4-benzoyl)phenylthiobenzoate BpSBzhas been found to be a type I photoinitiator Upon exposure to light it is cleavedinto free radicals (quantum yield 045) which initiate the polymerization ofmethyl methacrylate In contrast BpOBz (see Chart 101) is not cleaved Itforms a long-lived triplet state rather than free radicals [43]

10222 Bimolecular reactions of type II photoinitiatorsTypical type II initiators containing carbonyl chromophores are listed in Ta-ble 102 Upon photon absorption they form long-lived triplet states which donot undergo -cleavage reactions because the triplet energy is lower than thebond dissociation energy The triplet species can however react with suitableco-initiators (see Table 103) For example benzophenone and other diaryl ke-tones abstract hydrogen atoms from other compounds such as isopropanol pro-vided that the triplet energy exceeds the bond dissociation energy of the CndashHbond to be broken

Type II initiators containing carbonyl groups can also undergo electron-trans-fer reactions which lead to hydrogen abstraction after an intermediate exciplex(excited complex) has been formed between the diaryl ketone radical anion andthe amine radical cation as illustrated in Scheme 103

10223 Macromolecular photoinitiatorsBoth type I and type II initiator moieties (see Chart 102) can be chemically in-corporated into macromolecules as pendant groups through the copolymeriza-tion of conventional monomers and monomers containing the initiator moi-eties In the curing of surface coatings the use of macromolecular photoinitia-tors provides for a good compatibility of the initiator in the formulation More-over the migration of the initiator to the surface of the material is preventedwhich results in low-odor and non-toxic coatings

In this context linear polysilanes are also worthy of note As reported in Sec-tion 742 (see Scheme 717) light absorption induces the formation of silyl radi-cals by main-chain scission in addition to the extrusion of silylene Free radical


Chart 101 Chemical structures of BpSBz and BpOBz


Table 102 Chemical structures of typical type II photoinitiators


Benzophenone derivatives

Thioxanthone derivatives

12-Diketones (benzils andcamphorquinone)

-Keto coumarins

Anthraquinones

Terephthalophenones

Water-soluble aromatic ketones

Table 103 Chemical structures of amines functioning asco-initiators for type II free radical photoinitiators


Methyl diethanolamine

Triethanolamine

Ethyl 4-(dimethylamino)benzoate

n-Butoxyethyl 4-(dimethylamino)benzoate

chain polymerization is initiated if polysilanes are photolyzed in the presence ofunsaturated monomers such as methyl methacrylate and styrene (seeScheme 104) [44] As in the case of benzoin the quantum yield for initiation13i is of the order of 01 13i represents the number of kinetic chains initiatedper photon absorbed by the initiator The rate constants for the addition of silylradicals to unsaturated compounds are quite large (8107 and 2108 mndash1 sndash1 formethyl methacrylate and styrene respectively) [45]

10224 Photoinitiators for visible lightAt present visible-light-sensitive polymerizable systems are used for special ap-plications in conjunction with visible-light-emitting lasers of low cost and excel-lent performance Typical such applications are maskless photoimaging pro-cesses such as laser direct imaging LDI and computer-assisted design CAD sys-tems which are used for the imaging of printed circuit boards Additional visiblelight applications include the production of holograms and color printing [2ndash4]In the literature a large number of photoinitiator systems appropriate for visi-ble light exposure have been proposed Of importance for practical applicationsare some organometallic initiators various dyeco-initiator systems and some-diketones which are dealt with in the following sections


Scheme 103 Generation of reactive free radicals with the aidof type II initiators exemplified by the reaction of a triplet-excited diaryl ketone with a tertiary amine

Scheme 104 Initiation of the polymerization of unsaturatedcompounds by reaction with photogenerated macrosilylradicals

102241 Metal-based initiatorsThere is a large group of metal-based compounds capable of initiating the freeradical photopolymerization of unsaturated compounds (see Table 104) [23 24]

By virtue of their absorption characteristics many of the compounds listed inTable 104 can be employed in conjunction with visible light sources As the re-search in organometallic chemistry gained momentum the potential advantagesof organometallic complexes as photoinitiators were also explored and two suchcompounds a ferrocenium salt and a titanocene were commercialized (seeChart 103)


Chart 102 Chemical structures of photosensitive moietiescontained in typical macromolecular photoinitiators

Table 104 Typical metal-based photoinitiators [9 24 46]

Class Example a)

Transition metal ions Fe2+ V2+ V3+ V4+ UO22+

Transition metal inorganic complexes L2VOCl L3Mn L3Fe(SCN)3 L3Ru2+

Transition metal organometallic complexesincluding ferrocenium salts and titanocenederivatives

In conjunction with a co-initiator such as CCl4Mn2(CO)10 Fe(CO)5 Cr(CO)6 W(CO)6Mo(CO)6 Mo(CO)5Py CpMn(CO)3

In conjunction with hydroperoxides(6-arene)(5-cyclopentadienyl)iron(II) hexa-fluorophosphate b)

bis(5-cyclopentadienyl)-bis[26-difluoro-3-(1H-pyrr-1-yl)phenyl]titanium b)

Non-transition metal complexes Al(C2H5)3

a) L ligand such as acetylacetonyl (acac) Cp cyclopentadienylb) see Chart 103

When the ferrocenium salts are applied in conjunction with alkyl hydroperox-ides such as cumyl hydroperoxide they yield on exposure to light reactive freeradicals as shown in Scheme 105

The fluorinated titanocene presented in Chart 103 is a very effective photoini-tiator that functions without a co-initiator when irradiated with visible light Itis thermally stable (decomposition at 230 C) and absorbs light up to 560 nmwith maxima at 405 and 480 nm [2] According to mechanistic studies the com-


Chart 103 Chemical structures of (6-arene)(5-cyclopenta-dienyl)iron(II) hexafluorophosphate (left) and bis(5-cyclo-pentadienyl)-bis[26-difluoro-3-(1H-pyrr-1-yl)phenyl]titanium(right)

Scheme 105 Generation of free radicals upon irradiation of aferrocenium salt in the presence of an alkyl hydroperoxide

Scheme 106 Photoinitiation of the free radical polymerizationof an alkyl acrylate with the aid of a fluorinated titanocene [47]

plex undergoes an isomerization upon absorption of a photon In the presenceof an unsaturated monomer the resulting coordinatively unsaturated isomerundergoes a ligand-exchange reaction to yield a biradical capable of initiatingthe polymerization of further monomer molecules (see Scheme 106) [47]

102242 Dyeco-initiator systemsDye molecules in an electronically excited state are capable of undergoing elec-tron-transfer reactions with appropriate compounds denoted as co-initiators [28 12 15] The free radical ions formed by electron transfer or the free radicalsformed by the decomposition of the radical ions can initiate the polymerizationof monomers In principle the excited dye molecule can be reduced or oxidizedie it can accept an electron from the co-initiator CI or it can transfer an elec-tron to the CI [see Eqs (10-1) and (10-2)]

D CI D CI D CI 10-1

D CI D CI D CI 10-2

The electron transfer is thermodynamically allowed if the free energy G cal-culated by the Rehm-Weller equation [Eq (10-3)] is negative

G F13Eox12 Ered

12 ES Ec 10-3


Table 105 Chemical structures of typical photoreducible dyes

Family Denotation Chemical structure max (nm) a)

Acridines Acriflavin 460

Xanthenes Rose Bengal 565

Thiazenes Methylene blue 645

Cyanines Cyanine dye 490ndash700depending on n

a) Maximum of absorption band

Here F is the Faraday constant Eox12 and Ered

12 are the oxidation and reductionpotentials of the donor and acceptor respectively ES is the singlet-state energyof the dye and Ec is the coulombic stabilization energy Typical dyes and co-initiators are presented in Tables 105 and 106 respectively

For practical applications initiator systems functioning on the basis of dye re-duction are most important Scheme 107 illustrates how free radicals areformed with the aid of a co-initiator of the tertiary amine type In this case theamino radical cation formed by electron transfer loses a proton to give an -aminoalkyl radical which initiates the polymerization

102243 Quinones and 12-diketonesIn conjunction with hydrogen donors such as dimethylaniline and triethyla-mine benzils and various quinones such as anthraquinone 910-phenanthrenequinone and camphor quinone (see Chart 104) can be used as visible-light-sensitive photoinitiators [8] Some of these compounds are used to cure dentalrestorative systems (see Section 113) Another application concerns the curingof waterborne pigmented latex paints which do not contain volatile organic com-pounds (VOCs) [48]


Table 106 Chemical structures of typical co-initiatorsemployed in dye-sensitized free radical polymerization [2]

Family Chemical structure Denotation

AminesTriethanolamineN-phenylglycine

Phosphines and arsines Triphenylphosphinetriphenylarsine

Borates Triphenylbutylborate

Organotin compounds Benzyltrimethylstannane

Heterocyclic compounds Oxazole thiazole


Sche

me

107

Gen

erat

ion

offr

eera

dica

lsby

the

phot

ored

uctio

nof

met

hyle

nebl

uew

ithtr

ieth

anol

amin

e[2

]

Cl

10225 Inorganic photoinitiatorsInorganic materials such as titanium dioxide TiO2 and cadmium sulfide CdScan initiate the polymerization of unsaturated compounds upon exposure tolight [49ndash51] For the photoinitiation of the polymerization of methyl methacry-late by nanosized titanium dioxide [49 50] the mechanism presented inScheme 108 has been proposed Accordingly electrons released upon absorp-tion of light by the TiO2 particles are trapped at the hydrated surface of the par-ticles by Ti4(+)OH groups Ti3(+)OH formed in this way can react with molecularoxygen to form O2

(ndash) The latter combines with H(+) to yield HOO When twoHOO radicals combine H2O2 is formed which can react with O2

(ndash) This reac-tion yields the polymerization initiator ie very reactive OH radicals Actuallythis is a photocatalytic mechanism since the inorganic particles are not con-sumed during the process


Chart 104 Chemical structures of quinones and 12-diketones

Scheme 108 Generation of reactive free radicals during theabsorption of light by titanium dioxide

103Photoinitiation of ionic polymerizations

1031Cationic polymerization

10311 General remarksThe virtues of photoinitiated cationic polymerization are rapid polymerizationwithout oxygen inhibition minimal sensitivity to water and the ability to poly-merize vinyl ethers oxiranes (epoxides) and other heterocyclic monomers (seeTable 107) that do not polymerize by a free radical mechanism

In analogy to free radical polymerizations (see Scheme 101) cationic poly-merizations proceed as chain reactions involving initiation and propagationHowever in many cases there is no termination by neutralization and thegrowing chains are only terminated by nucleophilic impurities contained in the


Table 107 Chemical structures of monomers polymerizable by a cationic mechanism [2 7]

Monomer Polymer

103 Photoinitiation of ionic polymerizations 289

Table 108 Chemical structures of typical cationic photoinitiators [2 27 52 53]

Class Chemical structure a)

Diazonium salts

Diaryl iodonium salts

Triaryl sulfonium salts

5-Arylthianthrenium salts

Dialkylphenacyl sulfonium salts

N-Alkoxy pyridinium and isoquinolinium salts

Phosphonium salts

Ferrocenium salts

Phenacyl anilinium salts

Triaryl cyclopropenium salts

Sulfonyloxy ketones

Silyl benzyl ethers

a) X(ndash) denotes a non-nucleophilic anion such as BF4(ndash)

PF6(ndash) AsF6

(ndash) SbF6(ndash) CF3SO3

(ndash) CF3(CF2)3SO3(ndash) (C6F5)4B(ndash) (C6F5)4Ga(ndash)

system Cationic chain reactions are photoinitiated with the aid of special initia-tors Typical cationic photoinitiators are listed in Table 108

10312 Generation of reactive cationsReactive cations can be generated via three different routes (a) by direct photo-lysis of the initiator (b) by sensitized photolysis of the initiator and (c) by freeradical mediation These routes are described below

103121 Direct photolysis of the initiatorCrivellorsquos pioneering work on onium salt-type photoinitiators (sulfonium and io-donium salts) gave great impetus to investigations of cationic polymerizations[5 6] A common feature of mechanisms proposed in relation to onium salt-type initiators of the general structure (AndashB)(+)X(ndash) is the generation ofBroslashnsted acids (superacids) of the structure H(+)X(ndash) based on non-nucleophilicanions X(ndash) These superacids play a prominent role in the initiation processHowever radical cations A(+) formed by light-induced bond cleavage may alsoreact with the polymerizable monomers According to the general mechanism


Scheme 109 Photolysis of an onium ion (AndashB)(+)

Scheme 1010 Photolysis of a diaryl iodonium ion involvingboth heterolytic (a) and homolytic (b) ArndashI bond rupture

of the photolysis shown in Scheme 109 the radical cation A(+) may abstract ahydrogen from surrounding molecules RH The resulting cation AH(+) then re-leases a proton

The detailed mechanism of the photolysis of a diaryl iodonium ion presentedin Scheme 1010 may serve here as a typical example since the scope of thisbook does not permit the discussion of mechanistic details concerning thephotolysis of all of the initiators compiled in Table 108 Details concerning thephotolysis of initiators and mechanisms of the initiation of cationic polymeriza-tions are available in review articles [2 27]

Both the initially formed radical cation and the proton are potential initiatingspecies for the reaction with a polymerizable monomer M (see Scheme 1011)

103122 Sensitized photolysis of the initiatorIf onium salts do not or only weakly absorb light at gt 300 nm then photosen-sitizers PS that absorb strongly at long wavelengths may be employed in con-junction with the onium salts In most cases energy transfer from PS to(AndashB)(+) can be excluded However PS can be oxidized by the onium ion ieradical cations PS+ can be formed by electron transfer from the electronicallyexcited photosensitizer PS to the onium ion (see Scheme 1012) provided thatthe free energy G of this reaction has a sufficiently high negative value

Regarding the cationic polymerization of an appropriate monomer three initi-ation routes are feasible (see Scheme 1013) (a) PS(+) reacts directly with M (b)PS(+) abstracts a hydrogen from a surrounding molecule RH to form the pro-ton-releasing PSH(+) ion (c) PS(+) combines with radical B thus forming thecation BndashPS(+) Protons released from PSH(+) ions as well as BndashPS(+) ions arelikely to add to M


Scheme 1011 Reactions of a radicalcation A(+) and a proton H(+) witha polymerizable monomer

Scheme 1012 Oxidation of an electronically excited sensitizer PS by an onium ion (AndashB)(+)

Derivatives of anthracene and carbazole are typical electron-transfer photosen-sitizers Of practical interest are derivatives containing cationically polymeriz-able epoxide groups (see Chart 105) [54] During the ring-opening photopoly-merization of epoxides these sensitizers are covalently incorporated into thepolymeric network and cannot be removed by extraction Therefore the poten-tial risk of toxic effects of the sensitizers is strongly diminished

103123 Free-radical-mediated generation of cations

1031231 Oxidation of radicalsA large number of carbon-centered free radicals which are formed by photolysisor thermolysis of commercially available free radical initiators can be oxidizedby onium ions (A-B)(+) by reaction according to Eq (10-4)


Scheme 1013 Possible initiation routes in the cationic polymerizationof monomer M Initiating system onium saltsensitizer

Chart 105 Typical electron-transfer photosensitizers that maybe applied in conjunction with onium salts [54]

C

AB C

AB 10-4

Carbocations generated in this way can add directly to appropriate monomers(eg tetrahydrofuran cyclohexene oxide n-butyl vinyl ether) or can formBroslashnsted acids by abstracting hydrogen from surrounding molecules Thismethod which is commonly referred to as free-radical-promoted cationic polymer-ization is quite versatile because the user may rely on a large variety of radicalsources Some of them are compiled in Table 109

A sufficiently high negative value of the free energy G is required for theoccurrence of reaction according to Eq (10-4) G in units of kJ molndash1 can beestimated with the aid of Eq (10-5) the modified RehmndashWeller equation

G fcEox12 Ered

12 10-5

Here Eox12 and Ered

12 denote the half-wave potentials in units of V of oxidationand reduction of the carbon-centered radical and of the onium ion (AndashB)(+) re-


Table 109 Free radicals that may be employed in free-radical-promoted cationic polymerizations

Photoinitiator Electron-donating free radical Generation of radical

Direct

Benzoin

Direct

Phenylazotriphenylmethane

Direct

PolysilanesIndirect

BenzophenoneRHIndirect

Acylphosphine oxidesCH2=CHR

spectively The conversion factor fc is equal to 97 kJ molndash1 Vndash1 On the basis ofthe reduction potentials listed in Table 1010 it becomes evident why 2-hydroxy-propyl radicals are oxidized much more efficiently by N-ethoxypyridinium anddiphenyliodonium ions than by triphenylsulfonium ions

1031232 Addition-fragmentation reactionsThe addition of a free radical to the carbon-carbon double bond of an allylicgroup that forms part of an onium ion can induce disintegration of the oniumsalt thus giving rise to the release of an inert compound and a reactive radicalcation Allylic compounds employed for this purpose are presented inChart 106 and the reaction mechanism for a typical case is presented inScheme 1014 [55]

In this case cationic polymerization is initiated by direct addition of photo-generated reactive radical cations to the appropriate monomers AlternativelyBroslashnsted acids may be formed through reaction of the radical cations with hy-drogen-donating constituents of the formulation and then the initiation step in-volves the addition of protons to monomer molecules The method discussedhere has the advantage that virtually all kinds of radicals may be operative inthe initiation process Therefore the polymerization can be elegantly tuned tothe wavelength of the light by choosing radical sources with a suitable spectralresponse


Table 1010 The importance of the reduction potential withregard to the reaction of onium ions with 2-hydroxypropylradicals [26]

Species Eox12(V) Ered

12(V) Eox12ndash Ered

12V

ndash12

ndash11 ndash01

ndash07 ndash05

ndash05 ndash07

ndash02 ndash10

1032Anionic polymerization

10321 General remarksThe possibility that photoinitiated polymerization can occur through an anionicmechanism has long been overlooked Even today literature reports on anionicphotopolymerization are rare and there are no important commercial applica-tions of which the author is aware However this situation might change sinceextensive research on photoinduced base-catalyzed processes using photolatentamines has opened up new application areas [1 3 56]

10322 Generation of reactive species

103221 Photo-release of reactive anionsThe compounds listed in Table 1011 have been found to photoinitiate the poly-merization of neat ethyl or methyl 2-cyanoacrylate CA that readily polymerizeby an anionic mechanism Therefore this has been taken as evidence for theoccurrence of an anionic mechanism [57ndash59]

The essential step in the proposed initiation mechanism is the photoinducedrelease of a reactive anion which readily adds to the monomer The polymer isthen formed through the repetitive addition of CA to the growing anionic chain(see Scheme 1015)


Chart 106 Allylic compounds employed in addition-fragmentation reactions

Scheme 1014 Addition of a radical R to the S-[2-(ethoxycarbonyl)allyl]tetrahydrothiophenium ion [55]

103222 Photo-production of reactive organic basesIn the context of the anionic polymerization of CA derivatives as considered inSection 103221 it is notable that the polymerization of cyanoacrylates is alsophotoinitiated by substituted pyridine pentacarbonyl complexes of tungsten orchromium ie M(CO)5L with M= Cr or W and L = 2- or 4-vinylpyridine [60]Photo-released pyridine adds to CA and the resulting zwitterion initiates theanionic chain polymerization (see Scheme 1016)

Substances that release reactive bases or other reactive species upon exposureto light are often referred to as photolatent compounds or in the context of the


Table 1011 Chemical structures of anionic photoinitiators [57ndash59]

Denotation Chemical structure Released anion(assumed)

PotassiumReineckate

CrNH32NCS4 K(NCS)

Platinum(II) acetyl-acetonate (Pt(acac)2)

acac a)

Benzoylferrocenedibenzoylferrocene

b

Crystal violetleuconitrile (CVCN)

CN

Malachite greenleucohydroxide(MGOH)

HO

a) acac acetylacetonateb) Forms in the presence of trace amounts of water

Scheme 1015 Photoinitiation of the polymerization of ethyl2-cyanoacrylate by potassium reineckate [57]

initiation of polymerizations as photolatent initiators Actually the photogenera-tion of organic bases is an important tool in inducing the polymerization ofmonomers of the oxirane type Relevant research has been focused on thephotogeneration of amines with the aim of developing a novel technique to cureepoxidized resins through intermolecular cross-linking [2 3] Strong organicbases for instance tertiary amines or amidine bases function as curing agentsScheme 1017 shows how tertiary amines act in the presence of polyols (oligo-mers bearing hydroxyl groups) [56] After ring-opening is achieved by nucleo-philic attack of the amine at a ring carbon a proton is transferred from thepolyol to the oxygen The resulting alkoxide then adds to the ring carbon of an-other molecule and thus starts the anionic chain propagation

Free tertiary amines can be obtained from various low molar mass com-pounds by irradiation with UV light Relevant earlier work has been reviewed[3 56] According to more recent reports 5-benzyl-15-diazabicyclo[430]nonaneis a very effective photolatent initiator [1 3] It releases 15-diazabicyclo[430]-non-5-ene DBN a bicyclic amidine possessing a high basicity (pKa = 12ndash13) dueto the strong conjugative interaction between the two nitrogens The suggestedmechanism is depicted in Scheme 1018


Scheme 1016 Initiation of the polymerization ofcyanoacrylate with the aid of photo-released pyridine [60]

Scheme 1017 Mechanism of the initiation of the anionicpolymerization of epoxides by a tertiary amine in conjunctionwith a polyol [56]

Also polymeric amines have been generated A typical system is presented inScheme 1019

Actually in the conventional manufacture of polyurethane-based coatingsamine-catalyzed cross-linking is a widely used method Curing of ready-to-useformulations occurs within several hours and is difficult to control In contrastphoto-triggered curing can be performed on demand and the working windowcan be extended to a full day with formulations containing a photolatent com-pound such as the DBN-releasing initiator [1]

104Topochemical polymerizations

1041General remarks

One of the most intriguing phenomena in the field of photopolymerization con-cerns the light-induced solid-state conversion of certain low molar mass com-pounds into macromolecules Based on Schmidtrsquos pioneering work on the di-merization of cinnamic acid and its derivatives by [2+2] photocycloaddition [6162] the light-induced solid-state polymerization of diacetylenes and dialkeneswas discovered by Wegner [63] and Hasegawa [64] respectively In these casesthe polymerization proceeds under crystal-lattice control The reactivity of thestarting compound and the structure of the resulting product are governed bythe molecular geometry in the reactant crystal and the reaction proceeds with aminimum of atomic and molecular movement These criteria correspond to theterm topochemical reaction In many cases the topochemical polymerization pro-


Scheme 1018 Photoinduced release of DBN from 5-benzyl-15-diazabicyclo[430]nonane [1]

Scheme 1019 Photoinduced generation of pendant tertiaryamine groups on polymethacrylate chains [56]

ceeds homogeneously by a crystal-to-crystal transformation Therefore polymersingle crystals which are otherwise difficult to obtain can be obtained by topo-chemical photopolymerization

1042Topochemical photopolymerization of diacetylenes

The discovery of the photopolymerization of crystalline diacetylenes such as hexa-35-diyne-16-diol and other derivatives (see Chart 107) [2 30 63] initiated scien-tific and technical developments extending to various fields [31ndash33 35 65 66]

First of all basic research concerning chemical reactions in the solid statewas stimulated As a result various applications became feasible since the di-acetylene polymerization principle turned out to be applicable to various otherorganized structures including Langmuir-Blodgett films liposomes vesiclesand self-assembled monolayers on metal oxide or graphite surfaces A typical ex-ample concerns the photopolymerization of self-ordered monomolecular layersof pentacosadiynoic acid CH3(CH2)11ndashCCndashCCndash(CH2)8COOH and nonaco-sadiynoic acid CH3(CH2)15ndashCCndashCCndash(CH2)8COOH on a graphite substrate[68] Scheme 1020 depicts the assembly of the diacetylene molecules and thesubsequent photopolymerization at 254 nm

An exciting feature of such polymerized monolayers is the color change fromblue to red that accompanies conformational changes in conjugated polydiacety-lenes induced by changes in temperature or pH or by mechanical stress Thisphenomenon has been exploited in the construction of direct sensing devices[70ndash75] The latter consist of functionalized polydiacetylene bilayers with cova-lently attached receptors Binding of biological entities (large molecules or cells)provides a mechanical stimulus It causes conformational changes in the poly-diacetylene layers (side-chain disordering and disruption of main-chain pack-ing) resulting in a chromatic shift [67] This method has been exploited for ex-ample in the direct colorimetric detection of an influenza virus [70] and of cho-lera toxin [71] as well as of biochemical substrates such as glucose [72]

104 Topochemical polymerizations 299

Chart 107 Typical diacetylene derivatives capable of under-going topochemical photopolymerization Left classicalexamples [30] Right Self-assembling bolaamphiphilicdiacetylenes [67]

