phthalocyanines and related compounds: nonlinear optical response and photoinduced electron transfer...

CHAPTER 1

Phthalocyanines and RelatedCompounds: Nonlinear OpticalResponse and PhotoinducedElectron Transfer Process

Yu Chen,1 Mohamed E. EI-Khouly,2�3 James J. Doyle,4

Eleni G. A. Notaras,4 Werner J. Blau,5 Seán M. O’Flaherty5

1Department of Chemistry, Lab for Advanced Materials, East China University of Scienceand Technology, 130 Meilong Road, Shanghai 200237, P. R. China2Department of Chemistry, Faculty of Education, Kafr El-Sheikh, Tanta University, Egypt3Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,COE, Katahira 2-1-1, Sendai 980-8577, Japan4Molecular Electronics and Nanotechnology, School of Physics, Trinity College,Dublin, Republic of Ireland5European Synchrotron Radiation Facility, Grenoble, France

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Theoretical Background for Linear and Nonlinear Absorption . . . . . . . 4

2.1. Linear Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2. Two-Photon Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3. Excited-State Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4. Optical-Limiting Response . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3. NLO and OL Response in Solutions and in the Solid States . . . . . . . . 104. Photoinduced Electron Transfer Process . . . . . . . . . . . . . . . . . . . . . 15

4.1. Porphyrin-Fullerene Systems . . . . . . . . . . . . . . . . . . . . . . . . . 164.2. Chlorophyll-Fullerene Systems . . . . . . . . . . . . . . . . . . . . . . . . 204.3. Phthalocyanine/Naphthalocyanine-Fullerene Systems . . . . . . . . . . 214.4. Porphyrin/Naphthalocyanine-Fullerene

Supramolecular Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 225. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

ISBN: 1-58883-095-0Copyright © 2006 by American Scientific PublishersAll rights of reproduction in any form reserved.

1

Handbook of Organic Electronics and PhotonicsEdited by Hari Singh Nalwa

Volume 0: Pages (1–31)

2 Phthalocyanines and Related Compounds

1. INTRODUCTIONThe realization of photonic technologies depends on two main factors:(1) the existence and understanding of appropriate optical techniques, and(2) suitable optical materials for the fabrication of optical circuits, interconnects, anddevices.

The first factor has already been realized with the development of the laser and the theoryof nonlinear optics. The second factor required for photonic technologies includes suitablematerials, which can facilitate efficient nonlinear optical (NLO) interactions of high conver-sion efficiency and which can be fabricated into useful devices. NLO materials have potentialapplications in optical signal processing, switching, frequency generation (making use of pro-cesses such as harmonic generation, frequency mixing, and optical parametric oscillation),and may also contribute to optical data storage, optical communication, and image process-ing [1]. Such materials are vital for high-speed information processing and for dynamic orpermanent optical information storage applications.The field of nonlinear optics had its beginnings in 1875 with the publication by Kerr of his

observations of an electric field-induced change in the refractive index of CS2, dependent onthe square of the field amplitude, now known as the Kerr effect [2]. This was followed in 1893by a similar, but linear, electric field effect in quartz known as the Pockels (linear electroop-tic) effect [3, 4]. Franken and coworkers opened up the area of NLO with the observationof second-harmonic generation (SHG) in a quartz crystal [5]. SHG in an organic material(benzpyrene) was first observed by Rentzepis and Pao in 1964 [6]. Bloembergen (Nobel Prizewinner, 1981) et al. further advanced this ever-emerging research field by exploring a largerange of NLO responses of materials. Basically, large and fast intrinsic nonlinearities, highdamage threshold limits, and potential ease of processing of organic NLO molecules areof extreme interest. Many forms of materials have been studied including crystals, organicglasses, Langmuir-Blodgett structures, vapor deposited films, clathrate complexes, phthalo-cyanines, porphyrins and polymer films, etc. [7].Currently research in this area thus involves the design and optimization of suitable mate-

rials to perform nonlinear optical processes under laser irradiation. In comparison withinorganic materials, the conjugated organic materials are of a rapidly growing promisingfield of research due to their relatively low cost, high laser-damage thresholds, fast responsetimes, small dispersion in the index of refraction from dc to optical frequencies, virtuallyendless possibilities of structural modification, high nonlinear optical susceptibilities andeasy-in processability, which make them very suitable for their applications in photonic andbiophotonic devices [8–15]. Although too many papers and patents already in existence overthe past 10 years concerning development of organic and polymeric nonlinear optical mate-rials, the design of these materials that simultaneously possess the physical, mechanical, andNLO properties required for specific applications is still a challenge. To obtain device-qualitymaterials, three stringent issues must be addressed:(1) Design and optimization of novel conjugated organic and polymeric functional materialsto perform nonlinear optical processes under laser irradiation;(2) Maintenance of long-term stability toward environment factors such as heat, light, oxy-gen, moisture, and chemicals during the fabrication and measurement of NLO devices; and(3) Effective suppression of the aggregation of conjugated materials (e.g., phthalocyaninesand porphyrins in solutions and in solid states).

This aggregation is generally undesirable since strong intermolecular interactions usually addrelaxation pathways, shorten the excited-state lifetime, and reduce the effective nonlinearabsorption [16].The highly delocalized aromatic 18 �-electron system of phthalocyanines (Pcs) can give

rise to a strong NLO response. Similar to C60 and its organic and polymeric derivatives, Pcsare materials that optically limit nanosecond light pulses in a fairly wide range of the UV/Visspectrum via excited-state absorption processes. Pcs have been extensively investigated assome of the most promising NLO materials due to their architectural flexibility, which allowstailoring of their physical, optoelectronic and chemical parameters in a broad range, and

Phthalocyanines and Related Compounds 3

exceptional stability and processability features [17–19]. The nonlinear optical absorptionmechanism of Pcs in the optical region comprised between Q- and B-bands involves thepopulation of excited states, which absorb more effectively than the ground state. This givesrise to the phenomenon of Reverse Saturable Absorption (RSA) as a consequence of multi-photon absorption [17–20]. It has been shown that phthalocyanine compounds exhibit RSAbecause of the occurrence of intersystem crossing from the lowest excited singlet state (S1)to the lowest triplet state (T1) and the subsequent increase in the population of the stronglyabsorbing T1 state with nanosecond dynamics.The vast possibilities to chemically or physically modify Pcs allow tailoring the speed and

magnitude of NLO responses to the target application. About 70 different elements havebeen incorporated into the phthalocyanine core. Many of the central metal elements haveone or two sites which can coordinate to a variety of axial ligands. The introduction of adiversity of peripheral substituents and axial substituents—with many possibilities to alterthe substitution pattern with respect to the number and position of the substituents—has astrong influence on the electronic absorption spectra and optical nonlinearities [16, 21–24].Other factors that affect optical nonlinearities are the length of �-electron conjugation, thecrystal structure of the compounds, and the thin film fabrication techniques employed (iffilms can be made).The third-order NLO properties with phthalocyanines were first reported in 1987 for

the peripherally unsubstituted chlorogallium phthalocyanine (PcGaCl) and fluoroaluminumphthalocyanine (PcAlF) [25]. The ��3� value for PcGaCl was half that of PcAlF at 1.064 �m.After this, the ��3� values of a various Pcs, including PcAlCl, PcInCl, and PcTiO, were mea-sured using (THG) techniques [26]. Comparison of ��3� values of Pcs with and withoutaxial ligands [17] indicates that Pcs with an axial ligand, for example, PcVO, PcTiO, PcAlF,PcGaCl, PcInCl show a large ��3� in THG experiments. Axial substitution in Pc complexeshas provoked relevant changes in the electronic structure of the molecule by altering the�-electronic distribution due to the dipole moment of the central metal-axial ligand bond[27–35]. Enlarged third-order optical nonlinearities in the charge-transfer complexes such asPcCu-C60 and others have been observed [36, 37]. A supramolecule based on a phthalocyanineand an anthraquinone unit also exhibits an enhancement of the molecular hyperpolarizabilityas a result of the charge transfer between the two chromophores after optical excitation [38].Studies of the NLO properties of Pcs have resulted in the identification of materials that