At ambient temperatures the polymerization of diacetylenes proceeds as achain reaction by 14-addition and results in alternating ene-yne polymer chainswith exclusive trans selectivity The quantum yield for initiation is low (ca 001)[31] Upon absorption of a photon by a diacetylene moiety of one of the mole-cules in the assembly or crystal an excited diradical state with an unpaired elec-tron at either end is generated Subsequently the radical sites undergo thermaladdition reactions with neighboring diacetylene moieties The resulting dimerspossess reactive radical sites at their ends which are capable of inducing chaingrowth [31 34 76] There is experimental evidence (ESR) that dicarbenes (seeChart 108) are also involved in the polymerization if chains become longerthan five repeating units [31 32]

An essential prerequisite for the topochemical polymerization of diacetylenesis a packing of the monomer molecules at a distance of d = 47ndash52 Aring and a tiltangle of about 45 between the molecular axis and the packing axis [35]


Scheme 1020 Schematic representation of the polymerizationof assembled functional diacetylenes by 14-addition uponexposure to UV light R1 and R2 denote functionalized alkylchains [69]

Chart 108 Structures of diradicals and dicarbenes involved inthe topochemical photopolymerization of diacetylenes [32]

1043Topochemical photopolymerization of dialkenes

The photopolymerization of diolefinic crystals was discovered in the case of the[2+2] photocyclopolymerization of 25-distyrylpyrazine (DSP) crystals and wasnamed four-center-type polymerization (see Scheme 1021) [36 37]

Chart 109 presents four other dialkenes that are amenable to topochemicalphotopolymerization

Notably the polymerization of dialkenes proceeds as a stepwise process andnot as a chain reaction In other words the addition of each repeating unit tothe chain requires the absorption of a further photon (see Scheme 1022)


Scheme 1021 Four-center-type photopolymerization of crystalline 25-distyrylpyrazine [37]

Chart 109 Dialkenes capable of undergoing topochemicalpolymerization upon exposure to UV light [37]


Scheme 1022 Stepwise [2+2] photocyclopolymerization of a dialkene [36 37]

References

1 K Dietliker T Jung J Benkhoff HKura A Matsumoto H Oka D Hristo-va G Gescheidt G Rist New Develop-ments in Photoinitiators MacromolSymp 217 (2004) 77

2 K Dietliker Photoinitiators for Free Radi-cal and Cationic Polymerization Vol IIIin PKT Oldring (ed) Chemistry andTechnology of UV and EB Formulations forCoatings Inks and Paints SITA Technol-ogy London (1991)

3 K Dietliker T Jung J Benkhoff Photo-latent Amines New Opportunities in Ra-diation Curing Techn Conf Proc UV ampEB Technol Expo amp Conf Charlotte NCUSA (2004) p 217

4 K Dietliker A Compilation of Photoinitia-tors Commercially Available for UV TodaySITA Technology Edinburgh (2002)

5 J V Crivello Latest Developments in theChemistry of Onium Salts Chapter 8 inVol III of [10]

6 J V Crivello The Discovery and Develop-ment of Onium Salt Cationic Photoinitia-tors J Polym Sci Part A Polym Chem37 (1999) 4241

7 J V Crivello K Dietliker Photoinitiatorsfor Free Radical Cationic and AnionicPhotopolymerization Wiley New York(1998)

8 I Reetz Y Yagci MK Mishra Photoini-tiated Radical Vinyl Polymerization inMK Mishra Y Yagci (eds) Handbookof Radical Vinyl Polymerization DekkerNew York (1998)

9 J P Fouassier (ed) PhotoinitiationPhotopolymerization and PhotocuringFundamentals and Applications HanserMuumlnchen (1995)

10 J P Fouassier J F Rabek (eds) Radia-tion Curing in Polymer Science and Tech-nology Elsevier Applied Science London(1993)

11 CG Roffey Photogeneration of ReactiveSpecies for UV Curing Wiley New York(1997)

12 G Oster NL Yang Photopolymerizationof Vinyl Monomers Chem Rev 68 (1968)125

13 NS Allen (ed) Photopolymerization andPhotoimaging Science and TechnologyElsevier Applied Science London (1989)

14 NS Allen Photoinitiators for Photocur-ing in J C Salamone (Ed) Concise Poly-meric Materials Encyclopedia CRC PressBoca Raton FL USA (1999) p 1047

15 HJ Timpe S Jokusch K Koumlrner Dye-Sensitized Photopolymerization Chapter13 in Vol II of [10]

16 A B Scranton CN Bowman R WPfeiffer (eds) Photopolymerization ACS

References 303

Symposium Series 673 AmericanChemical Society Washington DC(1996)

17 SP Pappas (ed) UV Curing Scienceand Technology 2nd ed Technology Mar-keting Corp Stamford CT USA (1985)

18 SP Pappas (ed) Radiation CuringScience and Technology Plenum PressNew York (1992)

19 R S Davidson Polymeric and Polymerisa-ble Free Radical Photoinitiators J Photo-chem Photobiol A Chem 69 (1993)263

20 HF Gruber Photoinitiators for Free Radi-cal Polymerization Prog Polym Sci 17(1993) 953

21 C Decker Photoinitiated Crosslinking Po-lymerization Prog Polym Sci 21 (1996)593

22 T Yamaoka K Naitoh Visible LightPhotoinitiation Systems Based on ElectronTransfer and Energy Transfer Processes inV V Krongauz AD Trifunac (eds) Pro-cesses in Photoreactive Polymers Chapmanamp Hall New York (1995)

23 D Billy C Kutal Inorganic and Organo-metallic Photoinitiators Chapter 2 in [18]

24 A F Cunningham V Desobry Metal-Based Photoinitiators Chapter 6 of Vol IIin [10]

25 W Schnabel Cationic Photopolymeriza-tion with the Aid of Pyridinium-Type SaltsMacromol Rapid Commun 21 (2000)628

26 W Schnabel Photoinitiation of Ionic Poly-merizations Chapter 7 in NS Allen MEdge I R Bellobono E Selli (eds) Cur-rent Trends in Polymer PhotochemistryHorwood New York (1995)

27 Y Yagci I Reetz Externally StimulatedInitiator Systems for Cationic Polymeriza-tion Prog Polym Sci 23 (1998) 1485

28 R Lazauskaite J V Grazulevicius Cat-ionic Photopolymerization Chapter 7 ofVol 2 in HS Nalwa (ed) Handbook ofPhotochemistry and Photobiology Ameri-can Scientific Publishers StevensonRanch CA USA (2003)

29 V Strehmel Epoxies Structures Photoin-duced Cross-Linking Network Propertiesand Applications Vol 2 p 2 in HSNalwa (ed) Handbook of Photochemistryand Photobiology American Scientific

Publishers Stevenson Ranch CA USA(2003)

30 G Wegner Solid-State PolymerizationMechanisms Pure amp Appl Chem 49(1977) 443

31 H Sixl Spectroscopy of the IntermediateState of the Solid-State Polymerization Re-action in Diacetylene Crystals Adv PolymSci 63 (1984) 49

32 H Baumlssler Photopolymerization of Poly-diacetylenes Adv Polym Sci 63 (1984) 1

33 D Bloor R R Chance (eds) Polydiacety-lenes Synthesis Structures and ElectronicProperties Nijhoff Dordrecht (1985)

34 M Schwoumlrer H Niederwald Photopoly-merization of Diacetylene Single CrystalsMakromol Chem Suppl 12 (1985) 61

35 V Enkelmann Structural Aspects of theTopochemical Polymerization of Diacety-lenes Adv Polym Sci 63 (1984) 91

36 M Hasegawa Photopolymerization ofDiolefin Crystals Chem Rev 83 (1983)507

37 M Hasegawa Product Control in Topo-chemical Photoreactions Chapter 10 inNS Allen M Edge I R Bellobono ESelli (eds) Current Trends in PolymerPhotochemistry Horwood New York(1995)

38 CE Hoyle JF Kinstle (eds) RadiationCuring of Polymeric Materials ACS Sym-posium Series 417 American ChemicalSociety Washington DC (1990)

39 G Odian Principles of Photopolymeriza-tion Wiley New York (1991)

40 C Carlini L Angiolini Polymers as FreeRadical Photoinitiators Adv Polym Sci123 Springer Berlin (1995)

41 A Kajiwara Y Konishi Y MorishimaW Schnabel K Kuwata M KamachiMacromolecules 26 (1993) 1656

42 T Sumiyoshi W Schnabel A Henne PLechtken Polymer 26 (1985) 141

43 A Wrzynszczynski J Bartoszewicz G LHig B Marciniak K Paczkowski JPhotochem Photobiol Chem 155(2003) 253

44 R West AR Wolff D J Peterson J Ra-diat Curing 13 (1986) 35

45 C Peinado A Alonso F Catalina WSchnabel Macromol Chem Phys 201(2000) 1156


46 C Badarau Z Y Wang Macromolecules36 (2003) 6959

47 J Finter M Riedicker O Rohde B Rot-zinger Makromol Chem MakromolSymp 24 (1989) 177

48 C Bibaut-Renauld D Burget J PFouassier CG Varelas J Thomatos GTsagaropoulos L O Ryrfors O J Karls-son J Polym Sci Part A Polym Chem40 (2002) 371

49 C Dong X Ni J Macromol Sci Part APure amp Appl Chem A 41 (2004) 547

50 A J Hoffman G Mills H Yee MRHoffmann J Phys Chem 96 (1992)5540 and 5546

51 IG Popovic L Katzikas U Muumlller J SVelickovic H Weller Macromol ChemPhys 195 (1994) 889

52 J V Crivello J Ma F Jiang J PolymSci Part A Polym Chem 40 (2002)3465

53 H Li K Ren DC Neckers Macromole-cules 34 (2001) 8637

54 J V Crivello M Jang J Macromol SciPure Appl Chem A42 (2005) 1

55 S Denizligil Y Yagci CM McArdlePolymer 36 (1995) 3093

56 A Mejiritski AM Sarker B WheatonDC Neckers Chem Mater 9 (1997)1488

57 C Kutal P A Grutsch DB Yang Mac-romolecules 24 (1991) 6872

58 Y Yamaguchi B J Palmer C Kutal TWakamatsu DB Yang Macromolecules31 (1998) 5155

59 V Jarikov DC Neckers Macromole-cules 33 (2000) 7761

60 R B Paul J M Kelly DC Pepper CLong Polymer 38 (1997) 2011

61 MD Cohen GM Schmidt J ChemSoc (1964) 1006

62 G M Schmidt Pure amp Appl Chem 27(1971) 647

63 G Wegner Z Naturforsch 24B (1967)824

64 M Hasegawa Y Susuki J Polym Sci B5 (1967) 813

65 V Enkelmann G Wegner K NovakK B Wagner J Am Chem Soc 115(1993) 1678

66 C Bubeck B Tieke G Wegner BerBunsenges Phys Chem 86 (1982) 495

67 J Song J S Cisar CR Bertozzi J AmChem Soc 126 (2004) 8459

68 Y Okawa M Aono Nature 409 (2001)683

69 J-M Kim E-K Ji S M Woo H LeeD J Ahn Adv Mater 15 (2003) 1118

70 DH Charych JO Nagy W SpevakMD Benarski Science 261 (1993) 585

71 DH Charych Q Cheng A ReichertG Kuzienko M Stroh J O Nagy WSpevak RC Stevens Chem Biol 3(1996) 113

72 Q Cheng RC Stevens Adv Mater 9(1997) 481

73 K Morigaki T Baumgart A Offenhaumlu-ser W Knoll Angew Chem Int Ed 40(2001) 172

74 TS Kim KC Chan R M Crooks JAm Chem Soc 119 (1997) 189

75 Q Huo K C Russel RM LeblancLangmuir 15 (1999) 3972

76 W Neumann H Sixl Chem Phys 58(1981) 303

111General remarks

Photopolymerization is the basis of some very important practical applicationsfor instance in the areas of surface coating and printing plates In these caseslow molar mass liquid compounds are converted into rigid intermolecularlycross-linked materials that are insoluble in solvents The relevant technologicalprocesses are denoted by the term curing Detailed information is available invarious books and review articles [1ndash15] In contrast to thermal curing photocur-ing can be performed at ambient temperatures with solvent-free formulationsie volatile organic compounds (VOCs) are not released In many cases photocur-ing processes that proceed within a fraction of a second have replaced conven-tional thermal curing of solvent-containing formulations

The main industrially applied photocuring processes are based on four chemi-cal systems that are converted into three-dimensional networks upon irradiation[16] (1) Unsaturated maleicfumaric acid-containing polyesters (UPEs) dissolvedin styrene (2) acrylatemethacrylate systems (3) thiolene systems and (4) ep-oxide- or vinyl ether-containing systems In the case of systems (1)ndash(3) free radi-cal polymerizations are operative while in case (4) cationic species are involved(see Chapter 10) Regarding thiolene systems the mechanism of free radicalthiolalkene polymerization outlined in Scheme 111 is assumed to be operative[17] Here the chemistry depends upon the rate of hydrogen transfer from thethiol being competitive with the rate of alkene polymerization By employingpolyfunctional thiol compounds very tough abrasion-resistant coatings areformed [8]

Industrially applied polymerizable formulations are composed of mixtures ofmono- and multifunctional monomers and oligomers (see Table 111) contain-ing a photoinitiator and if required also additives such as polymers (pre-poly-mers resins) and pigments Table 112 presents as a typical example the com-position of a formulation applied for microfabrication (see Section 114)

Whether radical or cationic initiators are employed depends on the kind ofmechanism (free radical or cationic see Sections 102 and 103) according towhich the monomers polymerize Industrial applications of photocuring are ex-tremely varied and include the coating of metals (automotive varnishes) the

305

11Technical developments related to photopolymerization

11 Technical developments related to photopolymerization306

Scheme 111 Free radical thiolalkene polymerization [8]

Table 111 Typical di- and trifunctional compounds used for photocuring

Class Chemical structure Mode ofpolymerization

Trifunctionalacrylates

Free radical

Trimethylolpropane triacrylate Pentaerythritol triacrylate

Oligomericdiacrylates

Free radical

X Polyester Polyether Polyurethane Polysiloxane

ThiolEnes Free radical

Difunctionalepoxides

Cationic

Epoxidizedsiloxanes

Cationic

Difunctionalvinyl ethers

Cationic


production of printed circuit boards and the generation of 3-D models Someof the applications are described in more detail in the following sections

112Curing of coatings sealants and structural adhesives

1121Free radical curing

11211 Solvent-free formulationsUV curing of coatings was first commercially applied about four decades ago inthe wood and furniture industries It opened the door to significant savings bydelivering shorter production times improved product quality (better gloss)lower energy and equipment costs as well as environmental friendliness be-cause of the greatly reduced VOC emission Today UV curing is widely usedand all sorts of substrates including paper plastic and metal are coated by em-ploying this technique as can be seen in Table 113

Important commercial applications include clear coatings for paper in partic-ular overprint varnishes as commonly applied to magazines and consumergood packaging Paper coatings are applied at extremely high speeds typically5 m sndash1 and the coated products are immediately ready for testing or shipmentSuch high-performance applications require a fast curing speed in conjunctionwith a conversion of reactive groups closely approaching 100 In this contextthe reactivity of the monomers and the viscosity of the formulation are of greatimportance Regarding polyester acrylate-based formulations for examplemonomers bearing carbamate or oxazolidone groups (see Chart 111) proved toplay a key role in allowing a remarkable level of performance [20] These mono-mers are very reactive and ensure a slow increase in the viscosity of the formu-lation with conversion

112 Curing of coatings sealants and structural adhesives 307

Table 112 Composition of a formulation applied for microfabrication [18]

MonomerOligomer wt

Alkoxylated trifunctional acrylate 10Tris(2-hydroxyethyl) isocyanurate triacrylate 10Trifunctional methacrylate 11Ethoxylated trimethylolpropane triacrylate 10Triethyleneglycol diacrylate 11Isobornyl acrylate 25Trimethylolpropane trimethacrylate 275Brominated urethane acrylate (oligomer) 75Aliphatic polyester-based urethane dimethacrylate (oligomer) 825Aromatic urethane acrylate (oligomer) 27

Silicones have the advantage of softness biological inertness good substratewettability and superb permeability of gases Therefore UV-cured silicones aresuitable for various interesting product applications [21] including ophthalmicdevices (hard and soft contact lenses intraocular lens implants) gaskets seal-ings and optical fiber coatings Photocurable formulations appropriate for thefabrication of such products contain siloxane derivatives bearing unsaturatedcarbon-carbon double bonds (see Chart 112)


Table 113 Typical commercial applications of radiation-curedcoatings in major industries [19]

Industrial sector Mode of application

Furniture andconstruction

Hardwood flooring PVC flooring wood and metal furniture particleboard sealer galvanized tubing fencing etc

Electronics andtelecommunications

Electrical conductor wire printed circuit board coatings opticalfibers magnetic media coatings computer disc clearcoats coatingsfor metallized substrates

Printing andpackaging

Inks release coatings overcoats for graphic art magazine coverscoatings on beverage cans coatings on non-food packaging barriercoatings DVD laminates

Automotive Headlamps printed dashboard components refinishing coatingsConsumer goods Release coatings for adhesives pressure-sensitive adhesives leather

coatings coatings on plastic housings (cell phones computers etc)eyeglass lenses mirror coatings

Chart 111 Chemical structures of acrylates containingcarbamate (left) or oxazolidone groups (right) [20]

Chart 112 Chemical structures of typical siloxane-derived monomers [21]

11212 Waterborn formulationsWaterborn formulations have been developed with the aim of extending the appli-cability of radiation curing Representing a clear departure from the concept of sol-vent-free systems waterborn formulations seem to be especially advantageous forthe radiation curing of wood coatings Formulations in the form of aqueous emul-sions can be easily thinned by the addition of further water Moreover emulsionscan be easily dispensed onto the substrate eg by spraying The resultant coatingspossess good matting properties and adhere tightly to the substrate due to reducedshrinkage during curing Naturally the use of waterborn systems necessitates adrying step following the radiation curing process High-frequency near-infraredor microwave heating can be applied for this purpose [22]

1122Cationic curing

While in the early days acrylate-based systems cured by a free radical mecha-nism were overwhelmingly employed for surface coatings nowadays epoxide-based systems cured cationically are also used to an increasing extent Epoxide-based formulations yield excellent overprint varnishes on tin-free steel and alu-minum for rigid packaging especially in the production of steel food cans andaluminum beer and beverage cans The cured films exhibit good adhesion flex-ibility and abrasion resistance and the high production rates (up to 1600 cansper minute) are astounding [12]

Since coatings containing cycloaliphatic epoxides tend to be brittle other com-pounds such as oligomeric polyols are frequently added as flexibilizing agents Al-cohols can react with the oxonium ions formed by the addition of protons to ep-oxide groups (see Scheme 112) and are thereby copolymerized with the epoxides

When alcohols add to the growing polymer chains protons are produced inequivalent amounts Since these protons can in turn react with epoxidegroups the addition reaction represents a chain-transfer process The use of di-functional alcohols results in an extension of the polymer chains whilst poly-functional alcohols contribute strongly to the formation of a three-dimensionalpolymer network Chart 113 depicts the structure of part of such a three-dimen-sional network

A major difference between cationic and free radical curing is the degree ofshrinkage caused by the polymerization Cationic ring-opening polymerization


Scheme 112 Formation of oxonium ions by the addition ofprotons to epoxides and their subsequent reaction withalcohols

leads to a shrinkage of 1ndash2 as compared to 5ndash20 for radical polymerizationof double bonds A lower degree of shrinkage implies a stronger adhesion ofthe coating to the substrate

1123Dual curing

Coatings protecting three-dimensional objects can be produced by dual-curingmethods using chemical systems that are converted in two separate stages ofpolymerization or polycondensation [23] Regarding the coating of three-dimen-sional objects problems often arise from shadow areas that cannot be reachedby the incident UV light and therefore remain uncured Similar problems arisein the case of UV-curing of coatings on porous substrates such as wood and ofthick pigmented coatings where pigment particle screening prevents the pene-tration of light to deep-lying layers In all of these cases and also in the case ofautomotive top coatings dual curing is successfully employed in industrial pro-cesses [24 25] A typical dual-curing method combining UV irradiation andthermal treatment operates with substances bearing two types of reactive func-tions for example UV-curable acrylate groups and thermally curable isocyanategroups associated with a polyol [24 26] Typical oligomers bearing both acrylateand isocyanate groups are shown in Chart 114

First UV irradiation initiates with the aid of an appropriate initiator the poly-merization of acrylate groups and then heating causes the isocyanate groups toreact with hydroxyl groups The latter reaction results in the formation of ure-thane linkages (see Scheme 113)

Polyols used for this purpose include trimethylol propane and propylene gly-col Systems containing urethane-acrylate oligomers bearing doubly-functional-ized isocyanate groups are commercially available [24] The chemical structure


Chart 113 Structure of part of a network formed by thereaction of a difunctional epoxide with a trifunctional polyol

of a three-dimensional network formed from a dual-cure acrylic urethane sys-tem is presented in Chart 115

Another mode of dual curing involves the simultaneous occurrence of freeradical and cationic radiation-induced cross-linking polymerization of formula-tions containing appropriate initiators [20 23 28] This method which is calledhybrid curing leads to coatings with unique properties A typical hybrid-cure sys-tem contains a diacrylate and a diepoxide the former polymerizing by a freeradical and the latter by a cationic mechanism Exposure of the system to in-


Chart 114 Chemical structures of oligomers bearing acrylateand isocyanate groups isocyanato-allophane acrylate (left)and isocyanato-urethane acrylate (right) [24]

Scheme 113 Formation of urethane linkages by the reactionof isocyanate with hydroxyl groups

Chart 115 Part of a three-dimensional network formed by UVirradiation and thermal treatment of a dual-cure acrylicurethane system [27]

tense UV radiation results in the formation of interpenetrating networks (IPNssee Chart 116)

Often IPN polymers combine the main features of the different networksFor example elasticity and rigidity are combined in the case of interpenetratingnetworks formed from a vinyl ether and an acrylate respectively

113Curing of dental preventive and restorative systems

Radiation-cured dental adhesives began replacing amalgam fillings in the early1970s The growth of cosmetic dentistry created new applications and at presentdental adhesives comprise a major portion of all radiation-cured adhesives interms of market value Photocurable dental preventive and restorative formula-tions are composed of a mixture of monomeric and oligomeric esters ofmethacrylic and acrylic acid a filler such as ultrafine silica and a free-radical-typeinitiator system [29ndash36] In the early days curing was initiated at 360 nm withbenzoin and its derivatives or benzil ketals serving as photoinitiators Nowadays


Chart 116 Segments of network structures formed by theradical polymerization of a diacrylate (top) and the cationicpolymerization of a biscycloaliphatic diepoxide (bottom) [27]

curing is accomplished with visible light eg with 488 nm light emitted by anargon-ion laser using 12-diketoneamine initiator systems (see Subsection102243) The diketones include camphor quinone CQ (177-trimethylbicy-clo[221]heptane-23-dione) and 1-phenyl-12-propanedione PPD Appropriateamines include dimethylaminoethyl methacrylate (A-1) NN-dimethyl-p-tolui-dine (A-2) p-NN-dimethylaminobenzoic acid ethyl ester (A-3) and N-phenylgly-cine (A-4) The chemical structures are presented in Chart 117 N-Phenylglycine(A-4) is reportedly less biologically harmful than the other amines [31]

Chemical structures of typical polymerizable compounds employed for the gen-eration of the polymeric matrix of dental formulations are presented in Chart 118

Polymerized acrylate- and methacrylate-based resins are characterized by ex-cellent aesthetics and good mechanical strength Shortcomings include incom-plete conversion lack of durable adhesion to tooth structure and most impor-tantly polymerization shrinkage The latter results from a volume contractionreflecting the conversion of van der Waals distances between free monomerunits to the distances of covalent bonds linking these units in the polymerchain To avoid multilayer application this problem can be overcome by em-ploying non-shrinking formulations containing oxaspiro monomers such as M-7and M-8 (see Chart 119) together with diepoxides that undergo ring-openingpolymerization initiated by cationically functioning photoinitiators upon expo-sure to visible light Methacrylate-substituted oxaspiro monomers such as M-9polymerize by a simultaneous free radical and cationic dual-photo-cure processto yield cross-linked ring-opened structures These aspects are discussed in areview by Antonucci et al [30]

114Stereolithography ndash microfabrication

Stereolithography is a technique widely adopted in industry in conjunction withcomputer-aided design CAD and computer-aided manufacturing CAM ie micro-machining [32 37 38] Stereolithography allows the fabrication of solid plasticthree-dimensional (3-D) prototypes or models of products and devices fromCAD drawings in a matter of hours Rapid prototyping by means of stereo-

114 Stereolithography ndash microfabrication 313

Chart 117 Chemical structures of diketones and aminesserving as co-initiators in the curing of dental formulations


Cha

rt11

8C

hem

ical

stru

ctur

esof

typi

cal

poly

mer

izab

leco

mpo

unds

cont

aine

din

dent

alfo

rmul

atio

ns[3

0]

lithography is used everywhere from designing automotive and airplane parts todesigning artificial hips and other replacement joints The designer simply digi-tizes the plan punches it into a computer and gets a prototype within hoursThe procedure involves hitting a photosensitive liquid contained in a vat with alaser beam Under computer guidance the beam outlines a shape Whereverthe light strikes the liquid rapid polymerization occurs and thus the liquidsolidifies Since this process is restricted to a thin layer a three-dimensionalplastic model is built-up in a layer-by-layer growth procedure This is accom-plished by steadily lowering a movable table in the vat or by continuouslypumping monomer into the vat from an external reservoir Both procedures aredepicted schematically in Fig 111