are among the most promising optical limiting (OL) materials available. It is well-known thatintense laser beams can easily damage delicate optical instruments—especially the humaneye—and consequently researchers in the field of OL have invested much effort into theresearch of materials and processes in an attempt to afford some measure of protection fromsuch beams. In order to have some independent ‘yardstick’ or gauge by which the efficacyof OL materials under investigation might be ‘weighed,’ quantified, and compared, variousparameters have been defined and quoted in the literature. Some authors have referred tothreshold fluence, which is defined as that fluence at which the transmission drops to onehalf of its linear value. Justus et al. [39] reported a value of 0.33 J cm−2 for C60; Qu et al.[40] reported a value of 27 mJ/cm−2 for [C12H25O]8PcPb; and Sun et al. [41] quoted 1 J cm−2

for multi-walled carbon nanotubes suspended in H2O, and 1.1 J cm−2 for C60 in toluene at532 nm, where these samples had linear transmissions of∼50%. Vivien et al. [42] also adoptedthis approach when discussing the wavelength dependence of the optical limiting in single-walled carbon-nanotube suspensions, reporting threshold fluences of 150 mJ cm−2 for H2Osuspensions, and 40 mJ cm−2 for chloroform suspensions under 532 nm nanosecond irradia-tion. Alternatively, the saturation threshold fluence also may be used to quantify the opticallimiting response. This is the fluence at which the transmitted light reaches saturated value.Tutt and Kost [43] reported a saturation fluence of 1 J cm−2 for an 80% transmitting C60solution, and Perry et al. [44] reported 0.47 J cm−2 for tBu4PcInCl. McLean et al. introducedyet another fluence parameter, F c, as a figure of merit for optical limiting in the nanosec-ond case [45]. It was defined as Fc = h/��ex − �0�, where h is the photon energy and �exand �0 are the excited triplet state and ground state absorption cross sections, and 60 mJcm−2 was quoted for a 55% transmitting solution of C60. It should be noted that reports onsuspensions or solutions of materials where certain linear transmissions are quoted—instead


of the mass or molar concentration—add further ambiguity when attempting to estimate amolecular property from the macroscopic response. Despite frequent use of the numerousfluence parameters cited by various authors, the ratio of the excited-state absorption crosssection to that of the ground-state, �ex/�0, appears to have been widely accepted as being anexcellent indicator of limiting action. However, Shirk et al. [16] argue that the �ex/�0 ratiodoes not characterize the strength of the nonlinear absorption, and suggest that the cross-section difference (�ex −�0) is a more useful indicator of limiting action. In phthalocyanines,the largest �ex/�0 ratio reported was 33 [16] for tBu4PcInCl under 532 nm irradiation. Perryet al. [44] reported 30 ± 6 for the same compound. A �ex/�0 ratio of ∼30 at 532 nm hasbeen reported by Wood et al. [46] for a tetrabenzoporphyrin with Zn at the centre metal ofthe porphyrin structure. There clearly exists some disagreement as to how to quantitativelydefine the magnitude of the limiting action of nonlinearly active materials. This is partly dueto the different dissipating mechanisms that may be involved in the optical limiting action,partly due to differences in experimental techniques employed to measure the response, andpartly due to differences in the opinions of the various authors.

2. THEORETICAL BACKGROUND FOR LINEARAND NONLINEAR ABSORPTION

2.1. Linear Absorption

Under ambient light irradiation (low intensity), materials typically exhibit a constant linearabsorption coefficient, 0, usually described by the Lambert-Beer law as:

dI

z= − 0I (1)

This simple differential equation can be solved by integrating with limits on dz going fromz = 0 → L and with limits on dI going from I = IIn → IT . If one defines the transmissionas a function of the incident intensity as T �IIn� = IT /IIn, then the solution to Eq. (1) is