Rapid prototyping is an ldquoadditiverdquo process combining layers of plastic to cre-ate a solid object In contrast most machining processes (milling drillinggrinding) are ldquosubtractiverdquo processes that remove material from a solid block

Stereolithography also allows the creation of tiny parts of micrometer dimen-sions including microgears that may be employed for the construction of micro-machines such as micropumps and micromotors artificial organs surgical oper-ating tools etc [39] Also polymeric three-dimensional photonic crystals ie poly-meric materials consisting of periodic microstructures such as m-sized rods can


Chart 119 Oxaspiro monomers used in non-shrinking dental formulations [30]

Fig 111 Schematic depiction of the stereolithographiccreation of solid plastic three-dimensional (3-D) prototypeswith the aid of a movable table (a) or by monomer pumping (b)

be generated by the laser microfabrication technique Due to the presence of per-iodic microstructures these materials possess photonic band gaps ie wavelengthregions in which propagating modes are forbidden in all directions This offers thepossibility to manipulate and control light [18 40ndash44] The development of thisfield pertains to two-photon polymerization which relies on the simultaneous ab-sorption of two photons by appropriate photoinitiators by way of a virtual electron-ic excitation state (see Section 3323 and Fig 36) In contrast to single-photon ab-sorption whereby the absorbed dose rate Drabs is proportional to the incident in-tensity I0 (DrabsI0) Drabs is proportional to I0

2 in the case of two-photon absorp-tion (DrabsI0

2) This implies that photopolymerization can be confined tovolumes with dimensions of the order of the wavelength of the light as no out-of-focus absorption and thus polymerization can occur Free radical two-photonpolymerization has been performed with the aid of commercially available photo-initiators such as phosphine oxides or 2-benzyl-2-dimethylamino-1-(4-morpholino-phenyl)butan-1-one as shown in Scheme 114 [18 43 44]

Research has also been devoted to cationic two-photon photopolymerizationusing conventional initiator systems such as isopropylthioxanthone (ITX)diaryliodonium salt with ITX serving as the photosensitizer [45 46] Mode-locked op-erated Ti sapphire laser systems emitting femtosecond light pulses at 600 710or 795 nm were employed in these studies

115Printing plates


Printing processes use printing plates to transfer an image to paper or other sub-strates The plates may be made of different materials The image is applied tothe printing plate by means of photomechanical photochemical or laser en-graving processes For printing the plates are attached to a cylinder Ink is ap-plied to the image areas of the plate and transferred to the paper or in the caseof offset printing to an intermediate cylinder and then to the paper

During the past decades photosensitive polymer printing plates have largelydisplaced the classical letterpress printing plates made of metals such as lead[47ndash51] This technological revolution commenced in the 1950s [52] when the Dy-


Scheme 114 Main pathway of the photolysis of a 4-morpholinophenyl amino ketone following two-photonabsorption at exc = 600 nm [18]

cryl (DuPont) and Nyloprint (BASF) letterpress plates entered the market [47] Theletterpress technique based on light-sensitive polymer printing plates is used toprint newspapers paperback books business stationary postage stamps adhesivelabels and many other items Print runs of 500000 or more can be easily achievedThe letterpress printing plates are relief-structured ie the printing areas areraised above the non-printing areas During printing ink dispensed on the raisedareas is transferred to the substrate Depending on the printing mode the reliefdepth ranges from 02 to several mm Instead of stiff printing plates relief plateson a flexible support are employed in a special relief printing technique termedflexography This technique can also be used for coarser and larger-scale work suchas in corrugated board printing Besides letterpress printing which is consideredin the following subsections photosensitive systems are also employed in otherprinting modes such as gravure and screen printing [48 49 51]

1152Structure of polymer letterpress plates

As can be seen in Fig 112 polymer letterpress plates consist of various layers aprotective cover layer a photosensitive layer an adhesion layer and a support layer

1153Composition of the photosensitive layer

The photosensitive layers of early plates were composed of acrylatemethacrylateand acrylated cellulose acetate mixtures Other printing plates contained polya-mides or nylon derivatives as binders Generally printing plates contain a mixtureof reactive monomers and multifunctional oligomers (pre-polymers) polymericbinders and photoinitiators with exceptional cure depth The original photoinitia-tors were benzoin derivatives Later anthraquinone and other systems were usedBoth free radical polymerization and cationic polymerization are applicable [49]

1154Generation of the relief structure

The printing plate covered with the polymerizable material often incorrectly re-ferred to as the photopolymer is irradiated through a film negative to initiatephotocuring Thereby the areas of the photosensitive layer corresponding to the

115 Printing plates 317

Fig 112 Schematic depiction of thestructure of a typical polymer letter-press plate Adapted from Frass et al[49] with permission from Wiley-VCH

transparent regions of the negative film are polymerized and become insolublein the developer The relief structures generated in this way are required to pos-sess a high cross-linking density so as to provide for sufficient hardness andheat and water resistance Following irradiation the plate is developed with anappropriate liquid (mostly water or alcoholwater mixtures) washed dried andif necessary re-exposed A modern technique employs solvent-free thermal de-velopment [53] the irradiated plate is fixed onto an internally heated drum in aprocessor heated to around 50 C At this temperature the unexposed monomerforms a fluid that can be lifted from the plate with a fleece that is pressedagainst the plate In 10ndash12 revolutions a relief depth of 06ndash09 mm is reachedat which point the plate is ejected Recently printing plate fabrication tech-niques employing computer-to-plate digital laser exposure have been introducedthus rendering the negative film process obsolete [54] These techniques rely onthe use of infrared lasers particularly fiber lasers emitting at = 1110 nm [55]Digital imaging of photopolymer plates requires a special plate compositionThe photosensitive material adhered to the substrate layer is coated with alayer of carbon black only a few m thick The black layer is then ablated by theIR laser beam resulting in a digital image on the surface of the plate Theprinting plate is subsequently processed in much the same way as conventionalphotopolymer plates by exposure to UV light washout drying and finishingComputer-to-plate printing is also accomplished with printing plates bearing aheat-sensitive mask layer containing IR absorbers Prior to UV exposure theseplates are irradiated with a computer-guided IR laser in order to generate amask through imagewise exposure [56]

116Curing of printing inks

UV curing of inks is employed in flexographic and offset printing [57] Besidespigments appropriate inks typically contain unsaturated polymers based onpolyacrylates and polyesters photoinitiators and additives [58] The ink is curedafter printing by exposing the printed items to UV light Since the ink hardenswithin a fraction of a second printing speeds of up to 300 m minndash1 can be at-tained UV-cured printing inks are superior to water-based thermally cured inksdue to their higher gloss and better fastness ie abrasion resistance


117Holography

1171Principal aspects

The fact that photopolymerization can be used to record volume phase holo-grams is the basis of various commercial products made for instance by Du-Pont LucentInPhase and Polaroid Therefore the basic principles of hologra-phy are briefly described here although other methods for writing hologramshave been dealt with previously in the context of photorefractivity (Section 45)and photochromism (Section 582) There are various books that deal with thegeneral area of holography [59ndash63]

The term holography derives from the Greek words holos (whole) and graphein(write) and denotes whole or total recording A hologram is a two-dimensionalrecording but produces a three-dimensional image Holography invented by Ga-bor (Nobel Prize in 1971) [64] involves recording the complete wave field scat-tered by an object that is to say both the phase and the amplitude of the lightwaves diffracted by the object are recorded This is in contrast to conventionalimaging techniques such as photography which merely permit the recordingof the intensity distribution in the original scene and therefore all informationon the relative phases of the light waves coming from different points of the ob-ject is lost Since recording media respond only to the light intensity hologra-phy converts phase information into intensity variations This is accomplishedby using coherent illumination in conjunction with an interference techniqueFigure 113 depicts schematically how a hologram is written

Light generated by a laser simultaneously falls on the object and a mirrorThe light waves diffracted from the object and those reflected by the mirror pro-

117 Holography 319

Fig 113 Recording of a hologram of an object by generatingan interference pattern on the detection plate

duce an interference pattern on the detection plate by generating a local refrac-tive index modulation (phase hologram) or an absorption coefficient modulation(amplitude hologram) After processing the image can be reconstructed by illu-minating the hologram with only the reference light beam As demonstrated inFig 114 light diffracted by the hologram appears to come from the original ob-ject

The quality of a hologram is characterized by the efficiency factor = IIRwhere I and IR are the intensities of the diffracted beam and the incident refer-ence beam respectively

The term volume holography refers to recording plates with a thickness of upto a few millimeters In such voluminous matrices data storage in three dimen-sions is possible This implies an enormous increase in storage capacity in com-parison with other methods If multiplexing techniques (see Section 121) are ap-plied thousands of holograms can be superimposed in the same plate

Photopolymerizable systems appropriate for recording holograms are oftenand sometimes also in this book incorrectly referred to as photopolymersalthough their essential components are monomers and not polymers They typ-ically comprise one or more monomers a photoinitiator system an inactivecomponent (binder) and occasionally substances that serve to regulate pre-expo-sure shelf-life or viscosity The resulting formulation is typically a viscous fluidor a solid with a low glass transition temperature For exposure the formulationis coated onto a solid or flexible substrate or dispensed between two opticallyflat glass slides Detailed information on the topic of polymers in holography isavailable in various reviews [65ndash72]


Fig 114 Reconstruction of the image of an object recordedin a hologram by illuminating the detection plate with the ref-erence light wave

1172Mechanism of hologram formation

The formation of a hologram in a formulation containing polymerizable mono-mers is due to the generation of a refractive index grating [73] When the holo-graphic formulation is exposed to a light interference grating the dispersedmonomer polymerizes rapidly in the regions of high intensity ie in the brightregions Since the monomer concentration is depleted in these regions concen-tration gradients are generated which cause component segregation ie the gradi-ents drive the diffusion of the monomer from the dark into the depleted brightregions where it polymerizes Ultimately the bright regions are characterizedas areas of high concentration of newly formed polymer and the dark regionsas areas of high binder concentration Since the two materials differ in their re-fractive indices a phase grating recorded in real time results To increase theefficiency the hologram may be heated for a short period to temperatures of100ndash160 C [73] Further monomer diffusion leading to an increased refractiveindex modulation is believed to occur during the heating step Any unreactedmonomer can be finally converted by briefly exposing the plate to incoherentUV light (360ndash400 nm) No wet-processing is required with modern holographicformulations

1173Multicolor holographic recording

Color holography allows the addition of life-like color to holographic images Aswell as full-color display holograms multi-wavelength holographic optical ele-ments can also be made with the aid of color holography [74ndash76] The phenom-enon of color mixing employed in color photography is utilized to generate colorholograms By utilizing three recording wavelengths usually red green andblue which are simultaneously incident on the holographic plate the impres-sion of a wide variety of colors is created Actually the image of an object ob-tained from a color hologram is the superposition of the images of three holo-grams written with three laser beams Typical laser wavelengths are 647 nm(red) 532 nm (green) and 476 nm (blue) If photopolymerizable formulationsare employed color holograms can be created by writing the holograms in asingle holographic plate containing polymerization-initiating systems that aresufficiently sensitive at the specific wavelengths of the laser beams Alterna-tively color holograms can be created by employing multiple-layer holographicplates composed of wavelength-specific photopolymer layers (see Fig 115) [75]

After recording the plates are commonly subjected to a thermal treatment toincrease the refractive index modulation and flood-exposed to UV light to fixthe hologram Wet-processing is not required [75] For image retrieval the holo-graphic plate is simultaneously exposed to the three laser beams whereby thecolored image of the object is formed by the additive mixture of the individualholograms Image retrieval with white light is possible provided that the co-

117 Holography 321

lored hologram was written with the reference beams incident on the reverseside of the plate

1174Holographic materials

For many years the most widely used holographic materials were silver halidephotographic emulsions and dichromated gelatin Upon exposure to light gelatinlayers containing a small amount of a dichromate such as (NH4)2Cr2O7 becomeprogressively harder since photochemically produced Cr3+ ions form localizedcross-links between carboxylate groups of neighboring gelatin chains This re-sults in a modulation of the refractive index The drawbacks of these materialsare the need for wet-processing high grain noise and environmental sensitivityDuring the last decades various polymeric formulations have emerged as alter-natives for practical holographic applications [72] Although the precise compo-sitions of relevant commercial formulations are not disclosed by the producersit is generally agreed that in most cases acrylate- and methacrylate-based mono-mers are used as polymerizable components [66] In typical holographic storagestudies the formulation comprises a difunctional acrylate oligomer N-vinyl car-bazole and isobornyl acrylate [77] In these cases the polymerization proceedsby a free radical mechanism and initiator systems operating in the visible ornear-IR wavelength region are employed Multifunctional monomers are oftenadded to the formulation so as to produce a molecular architecture that consistsof a cross-linked polymer network which improves dimensional stability andimage fidelity

Moreover cationically polymerizable epoxide monomers capable of under-going ring-opening polymerization (see Chart 1110) are used in volume holo-graphic recording [78 79]

Actually volume shrinkage is an important drawback regarding hologram re-cording based on vinyl monomer polymerization On the other hand no vol-ume shrinkage or even a slight volume increase occurs upon polymerization ofepoxide monomers Therefore in holographic formulations containing both


Fig 115 Structure of a holographic three-layer plateemployed for color holographic recording Adapted from Troutet al [74] with permission from the International Society forOptical Engineering (SPIE)

types of monomers volume shrinkage is largely reduced This is especially thecase if prior to recording a rather stable matrix is formed by in situ polymer-ization of the epoxide monomer Thereby a cross-linked network is formed inthe presence of the unreacted acrylate monomer which is ready for subsequentholographic recording [67]

Electrically switchable holograms can be generated with formulations containinga liquid-crystalline monomer A typical example is given in Chart 1111

During recording a highly cross-linked polymer is formed in the bright re-gions of the interference grating Since it retains the initial order of the nematicmonomer the refractive index remains essentially unchanged However uponapplication of an electric field the mobile monomeric regions corresponding tothe dark regions are selectively reoriented resulting in a large refractive indexchange By repeatedly switching the electric field on and off the hologram isalso switched on and off [80] Alternatively electrically switchable hologramscan be made by using formulations containing an unreactive liquid crystal anda non-liquid-crystal monomer As the monomer diffuses from the dark regionsto the bright regions to polymerize there the liquid crystal is forced into thedark regions There it undergoes phase separation appearing as droplets Theresulting so-called holographic polymer-dispersed liquid crystal (H-PDLC) canalso be switched on and off by switching of the applied electric field [81ndash83]

1175Holographic applications

Holography has found a remarkably wide range of applications Several compa-nies produce photopolymer holograms for use in graphic arts security andgoods authentication devices Photopolymer holograms have the capability to of-fer bright and easily viewable displays for cell phones and other consumer elec-tronics products as well as unique eye-catching 3D color images that can be at-tached to a variety of products Additional application fields include holographic

117 Holography 323

Chart 1110 Structures of typical epoxide monomers employed in volume holography [78]

Chart 1111 Chemical structure of a liquid-crystal-forming monomer [80]

optical elements particle size analysis high-resolution imaging multiple imag-ing stress analysis and vibration studies The importance of holography in in-formation storage and processing is dealt with in Section 1232 Actually poly-meric holographic formulations are promising materials for write-once-read-many (WORM) and read-only-memory (ROM) data storage applications becauseof their good light sensitivity good image stability format flexibility large dy-namic range and relatively low cost There are various formulations that yieldimages directly upon exposure to light ie images are developed in real time

118Light-induced synthesis of block and graft copolymers


The copolymerization of monomers of different chemical nature often resultsin polymers possessing a specific combination of physical properties and istherefore of interest for the development of novel high-tech devices This ap-plies in particular to block and graft copolymers of the general structures indi-cated in Chart 1112

Block copolymers are composed of long chain segments of repeating units oftypes A or B whereas graft copolymers are composed of chains of repeatingunits A onto which side chains composed of repeating units B are graftedBoth types of copolymers can be synthesized by means of photochemical meth-ods based on free radical or cationic mechanisms For practical applications cat-ionic polymerizations are less attractive than free radical polymerizationsTherefore only the latter will be dealt with in the following subsections

Most of the known photochemical procedures for the synthesis of block andgraft copolymers are based on the modification of already existing polymerswith photolabile groups incorporated at defined positions ie at the chain endat side chains or in the main chain (see Chart 1113) [84]

Upon absorption of light the photolabile groups can dissociate into pairs offree radicals capable of initiating the polymerization of a monomer present inthe system (see Scheme 115)

Typical chromophoric groups that have been chemically incorporated into orattached to linear macromolecules for the purpose of photosynthesizing block


Chart 1112 General chemical structures of block copolymersand graft copolymers consisting of monomer units A and B

or graft copolymers are compiled in Table 114 (see also Chart 102 in Sec-tion 10223) Macromolecules bearing photolabile groups are occasionally alsotermed macroinitiators [85]

Apart from the photoreactions of dithiocarbamate groups (last entry in Ta-ble 114) no details on the radical-generating photoreactions referred to in Ta-ble 114 are given here These can be found in [84ndash86] Dithiocarbamate groupsplay a special role with regard to the photoinitiation of polymerizations This is

118 Light-induced synthesis of block and graft copolymers 325

Chart 1113 General structures of polymers bearingphotolabile groups at the chain end (a) at side chains (b) orincorporated into the main chain (c)

Scheme 115 Formation of block and graft copolymersfollowing the photodissociation of chromophoric groups Forthe sake of simplicity chain-termination reactions are notincluded

due to the fact that the sulfur-centered radical is much less reactive than the car-bon-centered radical and hence does not react with vinyl monomers but ratheracts as a terminator of growing macroradicals Thus polymerizations initiatedby the photolysis of polymeric dithiocarbamates result in macromolecules pos-sessing the original end groups (see Scheme 116) Initiators behaving in thisway were termed iniferters by Otsu as an acronym for initiator-transfer-agent-ter-minator [87]

Block and graft copolymerization can also be initiated in indirect modesHere light is absorbed by independent initiator molecules that are present inthe reaction system but are not incorporated into the polymer Reactive speciesformed in this way interact with the polymer so as to generate free radical sites


Table 114 Photolabile groups chemically incorporated intolinear polymers at in-chain lateral or terminal positionsgiving rise to the formation of reactive free radicals [84]

Photolabile groups a) Free radicals

Carbonyl groups

Keto oxime ester groups

Benzoin methyl ether groups

N-Nitroso groups

Disulfide groups

Phenyl sulfide groups

Dithiocarbamate groups

a) R1 denotes a macromolecular substituent

on the latter that are capable of reacting with monomer molecules Such sys-tems are presented in Table 115 Of general importance is the system based onhydrogen abstraction from the trunk polymer by excited aromatic carbonylgroups

The methods described above commonly do not lead to pure products In-stead mixtures composed of starting material and copolymer are obtainedMoreover homopolymer is produced if one of the free radicals released fromthe initiator radical pair is of low molar mass (see Scheme 115) These are ser-ious drawbacks for practical applications regarding the production of novel poly-meric materials based on block copolymers However there is important techni-cal potential with respect to photografting of surfaces of polymeric articles Havingbeen widely explored by many investigators during the last decades [88 89] this


Scheme 116 Formation of a diblock copolymer with the aid of a photoiniferter

Table 115 Indirect generation of free radical sites at lateral or terminal positions of linear polymers

Precursor reaction Attack of polymer Product free radicals

a

a

a) Refers to aromatic carbonyl compounds such as benzophenone or anthraquinone

field is still attractive to many researchers [90ndash96] It is the subject of the follow-ing subsection in which some interesting applications are described

1182Surface modification by photografting

Photografting can change the surface properties of polymeric articles For exam-ple photografting can impart hydrophilicity to hydrophobic surfaces of polyal-kenes and bring about antifogging antistatic and antistaining properties andimprovements in dyeability adhesiveness printability and biocompatibilityPhotografting competes with other techniques of surface modification includ-ing corona discharge plasma treatment chemical oxidation and coating Photo-grafting has the advantage over these methods that a large variety of propertychanges can be imparted to plastic articles by grafting monomers of quite differ-ent chemical nature onto the same polymer Surface grafting can also be accom-plished with high-energy radiation of low penetration depth including electronbeam radiation and soft X-rays Photografting is advantageous over high-energyradiation grafting in that it is virtually restricted to a very thin surface layer andin that it can be applied with rather little effort with respect to the radiationsources Polyalkenes and other polymers that are produced industrially in largequantities lack chromophoric groups capable of absorbing UV light emittedfrom commonly available light sources To circumvent this problem proceduresbased on the adsorption of monomers and initiators by pre-soaking have been


Fig 116 Schematic depiction of surface photograftingprocesses (a) continuous grafting [91] (b) immersiongrafting [97] (c) vapor-phase grafting Adapted from Ogiwaraet al [98] with permission from John Wiley amp Sons Inc

elaborated For example acrylic acid acrylamide vinylpyridine or glycidyl acry-late can be grafted onto low-density polyethylene or linear polyesters in layersranging from 2 to 8 nm in a continuous process using benzophenone as a hy-drogen abstraction-type initiator As shown in Fig 116a the polymer foil isdrawn from a roll through a solution of initiator and monomer to a reactionchamber for irradiation at 250 nm and is subsequently reeled up [91] Fig-ure 116 also depicts batch processes ie immersion grafting (Fig 116 b) andvapor-phase grafting (Fig 116c) In the latter case the initiator-coated polymeris irradiated in an atmosphere of the monomer

The aim within the frame of this book is not to survey the plethora of publi-cations devoted to surface photografting Typical work published in recent yearsis compiled in Table 116 which demonstrates that the enhancement of hydro-philicity and wettability of hydrophobic polymers and the improvement of adhe-sion of polymers to various substrates are still major research topics (see also[99]) Moreover the grafting of ultrafine inorganic particles such as nanosizedsilica and titania with vinyl monomers is an attractive subject Relevant earlierwork on surface photografting has been reviewed by Yagci and Schnabel [84]

References 329

Table 116 Surface grafting of monomers recent investigations

Substrate Monomers Remarks Refs

Low-density polyethylene Acrylic acid acrylamidevinyl pyridine glycidylacrylate

Enhanced hydrophilicityand dye adsorptionadhesion to differentsubstrates

[91 92]

Low-density polyethylenePolypropylene

Acrylic acid hydroxypropylacrylate

Enhanced hydrophilicitywettability

[100]


Maleic anhydridevinyl acetatemaleicanhydride

Enhanced hydrophilicity [101][102]

Polyurethane Methacrylic acid Enhanced hydrophilicityenhanced biological cellcompatibility

[103]

Ultrafine inorganic parti-cles (silica titania)

Acrylic acid acrylamideacrylonitrile styrene

Grafted materials givestable dispersions inappropriate liquids

[104][105]

References

1 CG Roffey Photopolymerization of Sur-face Coatings Wiley Chichester (1982)

2 SP Pappas (ed) UV Curing Scienceand Technology Vols I and II Technology

Marketing Corp Norwalk CT USA(1978) and (1985)




5 PK T Oldring (ed) Chemistry and Tech-nology of UV and EB Formulations forCoatings Inks and Paints Volumes IndashIVSITA Technology London (1991)

6 H Boumlttcher (ed) Technical Applicationsof Photochemistry Deutscher Verlag fuumlrGrundstoffindustrie Leipzig (1991)


8 NS Allen M Edge UV and ElectronBeam Curable Pre-Polymers and DiluentMonomers Classification Preparation andProperties in Vol I of [7] p 225

9 R Mehnert A Pincus I Janorski RStowe A Berejka UV and EB Technologyand Equipment SITA Technology London(1998)

10 (a) G Webster Prepolymers amp ReactiveDiluents SITA Technology London(1998) (b) G Webster G Bradley CLowe A Compilation of Oligomers andMonomers Commercially Available for UVToday SITA Technology London (2001)