T = exp �− 0L� = exp �−�0NL� (2)

where L denotes the sample length, �0 represents the absorption cross section of the groundstate to first excited singlet state transition (S0 → S1 transitions), and N represents thetotal population under excitation. Thus, in this operating regime the transmission, T , isindependent of the excitation intensity. In this regime the ground state population nevergets significantly depopulated and consequently, as the population has to be conserved, theexcited state never achieves a significant population either. Thus, the light irradiating thesample experiences an approximately homogeneous population distribution as it propagatesthrough the medium. As the excitation intensity increases to levels where the photon densityis comparable to the population density, an induced regime transition may occur where theabsorption coefficient is depleted or enhanced corresponding to increases or decreases in thetransmission, respectively. The resultant regime depends upon the absorption cross sectionsof the ground- and excited-state levels and the lifetimes of the various allowable transitionsin the system.For phthalocyanines, their electronic absorption spectra are characterized by intense

Q-band in the red end of the visible spectrum of light between 600–700 nm, with a molarabsorption often exceeding 105 cm2/mol, and a B-band at 300–400 nm in the blue end ofthe visible spectrum. In the spectra of metal phthalocyanine solutions, the intense Q-bandarises from a doubly degenerate �–�∗-transition between the A1g(a21u) ground state to thefirst excited single state, which has Eu(a11ue

1g) symmetry. The second allowed �–�∗-transition

(B-band) is due to a transition between either an a2u or a b2u orbital to the eg orbital(LUMO). In the case of metal-free phthalocyanine, all states are non-degenerated due tothe reduced D2h molecular symmetry. The Q-band transition is polarized in either the x or ydirection, and is therefore split into two bands. In the spectra of phthalocyanines with opend-shell metal as central atoms, metal to ligand or ligand to metal charge transfer transitionscan be observed [47]. Intermolecular interactions like aggregation give rise to effects like


band broadening, blue shift of the Q- and B-band or to an observed splitting of the Q-band[48–51]. In the solid state, coupling between dye molecules creates an exciton band structurewith much broader absorption bands, often covering about half the solar spectrum. In con-trast to phthalocyanines, naphthalocyanine compounds, which are almost transparent in thered light region (600–700 nm) where phthalocyanines have a considerable linear absorptioncoefficient due to the Q-bands, have a B-band in the region of 350 nm, but conversely, theQ-bands are red-shifted by about 100 nm to around 800 nm. The UV/Vis absorption spectraof the highly conjugated porphyrin compounds usually exhibit an intense feature (extinctioncoefficient > 200000) at about 400 nm (B-band), followed by several weaker absorptions(Q-bands) at higher wavelengths (from 450 to 800 nm). Figure 1 compares electronic absorp-tion properties of porphyrin, phthalocyanine, and naphthalocyanine.

2.2. Two-Photon Absorption

Two-photon absorption is the simultaneous absorption of two photons of energy, h, tospan an energy bandgap of approximately equal—but more likely slightly less than—2 h.One can consider an incident optical beam, with frequency irradiating a material with,for example, an S0 → S1 transition of energy ≈2 h with sufficient intensity such that twophotons may be simultaneously absorbed through a virtual energy level lying between S0and S1. This hypothetical system is sketched in Figure 2. After a small vibrational relaxation,the excited population may then occupy the S1 orbital from where it may subsequently re-radiate or irrationally relax back to the ground state. This is the same principle by whichSHG operates, where two photons of frequency, , are absorbed simultaneously in a crystaland re-radiated as a single photon of frequency 2 h or with wavelength equal to half thatof the initial excitation.

2.3. Excited-State Absorption

Saturable absorption refers to the case of excited-state absorption (ESA), where the trans-mission increases with incident intensity, and indicates that the ground state absorption hasbleached. Conversely, the opposite situation, where the transmission decreases with increas-ing incident intensity, is termed reverse saturable absorption (RSA). This occurs when theexcited-state absorption cross-section is greater than that of the ground state, and was ini-tially discussed by Guiliano and Hess in 1967 [52].Hercher conducted a general analysis of saturable absorbers using a three-level scheme

[53]. Under the steady state approximation, where the populations of energy levels remainconstant for the duration of the pulse, he derived a general expression for the absorptioncoefficient in terms of a parameter called the saturation intensity, ISat:

= 0

1+ IISat

(3)

Abs

orba

nce

800700600500400

Wavelength / nm

ZnPc

ZnNc

ZnOEP

ZnTPP

Figure 1. Steady-state absorption spectra of zinc tetraphenylporphyrin (ZnTPP), zinc octaethylporphyrin (ZnOEP),zinc phthalocyanine (ZnPc), and zinc naphthalocyanine (ZnNc).


S1

S0

hνVirtual level

Vibrationalrelaxation

hν

Figure 2. Sketch of two-photon absorption process. In the hypothetical system, the S0 → S1 transition is of energy≈2 h, where h is the energy of a single-incident photon.

where ISat = h/��0�10�, and �10 refers to the excited-state relaxation time of the bleachingtransition. ISat is the intensity at which the absorption coefficient drops to one-half its linearvalue. Hercher also investigated the effect of excited-state absorption on the transmissionof a saturable absorber, noted that it precluded the possibility of the absorption bleachingtotally, and termed this “residual absorption.”To discuss the importance of the various parameters that influence RSA in the phthalocya-

nine system, a general model of a five-level system, such as that shown in Figure 3, has beeninvestigated, and approximations suitable for the phthalocyanine system under nanosecondirradiation have been applied. Laser rate equations were used to simulate the excitationand subsequent relaxation of the system. The vibrational levels of the electronic states areignored, and for the sake of simplicity the laser pulse width is assumed to be longer thanany of the lifetimes associated with the levels. As such, this is still a complicated system, andin an effort to further simplify matters it can be assumed that relaxation from states S2 andT2 is very rapid, so that the population of these two levels may be neglected.Generally, after initial excitation of this five-level system the first excited singlet state S1

is populated. From here the electrons may be subsequently excited into S2 within the pulsewidth of the laser. Once in S2 they rapidly relax to S1 again. From S1 the population mayundergo an intersystem crossing to the first excited triplet T1 with a time constant �isc, andthereafter be excited into T2. Similarly, to S2 this state relaxes rapidly and the populationis exchanged between T1 and S1 cyclically since the lifetime of T1 (�ph) is very long incomparison to �isc. Furthermore, stimulated emission from S1 is excluded due to the smallfluorescence quantum yield. The system now reduces to the following set of three differentialrate equations:

�n1

�t= −�0I

hn1 +

n2

�10+ n3

�ph(4)

S2

S1

S0

T2

T1

n1

n2

n3

σS

σex(σT)

σ0

τisc

τ10

τph

τnr

τnr

Figure 3. Generalized five-level system used in deriving the excited-state absorption model used to simulate RSAin the phthalocyanine system. Si represents singlet levels and Ti represents triplet levels. Solid arrows imply anexcitation resulting from photon absorption, and jagged arrows represent relaxations.


�n2

�t= �0I

hn1 −

n2

�isc− n2

�10(5)

�n3

�t= n2

�isc− n3

�ph(6)

where n1, n2, and n3 refer to the populations of S0, S1, and T1, respectively. Attenuation ofthe laser beam is governed by a propagation equation where the absorption coefficient nowincludes the excited-state absorption from S1 and T1:

�I

�z= − I = −��0n1 + �Sn2 + �T n3�I (7)

Under the steady state approximation, which is valid when the pulse width is much longerthan any relaxation time, all the time derivatives may be set to zero. This is a valid assumptionfor nanosecond pulses, as the lifetimes in phthalocyanines are typically on the order ofpicoseconds [16]. In this case, the equation can be easily solved analytically and the intensity-dependent absorption coefficient becomes:

�I� = �0N

1+ IISat

(1+ �S

�T· �isc�ph

· I

ISat+ �T�0

· I

ISat

)(8)