11 C Lowe G Webster S Kessel I McDo-nald Formulation SITA Technology Lon-don (1999)


13 R S Davidson Radiation Curing RapraTechnology Shawbury Shrewsbury(2001)

14 K Dietliker A Compilation of Photoinitia-tors Commercially Available for UV TodaySITA Technology London (2002)

15 DC Neckers W Jager Photoinitiationfor Polymerisation SITA Technology Lon-don (1999)

16 P Dufour State-of-the-Art and Trends inthe Radiation Curing Market in Vol I of[7] p 1

17 A F Jacobine Thiol-Ene PhotopolymersChapter 7 in Vol III of [7]

18 LH Nguyen M Straub M Gu AdvFunct Mater 15 (2005) 209

19 Information of Radiation Curing CenterSpecialChem SA httpwwwspecial-chem4coatingscomtcradiation2Dcur-ing

20 C Decker Photoinitiated Curing of Multi-functional Monomers Acta Polym 45(1994) 333

21 A F Jacobine ST Nakos PolymerizableSilicone Monomers Oligomers and ResinsChapter 5 of [3]

22 HH Bankowsky P Enenkel M LokaiK Menzel Paint amp Coatings Ind April(2000)

23 S Peeters Overview of Dual-Cure and Hy-brid-Cure Systems in Radiation CuringChapter 6 in Vol III of [7]

24 E Beck B Bruchmann Proc XXVII FA-TIPEC Congress Aix-en-Provence(2004) Vol I p 41

25 J Ortmeier Proc Conf Automotive Cir-cle International Bad Nauheim (2003)p 54

26 K Studer C Decker E Beck RSchwalm Prog Org Coat 48 (2003)101

27 C Decker F Masson R SchwalmMacromol Mater Eng 288 (2003) 17


29 M Braden RL Clarke J Nicholson SParker Polymeric Dental MaterialsSpringer Berlin (1997)

30 J M Antonucci JW Stansbury Molecu-larly Designed Dental Polymers in R Ar-shady (ed) Desk Reference of FunctionalPolymers Synthesis and ApplicationsAmerican Chemical Society WashingtonDC (1997) p 719

31 Z Kucybala M Pietrzak J PaczkowskiLA Lindeacuten J F Rabek Polymer 37(1996) 4585

32 J Jakubiak X Allonas JP Fouassier ASionkowska E Andrzejewska LA Lin-deacuten J F Rabek Polymer 44 (2003) 5219

33 Y J Park HH Chae HR Rawls Den-tal Mater 15 (1999) 120

34 J-F Roulet Degradation of Dental Poly-mers Karger Basel (1987)

35 LA Lindeacuten Photocuring of PolymericDental Materials and Plastic Composite Re-sins Chapter 13 in Vol IV of [7]

References 331

36 LA Lindeacuten Proceed RadTech EuropeMediterranean Conference RadTech Eu-rope Fribourg Switzerland (1993)p 557

37 J Jakubiak JF Rabek Poliymery 45(2000) 759 and 46 (2001) 5219

38 PF Jacobs (ed) Rapid Prototyping andTechnology Fundamentals of Stereolithogra-phy Society of Manufacturing EngineersDearborn (1992)

39 K Yamaguchi T Nakamoto Manufactur-ing of Unidirectional Whisker ReinforcedPlastic Microstructures in N Ikawa TKishinami F Kimura (eds) Rapid Prod-uct Developments Chapman amp Hall Lon-don (1997) p 159

40 V Mizeikis HB Sun A MarcinkeviciusJ Nishii S Matsuo S Juodkazis HMisawa J Photochem Photobiol AChem 145 (2001) 41


42 (a) T-C Lin S-J Chung K-S Kim XWang G S He J Swiatkiewicz HEPudavar P N Prasad Organics and Poly-mers with High Two-Photon Activities andTheir Applications in KS Lee (ed) Poly-mers for Photonics Applications IISpringer Berlin (2002) (b) B StrehmelV Strehmel Two-Photon Physical Organ-ic and Polymer Chemistry Theory Tech-niques Chromophore Design and Applica-tions in D C Neckers WS Jenks TWolff (eds) Advances in Photochemistry29 Wiley-Interscience New York (2007)p 111

43 R A Borisov GN Dorojkina N I Koro-teev VM Kozenkov SA MagnitskiiDV Malakov AV Tarasishin AMZheltikov Appl Phys B 67 (1998) 765

44 K D Belfield X Ren E W van StrylandDV Hagan V Dobikovsky E J MiesakJ Am Chem Soc 122 (2000) 1217

45 Y Boiko JM Costa M Wang SEsener Proc SPIE 4279 (2001) 212

46 SM Kuebler BH Cumpson S Anan-thavel S Barlow J E Ehrlich LL Et-sine A A Heikal D McCord-MaughonJ Qin H Rocel M Rumi SR MarderJ W Perry Proc SPIE 3937 (2000) 97


48 J Bendig H-J Timpe Photostructuringin H Boumlttcher (ed) Technical Applica-tions of Photochemistry Deutscher Verlagfuumlr Grundstoffindustrie Leipzig (1991)p 172

49 W Frass H Hoffmann B BronstertImaging Technology in B Elvers S Haw-kins M Ravenscroft G Schulz (eds)Ullmannrsquos Encyclopedia of IndustrialChemistry VCH Weinheim (1989) VolA 13 p 629

50 HR Ragin Radiation-Curable Coatingswith Emphasis on the Graphic Arts Chap-ter 7 of [3]

51 H Vollmann W Frass D Mohr AngewMakromol Chem 145146 (1986) 411

52 L Plambeck US Patent 2760863(1956)

53 The Seybold Report on Publishing Sys-tems 30 (2000) 5

54 Newsletter for the Flexographic Pre-Press Platemakers Association httpfppanetnewslettersummer03mem-bershtm

55 Flexo and Gravure International March(2001)

56 Y Ichii S Ichikawa S Tanaka Jap Pat-ent (Toray Industries) JP 2005326442(2005)

57 R Kuumlbler Printing Inks in B Elvers SHawkins W Russey G Schulz (eds)Ullmannrsquos Encyclopedia of IndustrialChemistry VCH Weinheim (1993) VolA 22 p 143

58 A J Bean Radiation Curing of PrintingInks Chapter 8 of [3]

59 HM Smith Principles of HolographyWiley New York (1969)

60 HJ Caulfield (ed) Handbook of OpticalHolography Academic Press New York(1979)

61 N Abramson The Making and Evalua-tion of Optical Holograms AcademicPress London (1981)

62 HJ Coufal D Psaltis G T Sincerbox(eds) Holographic Data StorageSpringer Berlin (2000)

63 P Hariharan Optical Holography Princi-ples Techniques and Applications 2nd Edi-tion Cambridge University Press Cam-bridge (1996)

64 D Gabor Nature 161 (1948) 777


65 S Reich Photodielectric Polymers for Ho-lography Angew Chem Int Ed 16(1977) 441

66 R T Ingwall D Waldmann Photopoly-mer Systems in [62] p 171

67 I Dhar MG Schnoes HE Katz AHale MT Schilling AL Harris Photo-polymers for Digital Holographic DataStorage in [62] p 199

68 MB Sponsler (a) Photochemical andPhotophysical Processes in the Design ofHolographic Recording Materials Spec-trum 13 (2000) 7 (b) Hologram Switchingand Erasing Strategies with Liquid Crystalsin V Ramamurthy KS Schanze (eds)Optical Sensors and Switches Marcel Dek-ker New York (2001) p 363

69 DJ Lougnot Photopolymers and Hologra-phy Chapter 3 in Vol III of [7]

70 DJ Lougnot Self-Processing PhotopolymerMaterials for Holographic Recording CritRev Opt Sci Technol CR 63 (1996) 190

71 ML Schilling VL Colvin I Dhar A LHarris FC Schilling HE Katz T Wy-socki AL Hale LL Blyer C Boyd Ac-rylate Oligomer-Based Photopolymers forOptical Storage Applications Chem Ma-ter 11 (1999) 247

72 LV Natarajan R L Sutherland V Ton-diglia T J Bunning WW AdamsPhotopolymer Materials Development ofHolographic Gratings in J C Salamone(ed) Concise Polymeric Materials Encyclo-pedia CRC Press Boca Raton FloridaUSA (1999) p 1052

73 SH Stevenson ML Armstrong P JOrsquoConnor D F Tipton Proc SPIE 2333(1994) 60

74 T J Trout W J Gambogi S H Steven-son Proc SPIE 2577 (1995) 94

75 W J Gambogi W K Smothers K WSteijn Proc SPIE 2405 (1995) 62

76 K W Steijn Proc SPIE 2688 (1996) 12377 V L Colvin R G Larson A L Harris

ML Schilling J Appl Phys 81 (1997)5913

78 DA Waldman R T Ingwall PK DalMG Horner ES Kolb H-Y S Li R AMinns HG Schild Proc SPIE 2689(1996) 127 US Patent 5759721 (1998)

79 V Strehmel Epoxies Structures Photoin-duced Crosslinking Network Properties andApplications in HS Nalwa (ed) Hand-

book of Photochemistry and PhotobiologyVol 2 American Scientific Publ Steven-son Ranch CA USA (2003) p 102

80 J Zhang CA Carlen S Palmer MBSponsler J Am Chem Soc 116 (1994)7055

81 R T Pogue R L Sutherland MGSchmitt LV Natarajan VP TondigliaT J Bunning Appl Spectrosc 54 (2000)12A

82 CC Bowley G P Crawford Appl PhysLett 76 (2000) 2235

83 D Duca AV Sukhov C Umeton LiqCryst 26 (1999) 931

84 Y Yagci W Schnabel Light-Induced Syn-thesis of Block and Graft Copolymers ProgPolym Sci 15 (1990) 551

85 I Reetz Y Yagci MK Mishra Photoini-tiated Radical Vinyl Polymerization inMK Mishra Y Yagci (eds) Handbookof Radical Vinyl Polymerization DekkerNew York (1998) p 149

86 A E Muftuoglu MA Tasdelen Y YagciPhotoinduced Synthesis of Block Copoly-mers Chapter 29 in JP Fouassier (ed)Photochemistry and UV Curing NewTrends Research Signpost TrivandrumIndia (2006)

87 (a) T Otsu M Yoshida MakromolChem Rapid Commun 3 (1982) 127(b) A Sebenik Living Free-Radical BlockCopolymerization Using Thio-InifertersProg Polym Sci 23 (1998) 875

88 J C Arthur Photoinitiated Grafting ofMonomers onto Cellulose Substrates inNS Allen (ed) Developments in PolymerPhotochemistry ndash 1 Appl Science PublLondon (1980) p 69

89 J C Arthur Photografting of Monomersonto Synthetic Polymer Substrates in N SAllen (ed) Developments in PolymerPhotochemistry ndash 2 Appl Science PublLondon (1981) p 39

90 K L Mittal (ed) Polymer Surface Modifi-cation Relevance to Adhesion VSPUtrecht (1996)

91 B Raringnby Surface Photografting onto Poly-mers ndash A New Method in Adhesion Con-trol in Part 3 of [90]

92 B Raringnby Surface Modification and Lami-nation of Polymers by Photografting Int JAdhesion and Adhesives 19 (1999) 337

References 333

93 B Raringnby Photoinitiated Modification ofSynthetic Polymers Photocrosslinking andSurface Photografting in NS Allen MEdge I R Bellobono E Selli (eds) Cur-rent Trends in Polymer Photochemistry El-lis Horwood New York (1995) Chapter2

94 MJ Swanson GW Oppermann Photo-chemical Surface Modification Photograft-ing of Polymers for Improved Adhesion inPart 3 of [90]

95 J P Bilz C B Lottle (eds) Fundamentaland Applied Aspects of Chemically ModifiedSurfaces The Royal Chemical SocietyLondon (1999)

96 PA Dworjanyn J L Garnett Role ofGrafting in UV- and EB-Curing ReactionsChapter 6 of Vol I of [7]

97 S Tazuke M Matoba H Kimura TOkada in CE Carraher Jr M Tsuda(eds) Modification of Polymers ACS

Symp Series 121 Washington DC(1980)

98 Y Ogiwara M Kanda M Takumi HKubota J Polym Sci Lett Ed 19(1981) 457

99 NS Allen Polymer PhotochemistryPhotochem 34 (2003) 197

100 K Zahouilly Techn Conf ProceedRadTech 1 (2002) 1079

101 J Deng W Yang J Appl Polym Sci97 (2005) 2230


103 Y Zhu C Gao J Guan J Chen JBiomed Mater Res 67 A (2003) 1367

104 M Satoh K Shirai H Saitoh T Ya-mauchi N Tsubokawa J Polym SciPart A Polym Chem 43 (2005) 600

105 N Tsubokawa Y Shirai H TsuchidaS Handa J Polym Sci Part A PolymChem 32 (1994) 2327

Part IVMiscellaneous technical developments

121General aspects

The revolutionary development in computer technology during the last decadeshas been inextricably linked with the elaboration of novel data storage methodsand the invention of relevant devices Impetus for innovations in the data stor-age field has also come from the steadily increasing demand for larger storagecapacity in the disparate fields of scientific research industrial production anddaily life entertainment [1] At present optical storage techniques reliant onpolymeric recording media play a prominent role Actually polymers are beingused in various ways not only as disk substrates but also as surfacingsubbinglayers for the substrate protective and antistatic overcoatings etc

The history of modern storage media commenced with magnetic memorieswhich proved very reliable in terms of stability and recordingreading speedWhen they could no longer meet capacity requirements a new optical storagesystem consisting of a drive unit and a storage medium in rotating disk form

337

12Polymers in optical memories

Table 121 Characteristics of single-sided single-layer 12 cm disks [2-4]

Disk Format d a)

(mm)TP b)

(m) c)

(nm)NA d) CSt

e)

(GB)rtrans

f )

(Mb sndash1)

Compact Disk (CD) 12 16 780 045 065 01Digital Versatile Disk (DVD) 12 074 650 060 47 11HD-DVDg) 12 040 405 065 15 36Blu-ray Disk (BD) h) 12 032 405 085 25 36

a) Substrate thicknessb) Track pitchc) Laser wavelengthd) Numerical aperture of objective lensese) Storage capacity 1 Byte (B) =8 bits (b)f) Data transfer rateg) High Definition DVD developed by Toshiba and NEC within

the DVD Forumh) Developed by Blu-ray Disc Association

the compact disk CD (storage capacity 650 MB) was invented Then followingthe constantly increasing demand for larger storage capacity the digital versatiledisk DVD (storage capacity 47 GB) was developed At present disks having astorage capacity of about 25 GB manufactured with the aid of advanced tech-niques are poised to enter the market The characteristics of single-sided sin-gle-layer disks are listed in Table 121 Because of the given limit in informationstorage of these optical media novel storage systems emerging from a hybridtechnology (magneto-optical disks MO) or developed on the basis of solid im-mersion techniques or volume holography can be foreseen

122Current optical data storage systems

1221Compact disk (CD) and digital versatile disk (DVD)

Since its release in 1982 the compact disk has taken the world by storm andbillions of CDs have been manufactured [5 6] Most of them are of the read-only memory (ROM) type made from transparent polycarbonate (seeChart 121) and providing almost perfect resolution

In the cases of both CD-ROM and DVD the information is binary coded bit-wise in the form of pits and lands (see Fig 121) Commencing at the insidespirally arranged tracks of pits and lands are engraved into the disk

Standard stamper-injection molding is the most commonly used method formanufacturing compact disks [2 3] It comprises various steps which are de-picted schematically in Fig 122 First a plane glass substrate is coated with aphotoreactive layer which is patterned with a pitland structure by an appropri-ate technique such as photolithography (see Section 91) In the latter case thedisk is rotated at a constant linear velocity while being exposed along a spiralpath from the inside to the outer edge to a laser beam eg of an Ar laser emit-ting 442 nm light Since the exposure is intermittent subsequent developmentresults in a pitland structure of the tracks The master disk obtained in thisway is then electroformed to create a stamper for use in an injection-moldingprocess Disks generated in this way are first coated with a thin reflective metallayer (typically Al) and then with two layers a protective acrylic layer and a labellayer both of which are cured using UV light (see Section 112) Finally thedisks having a total thickness of 12 mm are packaged in jewel-boxes for ship-

12 Polymers in optical memories338

Chart 121 Chemical structure of polycarbonate used for compact disks

ping DVDs are also fabricated by injection molding In this case two 06 mmthick disks one of them containing the recording layer are glued together [7]

During reading coherent laser light shone onto the tracks is reflected by themetal in the case of lands and is scattered in the case of pits which corre-sponds to the photocell-aided recording of 0 or 1 respectively The maximumdisk storage capacity is set by the resolving power ie the size and the packingdensity of the pits This is limited by the wavelength of the laser light since thefocus of the laser beam used for writing and reading cannot be smaller than

122 Current optical data storage systems 339

Fig 121 Pitland structure of tracks of compact disks

Fig 122 Schematic depiction of the commonly used methodfor the manufacture of compact disks

the wavelength In the case of optical systems operated with conventionallenses the diameter of the laser spot at the recording medium is given byEq (12-1) It can be seen that is proportional to NA where k is a constantand NA is the numerical aperture of the objective lens

k

NA12-1

Past strategies for increasing the storage capacity of optical disks were based ona reduction of and an increase in NA as can be seen from Table 121 In prin-ciple a reduction in the spot size can be achieved with the aid of solid immer-sion lenses This as yet not practically exploited technique operating with ahemispherical or a Weierstrass superspherical lens placed near the recordingmedium (lt 100 nm) yields a reduced spot size as is evident from Eqs (12-2)and (12-3) respectively where n denotes the refractive index of the lens [8]

Hemispherical lens k

nNA12-2

Weierstrass superspherical lens k

n2NA12-3

In addition to read-only systems there are recordable (write-onceread-manyCD-R) and rewritable CD formats (CD-RW) which will not be treated here Cur-rent recordable storage systems are based on laser-induced pit formation in or-ganic dye films or a laser-induced amorphous-to-crystalline phase change in aninorganic alloy film Current rewritable optical recording methods involve phasechange recording and magneto-optical (MO) recording The latter is based onswitching the magnetization direction of perpendicularly magnetized domainsin a magnetic film [9]

1222Blue-ray disks

As blue diode lasers became available on a large scale [10] a new generation ofstorage disks with further increased storage capacity was developed by the Blu-ray Disc Association and by Toshiba and NEC within the DVD Forum [4] Ascan be seen in Table 121 a HD-DVD holds 15 GB and BDs hold 25 GB (single-layer DB) or 50 GB (dual-layer BD) Figure 123 shows as a typical example thecross-section of a novel disk type having a triple-layer structure one BD layerand a dual DVD layer to be read by a blue and a red laser respectively


123Future optical data storage systems

1231General aspects

Considering the currently applied optical and magnetic recording methodsthere are physical limitations to a further increase in storage capacity Near-fieldoptical recording with an expected recording density of more than 1 Tb inndash2 (ca19 GB cmndash2) thus exceeding that of Blue-ray disks by about two orders of mag-nitude might be a method to overcome these limitations Here the data bitsare written and read by using an optical near field generated near a nanometer-scale object In this case the size of the optical spot can be reduced to less than1 nm because it is not limited by light diffraction [8] Pioneering near-field re-cording experiments with chromophoric compounds embedded in a polymericmatrix yielded recording marks with a diameter smaller than 100 nm [11]Although the desired high recording density is realized in this way practical ap-plication is hampered by rather slow data transfer rates which are on the levelof the storage systems in current use

123 Future optical data storage systems 341

Fig 123 Schematic depiction of the cross-section of a Blu-rayDVD ROM disk having a triple-layer structure BD single layer25 GB DVD dual layer 85 GB

An interesting non-optical technique developed in another attempt to achievelarger data storage capacities relates to an atomic microscope-based data storagetechnique operating with very thin polymer films With this technique 30ndash40 nm-sized bit indentations with a similar pitch size are made by a single can-tilever in thin polymer films typically a 50 nm poly(methyl methacrylate) thinfilm resulting in a storage density of 8ndash10 GB cmndash2 [12] While this new tech-nique is also unlikely to lead to products on the market in the near futureholography seems to be more promising The principle of holography has beenoutlined in Section 117 and the applicability of holography as an optical stor-age method has been alluded to elsewhere (see Sections 352 45 and 582)Therefore in this chapter mainly application-related aspects are discussed

1232Volume holography

12321 Storage mechanismHolography offers the potential for data storage since a large number of holo-grams can be superimposed in one volume element of an appropriate matrix[13ndash15] Bit recording in three dimensions implies an enormous increase instorage capacity in comparison to the techniques described in Section 122since multiple pages of data can be stored in the same volume of holographicmaterial The storage mechanism is based on the generation of light-induced lo-cal changes in the refractive index (phase hologram) or in the absorption coeffi-cient (amplitude hologram) As outlined in Section 117 interference patternsare generated upon superimposing the light beam carrying the informationwith a reference beam Read-out is achieved with the aid of the reference beamA schematic set-up for recording phase holograms on an appropriate holo-graphic plate is depicted in Fig 124

A large storage capacity corresponding to a density ranging up to 06 GB cmndash2

(ca 60 GB per 12 cm disk) is feasible if thousands of holograms are superim-


Fig 124 Schematic depiction of a set-up for recordingholograms SLM Spatial light modulator

posed in the same disk This can be achieved by means of multiplexing ie byaddressing individual high-density data pages to holographic plates by changingthe angle wavelength or phase code of the reference beam [16] Figure 125shows a set-up operating on the basis of angular multiplexing ie by varyingthe angle between the writing and reference beams

The great success of ROM disks (CD and DVD) relies on the availability of in-expensive methods to mass-produce copies of recorded disks In this context amethod to replicate holographic disks containing page-formatted data with theaid of a replicator operating with ten reference beams is noteworthy [18]

12322 Storage materialsHolographic storage materials appropriate for commercial application have tofulfil various requirements the most important of which are as follows highstorage density (gt 1 GB cmndash2) fast writing time (ms) high sensitivity (mW)long memory (years) fast access time (s) and reversibility (gt 104 cycles) forwriteerase systems [19] In this context three categories of materials have been


Fig 125 Volume holography in conjunction with angularmultiplexing Set-ups for the recording of digital data (a) andthe retrieval of stored data (b) Adapted from Sincerbox [17]with permission from Springer

found appropriate for volume holography [20] inorganic crystals [21 22] inor-ganic glasses [23ndash25] and photopolymer systems [26ndash28] (see Table 122)

Of the polymeric systems the photopolymerizable systems (commonly re-ferred to as photopolymers) show the most promise (see Section 117) At pres-ent InPhase Technologies and Aprilis Inc are reported to commercialize ROMproducts with storage densities of 12 and 19 GB cmndash2 and negligible shrinkageduring writing [20] The readwrite speed is said to be comparable to that of anoptical disk A competitor in the race to the market is Polight Technologies Ltdwho are commercializing products based on inorganic glasses In this case therecording media are rewritable since the light-induced refractive index changesare reversible However chalcogenide glasses are much less sensitive comparedto polymerizable systems because the latter exhibit an amplification mecha-nism based on a chain reaction ie each absorbed photon induces the polymer-ization of a large number of molecules On the other hand there is no shrink-age problem in the case of inorganic glasses which moreover have a muchsmaller thermal expansion coefficient than polymers The latter is of impor-tance when the temperature soars in disk drives Photorefractive crystals whichwere the subject of much attention for a while do not compete with the othermaterials with regard to the commercialization of a product This is mainly dueto the fact that the light used to read holograms also erases them Therefore ininorganic crystals holograms have to be fixed after writing by heating Anotherdrawback in this case is the low photosensitivity [20] Finally the so-calledphotoaddressable polymers PAPs were considered as potential candidates fordata storing materials For instance PAP systems consisting of linear polymersbearing pendant liquid-crystalline side chains and azobenzene chromophores(see Sections 352 and 582) seemed to be very suitable for recording volumephase holograms [21] However even under favorable illumination conditionsthe writing time of holograms was found to be of the order of 100 ms (for somesystems of the order of several seconds) This writing speed is at least one orderof magnitude too long for technical applications


Table 122 Light-sensitive materials suitable for volume holography

Polymeric systems Inorganic crystals a) Inorganic glasses

Photopolymerizable systemsPhotorefractive systemsPhotochromic systems(Photoaddressable polymers)

LiNiO3 KNiO3 LiTaO3BaTiO3 SrxBa1-xNb2O6

b)Bi12TiO20

Chalcogenide glasses contain-ing group VI elements suchas As2S3 As2Se3

a) For the recording of holograms crystals are doped with Fe Cr Cu Mg or Znb) x varying from 0 to 1

1233Photo-induced surface relief storing

A novel optical recording method based on large-scale light-driven mass trans-port in films of azobenzene polymers has been proposed As outlined in Sec-tion 56 the phenomenon of light-induced mass transport is due to the photo-isomerization of azobenzene groups It can be utilized to inscribe narrow reliefstructures in the surfaces of appropriate polymer films by using light of the re-quisite wavelength The relief structures can be erased and rewritten On thisbasis a novel technique for high-density optical data storage has been developed[29] Since data can be stored at a recording density of up to 108 B cmndash2 by com-bining angular and depth gradation this method has potential for practical ap-plication

References 345

References

1 D Day M Gu A Smallridge Review ofOptical Data Storage in P Boffi D Pic-cini M C Ubaldi (eds) Infrared Holog-raphy for Optical CommunicationsSpringer Berlin (2003) p 1

2 EA LeMaster Compact Disc Manufactur-ing httpwwweewashingtoneduconselecW94edwardedwardhtm(1994)

3 K J Kuhn Audio Compact Disk ndash An In-troduction httpwwweewashingtoneduconselecCEkuhncdaudio95x6htm (1994)

4 Blu-ray FAQ httpwwwblu-raycomfaq

5 K C Pohlmann The CD ROM Hand-book A-R Editions Madison (1992)

6 C Sherman The Compact Disc Hand-book Intertext Publications New York(1988)

7 S Watson httpelectronicshowstuffworkscombblu-ray3 htm(2004)

8 T Matsumoto Near-Field Optical HeadTechnology for High Density Near-FieldOptical Recording in M Ohtsu (ed) Pro-gress in Nano-Electro-Optics IIISpringer Series in Optical Sciences Ber-lin 96 (2005) 93

9 HJ Borg R van Woudenberg Trends inOptical Recording J Magnetism MagnetMater 193 (1999) 519

10 S Nakamura S Fasol The Blue DiodeLaser Springer Berlin (1997)

11 M Irie High-Density Optical Memory andUltrafine Photofabrication in S KawataM Ohtsu M Irie (eds) Nano-OpticsSpringer Series in Optical Sciences Ber-lin 84 (2002) 137

12 P Vettiger M Despont U Duumlrig MLantz HE Rothuizen K G BinnigAFM-Based Mass Storage ndash The MillipedeConcept in R Waser (ed) Nanoelectron-ics and Information Technology Wiley-VCH Weinheim (2005) p 685

13 V A Barachevsky Organic Storage Mediafor Holographic Optical Memory State ofthe Art and Future Optical Memory andNeural Networks 9 (2000) 251 and ProcSPIE 4149 (2000) 205


15 R M Shelby Materials for HolographicDigital Data Storage Proc SPIE 4659(2002) 344

16 G Barbastathis D Psaltis Volume Holo-graphic Multiplexing Methods in [14]p 21

17 G T Sincerbox History and PhysicalPrinciples in [14] p 3

18 F Mok G Zhou D Psaltis HolographicRead-Only Memory in [14] p 399


19 L Lucchetti F Simoni Soft Materials forOptical Data Storage Rivista del NuovoCimento 23 (2000) 1

20 N Anscombe Holographic Data StorageWhen Will it Happen Photonics Spec-tra June (2003) 54

21 M Imlau T Bieringer S G Odoulov TWoike Holographic Data Storage in RWaser (ed) Nanoelectronics and Informa-tion Technology Wiley-VCH Weinheim(2005) p 657

22 K Buse E Kraumltzig Inorganic Photorefrac-tive Materials in [14] p 113

23 V I Minko I Z Indutniy PE Shepelia-vyi PM Litvin J Optoelectron AdvMater 7 (2005) 1429

24 A Feigel Z Kotler B Sfez A Arsh MKlebanov V Lyubin Appl Phys Lett 13(2000) 3221

25 S Ramachandran SG Bishop J PGuo D J Bradley Photon TechnolLett IEEE 8 (1996) 1041


27 I Dhar MG Schnoes HE Katz AHale ML Schilling A L Harris Photo-polymers for Digital Holographic DataStorage in [14] p 199

28 S Kawata Y Kawata Three-DimensionalOptical Data Storage Using PhotochromicMaterials Chem Rev 100 (2000) 1777

29 T Fukuda Rewritable High-Density Opti-cal Recording on Azobenzene Polymer ThinFilms Opt Rev 12 (2005) 126

131General aspects

The increasing desire to detect analytes (components of mixtures of com-pounds) in situ and in real time and to monitor continuously the chemicalchanges in industrial and biological processes has given impetus to interestingdevelopments in the field of chemical sensors also referred to as chemosensors[1ndash15] Chemosensing can be accomplished by measuring a chemical or physi-cal property of either a particular analyte or of a chemical transducer interactingwith a particular analyte For practical applications the latter type of chemicalsensor is most important Prominent in this context are highly fluorescent con-jugated polymers that possess a large number of receptor sites for analytes infact one receptor site per repeating unit Non-covalent binding of an analyte re-sults in a shift of the maximum of the emission spectrum or causes quenchingor enhancement of the fluorescence intensity A somewhat different type of che-mosensor comprises molecules in some cases supramolecules that recognizeand signal the presence of analytes on the basis of a 3R scheme ndash ldquorecognize re-lay and reportrdquo which is schematically depicted in Fig 131 The sensor systemconsists of a receptor site and a reporter site which are commonly covalentlylinked A non-covalent recognition event at the receptor site is communicated tothe reporter site which produces a measurable signal Energy transfer electrontransfer a conformational change in the molecular structure or a combinationof these processes constitutes the relay mechanism Commonly chemosensor

347

13Polymeric photosensors

Fig 131 Schematic depiction of chemical sensor action Anoptical or electrical signal reports the non-covalent binding ofan analyte to the receptor site

systems operating according to the 3R scheme consist of sensor molecules orgroups that are physically admixed or covalently linked to a polymer matrix

The magnitude of the signal generated by the sensor is normally proportionalto the concentration of the analyte Regarding practical applications optical che-mosensors that monitor changes in fluorescence intensity or to a lesser extentin optical absorption are much more prevalent as compared to chemosensorsthat monitor changes in electrical conductivity or electrical current

In many cases optical chemosensor devices consist of a probe called an op-tode in which modulation of the optical signal takes place and an optical linkconnecting the probe to the instrumentation The main parts of the latter are alight source a photodetector and an electronic signal-processing unit A sche-matic depiction of a typical optode is shown in Fig 132 This optode operateswith the aid of two fluorophores that undergo a change in fluorescent lightemission in the presence of O2 or CO2 Fluorophore I is admixed and fluoro-phore II is chemically linked to the polymer

In conclusion polymers play a versatile role in the field of chemosensorsMost interestingly certain polymers can actively serve as sensors This pertainsto certain strongly fluorescent conjugated polymers as pointed out above andto polymers employed as cladding for optical fibers in evanescent wave-basedsensors Moreover polymers are widely used as supports for transducers whichare either admixed or chemically linked to the polymer matrices Typical exam-ples are given in the following sections

13 Polymeric photosensors348

Fig 132 Structure of an optode for the detection of molecularoxygen and carbon dioxide Fluorophore I (O2) tris(22-bipyridyl)ruthenium(II) dichloride fluorophore II (CO2)1-hydroxypyrene-368-trisulfonate Adapted from Baldini et al[4] with permission from Springer

132Polymers as active chemical sensors

1321Conjugated polymers

Conjugated polymers are powerful fluorescent materials which makes themsuitable for applications as chemical sensors Chart 131 presents the structuresof some typical polymers that are applicable for the detection of analytes at lowconcentrations These polymers include poly(p-phenylene ethynylene) PPEpoly(p-phenylene vinylene) PPV polyacetylene and polyfluorene Those poly-mers bearing ionizable pendant groups are water-soluble polyelectrolytes

132 Polymers as active chemical sensors 349

Chart 131 Chemical structures of typical conjugated poly-mers used as chemical sensors for organic compounds

The sensing ability of conjugated polymers relies on the fact that non-covalentbinding of extremely small amounts of analytes can quench their fluorescenceThis phenomenon referred to as superquenching is due to the pronounced delo-calization of excitons formed in conjugated polymers upon light absorptionOwing to this delocalization excitons can rapidly travel along the polymer chainto quenching sites This mode of action is referred to as fluorescence turn-off sens-ing On the other hand fluorescence turn-on sensing is observed when an analyteis capable of selectively detaching a quencher previously non-covalently linkedto the polymer Examples of both mechanisms are described in the followingsubsections

13211 Turn-off fluorescence detectionConjugated polymer based chemosensors operating in the fluorescence turn-offmode are used to quickly detect trace amounts of certain organic substances inthe gas phase or in solution This is important in areas such as forensics orthe packaging and distribution of food etc An interesting example relates tothe fast detection of 246-trinitrotoluene vapor (see Chart 132) [16] TNT ispresent in about 80 of the 120 million landmines that are buried in over 70countries [17] A TNT sensor is based on a PPE polymer functionalized withpentiptycene groups (S-2 in Chart 131) An industrially developed portable land-mine detector operating in this way is reported to detect femtogram quantitiesof TNT in one second thus performing better than a TNT sniffer dog [18]

13212 Turn-on fluorescence detectionChemical sensors based on the turn-on fluorescence mode are used to selec-tively detect certain proteins and carbohydrates [12] Moreover the activity ofprotease enzymes playing important roles in regulating biological systems suchas thrombin (blood coagulation) or caspace (apoptosis) can be detected in thisway Scheme 131 illustrates how avidin a glycoprotein of molar mass66104 g molndash1 that is present in raw egg white is detected with the aid of ananionic PPV polymer to which cationic biotin-tethered viologen is linked byelectrostatic interaction The adduct does not fluoresce Upon addition of avidinhowever the fluorescence is restored since the biotin group is bound verytightly within the active site of avidin [19]

Another example is related to enzyme activity Scheme 132 demonstrateshow turn-on fluorescence can be used to monitor protease activity [20 21]


Chart 132 Chemical structure of 246-trinitrotoluene TNT anexplosive constituent of landmines

Here a protein functionalized with a quencher Q is linked to the polymer byelectrostatic interaction so that initially fluorescence is quenched When addedprotease cleaves a specific bond in the peptide chain the quencher is releasedinto solution and fluorescence is restored


Scheme 131 Detection of avidin by turn-on fluorescenceAdapted from Chen et al [19] with permission from theNational Academy of Sciences USA

Scheme 132 Detection of protease by turn-on fluorescenceAdapted from Kumaraswani et al [21] with permission fromthe National Academy of Sciences USA

13213 ssDNA base sequence detectionConjugated polymers also permit the detection of DNA hybridization (pairingof complementary DNA single strands ssDNAs) and thus act as ssDNA sequencesensors [22] These sensors comprise an aqueous solution containing CP a cat-ionic conjugated polymer (eg S-5 in Chart 131) and ssDNA-FL a single-stranded DNA with a known base sequence and labeled with a chromophoresuch as fluorescein FL CP and ssDNA do not interact Irradiation with light ofrelatively short wavelength that is not absorbed by FL causes the fluorescence ofCP Upon addition of an ssDNA with a specific base sequence complementaryto that of the probe ssDNA-FL hybridization occurs The double-strand thusformed becomes electrostatically linked to CP thus allowing energy transferfrom electronically excited CP to FL (see Scheme 133) The characteristic fluo-rescence of the FL groups generated in this way signals hybridization The FLfluorescence is not observed upon the addition of non-complementary ssDNARelative to the CP emission the FL emission spectrum is shifted to the long-wavelength region and can therefore be reliably detected Recent research onstrand-specific DNA detection with cationic conjugated polymers has been con-cerned with their incorporation into DNA chips and microarrays [23 24]

13214 Sensors for metal ionsAs the recognition of possible effects of metal ions is of paramount importancewith regard to human health considerable effort has been directed towards thedevelopment of suitable chemosensors [14 15 25 26] Interesting work in thisfield concerns sensors based on regiospecific polythiophenes with substitutedcrown-ether macrocycles such as S-8 in Chart 131 Depending on the ring sizeof the macrocycle substituent these polymers display selectivity for specific alka-li metal cations Accommodation of ions causes a substantial blue shift of themaximum of the emission spectrum Similarly calix[4]arene-substituted poly-(phenylene bithiophene)s exhibit selectivity towards certain metal ions For ex-ample S-9 in Chart 131 selectively binds sodium ions as indicated by a blueshift of the maximum of the emission spectrum [26] Certain conjugated poly-mers bearing pendant amino groups are capable of selectively binding divalentcations such as Ca2+ Zn2+ and Hg2+ in aqueous solution [27] This applies forexample to polymer S-10 in Chart 131 which bears pendant NNN-trimethyl-ethylenediamino groups Chelation of the cation results in a pronounced in-crease in the fluorescence intensity in particular in the case of Hg2+ The aug-mented light emission may be rationalized in terms of the 3R scheme (see Sec-


Scheme 133 Energy transfer from an electronically excited conjugated polymer to fluorescein

tion 131) with photoinduced electron transfer PET as the relay mechanismRapid intramolecular electron transfer from the nonbonding electron pair at theN atom of the receptor site to the excited reporter site quenches the fluores-cence in the absence of the analyte Cation binding prevents PET

13215 Image sensorsLarge-area (1515 cm) full-color image sensors can be made on the basis ofphotoinduced charge generation in conjugated polymers (see Chapter 2) [2829] Figure 133 shows the structure of a thin-film sandwich device in the metalpolymerITO configuration

In typical work of Yu et al [29] the arrays were fabricated on ITO glass sub-strates The ITO glass layer was patterned by photolithography into perpendicu-lar rows of electrode strips (width 450 m spacing 185 m) The polymer filma blend of poly(3-octyl thiophene) and fullerene PCBM[66] (see Section 63)was spin-cast onto the substrate

Such microfabricated array devices are suitable for linear or two-dimensional(2D) digital optical cameras In principle they may also be actively used as elec-troluminescent devices

1322Optical fiber sensors

Besides acting as wave guides in sensor devices (see Fig 132) optical fibers playan important role as actively functioning sensing elements in evanescent field ab-sorption sensors In this case part of the fiber cladding is replaced by a modifiedsolvent-repellent polymer which when inserted into a solution is capable of se-lectively adsorbing specific analytes [4] The working principle of evanescent fieldabsorption sensors is based on the interaction of the analyte with the evanescentfield generated when light passes through the core of an optical fiber The lighttravels down the core as a result of numerous total internal reflections at thecorendashcladding interface Optical interference occurs between parallel wavefrontsduring the succession of skips along the core resulting in a standing wave andan electromagnetic evanescent field that penetrates the corendashcladding interfaceIn other words some of the radiation at the corendashcladding interface penetratesa certain distance into the cladding The depth of penetration dp is defined as


Fig 133 Structure of a largeimage sensor device operated witha polythiophenefullerene blendAdapted from Yu et al [29] withpermission from Wiley-VCH

the distance into the cladding over which the evanescent field is reduced to 1e ofits interface value dp can be calculated according to Eq (13-1)

dp

2n2

1 sin2 n22

13-1

where is the wavelength of light propagating down the fiber n1 and n2 arethe refractive indices of the core and the surrounding cladding respectively and is the angle of incidence at the corendashcladding interface Typical values of dp

are of the order of the light wavelength The strength of the evanescent fieldis reduced if it interacts with absorbing species The penetrating light is thenabsorbed and the intensity of the light passing through the fiber is attenuatedThis reduction in intensity can be measured and related to the chromophoreconcentration at the core interface Fiber evanescent field absorption (FEFA)spectroscopy offers advantages over conventional absorption spectroscopy usingcuvettes ie the effective absorption path length can be made very small andthe technique can be applied to strongly absorbing chromophores Moreoverdue to the low value of dp FEFA is insensitive to scattering particles thus per-mitting light absorption measurements in turbid water [30] The FEFA tech-nique is quite versatile measurements in aqueous solutions can be readily per-formed with optical fibers made of poly(methyl methacrylate) PMMA aftercomplete removal of the cladding over the length that is to be immersed in thesolution In this case the solution behaves as cladding and the evanescent fieldpenetrates into the liquid [31] The sensing sensitivity can be increased by coil-ing the fiber eg to a length of 15 m on a Teflon support of radius 15 cmCoupling of a coiled polysiloxane-cladded fiber with a near-infrared spectrome-ter operated in the 10ndash22 m range permits the recognition of organic com-pounds in mixtures such as chloroform in carbon tetrachloride or toluene in cy-clohexane [32]

1323Displacement sensors

The working principle of displacement sensors is the swelling and shrinking ofpolymer beads located at the end of polymer fibers as a function of analyteconcentration Variations in the bead volume due to changes in analyte concen-tration alter the intensity of probe light guided through the bead to a reflectorTypical optode types operating in this way are listed in Table 131 Owing to thefragility of the beads there are problems related to the reproducibility and thedurability of these sensors [4]


133Polymers as transducer supports

A large number of optodes developed for the selective detection of inorganic an-ions and cations so-called ion-selective optodes (see Table 132) consist of poly-mer membranes that contain transducers The latter are mostly physically ad-mixed but in some cases they are covalently bound to the polymer matrix Mostof these optodes [7 8] are based on poly(vinyl chloride) plasticized with DOSBBPA DOP o-NPOE or other plasticizers (see Chart 133) Typically mem-branes are composed of 33 wt PVC 66 wt plasticizer and 1 wt ionophore(analyte-complexing agent) and lipophilic salt (ion-exchanger) Other polymersoccasionally employed in hydrophobic optodes include polysiloxanes and poly(vi-

132 Polymers as transducer supports 355

Table 131 Displacement sensor systems based on reversible swelling

Analyte Polymer Refs

Protons in water (pH) Polystyrene bearing amino groups [33]Ions (ionic strength) Sulfonated polystyrene sulfonated dextran [34]Water in organic liquids Polystyrene bearing quaternary ammonium

groups[35]

Hydrocarbons in water Poly(methyl trifluoropropyl siloxane)poly(dimethyl siloxane)poly(styrene-co-butyl methacrylate)

[36 37]

Chart 133 Plasticizers used in PVC-based optodesDOS dioctyl sebacate BBPA bis(1-butylpentyl) adipateDOP dioctyl phthalate o-NPOE o-nitrophenyl octyl ether

Table 132 Typical optode-detectable analytes [7]

Analyte class Analytes

Inorganic cations H+ Li+ Na+ K+ Mg2+ Ca2+ Ag+ Zn2+ Hg2+ Pb2+ NH4

Inorganic anions CO32ndash SCNndash NO2

ndash Clndash Indash

Organic cations Ammonium ions of 1-phenylethylamine octylamineOrganic anions Salicylate guanosine triphosphate heparinNeutral analytes H2O NH3 SO2 O2 ethanol

nylidene chloride) Polyacrylamide or other hydrogel-forming polymers are usedin the case of hydrophilic membrane-based optodes

Many of the optodes referred to here employ sensors operating on the basis ofthe 3R scheme (see Section 131) the relay mechanism being photoinduced elec-tron transfer PET Due to their applicability in various chemical and biologicalprocesses they have received much attention in recent years [1 7 8 10] Of notein this context are sensors that become fluorescent upon complexation of an ana-lyte because the binding of the analyte within the sensor prevents the PET thatsuppresses fluoresence in the absence of the analyte [38] Anthryl aza-crown-ca-lix[4]arene a K+-selective sensor (see Chart 134) exhibits such behavior It selec-tively binds potassium ions and this triggers a substantial increase in anthryl flu-orescence through disruption of the PET quenching process [9 39]


Chart 134 Chemical structure of N-(9-methyl-anthracene)-2527-bis(1-propyloxy) calix[4]arene azacrown-5 used as aselective potassium ion sensor [39]

References

1 V Ramamurthy K S Schanze (eds) Op-tical Sensors and Switches Marcel DekkerNew York (2001)

2 Y Osada DE Rossi (eds) Polymer Sen-sors and Actuators Macromolecular Sys-tems ndash Material Approach Springer Ber-lin (2000)

3 J Wackerly Conjugated Polymers as Fluo-rescence-Based Chemical Sensorswwwscsuiuceduchemgradprogramchem435fall0406_Wackerly_Abstractpdt

4 F Baldini S Bracci Polymers for OpticalFiber Sensors Chapter 3 of [2] p 91

5 BR Eggins Chemical Sensors and Bio-sensors Wiley Chichester (2002)

6 A Mulchandani OA Sadik (eds)Chemical and Biological Sensors for Envi-ronmental Monitoring ACS Symposium

Series 762 American Chemical SocietyWashington DC (2000)

7 E Bakker P Buumlhlmann E Pretsch Car-rier-Based Ion-Selective Electrodes and BulkOptodes 1 General Characteristics ChemRev 97 (1997) 3083

8 P Buumlhlmann E Pretsch E Bakker Car-rier-Based Ion-Selective Electrodes and BulkOptodes 2 Ionophores for Potentiometricand Optical Sensors Chem Rev 98(1998) 1593

9 J B Benco HA Nienaber WGMcGimpsey Optical Sensors for BloodAnalytes The Spectrum 14 (2002) 1

10 A P de Silva HQN Gunaratne TGunnlaugsson A J M Huxley CPMcCoy JT Rademacher T E Rice Sig-naling Recognition Events with FluorescentSensors and Switches Chem Rev 97(1997) 1515

References 357

11 CM Rudzinski DG Nocera Buckets ofLight Chapter 1 of [1]

12 D Whitten R Jones T Bergstedt DMcBranch L Chen P Heeger FromSuperquenching to Biodetection BuildingSensors Based on Fluorescent Polyelectro-lytes Chapter 4 of [1]

13 T Ishii M Kaneko PhotoluminescentPolymers for Chemical Sensors in R Ar-shady (ed) Desk Reference of FunctionalPolymers Syntheses and ApplicationsAmerican Chemical Society WashingtonDC (1997) Chapter 43

14 L Dai P Soundarrajan T Kim Sensorsand Sensor Arrays Based on ConjugatedPolymers and Carbon Nanotubes PureAppl Chem 74 (2002) 1753

15 TM Swager The Molecular WireApproach to Sensory Signal AmplificationAcc Chem Res 31 (1998) 201

16 J-S Yang TM Swager J Am ChemSoc 120 (1998) 5321 and 11864

17 J Yinon Anal Chem (2003) 99A18 M La Grone C Cumming M Fisher

M Fox S Jacob D Reust M RockleyE Towers Proc SPIE 4038 (2000) 553

19 L Chen DW McBranch H-L Helge-son R Wudl D Whitten Proc NatlAcad SciUSA 96 (1999) 12287

20 MR Pinto K S Schanze Proc NatlAcad SciUSA 101 (2004) 7505

21 S Kumaraswany T Bergstedt X Shi FRininsland S Kushon W Xia K LeyK Achyuthan DW McBranch D Whit-ten Proc Natl Acad SciUSA 101(2004) 7511

22 BS Gaylord A J Heeger G C BazanJ Am Chem Soc 125 (2003) 896

23 B Liu G C Bazan Proc Natl AcadSciUSA 102 (2005) 589

24 H Xu H Wu F Huang S Song WLi Y Cao C Fan Nucl Acid Res 33(2005) e83

25 J Li Y Lu J Am Chem Soc 122(2000) 10466

26 K B Crawford MB Goldfinger TMSwager J Am Chem Soc 120 (1998)5178

27 L-J Fan Y Zhang WE Jones Jr Mac-romolecules 38 (2005) 2844

28 D Pede E Smela T Johansson M Jo-hansson O Inganaumls Adv Mater 10(1998) 233

29 G Yu J Wang J McElvain A J HeegerAdv Mater 10 (1998) 1431

30 DW Lamb Y Bunganaen J LouisG A Woolsey R Oliver G White Mar-ine and Freshwater Research 55 (2004)533

31 PG Leye M Boerkamp A ErnestDW Lamb J Phys Conf Series 15(2005) 262

32 MD Degrandpre LW Burgess ApplSpectrosc 44 (1990) 273

33 Z Shakhsher R W Seitz Anal Chem66 (1994) 1731

34 MF McCurley R W Seitz Anal ChimActa 249 (1991) 373

35 M Bai R W Seitz Talanta 41 (1994)993

36 G Kraus A Brecht V Vasic G Gaug-litz Fresen J Anal Chem 348 (1994)598

37 G Gauglitz A Brecht G Kraus WNahm Sensor Actuat B 11 (1993) 21

38 HF Ji R Dabestani G M Brown JAm Chem Soc 122 (2000) 9306

39 J B Benco HA Nienaber K DennenW G McGimpsey J Photochem Photo-biol A Chem 152 (2002) 33

141General aspects

Photocatalysts are substances that initiate chemical reactions under the influ-ence of light without being consumed during the process Although the field ofphotocatalysts is largely dominated by inorganic substances such as titanium di-oxide [1-4] polymers also have roles to play in particular as catalyst-supportingmaterials However there are also some interesting developments concerningspecial polymers that function as active photocatalysts These developments per-tain not only to certain conjugated polymers but also to polymers bearing pen-dant aromatic groups In general a photocatalytic process commences with theabsorption of photons by the catalyst Subsequent chemical alterations in thesurrounding substrate molecules are the result of interactions with relativelylong-lived excited states or electrically charged species formed in the catalystTypical polymeric photocatalysts and mechanistic aspects are presented in thefollowing subsections

142Polymers as active photocatalysts


It has been shown in Chapters 2 and 6 that conjugated polymers are quite ver-satile with regard to practical applications For example they play an outstand-ing role in the fields of organic light-emitting diodes and photovoltaic devices(see Sections 62 and 63 respectively) Here their photocatalytic capability ishighlighted by referring to the fixation of carbon dioxide CO2 a process ofquite general importance since methods of fixation of carbon dioxide are

359

14Polymeric photocatalysts

Chart 141 Chemical structure of PPP

needed to prevent the uncontrolled release of this greenhouse gas into the at-mosphere [5] The process reported here operates with a solution of benzophe-none and triethylamine TEA in dimethylformamide containing dispersedpoly(p-phenylene) PPP the structure of which is shown in Chart 141 Upon ex-posure to visible light (gt 400 nm) PPP catalyzes the photoreduction of benzo-phenone yielding benzhydrol and benzopinacol (Scheme 141a) If the system issaturated with CO2 diphenylglycolic acid is formed ie CO2 is fixed (Sche-me 141 b)

The somewhat simplified reaction mechanism shown in Scheme 142 is basedon the photogeneration of electronhole pairs in PPP While the holes reactwith triethylamine present in the system the electrons remain in the polymeras delocalized anion radicals They react with benzophenone to form the diphe-nylcarbinol anion and the latter eventually reacts with CO2 The CO2 fixation isstrongly enhanced by the presence of tetraethylammonium chloride The softonium cations are thought to stabilize the diphenylcarbinol anion the precursorof the final product

14 Polymeric photocatalysts360

Scheme 141 PPP-catalyzed photoreactions of benzophenonein the absence (a) and in the presence of CO2 [5]

Scheme 142 Simplified reaction mechanism of the PPP-cata-lyzed photofixation of CO2 in benzophenone [5]

1422Linear polymers bearing pendant aromatic groups

This type of reaction has been pioneered by Guillet et al using poly(sodium sty-rene sulfonate-co-2-vinylnaphthalene) a copolymer consisting in this case ofabout equal parts of the respective monomers (see Chart 142) [6]

In aqueous solution this copolymer adopts a pseudo-micellar conformationie the macromolecules form hydrophobic microdomains capable of solubilizingorganic compounds that are sparingly soluble in water Table 141 presents typi-cal systems explored in this work

The reaction mechanism depends on the system and may be based on energyor electron transfer between the naphthalene moieties of the copolymer and thesubstrate molecule In the case of oxidations singlet oxygen generated by en-ergy transfer from the naphthalene moiety to 3O2 may be involved Typical re-action mechanisms are presented in Schemes 143 and 144

142 Polymers as active photocatalysts 361

Table 141 Reactions photocatalyzed by poly(sodium styrene sulfonate-co-2-vinyl-naphthalene) in aqueous solution under solar irradiation [6]

Process Products Reaction mechanism

Oxidation of cyanide CNndash NCOndash Electron transfer

Oxidation of styrene Singlet oxygen reaction

Photodechlorination of hexachlorobiphenyl Electron transfer

Photosynthesis of previtamin D3 Isomerization of 7-dehydrocholesterol

Chart 142 Chemical structures of the base unitsof poly(sodium styrene sulfonate-co-2-vinylnaphthalene)

143Polymers as supports for inorganic photocatalysts

Certain inorganic materials can be employed as photocatalysts for the synthesisor degradation of compounds in heterogeneous systems Relevant devices con-tain for example films incorporating immobilized photocatalyst particles Typi-cally titania TiO2 is used for the treatment of water contaminated with chemi-cal pollutants andor bacteria [9] The contaminants are oxidized by reactive spe-cies ie hydroxyl and superoxide radicals generated by reaction of electronholepairs with O2 and water adsorbed at the particle surface Electronhole pairs areformed when UV light (lt 400 nm) is absorbed by titania (see Scheme 145)

Titania is especially suitable as a photocatalyst because it is highly catalyti-cally active yet chemically and biologically inert photostable and cheap Thephotocatalytic efficiency of inorganic particles depends strongly on their specificsurface area and their accessibility since only substrate molecules in close con-tact with the particle surface can undergo chemical alterations Both require-ments ie large surface area and accessibility can be very well fulfilled by usingnanoparticles embedded in polymer films of high porosity as has been demon-strated in the case of titania [10 11] For example photocatalytic porous filmscontaining nanocrystalline anatase the active TiO2 modification have been pre-pared on polycarbonate and poly(methyl methacrylate) substrates [10] In an-


Scheme 143 Singlet oxygen-mediated oxidation of styrenephotocatalyzed by poly(sodium styrene sulfonate-co-2-vinyl-naphthalene) N denotes the naphthalene moiety of thecopolymer and Ph the phenyl group of styrene [7]

Scheme 144 Oxidation of cyanide ions photocatalyzed bypoly(sodium styrene sulfonate-co-2-vinylnaphthalene) Ndenotes the naphthalene moiety contained in the copolymeras a pendant group [8]

other case photocatalytic films consisting of layers of cationic poly(allylaminehydrochloride) anionic poly(acrylic acid) (see Chart 143) and positively chargedTiO2 nanoparticles were fabricated in a layer-by-layer self-assembling method[11] Besides the fact that polymer films are flexible the advantages of usingpolymer-supported catalysts for the synthesis or degradation of compounds in-clude reagent stability suitability for automation ease of work and reducedcontamination in the final product

The performance of polymer-coated TiO2 particles in an aqueous environmentis also noteworthy The presence of Nafion adlayers (see Chart 143) ensuresthat the surface charge on the TiO2 particles is highly negative over the entirepH range As a consequence the photocatalytic degradation PCD of cationicsubstrates is enhanced while that of anionic or neutral substrates is not signifi-cantly retarded [12] In contrast the efficiency and rate of PCD are much morepH-dependent in the case of naked TiO2 particles which are positively chargedat low pH and negatively charged at high pH due to the presence of TiOH2

+

and TiOndash groups respectivelyFrom a survey of the patent literature it is inferred that industrial research

and development is focused to a significant extent on polymer-supported photo-catalysts While most of the numerous patents deal with titania a few are de-voted to other materials such as ruthenium complexes or iridium oxide Novelapplications concerning the deodorization of air in automobiles with the aid ofpolytetrafluoroethylene-supported photocatalysts are noteworthy [13 14]

143 Polymers as supports for inorganic photocatalysts 363

Chart 143 Polymers employed as supports for inorganic photocatalysts

Scheme 145 Photogeneration ofoxidizing species upon irradiation oftitania with UV light


References

1 J M Herrmann Catalysis Today 53(1999) 115

2 MR Hoffmann S T Martin W ChoiDW Bahnemann Chem Rev 95 (1995)69

3 DF Olis H Al-Ekabi (eds) Photo-catalytic Purification and Treatment ofWater and Air Elsevier Amsterdam(1993)

4 N Serpone E Pelizetti (eds) Photocata-lysis Fundamentals and Applications Wi-ley New York (1989)

5 Y Wada T Ogata K Hiranaga H Yasu-da T Kitamura K Murakoshi S Yana-gida J Chem Soc Perkin Trans 2(1998) 1999

6 J E Guillet Biomimetic Polymer Catalystsfor Important Photochemical ReactionsCan Chem News 52 (2000) 16

7 M Nowakowska J E Guillet Macromol-ecules 24 (1991) 474

8 M Nowakowska NA D Burke J EGuillet Chemosphere 39 (1999) 2249

9 J MC Robertson PK J RobertsonLA Lawton J Photochem PhotobiolA Chem 175 (2005) 51

10 M Langlet A Kim M Audier J MHerrmann J Sol-Gel Sci Tech 25(2002) 223

11 T-H Kim B-H Sohn Appl Surf Sci201 (2002) 109

12 H Park W Choi J Phys Chem B109(2005) 11667

13 K Yamamoto K Sakaguchi J AsanoPatent JP 2000300984 (2001)

14 T Hiyori T Domoto Patent JP2000296168 (2001)

aabsorbance (extinction optical density) 7absorption of light 5 14ndash photoinduced absorption 41ndash T-T absorption 41acetophenonesndash type I free radical photoinitiators 278acrylate- and methacrylate-based monomersndash volume holography 324acrylonitrilebutadienestyrene (ABS) co-

polymerndash photodegradation 199O-acyl--oximo ketonesndash type I free radical photoinitiators 278acylphosphine oxidesndash type I free radical photoinitiators 278acylphosphonatesndash type I free radical photoinitiators 278N-alkoxy pyridinium and isoquinolinium

saltsndash cationic photoinitiators 290Alzheimerrsquos disease 224amine-catalyzed cross-linkingndash photo-triggered curing 298ndash polyurethane-based coatings 298amines 315ndash curing of dental formulations 315amino ethersndash reaction with alkyl peroxyl or acyl peroxyl

radicals 264amplified spontaneous emission 44angular multiplexingndash volume holography 345anionic polymerizationndash photo-production of reactive organic

basesndash ndash amidine bases 297ndash ndash tertiary amines 297ndash photo-release of reactive anions 296anisotropic contraction 131

anisotropyndash generation by trans-cis-trans isomeriza-

tion 124antenna effect 17anthraquinonesndash type II free radical photoinitiators 280anthryl aza-crown-calix[4]arenendash potassium ion sensor 357antioxidantsndash radical scavengers 257apoptosis 223ndash turn-on fluorescence detection 352applications of NLO polymersndash optical limiters 100ndash phase conjugation 100ndash transphasor the optical transistor 100aromatic amino acidsndash phenylalanine (Phe) 209ndash tryptophan (Trp) 209ndash tyrosine (Tyr) 209aromatic ketonesndash water-soluble 280aspect ratio 236atomic force microscopy (AFM)ndash detection of surface gratings 133autoacceleration 199automotive accessories 310ndash photocured coatings 310automotive applicationsndash polymer optical fibers 169autooxidationndash polymers 199 200autoretardation 199avidinndash turn-on fluorescence detection 353azobenzene compoundsndash isomerization quantum yields 125azobenzene groups 115ndash in polyamides 117

365

Subject Index

ndash ndash conformational change 119ndash in polyimides 125 135ndash in polymer films 123ndash in polymers 116ndash in polypeptides 119azobenzene-modified polymersndash surface gratings 133

bBDndash blu-ray disk 339benzoylferrocenendash anionic photoinitiators 296benzyl ketalsndash type I free radical photoinitiators 278benzoin and benzoin ethersndash type I free radical photoinitiators 278benzophenone derivativesndash type II free radical photoinitiators

280benzotriazolesndash UV absorbers 258bioluminescence 207biopolymer structures 208bipolarons 55birefringence 73 124ndash light-induced 123birefringent modulator 96 97bisazides 189ndash poly(cis-isoprene) 188ndash photo-cross-linking of linear poly-

mers 188bisphenol A polycarbonate 68blepharisminsndash photosensors 211block copolymers 326ndash formation 327ndash structures 327blood coagulationndash turn-on fluorescence detection 352blue diode lasers 342blue-ray disksndash storage capacity 342blu-ray disk 339bond cleavage 177bond dissociation energies 177bovine serum albuminndash optical absorption 209Bragg condition 160Bragg reflector 160Bragg wavelength filters 96Broslashnsted (protonic) acidsndash photogeneration 240

business stationaryndash polymer printing plates 319

cCADndash computer-aided design 315cadmium sulfide CdSndash inorganic photoinitiators 286CaF2

ndash lens material at 157 nm 246calf thymus DNAndash optical absorption 209CAMndash computer-aided manufacturing 315cancer 211ndash photochemotherapy 223cansndash aluminum beer and beverage cansndash ndash photocured coatings 311ndash food cansndash ndash photocured coatings 311carbamate containing acrylatesndash photocured coatings 310carbohydratesndash turn-on fluorescence detection 352carbonyl groupsndash photoreactions 182carotenoidsndash photoreceptors 209 210cationic polymerization 288ndash chemical structures of monomers 289CDndash compact disk 339CD-ROM 340cellulose 208ndash photoreactions 221chain breakersndash radical scavengers 257chain polymerization 275chain reactionsndash dehydrochlorination of PVC 197ndash photo-oxidation of polymers 201ndash polymerization 275ndash ndash of diacetylenes 300ndash topochemical 300chain terminators 262ndash radical scavengers 257chalcogenide glasses 346charge carriersndash bipolarons 54ndash dissociation of excitons 56ndash drift mobility 60ndash generation 55

Subject Index366

ndash polarons 54 55ndash quantum yields 57 58ndash radical cations 55ndash transport 60ndash transport in amorphous polymers 64ndash ndash disorder concept 64ndash ndash hopping mechanism 64charge-coupled device (CCD) 41charge generation layersndash xerography 145charge hopping 52charge-transfer molecules 88charge-transport layersndash xerography 146chemical amplification resists 239chemical sensor actionndash schematic depiction 349chemosensing 349chiralityndash enantioselective induction 32chiral molecules 23chlorophylls 217ndash photoreceptors 211chromophoresndash electro-optically active 98chromophoric groups 6 177circadian rhythm 217circular birefringence 24circular dichroism 24 25ndash circular dichroism spectroscopy 25circular dichroism (CD) spectrandash polypeptide structures 120circular dichroism spectroscopyndash characterization of the chirality 32ndash nucleic acids 32ndash polypeptides 32ndash proteins 32ndash spectra of PMBET 34ndash spectra of polyisocyanate PICS 34cis-trans isomerization 54Claisen rearrangement 242claddings of optical fibersndash polymers 170clear coatings for paper 309cleavage of chemical bondsndash polystyrene 178ndash poly(methyl methacrylate) 178CO2 fixation 362coatingsndash radiation-curedndash ndash commercial applications 310coil helix transitionndash in poly(L-glutamic acids) 119

co-initiatorsndash type II free radical photoinitiators 280collagen 214ndash thermal denaturation 31color hologramsndash holography 323color mixingndash holography 323command surfaces 127compact disk 339ndash manufacture 341ndash storage capacity 340computer-aided design CAD 315computer-aided manufacturing CAM 315computer-assisted design CADndash photoinitiators for visible light 281computer chip fabrication 236conjugated polymers 156ndash absorption spectra 12ndash chemical sensors 351ndash chemosensors 349ndash exciton model 12ndash laser materials 157ndash photocatalysts 361constructionndash photocured coatings 310consumer goodsndash photocured coatings 310contact lenses 310contact printingndash photolithography 232copper wire cables 168copying machinesndash xerography 143cornea reprofiling and sculpting 254Cotton effect 24cross-linkingndash [2+2] cycloadditionndash ndash poly(vinyl cinnamate) 185ndash cleavage of phenolic OH groups 192ndash cycloaddition of C=C bondsndash ndash poly(vinyl cinnamate) 184ndash intermolecular cross-links 183ndash mechanism 183ndash photoacid-catalzyedndash ndash epoxide groups 242ndash photogenerated reactive species 188ndash photopolymerization 186ndash polymerization of reactive moieties in

pendant groups 186ndash quantum yields 194ndash thick polymer films 184ndash triplet nitrene 190

Subject Index 367

cryptochromesndash photoreceptors 209crystal violet leuconitrile (CVCN)ndash anionic photoinitiators 296curing 307ndash cationic curing 311ndash dual curing 312ndash free radical curing 309ndash of inks 320cyanide ionsndash photocatalyzed oxidation 364[2+2] cycloaddition 185ndash DNA dimeric photoproducts 212cycloaliphatic structuresndash in random copolymers 244cystine bridgesndash rupture 216cytochromes 217cytoskeleton 223

d3D color imagesndash holography 325data transfer rate 339degenerate four-wave mixing (DFWM) 86dendritic polymers 19 fdental formulationsndash curing 315ndash photocurable formulations 314ndash polymerizable compounds 316dental preventive and restorative systemsndash photocuring 314deodorization of air 365deoxyribonucleic acid (DNA) 208ndash photoreactions 211ndash thermal denaturation 31depletionndash of stabilizers 267desktop printingndash xerography 143detrimental degradationndash of unstabilized commercial polymeric

products 182Dexter mechanism 15diacetylenes 299ndash bolaamphiphilic diacetylenes 300ndash polymerization 300ndash topochemical photopolymerization

300dialkenesndash stepwise [2+2] photocyclopolymeriza-

tion 302diarylethenes 114

diazonium saltsndash cationic photoinitiators 290dibenzoylferrocenendash anionic photoinitiators 296dicarbenesndash diacetylene polymerization 301dichromated gelatine 324digital optical camerasndash image sensors 355digital versatile disks 339ndash storage capacity 340diglycol diallylcarbonate resinndash POFs 169diketonesndash curing of dental formulations 31512-diketones (benzils and camphorquinone)ndash type II free radical photoinitiators 280diphenyliodonium saltsndash photolysis 241dipole moment 6ndash aligning of permanent dipole mo-

ments 78ndash electric field dependence 74ndash hyperpolarizabilities and 74ndash linear polarizability 74diradicalsndash diacetylene polymerization 301displacement sensors 357ndash swelling and shrinking of polymer

beads 356displays for cell phonesndash holography 325dissolution inhibitor 236distributed Bragg reflector DBR 159distributed Bragg reflector device 16125-distyrylpyrazinendash four-center-type photopolymeriza-

tion 301ndash four-center-type polymerization 303disulfide bridgesndash proteins 215DNA 207 209ndash dimeric photoproducts 212ndash photodimers 213ndash repair of dimer lesions 213ndash sequence-selective photocleavage 226ndash strand cleavage 226DNA lesions 212DNA photolyases 219DNA strandsndash sequence-specific cleavage 227dopantsndash dinitrobenzene 69

Subject Index368

ndash fullerene C60 69ndash in photoconducting polymeric sys-

tems 50ndash isopropylcarbazole (ICP) 67ndash phenylcarbazole (PhC) 67ndash tetracyanoquinone (TCNQ) 69ndash trinitrofluorenone (TNF) 68ndash triphenylamine (TPA) 67doped polymers 49ndash dopants 67ndash hole mobility 67ndash photoconductivity 66 68ndash quantum yields of charge carrier genera-

tion 67ndash temperature dependence of the hole mo-

bility 68DRAMndash dynamic random access memory 234dual-cure acrylic urethane system 313dual curingndash coatings protecting three-dimensional ob-

jects 312ndash method combining UV irradiation and

thermal treatment 312ndash oligomers bearing acrylate and isocyanate

groups 312dual-layer photoreceptors 145ndash charge generation layer 144ndash charge transport layer 144dual-layer systemsndash xerography 143DVDndash digital versatile disk 339Dycrylndash letterpress plates 318dyeco-initiator systemsndash photoinitiators for visible light 281dye-sensitized free radical polymerizationndash co-initiators 286dynamic random access memory

(DRAM) 234

eEFISH method 79ndash electric field-induced second harmonic

generation 79elastin 214electrical-to-optical signal transducers

96electroluminescencendash polymer-based 148ndash quantum yields 152electroluminescence spectra

ndash oriented substituted poly(p-pheny-lene) 155

electronhole pairs 53ndash dissociation 55ndash organic solarcells 165ndash PPP 362electronicsndash photocured coatings 310electron-spin resonance (ESR) 54electron transition 9electro-optic (EO) phenomena 73 ffelectrophotography ndash xerographyndash photoreceptors 143ellipticityndash mean residue weight ellipticity 25ndash molar ellipticity 25enantiomers 23energy migration 16 17energy quenchers 257ndash light stabilizers 260energy quenching 177energy transfer 14 17 38ndash Dexter mechanism 15ndash Foumlrster mechanism 15ndash long-range interaction 15ndash short-range interaction 15enzymesndash inactivation 215EO (electro-optic) materials 73EO modulators 73epoxide monomersndash volume holography 324epoxidepolyol formulationsndash photocured coatings 312epoxidesndash photo-cross-linkingndash ndash stereolithography 186ndash ndash surface coating 186ndash ndash volume holography 186ESCAPndash Environmentally Stable Chemical Ampli-

fication Positive Photoresist 241Escherichia colindash resurrection of UV-killed 219ESIPT 260 268ndash excited-state intramolecular proton trans-

fer 259ethylene propylene diene copolymers

(EPDM elastomers)ndash photo-cross-linking 191EUVndash extreme ultraviolet radiation

( = 13 nm) 234

Subject Index 369

evanescent field absorption sensorsndash optical fiber sensors 355excimer emission 17excimers 16excited molecules 10ndash annihilation 16ndash deactivation by chemical reactions 21ndash excimers 16ndash intermolecular deactivation 14ndash intramolecular deactivation 13exciton concept 52exciton model 12excitons 52 152ndash CT excitons 53ndash dissociation 56ndash emission 56ndash Frenkel excitons 53ndash organic solar cells 165ndash Wannier excitons 53exposure characteristic curves 238extinction coefficient 7 9 11

ffatigue resistancendash photochromic systemsndash ndash diarylethenes 137ndash ndash fulgides 137femtosecond spectroscopy 43Fermi level 51ferrocenium saltsndash cationic photoinitiators 290ndash photoinitiators 283fiber evanescent field absorption (FEFA)

spectroscopy 356fiber-optic sensors 169fiber-optic systemsndash high-bandwidth 168fiber-to-the-home systems 169fibroin (silk) 214filmsndash Langmuir-Blodgett (LB) film 22flash photolysis 39flavin adenine dinucleotide FAD 219flavinsndash photoreceptors 210flexographic printing 320fluorescence 10 13 14ndash depolarization 28 29fluorescence turn-off sensing 352fluorescence turn-on sensing 352fluorine-containing polymersndash F2 (157 nm) lithography 245Foumlrster mechanism 15

four-center-type photopolymerization 301four-center-type polymerization 303Fourier-transform infrared (FTIR) spec-

tra 36Fowler-Nordheim (FN) tunnellingndash OLEDs 151Franck-Condon factor 6free radical polymerizationndash two-photon absorption 99free-radical-promoted cationic polymeriza-

tion 293free radicals 178ndash generation 182Frenkel excitons 53ndash in polysilanes 53fulgides 114fulgimides 114fullerene derivativesndash organic solar cells 166furniturendash photocured coatings 310

ggaskets 310gel dose Dgel

ndash cross-linking 195geminate electronhole pairs 57generation of light 146glow discharge 70graded-index polymer optical fibers 170graft copolymers 326 327

hhalogenated compoundsndash type I free radical photoinitiators 278HALSsndash hindered amine light stabilizers 262HASsndash chemical structures 263ndash hindered amine stabilizers 262ndash oxidation of 264HD-DVD 342ndash high definition DVD 339hemispherical lens 342heterolytic bond cleavage 113hole mobility 62 63ndash electric field dependence 66ndash temperature dependence 66hologramsndash electrically switchable 325ndash reconstruction of the image 322ndash recording 321ndash set-up for recording 344

Subject Index370

hologram formationndash mechanism 323holographic disksndash replication 345holographic imagingndash photorefractive materials 111ndash time-gated holographic imaging 111holographic materials 324holographic plate 344holographic storage materialsndash volume holography 345holographic three-layer platendash color holographic recording 324holographyndash applications 325ndash volume phase hologramsndash ndash photopolymerization 321hostguest systems 156HRS (hyper-Rayleigh scattering) meth-

od 79hybrid curingndash dual curingndash ndash simultaneous free radical and cationic

cross-linking polymerization 313hydrogen abstraction 182hydrogen bondsndash destruction 31hydroperoxide decomposersndash alkyl and aryl phosphites 265ndash chemical structures 265ndash dialkyl dithiocarbamates 265ndash dithioalkyl propionates 265ndash dithiophosphates 265hydroperoxide groups 200ndash generation of hydroxyl radicals 221hydroxyalkylphenonesndash type I free radical photoinitiators 278o-hydroxybenzophenonesndash UV absorbers 258hydroxyl radicals 180hyperchromicity 31hyperpolarizability 74 77ndash electric field-induced second harmonic

generation EFISH 79ndash hyper-Rayleigh scattering HRS 79hyper-Rayleigh scattering HRS 80hypochromicity 31

iimage sensorsndash full-color sensors 355immunoglobulinsndash segmental motions 29

impurity chromophoresndash carbonyl groups 180ndash charge-transfer complexes 181ndash commercial polymer formulations 180ndash conjugated double bonds 181ndash double bonds 181ndash hydroperoxide groups 180ndash in commercial polyalkenes and poly(vinyl

chloride)s 181ndash metal ions 181ndash polynuclear aromatics 181index of refraction 74ndash electric field dependence 78influenza virusndash colorimetric detectionndash ndash polydiacetylene 300information density 231information storagendash holography 326infrared (IR) spectroscopyndash analysis and identification of poly-

mers 35inifertersndash initiator-transfer-agent-terminators 328initiation techniquesndash electrochemical initiation 275ndash high-energy radiation initiation 275ndash photoinitiation 275ndash thermochemical initiation 275injection of charges 150inorganic particlesndash surface grafting 331inorganic photocatalysts 364intermolecular cross-linking 178interpenetrating networksndash hybrid curing 313intraocular lens implants 310iodonium saltsndash cationic photoinitiators 290ndash photolysis 291IPNndash interpenetrating networks 313IPN polymers 314IR spectra of polymers 36IR spectrometers 36

jJablonski diagram 10

kkeratin (wool) 214Kerr effect 73-keto coumarins

Subject Index 371

ndash type II free radical photoinitiators 280Kleinman symmetry 77

l-lactalbumin 215Lambert-Beer law 7Langmuir-Blodgett (LB) film 22laser ablation 248ndash dopant-enhanced 250ndash generation of periodic nanostructures in

polymer surfaces 256ndash keratectomy 253ndash molecular mechanism 250ndash multi-photon absorption 250ndash plasma thrusters 256ndash plume 250ndash polymers designed for 251ndash synthesis of organic compounds 252laser direct imaging LDIndash photoinitiators for visible light 281lasers 156lasing mechanismndash Boltzmann equilibrium of states 158ndash population inversion 158ndash stimulated emission 158lasing threshold 159LDMSndash laser ablation 254ndash laser desorption mass spectrometry 254LED (light emitting diode)ndash multilayer polymer LED 149ndash single-layer polymer LED 149letterpress platesndash structure 319letterpress printing plates 318light attenuationndash in POFs 169light-driven mass transport 347light-emitting diodes LEDs 147light-harvestingndash in multiporphyrin arrays 21light sourcesndash extreme ultraviolet (EUV) sources 234ndash Hg discharge lamps 234ndash lasers 234light stabilizersndash bifunctional and trifunctional stabili-

zers 266ndash energy quenchers 257ndash radical scavengers 257ndash UV absorbers 257ligninsndash formation of quinoid structures 222

ndash optical absorption spectra 208ndash phenoxyl radicals 222ndash photoreactions 221ndash wood 207linear electro-optic effect (Pockels ef-

fect) 78linear polarizability 74liquid-crystal displays (LCDs)ndash polarized backlights 38liquid-crystalline copolymersndash forgery-proof storage systems 139liquid-crystalline polymersndash alignment 127ndash amplified photoalignment 126ndash birefringence 125ndash command surfaces 127ndash image storage 127ndash optical dichroism 125ndash photochromic amplification effect 127ndash trans-cis-trans isomerization of azoben-

zene groups 126liquid immersion lithographyndash photolithography 234lithium niobate 99lithographic process 232lithographyndash imprinting lithography 235ndash maskless lithography 235ndash photolithography 231local area networks (LANs)ndash polymer optical fibers 169luminancendashvoltage characteristicndash polymer LED 150luminescence 37ndash excimer emission 16ndash fluorescence 13ndash monomer emission 16ndash phosphorescence 13luminophores 28lysozymendash thermal denaturation 30

mMach-Zehnder (MZ) interferometer 96 97macroinitiators 327macromolecular photoinitiators 279 282macromoleculesndash photochromic transformationsndash ndash aggregation 117ndash ndash coil contraction 117ndash ndash coil expansion and contraction 116ndash ndash precipitation 117macroradicals 190 199

Subject Index372

ndash generation 191magneto-optical disk 340magneto-optical (MO) recording 342main-chain cleavagendash quantum yields 194main-chain scission 178malachite green leucohydroxide (MGOH)ndash anionic photoinitiators 296MALDIndash laser ablation 254ndash matrix-assisted laser desorptionioniza-

tion 254MALDI mass spectra 255maskndash photolithography 232maskless lithographyndash ion-beam lithography 235mass transportndash light-induced 132mechanical energyndash by light energy conversion 130mechanical machiningndash laser ablation 248media-oriented system transport (MOST)

devicesndash polymer optical fibers 169membranesndash photochromic transformationsndash ndash control of physical properties 122ndash photoresponsive behaviour 122metal-based photoinitiators 283metal ionsndash detection by chemosensors 354microcavityndash vertical cavity lasing device 160microfabrication 246 309 315microlithography 231micromachining 248 315ndash photomicrolithography 247microring laser 161microstructuresndash high aspect ratio 247mirrorndash conventional mirror 86ndash phase conjugate mirror 86molecular orbitals 7 8 9molecular wires 63monomer emission 17monomersndash surface grafting 3314-morpholinophenyl amino ketonendash two-photon absorptionndash ndash photolysis 318

multicolour holographic recordingndash holography 323multiplexingndash holography 322ndash volume holography 345

nnafionndash polymer support for inorganic photocata-

lysts 365nanofabrication 246naphthodianthronesndash photosensors 211near-field optical recordingndash recording density 343newspapersndash polymer printing plates 319nickel chelatesndash light stabilizers 261nitrenendash singlet nitrenendash ndash reactions 189ndash triplet nitrenendash ndash reactions 189o-nitrobenzyl ester photo-rearrangementndash nitronic acid 203nitronic acid 203nitroxyl (aminoxyl) radicals gt NndashO 262nitroxyl radicals 264ndash photolysis 268ndash reaction with polymers 265nonacosadiynoic acid 299non-conjugated polymersndash absorption of light 10ndash absorption spectra 11nonlinear optical materialsndash applications of NLO polymers 100ndash ndash optical data storage 99ndash ndash telecommunications 96ndash second-order NLO materials 87 89ndash third-order NLO materials 88nonlinear optical phenomena 73 ffndash second-order phenomena 79ndash third-order phenomena 82nonlinear optical (NLO) propertiesndash second-order optical nonlinearity 77ndash third-order optical nonlinearity 77Norrish reactions 268Norrish type I and II processes 260Norrish type I reaction 21 182 183Norrish type II reaction 182 183Novolak resists 236 237nucleases see photochemical nucleases 227

Subject Index 373

nucleic acids 207numerical aperture 233 339ndash of objective lenses 342Nyloprintndash letterpress plates 318

ooffset printing 320OLED (organic light emitting diode) dis-

plays 147OLEDsndash injection-limited conduction 151ndash polarized light 154ndash structure of a two-layer OLED 151ndash transport-limited conduction 151ndash white-light 155ndash ndash Pt-containing compounds 156oligopeptidesndash for optical storage 139Onsager theoryndash quantum yield of charge carrier genera-

tion 57optical absorption 9optical activity 23optical data storage 99ndash photochromic systemsndash ndash diarylethenes 137ndash ndash fulgides 137ndash ndash liquid-crystalline copolymers 138ndash ndash liquid-crystalline polyesters 138optical dichroismndash light-induced 123optical fiber cables 168optical fiber coatings 310optical fibers 167ndash information networks 168ndash step-index optical fibers 168optical fiber sensorsndash evanescent field absorption sensors 355optical limitersndash applications of NLO polymers 100optical memories 339optical near field recording 343optical phase conjugation (OPC) 86optical recording materials 126optical resonatorsndash feedback structuresndash ndash flat microdisks 159ndash ndash microrings 159ndash ndash microspheres 159optical rotary dispersion (ORD) 24optical storage techniquesndash blu-ray disk 339

ndash compact disk 339ndash digital versatile disk 339ndash high definition DVD 339ndash light-driven mass transport 347ndash near-field recording 343optical transistor 100optical waveguides 167optodesndash detection of molecular oxygen and carbon

dioxide 350ndash ion-selective optodes 357ndash polymer transducer supports 357organic light-emitting diodes OLEDs 147organometallic initiatorsndash photoinitiators for visible light 281orientation of polymersndash electric field-induced 75oscillator strength 7overprint varnishes 309ndash aluminum 311ndash tin-free steel 311oxaspiro monomersndash non-shrinking dental formulations 317ndash ndash curing of dental formulations 315oxazolidone containing acrylatesndash photocured coatings 310oxidationndash polymers 199oxyl radicals 200ndash reactions 201

ppackagingndash photocured coatings 310paperback booksndash polymer printing plates 319paper coatings 309Paterno-Buumlchi-type reaction 213ndash DNA dimeric photoproducts 212PBOCStndash acidolysis 241ndash poly(t-butoxycarbonyl oxystyrene) 240pentacosadiynoic acid 299pericyclic reactions (electrocycliza-

tions) 113peroxyl radicals 264ndash reactions 201phase conjugate mirror 86 87phase conjugationndash applications of NLO polymers 100phase controllers 96phase hologramsndash recording 344

Subject Index374

phenacyl anilinium saltsndash cationic photoinitiators 290phenylalanine (Phe) 209phenylglyoxylatesndash type I free radical photoinitiators 278phenyl salicylatesndash UV absorbers 258S-phenyl thiobenzoatesndash type I free radical photoinitiators 278phosphonium saltsndash cationic photoinitiators 290phosphorescence 10 13 14photoacid generators 243photoaddressable polymers 346photoalignmentndash liquid-crystalline compounds 128ndash of liquid-crystal molecules 23ndash of liquid-crystal systems 126ndash surface-assisted 129photocatalysts 361ndash inorganic materials 364photocatalytic polymer films 365photochemical nucleases 228photochemical reactionsndash amplification effects 178ndash polymers 178photochromic compounds 114photochromic eyewearndash photochromic lenses 136photochromic lensesndash indolinospironaphthoxazines 136ndash pyridobenzoxazines 136photochromic systems 346photochromic transformations 114ndash activation of second-order NLO proper-

ties 134ndash conformational changes in linear poly-

mers 115ndash data storage 137ndash heterolytic bond cleavage 113ndash pericyclic reactions (electrocycliza-

tions) 113ndash photoalignment of liquid-crystal sys-

tems 126ndash photochromic lenses 136ndash photocontrol of enzymatic activity 123ndash photoinduced anisotropy (PIA) 123ndash photomechanical effects 130ndash trans-cis (EZ) isomerization 113photochromism 113photoconductive polymers 49ndash produced by glow discharge 70ndash produced by heat 69

ndash produced by high-energy radiation 69ndash produced by plasma polymerization 70photoconductivity 49 ffndash electron conduction 61ndash hole conduction 61photocontrol of enzymatic activity 123photo-cross-linkingndash bisazides 188ndash co-polypeptide 185ndash intermolecular cross-links 183ndash mechanism 183ndash simultaneous cross-linking and main-

chain cleavage 193ndash thin filmsndash ndash photolithographic processes 184photocured coatingsndash waterborn formulations 311photocuring see also curingndash di- and trifunctional compounds 308ndash industrial applications 307ndash polymerizable formulations 307[2+2] photocycloaddition 299photodegradationndash polymers 196photodynamic therapy PDTndash cancer 223ndash sensitizers 224photo-Fries rearrangement 260ndash aromatic esters amides urethanes 202ndash polycarbonates 203photogeneration of charge carriers 50ndash dissociation of excitons 56photografting 330photoinduced absorption (PIA) 42photoinduced anisotropy (PIA) 123photo-induced surface relief storingndash recording density 347photoinitiation of cationic polymerizationsndash direct photolysis of the initiator 289ndash sensitized photolysis of the initiator 292photoinitiation of free radical polymeriza-

tions 276 277photoinitiation of ionic polymerizationsndash anionic polymerization 296ndash cationic polymerization 288ndash free radical-mediated generation of ca-

tionsndash ndash addition-fragmentation reactions 295ndash ndash oxidation of radicals 293photoinitiators 275ndash anionic photoinitiators 296ndash cationic photoinitiators 290ndash dyeco-initiator systems 284

Subject Index 375

ndash free radical polymerizationsndash ndash type I initiators 276ndash ndash type II initiators 276ndash inorganic photoinitiators 286ndash metal-based initiators 283ndash photoinitiators for visible light 281ndash quinones and 12-diketones 285ndash type I free radical photoinitiators 276 278ndash type II free radical photoinitiators 279

280photoionizationndash tryptophan 215ndash tyrosine 215photolatent compounds 297photolatent initiators 297 298photolithography 231ndash maskless lithography 235ndash phase-shifting transmission masks 234ndash projection optical lithography 233ndash soft lithography 246ndash zone-plate array lithography ZPAL 235photoluminescencephotolysisndash poly(methyl methacrylate) 179ndash polystyrene 179 180photo-mask productionndash electron-beam lithography 235ndash ion-beam lithography 235photomechanical effects 130 131 132ndash in hairy-rod type poly(glutamate)s 134ndash in monolayers 134photomorphogenic control functionsndash of photoreceptor proteins 219photon harvesting 16ndash role of anthracene groups 18ndash role of naphthalene groups 18photonic crystalsndash polymeric materials consisting of periodic

microstructures 317photopolymerizable systems 346photopolymerization 275ndash epoxides 186photopolymersndash holography 322photoreactivation 220ndash of organisms 219photorearrangements 204photoreceptor actionndash in biological processes 217photoreceptor proteins 210ndash regulatory action 217ndash transformation modes of chromo-

phores 217

photoreceptors 143 208photoreceptors dual layer 143photorefractive formulations 105ndash polymers 106photorefractive (PR) effect 103ndash applicationsndash ndash dynamic holographic interferome-

try 110ndash ndash holographic storage 110ndash ndash real-time processing 110ndash diffraction efficiencyndash ndash four-wave mixing technique 109ndash evidence for PR effectndash ndash two-beam coupling experiments 108ndash mechanism 104photorefractive systems 346photorefractivity 103ndash orientational photorefractivity 107photo-release of reactive anions 296photosensitizersndash nucleic acidprotein cross-linking 225photosensors 349photosynthesis 207photovoltaic (PV) cellsndash classical PV cellsndash ndash CdTe 162ndash ndash CuInSe2 162ndash ndash GaAs 162ndash ndash silicon 162ndash polymeric solar cells 163phytochrome kinase 218phytochrome interacting factor 218phytochromes 217ndash interdomain signal transmission 218ndash photoreceptors 209 210phytochromobilinndash photocycle 218PICUP (photo-induced cross-linking of un-

modified proteins) 223pigmentsndash light absorbers 257pinacol rearrangement 242pitland structurendash compact disks 340 341planar waveguidesndash polymeric 170plantsndash photomorphogenic processes 211plasma thrustersndash laser ablation 256platinum(II) acetyl-acetonate (Pt(acac)2)ndash anionic photoinitiators 296plume

Subject Index376

ndash laser ablation 250Pockels effect 73Pockels tensor 78POFs (polymer opticals fibers) 168 ffpolarization 75ndash electric field dependence 74polarized electroluminescencendash background illumination of liquid-crystal

displays 154polarized lightndash absorption 22ndash circularly polarized light 23 28ndash creation of anisotropy 23ndash degree of polarization 26ndash elliptically polarized light 24ndash emission 22 26ndash fluorescence 26ndash generation of anisotropy 124ndash generation of birefringence 124ndash linearly polarized light 22 23polarizing excitonic energy transfer EET 38polingndash electro-optical poling 93ndash Langmuir-Blodgett (LB) technique 93ndash optical poling 93ndash self-assembly techniques 93poly(4-acetoxy styrene)ndash photo-rearrangement 204polyacetylenendash chemical sensors 351poly(acrylic acid)ndash polymer support for inorganic photocata-

lysts 365polyacrylonitrilendash cross-linking 195ndash main-chain cleavage 195poly(allyl amine hydrochloride)ndash polymer support for inorganic photocata-

lysts 365polyaniline 51poly[bis(2-naphthoxy)phosphazene]

P2NP 69polycarbonatesndash compact disks 340ndash photo-rearrangement 204ndash POFs 169poly(cis-isoprene) 189ndash photolithography 236polydiacetylenesndash color change 300poly(dialkyl fluorine) 51poly(25-di-isopentyloxy-p-phenylene)

DPOPP 23

polyester acrylate-based formulationsndash coatings 309polyester with pendant azobenzene groupsndash holographically recorded gratings 138polyethylenendash surface grafting 331polyfluorenendash chemical sensors 351poly(glutamic acids)ndash coil helix transition 121poly(L-glutamic acids) 119ndash modified 120poly(4-hydroxystyrene)ndash photo-cross-linking 192polyimidesndash laser ablation 249ndash resists 237polyisocyanates 33ndash CD spectra 34poly(L-lysine)ndash CD spectra 26ndash circular dichroism 25polymer fibersndash information networks 168polymer filmsndash anisotropic contraction 131ndash chain alignment 154ndash light-induced dimensional altera-

tions 131ndash light-induced mass transport 132ndash photoinduced anisotropy (PIA) 123ndash surface relief gratings 132polymeric light sources 146polymeric materials 248polymer lasersndash conjugated polymers 156ndash electrically pumped 162ndash hostguest systems 156polymer LEDsndash hole and electron transport materi-

als 153ndash luminancendashvoltage characteristic 150polymer optical fibers (POFs) 169polymer optical waveguides 167polymersndash light-emitting diodes 148polymer single crystalsndash topochemical photopolymerization 299polymers bearing pendant aromatic groupsndash photocatalysts 363polymers in holography 322polymer transducer supportsndash polyacrylamide 357

Subject Index 377

ndash polysiloxanes 357ndash poly(vinylidene chloride) 357ndash PVC 357poly(methyl methacrylate) 11 37ndash POFs 169poly(methyl vinyl ketone) 11poly(phenyl vinylene)sndash BuEH-PPV 45ndash spectral narrowing 45poly(phenylene vinylene)sndash MEH-DSB 43ndash MEH-PPV 43poly(14-phenylene vinylene) 12poly(p-phenylene)s 32 51 362ndash ladder-type 51ndash m-LPPP 51 55poly(p-phenylene ethynylene) PPEndash chemical sensors 351poly(p-phenylene vinylene) PPV 51ndash chemical sensors 351ndash DOO-PPV 12ndash light-emitting diodes 147ndash MEH-PPV 27ndash PMCYHPV 12ndash PPFPV 12ndash PPV 12ndash solar cells 164poly(phenyl vinyl ketone) 11 42polypropylenendash surface grafting 331polysaccharides 207 208polysilanesndash main-chain cleavage 198ndash photodegradation 198polysilylene 51 57ndash main-chain cleavage 198ndash photodegradation 198poly(sodium styrene sulfonate-co-2-vinyl-

naphthalene)ndash photocatalyst 363polystyrene 11ndash excimer formation 17ndash POFs 169ndash segmental motions 29poly(thiophene)s 28 51ndash CD spectrum 33ndash PDMBT 32ndash PMBET 33ndash solar cells 164polyurethanendash surface grafting 331polyurethane-based coatings 298poly(uridylic acid)

ndash intra-chain hydrogen abstraction 227poly(vinyl acetate) 11poly(N-vinyl carbazole) 51 53 54poly(vinyl chloride)ndash dehydrochlorination 197ndash discoloration 196 197ndash photodegradation 196poly(vinyl cinnamate) 23positive resists 239potassiumndash anionic photoinitiators 296potassium ion sensor 358PPPndash active photocatalyst 361ndash poly(p-phenylene) 361printingndash photocured coatings 310printing inksndash curing 320printing plates 318ndash composition of the photosensitive

layer 319ndash generation of the relief structure 319projection optical lithography 233protease activity 353ndash turn-on fluorescence detection 352protein-nucleic acid assembliesndash photochemical cross-linking 223protein-protein assembliesndash photochemical cross-linking 223proteins 207 209ndash cross-linking 216ndash denaturation 214ndash photoreactions 214ndash rotational correlation 29ndash turn-on fluorescence detection 352proximity printingndash photolithography 232pterinsndash photoreceptors 210PTBVBndash poly(t-butyl-p-vinyl benzoate) 240

qquantum yield of photodecompositionndash [2+2] cycloreversion 213ndash purines 212ndash pyrimidines 212quantum yieldsndash cross-linking 194ndash electroluminescence 152ndash initiation of diacetylene polymeriza-

tion 301

Subject Index378

ndash main-chain cleavage 194ndash of charge generation 145ndash photoproducts of selected polymers 195quinones and 12-diketonesndash chemical structures 288

r3R schemendash chemosensing 349radiant flux of light same as intensity 7radical combination 180radical disproportionation 180radical scavengers 257 262read-only memory (ROM) 340rearrangementsndash o-nitrobenzyl ester rearrangement 202ndash photo-Fries rearrangement 202refractive indexndash complex refractive index 76ndash electric field-induced changes 74ndash imaginary part 76regioregularityndash poly(3-hexylthiophene) P3HT 62RehmndashWeller equation 285 293reineckatendash anionic photoinitiators 296repair of lesionsndash DNA photolyases 219resistsndash ArF (193 nm) lithography 242ndash chemical amplification resistsndash computer chip fabrication 236ndash F2 (157 nm) lithography 245ndash negative resists 238ndash photolithography 232ndash positive resists 238ndash sensitivity 238ndash ndash of deep UV resists 240Richardson-Schottky (RS) thermionic emis-

sionndash OLEDs 151ROMndash read-only memory 326 340rotational correlation timesndash proteins 29rotational diffusion constant 29ruby laser 74Russel mechanismndash combination of peroxyl radicals 200

ssacrificial consumptionndash of stabilizers 267sealings 310second harmonic generation (SHG) 74

76 82ndash photochromic activation 134second-order NLO materials 87ndash alignment of AD moieties 92ndash commercially available NLO poly-

mers 92ndash electric field-induced alignment (pol-

ing) 92ndash guest-host systems 89ndash NLO polymers 89 91ndash orientation techniques 92ndash poled polymer films 91ndash poling 93second-order NLO propertiesndash light-induced generation 135second-order optical nonlinearity 77self-focusingdefocusing 84sensitizersndash photochemotherapy of cancer cells 224shrinkagendash curing of dental formulations 315SIA International Roadmap 231signal modulators 96silicandash surface grafting 331siliconesndash UV-cured 310silver halide photographic emulsions 324silyl benzyl ethersndash cationic photoinitiators 290singlet-oxygenndash formation 202ndash reactions with unsaturated poly-

mers 202skin cancer 211soft lithographic process 246solar cellsndash donoracceptor heterojunctions 166ndash ndash CN-PPV 165ndash ndash MEH-PPV 165ndash flat-heterojunction organic solar

cells 165ndash performance characteristics 167ndash phase-separated polymer blends 165ndash p-n homojunction crystalline silicon solar

cells 163ndash semiconducting polymers 164solid immersion lenses

Subject Index 379

ndash hemispherical lenses 342ndash Weierstrass superspherical lenses 342solid immersion techniques 340solitonsndash negatively charged soliton 54ndash neutral soliton 54ndash positively charged soliton 54spectral narrowing 44spectroscopyndash time-resolved 38spectrumndash optical absorption 209spirooxazines 114spiropyran groupsndash in polypeptides 119spiropyrans 114ssDNA (single-strand DNA)ndash base sequence detection 354stabilizationndash light stabilizers 257stabilization of polymersndash by energy transfer 15stabilizers see also light stabilizersndash hydroperoxide decomposers 265ndash packages 266ndash sacrificial consumption and deple-

tion 267stencilsndash photolithography 232step-index polymer optical fibers 170stepwise [2+2] photocyclopolymeriza-

tion 302stepwise processes 303stereolithography 315 317storage capacity 339 340ndash blue-ray disks 342ndash HD-DVDs 342ndash volume holography 344storage materialsndash chalcogenide glasses 346ndash inorganic crystals 346ndash photoaddressable polymers PAPs 346ndash photopolymerizable systems 346ndash photopolymers 346ndash photorefractive crystals 346ndash volume holography 346storage systemsndash forgery-proof 139streak camera 41stress proteinsndash light-induced formation 216sulfonium saltsndash cationic photoinitiators 290

sulfonyloxy ketonesndash cationic photoinitiators 290superquenching 352surface grafting 331surface modificationndash photografting 330surface relief gratings 132susceptibilityndash linear electro-optic (EO) effect 81ndash second harmonic generation SHG 81susceptibility tensors 74 f 78synchrotron radiation 246

ttelecommunicationsndash photocured coatings 310terephthalophenonesndash type II free radical photoinitiators 280tertiary amines 298ndash initiators of anionic polymerizations 297thioanthrenium saltsndash cationic photoinitiators 290thioxanthone derivativesndash type II free radical photoinitiators 280third harmonic generation THG 78 83third-order NLO materialsndash conjugated compounds 88ndash polyacetylenes 88ndash polydiacetylenes 88 93ndash poly(phenylene vinylene)s 93ndash poly(p-phenylene)s 93ndash polysilanes 88ndash polythiophenes 93ndash susceptibilities 94 95ndash trans-polyacetylenes 93third-order optical nonlinearity 77third-order phenomena 82ndash degenerate four-wave mixing 83ndash electric field-induced second harmonic

generation 83ndash optical Kerr gate 83ndash third harmonic generation 83ndash two-photon absorption 83ndash Z-scan 83third-order susceptibilities 83threshold fluence 248time-of-flight (TOF) methodndash poly(methyl phenyl silylene) 62ndash determination of the mobility 60time-resolved optical absorption measure-

ments 39 fndash flash photolysis 39ndash Nd3+YAG laser 39

Subject Index380

ndash ruby laser 39ndash Tisapphire laser 39time-resolved spectroscopy 38 55ndash amplified spontaneous emission 44ndash fluorescence 44ndash luminescence 44ndash optical absorption 41ndash spectral narrowing 44 fTiO(F4-Pc)TTAndash dual-layer systemndash ndash xerography 145titaniandash photocatalyst 364ndash polymer-coated TiO2 particles 365ndash surface grafting 331titanium dioxide TiO2

ndash generation of reactive free radicals 288ndash inorganic photoinitiators 286titanocenesndash photoinitiators 283TMPndash 22-66-tetramethylpiperidine 262TMPOndash piperidinoxyl radical 264ndash reaction with alkyl radicals 264TNTndash 246-trinitrotoluene 352TNT sensor 352topochemical photopolymerization of diace-

tylenes 299topochemical polymerizations 299track pitch 339trans-cis (EZ) isomerization 113trans-coniferyl alcoholndash lignins 207trans-p-coumaryl alcoholndash lignins 207transphasor the optical transistorndash applications of NLO polymers 100trans-polyacetylene 51trans-sinapyl alcoholndash lignins 207triallyl cyanurate TACndash cross-linking enhancer 191triaryl cyclopropenium saltsndash cationic photoinitiators 290triarylmethanes 114135-triazinesndash UV absorbers 258trinitrofluorenone 54triphenylmethyl cationsndash photogeneration 118triphenylsulfonium salts

ndash photolysis 241triplet-triplet absorption 42tris(8-oxyquinolato)-aluminum Alq3

ndash electron conduction 61tryptophan (Trp) 209tungsten hexacarbonyl W(CO)6

ndash photoiniatorndash ndash photo-cross-linking 187two-photon absorption (TPA) 85two-photon polymerization 318type I free radical photoinitiatorsndash chemical structures 278type II free radical photoinitiatorsndash bimolecular reactions 279ndash chemical structures 280tyrosine (Tyr) 209

uUV absorbers 257 258UVAsndash UV absorbers 258UVVis spectroscopy 30

vVOC emission 309VOCsndash volatile organic compounds 307volume holography 340ndash holography 322ndash storage materials 345 346ndash storage mechanism 344volume shrinkagendash volume holography 324

wWannier excitons 53water-soluble aromatic ketonesndash type II free radical photoinitiators 280Weierstrass superspherical lens 342woodndash darkening 222ndash photoreactions 221ndash yellowing 222wool tendering 214WORMndash write-once-read-many 326

Subject Index 381

xxanthopsinsndash photoreceptors 210xerographic discharge method 58ndash quantum yields for charge carriers 59xerographyndash charge-generation systemsndash ndash pigment particles of dyes 145

yyellow proteinsndash photoreceptors 210yellowingndash wood 222

zZ-scan experiment 84 85

Subject Index382

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W Schnabel

Polymers and Light








W Schnabel

Polymers and Light

The Author









ISBN 978-3-527-31866-7


Preface XIII

Introduction 1



V

Contents








ContentsVI



References 112


References 139



Contents VII


References 171





References 205


ContentsVIII








References 268

Contents IX



References 302


ContentsX


References 329



References 345


References 356

Contents XI


References 364

Subject Index 365

ContentsXII



XIII

Preface





PrefaceXIV





1

Introduction




Introduction2



11Principal aspects


h En E0 1-1



M Mx My Mz 1-2

5



M

vvdv

edpede

ssds 1-3










(nm) a)max


Ethene 193 104

Ethyne 173 6103

Acetone 187271

103

15 n

Azomethane 347 5 n


10020

n

Amyl nitrite 219357

219357

n


f 875 102EM2 1-4



A lg10I0I cd 1-5



f 23 103c2mNe2F

d 432 109 F

d 1-6



















100ndash200 103




102ndash104



1ndash400


































D A D A 1-7


P A P A 1-8

P RH PH R 1-9
















M M M M 1-14














13S 1 13IDID0 1-15






Cha

rt1

4C

hem

ical

stru

ctur

eof

ade

ndri

tic21

-por

phyr

inar

ray

cons

istin

gof

20Z

npo

rphy

rin

units

atta

ched

toa

Zn-

free

porp

hyrin

foca

lco

re[2

122

]









C O RH C OH R

1-16



1-17

O O




171Absorption















13 100cd deg cm3 dm1 g1 1-18







2303Al Ar1804 deg 1-19

23034Al Ar rad 1-20







173Emission


P I II I

1-21














rt It ItIt 2It 1-22





rt 25exp6Drt 1-23


Dr 1r kTV 1-24











18Applications








18 Applications 31

N H O C N H O C












18 Applications 33













18 Applications 35









182Luminescence


18 Applications 37












18 Applications 39











18 Applications 41







18 Applications 43













References 45



References







































38 P Wahl CR Acad Sci 263 (1966)1525





References 47






































49

2Photoconductivity


221General aspects






Perylene dye

Azo dye

Quinone dye

Squaraine dye



2-1








PT Polythiophene






PANI Polyaniline


d p 2-2































Pr exp rc

r

exp eFr

2kT131 cos

2-4



kT e2

rc2-5



13cc 13cc0

4r2f rFTgrdr 2-6




gr 323 exp r2

2

2-7




13cc 1efI

Q

t

tt0

CefI

Ut

tt0

0

edfI

Ut

tt0

2-8












dtrF

2-9























DEH-PF 074

MEH-PPV 043

m-LPPP 016

P3HT 002

PAPS6 023




FT 0 exp Ea0 F12

kTeff

2-10









FT 0 exp 42

9

exp C2 2F12

132-11








Ea Epa Ed

a 2-12


FT 0 exp Ep

2kT 42

9

exp C 2 2

F12

132-13


25Doping






25 Doping 67

















Additive(3 mol)

EA a)


b)


c)













References






















References 71














































73



32Fundamentals



pi 0

j

ijEj

jk

ijkEjEk

jkl

ijklEjEkEl 3-3




PI P0

J

1IJ EJ

JK

2IJKEJEK

JKL

3IJKLEJEKEL 3-4




Plinear 1E 3-5


PX 11EX 12EY 13EZ


J

IJEJ 3-6

PZ 31EX 32EY 33EZ



1 11 12 1321 22 2331 32 33

3-7


32 Fundamentals 75



n n ik 3-8

2kc

3-9







L 1 2 3 4 5 6

JK 11 22 33 23 or 32 13 or 31 12 or 21


2 211

212

213

214

215

216

221

222

223

224

225

226

231

232

233

234

235

236

3-10



2 0 0 0 0

215 0

0 0 0 215 0 0

231

232

233 0 0 0

3-11


2ikj

2jkl

2jik

2kij

2kji

is valid [16]

215 is equal to


231 and



2ZZZ NFzzz cos3

3-12

2XXZ

2YYZ

2XZY

2YZY

2ZXX

2ZYY 1

2NFzzz cos sin2

3-13



32 Fundamentals 77





nIJ 12

n3IJrIJKEK 1

2n3

IJpIJKKE2K 3-14


nIJ 12

n3IJrIJKEK 3-15


2IJK 1

2n4

I rIJK 3-16






2333 and

2311 and are re-


n 12

n3er33Emod

2333Emod

ne NF cos3 Emod

ne3-17

n 12

n30r13Emod

2113Emod

n0

NF12

cos sin2

Emod

n03-18











3ZZZZ NF2F2

F0 z

5kT

3-20





I2 3ZZZZI2

E20 3-21







I2 g N1 2IJKsolvent

N2 2

IJKsolute

I2 3-22


IJK







I2 Kd2I2 3-23

K 512t4T2t2

0p2 sin2 An2

n22

3-24








3 3nonresonant

3resonant 3-25









P3 143XXXX3E3

3-26









n n0 n2I 3-27






n2 Tpv

2kI0L3-28


n2 122

cn03 3-29









1Tr

I0

I 1 2I0L 3-30





I4 2

4c2n2 32L2I1I3I2 3-31







341General aspects

























(nm) b)

(Debye)0

c)

(10ndash30 esu)

I 438 67 813

II 494 80 952

III 602 71 259

IV 698 104 359

V 680 83 479







b)

(C) c)

(m)r33

d)

(pm Vndash1)

Ber-1 100 155 42

3RDCYXY 15 mol 140 13 30

GT-P3 62 wt 180 1541 12

ROI-4 17 mol 215 13 16


tric field


































PVT 310ndash14 1907





PBT 1010ndash10 602

PBO 1010ndash10 602













2nL

n3ErL

3-32










FTC 1219 50

CLD 1342 70

GLD 1388 105


















References 101

References




























































n n3reESC

24-1

103

4Photorefractivity










G 0

2n sin131 22 4-2












n pE2 pV2

d2 4-3














0 ILsat

IL04-4


1L13ln0 ln 1 0 4-5


L dcos

4-6



n

44-7






Id

I04-8


sin2 fgdn

4-9


45Applications








45 Applications 111







References































113

5Photochromism


5 Photochromism114




(c) Spirooxazines

(d) Diarylethenes






521Solutions






5 Photochromism116







5 Photochromism118












5 Photochromism120






522Membranes



5 Photochromism122












n n n 5-1

5 Photochromism124



S A AA 2A 5-2










5 Photochromism126










5 Photochromism128









561Bulk materials






5 Photochromism130









5 Photochromism132





562Monolayers













5 Photochromism134







58Applications



5 Photochromism136






582Data storage




58 Applications 137




5 Photochromism138






References 139



References












5 Photochromism140








































References 141





























143
























PPV 540

PMPPV 560

MEH-PPV 590

PMCYH-PV 590

PDFPV 600

PPFPV 520












































TPD

PPV

PVK

PMPS


PBD

Alq3

PMA-PBD


















622Lasers












Ithresholdb)

(J cmndash2)Ref


01 [43]



45 [45]


= 400 nm= 150 ps

37 [46]


012 [47]








































































641General aspects





(mA cmndash2)Voc

b)

(V)ffill

c) mpd)

()cc

e) Ref





642Optical fibers


















840 lt10ndash8 10ndash4 101 104

1310 10ndash5 100 103 105

1550 10ndash3 101 105 106










References 171

References


































































References 173
































177

























Hydroperoxide group

Carbonyl group


Double bonds



Metal ions


13RH O2CTh 13RH O

2 CT R OOH ROOH 7-1






R C R COO

7-2



72Cross-linking








7-3































44-Diazidostilbene




RN3 h 1RN N2 7-41RN 3RN 7-5










P P PP 7-6
















7-7








1MwD

1Mw0

13S2

213X

Dabs

NA7-8








Dgel NA

13XMw07-9
















(102 )




(065) CO (069) CH4




7-10





742Polysilanes







75Oxidation





75 Oxidation 199


RX PH RXH P 7-11



POOHh PO OH 7-12


OH PH H2O P 7-13






P O2 POO

POO PH POOH P

POO POO a Products

POO P b POOP

P P c P P




75 Oxidation 201






7-14



7-15


77Rearrangements













References 205

References



























































207
















11-cis Retinal


Flavin

Phytochromes


Phytochromobilin





4-Hydroxycinnamate











Stentorin


Chlorophyll a



















(a) (b)











































RO OHh RO OH
























(nm)operation

(nm)



280 gt 300


290300 gt300


















References


















References 229



















































231









LWmin k1

NA9-1


NA n sin 9-2














157193









913Resists










































20 Negative


20 Negative


90 Positive

a)


















Imino sulfonates






[29][30]


[31]


[32]


[32 33]


























921General aspects





















































cm mvW

9-3










932UV absorbers







R1 H alkyl

R2 H alkyl phenyl

R3 H alkyl

R4 H butyl



Phenyl salicylates






Cyanoacrylates

Oxanilides



933Energy quenchers




















































References
























References 269





































































References 271































101Introduction



275





1021General remarks














R methyl ethyl

ethyl isopropyl

n-butyl isobutyl


Acetophenones


Phenylglyoxylates





Acylphosphonates
















-Keto coumarins

Anthraquinones

Terephthalophenones





Triethanolamine













Class Example a)

















D CI D CI D CI 10-1

D CI D CI D CI 10-2


G F13Eox12 Ered

12 ES Ec 10-3






















Sche

me

107

Gen

erat

ion

offr

eera

dica

lsby

the

phot

ored

uctio

nof

met

hyle

nebl

uew

ithtr

ieth

anol

amin

e[2

]

Cl













Monomer Polymer




Diazonium salts






Phosphonium salts

Ferrocenium salts



Sulfonyloxy ketones

Silyl benzyl ethers


PF6(ndash) AsF6























C

AB C

AB 10-4



G fcEox12 Ered

12 10-5

Here Eox12 and Ered





Direct

Benzoin

Direct


Direct

PolysilanesIndirect










12V

ndash12

ndash11 ndash01

ndash07 ndash05

ndash05 ndash07

ndash02 ndash10














PotassiumReineckate

CrNH32NCS4 K(NCS)


acac a)


b


CN


HO











1041General remarks


























References

















References 303
































































111General remarks





305







Free radical



Free radical




Cationic

Epoxidizedsiloxanes

Cationic


Cationic









MonomerOligomer wt

















1122Cationic curing








1123Dual curing


























Cha

rt11

8C

hem

ical

stru

ctur

esof

typi

cal

poly

mer

izab

leco

mpo

unds

cont

aine

din

dent

alfo

rmul

atio

ns[3

0]











115Printing plates



















117Holography





117 Holography 319













117 Holography 321













117 Holography 323























Carbonyl groups



N-Nitroso groups

Disulfide groups










a

a









References 329





[91 92]




[100]





[103]




[104][105]

References






































References 331





























64 D Gabor Nature 161 (1948) 777































References 333
















121General aspects



337



Disk Format d a)

(mm)TP b)

(m) c)

(nm)NA d) CSt

e)

(GB)rtrans

f )

(Mb sndash1)


















k

NA12-1



nNA12-2


n2NA12-3


1222Blue-ray disks




1231General aspects























b)Bi12TiO20





References 345

References































131General aspects


347





































dp

2n2

1 sin2 n22

13-1












groups[35]


[36 37]












References












References 357






























141General aspects





359




























+







References






















365

Subject Index








Subject Index366




Subject Index 367










Subject Index368








(DRAM) 234









( = 13 nm) 234

Subject Index 369










Subject Index370











Subject Index 371









Subject Index372







Subject Index 373









Subject Index374










Subject Index 375




phores 217




Subject Index376






DPOPP 23




Subject Index 377






tion 301

Subject Index378











Subject Index 379








Subject Index380










Subject Index 381




Subject Index382

Related Titles

Elias H-G


2008ISBN 978-3-527-31175-0



2008ISBN 978-3-527-31784-4

Elias H-G


2007ISBN 978-3-527-31174-3

Elias H-G


2007ISBN 978-3-527-31173-6

Elias H-G


2005ISBN 978-3-527-31172-9



2005ISBN 978-3-527-31092-0



Xanthos M (ed)




2004ISBN 978-3-527-31033-3

Elias H-G









W Schnabel

Polymers and Light

The Author









ISBN 978-3-527-31866-7


Preface XIII

Introduction 1



V

Contents








ContentsVI



References 112


References 139



Contents VII


References 171





References 205


ContentsVIII








References 268

Contents IX



References 302


ContentsX


References 329



References 345


References 356

Contents XI


References 364

Subject Index 365

ContentsXII



XIII

Preface





PrefaceXIV





1

Introduction




Introduction2



11Principal aspects


h En E0 1-1



M Mx My Mz 1-2

5



M

vvdv

edpede

ssds 1-3










(nm) a)max


Ethene 193 104

Ethyne 173 6103

Acetone 187271

103

15 n

Azomethane 347 5 n


10020

n

Amyl nitrite 219357

219357

n


f 875 102EM2 1-4



A lg10I0I cd 1-5



f 23 103c2mNe2F

d 432 109 F

d 1-6



















100ndash200 103




102ndash104



1ndash400


































D A D A 1-7


P A P A 1-8

P RH PH R 1-9
















M M M M 1-14














13S 1 13IDID0 1-15






Cha

rt1

4C

hem

ical

stru

ctur

eof

ade

ndri

tic21

-por

phyr

inar

ray

cons

istin

gof

20Z

npo

rphy

rin

units

atta

ched

toa

Zn-

free

porp

hyrin

foca

lco

re[2

122

]









C O RH C OH R

1-16



1-17

O O




171Absorption















13 100cd deg cm3 dm1 g1 1-18







2303Al Ar1804 deg 1-19

23034Al Ar rad 1-20







173Emission


P I II I

1-21














rt It ItIt 2It 1-22





rt 25exp6Drt 1-23


Dr 1r kTV 1-24











18Applications








18 Applications 31

N H O C N H O C












18 Applications 33













18 Applications 35









182Luminescence


18 Applications 37












18 Applications 39











18 Applications 41







18 Applications 43













References 45



References







































38 P Wahl CR Acad Sci 263 (1966)1525





References 47






































49

2Photoconductivity


221General aspects






Perylene dye

Azo dye

Quinone dye

Squaraine dye



2-1








PT Polythiophene






PANI Polyaniline


d p 2-2































Pr exp rc

r

exp eFr

2kT131 cos

2-4



kT e2

rc2-5



13cc 13cc0

4r2f rFTgrdr 2-6




gr 323 exp r2

2

2-7




13cc 1efI

Q

t

tt0

CefI

Ut

tt0

0

edfI

Ut

tt0

2-8












dtrF

2-9























DEH-PF 074

MEH-PPV 043

m-LPPP 016

P3HT 002

PAPS6 023




FT 0 exp Ea0 F12

kTeff

2-10









FT 0 exp 42

9

exp C2 2F12

132-11








Ea Epa Ed

a 2-12


FT 0 exp Ep

2kT 42

9

exp C 2 2

F12

132-13


25Doping






25 Doping 67

















Additive(3 mol)

EA a)


b)


c)













References






















References 71














































73



32Fundamentals



pi 0

j

ijEj

jk

ijkEjEk

jkl

ijklEjEkEl 3-3




PI P0

J

1IJ EJ

JK

2IJKEJEK

JKL

3IJKLEJEKEL 3-4




Plinear 1E 3-5


PX 11EX 12EY 13EZ


J

IJEJ 3-6

PZ 31EX 32EY 33EZ



1 11 12 1321 22 2331 32 33

3-7


32 Fundamentals 75



n n ik 3-8

2kc

3-9







L 1 2 3 4 5 6

JK 11 22 33 23 or 32 13 or 31 12 or 21


2 211

212

213

214

215

216

221

222

223

224

225

226

231

232

233

234

235

236

3-10



2 0 0 0 0

215 0

0 0 0 215 0 0

231

232

233 0 0 0

3-11


2ikj

2jkl

2jik

2kij

2kji

is valid [16]

215 is equal to


231 and



2ZZZ NFzzz cos3

3-12

2XXZ

2YYZ

2XZY

2YZY

2ZXX

2ZYY 1

2NFzzz cos sin2

3-13



32 Fundamentals 77





nIJ 12

n3IJrIJKEK 1

2n3

IJpIJKKE2K 3-14


nIJ 12

n3IJrIJKEK 3-15


2IJK 1

2n4

I rIJK 3-16






2333 and

2311 and are re-


n 12

n3er33Emod

2333Emod

ne NF cos3 Emod

ne3-17

n 12

n30r13Emod

2113Emod

n0

NF12

cos sin2

Emod

n03-18











3ZZZZ NF2F2

F0 z

5kT

3-20





I2 3ZZZZI2

E20 3-21







I2 g N1 2IJKsolvent

N2 2

IJKsolute

I2 3-22


IJK







I2 Kd2I2 3-23

K 512t4T2t2

0p2 sin2 An2

n22

3-24








3 3nonresonant

3resonant 3-25









P3 143XXXX3E3

3-26









n n0 n2I 3-27






n2 Tpv

2kI0L3-28


n2 122

cn03 3-29









1Tr

I0

I 1 2I0L 3-30





I4 2

4c2n2 32L2I1I3I2 3-31







341General aspects

























(nm) b)

(Debye)0

c)

(10ndash30 esu)

I 438 67 813

II 494 80 952

III 602 71 259

IV 698 104 359

V 680 83 479







b)

(C) c)

(m)r33

d)

(pm Vndash1)

Ber-1 100 155 42

3RDCYXY 15 mol 140 13 30

GT-P3 62 wt 180 1541 12

ROI-4 17 mol 215 13 16


tric field


































PVT 310ndash14 1907





PBT 1010ndash10 602

PBO 1010ndash10 602













2nL

n3ErL

3-32










FTC 1219 50

CLD 1342 70

GLD 1388 105


















References 101

References




























































n n3reESC

24-1

103

4Photorefractivity










G 0

2n sin131 22 4-2












n pE2 pV2

d2 4-3














0 ILsat

IL04-4


1L13ln0 ln 1 0 4-5


L dcos

4-6



n

44-7






Id

I04-8


sin2 fgdn

4-9


45Applications








45 Applications 111







References































113

5Photochromism


5 Photochromism114




(c) Spirooxazines

(d) Diarylethenes






521Solutions






5 Photochromism116







5 Photochromism118












5 Photochromism120






522Membranes



5 Photochromism122












n n n 5-1

5 Photochromism124



S A AA 2A 5-2










5 Photochromism126










5 Photochromism128









561Bulk materials






5 Photochromism130









5 Photochromism132





562Monolayers













5 Photochromism134







58Applications



5 Photochromism136






582Data storage




58 Applications 137




5 Photochromism138






References 139



References












5 Photochromism140








































References 141





























143
























PPV 540

PMPPV 560

MEH-PPV 590

PMCYH-PV 590

PDFPV 600

PPFPV 520












































TPD

PPV

PVK

PMPS


PBD

Alq3

PMA-PBD


















622Lasers












Ithresholdb)

(J cmndash2)Ref


01 [43]



45 [45]


= 400 nm= 150 ps

37 [46]


012 [47]








































































641General aspects





(mA cmndash2)Voc

b)

(V)ffill

c) mpd)

()cc

e) Ref





642Optical fibers


















840 lt10ndash8 10ndash4 101 104

1310 10ndash5 100 103 105

1550 10ndash3 101 105 106










References 171

References


































































References 173
































177

























Hydroperoxide group

Carbonyl group


Double bonds



Metal ions


13RH O2CTh 13RH O

2 CT R OOH ROOH 7-1






R C R COO

7-2



72Cross-linking








7-3































44-Diazidostilbene




RN3 h 1RN N2 7-41RN 3RN 7-5










P P PP 7-6
















7-7








1MwD

1Mw0

13S2

213X

Dabs

NA7-8








Dgel NA

13XMw07-9
















(102 )




(065) CO (069) CH4




7-10





742Polysilanes







75Oxidation





75 Oxidation 199


RX PH RXH P 7-11



POOHh PO OH 7-12


OH PH H2O P 7-13






P O2 POO

POO PH POOH P

POO POO a Products

POO P b POOP

P P c P P




75 Oxidation 201






7-14



7-15


77Rearrangements













References 205

References



























































207
















11-cis Retinal


Flavin

Phytochromes


Phytochromobilin





4-Hydroxycinnamate











Stentorin


Chlorophyll a



















(a) (b)











































RO OHh RO OH
























(nm)operation

(nm)



280 gt 300


290300 gt300


















References


















References 229



















































231









LWmin k1

NA9-1


NA n sin 9-2














157193









913Resists










































20 Negative


20 Negative


90 Positive

a)


















Imino sulfonates






[29][30]


[31]


[32]


[32 33]


























921General aspects





















































cm mvW

9-3










932UV absorbers







R1 H alkyl

R2 H alkyl phenyl

R3 H alkyl

R4 H butyl



Phenyl salicylates






Cyanoacrylates

Oxanilides



933Energy quenchers




















































References
























References 269





































































References 271































101Introduction



275





1021General remarks














R methyl ethyl

ethyl isopropyl

n-butyl isobutyl


Acetophenones


Phenylglyoxylates





Acylphosphonates
















-Keto coumarins

Anthraquinones

Terephthalophenones





Triethanolamine













Class Example a)

















D CI D CI D CI 10-1

D CI D CI D CI 10-2


G F13Eox12 Ered

12 ES Ec 10-3






















Sche

me

107

Gen

erat

ion

offr

eera

dica

lsby

the

phot

ored

uctio

nof

met

hyle

nebl

uew

ithtr

ieth

anol

amin

e[2

]

Cl













Monomer Polymer




Diazonium salts






Phosphonium salts

Ferrocenium salts



Sulfonyloxy ketones

Silyl benzyl ethers


PF6(ndash) AsF6























C

AB C

AB 10-4



G fcEox12 Ered

12 10-5

Here Eox12 and Ered





Direct

Benzoin

Direct


Direct

PolysilanesIndirect










12V

ndash12

ndash11 ndash01

ndash07 ndash05

ndash05 ndash07

ndash02 ndash10














PotassiumReineckate

CrNH32NCS4 K(NCS)


acac a)


b


CN


HO











1041General remarks


























References

















References 303
































































111General remarks





305







Free radical



Free radical




Cationic

Epoxidizedsiloxanes

Cationic


Cationic









MonomerOligomer wt

















1122Cationic curing








1123Dual curing


























Cha

rt11

8C

hem

ical

stru

ctur

esof

typi

cal

poly

mer

izab

leco

mpo

unds

cont

aine

din

dent

alfo

rmul

atio

ns[3

0]











115Printing plates



















117Holography





117 Holography 319













117 Holography 321













117 Holography 323























Carbonyl groups



N-Nitroso groups

Disulfide groups










a

a









References 329





[91 92]




[100]





[103]




[104][105]

References






































References 331





























64 D Gabor Nature 161 (1948) 777































References 333
















121General aspects



337



Disk Format d a)

(mm)TP b)

(m) c)

(nm)NA d) CSt

e)

(GB)rtrans

f )

(Mb sndash1)


















k

NA12-1



nNA12-2


n2NA12-3


1222Blue-ray disks




1231General aspects























b)Bi12TiO20





References 345

References































131General aspects


347





































dp

2n2

1 sin2 n22

13-1












groups[35]


[36 37]












References












References 357






























141General aspects





359




























+







References






















365

Subject Index








Subject Index366




Subject Index 367










Subject Index368








(DRAM) 234









( = 13 nm) 234

Subject Index 369










Subject Index370











Subject Index 371









Subject Index372







Subject Index 373









Subject Index374










Subject Index 375




phores 217




Subject Index376






DPOPP 23




Subject Index 377






tion 301

Subject Index378











Subject Index 379








Subject Index380










Subject Index 381




Subject Index382

Related Titles

Elias H-G


2008ISBN 978-3-527-31175-0



2008ISBN 978-3-527-31784-4

Elias H-G


2007ISBN 978-3-527-31174-3

Elias H-G


2007ISBN 978-3-527-31173-6

Elias H-G


2005ISBN 978-3-527-31172-9



2005ISBN 978-3-527-31092-0



Xanthos M (ed)




2004ISBN 978-3-527-31033-3

Elias H-G


polymers and light: fundamentals and technical applications

Documents