Noting that 0 = �0N (where N is the total number density of dissolved molecules), defining� = �T/�0 (noting that �T and �S are probably of approximately the same order when�isc �ph�, and as the triplet yield approaches unity for phthalocyanines anyway, one isin a position to eliminate the term with �S in the numerator. This effectively reduces thefive-level model to a three-level model, with the intensity-dependent absorption coefficientdefined as:

�I� ISat� �� = 0

1+ IISat

(1+ � I

ISat

)(9)

where, in this case, the excited-state absorption cross-section (�ex) is due almost entirelyto �T, and thus � = �T/�0. It can be noted that the intensity (I) and the pulse energydensity (defined as F = EPulse/(� �z�2), where EPulse is the energy per pulse and � �z�2

is the surface area through which the pulse is propagating at any position denoted by z),are directly related to each other. Consequently, the parameter I /ISat can be replaced withF /FSat in Eq. (9), where in this case, FSat is the energy density saturation.The changes in the normalized absorption coefficient ( �F � FSat� ��/ 0� with F /FSat are

displayed for different values of � in Figure 4. From these it is apparent that whenever theexcited-state absorption cross-section is greater than that of the ground state (i.e., � > 1),RSA is observed as an increase in the normalized intensity-(or energy-density) dependent

κ = 10

(a) (b)

Figure 4. (a) Plot of the normalized absorption coefficient �F � FSat� ��/ 0 against normalized pulse energy densityF /FSat for different values of �� = �ex/�0�. (b) Plot of �F � FSat� ��/ 0 against F /FSat�� = 10� for different valuesof FSat.


absorption coefficient ( �F � FSat� ��/ 0), with respect to increasing normalized pulse energydensity F /FSat. Whenever the converse is true, as in � < 1, saturable absorption is observed asa reduction in �F � FSat� ��/ 0 with increasing F /FSat. This model, though simple, reproducesthe gross effects of RSA and highlights the crucial role that excited-state absorption playsin the overall absorption coefficient. This steady-state model approximates to a dynamicmodel in the limit of temporally long pulse widths (i.e., nanosecond irradiation) when allother lifetimes in the material are of the order of picoseconds. A full dynamic model cannotbe approached analytically as the rate equations involved make up a set of coupled partialdifferential equations that must be numerically integrated over time and space.

2.4. Optical-Limiting Response

In 1939, Miles A. Dahlen wrote about phthalocyanines being the “first new chromophore ofcommercial importance developed in a quarter of a century” [54]. With the invention of thelaser in the 1960’s, [55] the merging of structural material engineering and quantum physicsbegan, which lead to such research areas as nonlinear optics. Numerous investigations beganinto novel NLO properties including nonlinear absorption, scattering, 2nd order and 3rdorder NLO properties, etc. [56]. As a consequence, diverse NLO processes including RSA[52] began to acquire much industrial attention and interest. It is this NLO property whichleads to the concept of Optical Limiting (i.e., strong attenuation of high intensity light andpotentially damaging light—such as focused laser beams—whilst allowing for the high trans-mission of ambient light) [57, 58]. Industrial interest in this field has been driven by the needto protect expensive optical equipment—most importantly the human eye. This delicate yetrobust “instrument” is a remarkable sensor, exhibiting multiple optical broadband responses,the ability to self-adjust to varying optical environments, and a dynamic range in excess of100 dB.As shown in Figure 5, the optical limiting curve can be plotted as input fluence (or energy)

versus output fluence (or energy) (see upper panel in Fig. 5) or, input fluence (or energy)versus transmittance (see lower panel in Fig. 5). An important term in the optical limitingmeasurement is the limiting threshold. It is defined as the input fluence (or energy) at whichthe transmittance is 50% of the linear transmittance. It is obvious that the lower the opticallimiting threshold, the better the optical limiting material. If different materials are measuredwith the same linear transmission, the experimental and system errors can be minimized.Importantly, the error from the nonlinear scattering can be minimized as well.Several mechanisms giving rise to NLO responses can operate simultaneously in a given

system. A perturbation of the electronic distribution in the material as a response to theelectric field of the incident (low intensity) light is reason for normal linear polarizationto occur. At high intensities, the electronic distribution no longer follows the applied field,resulting in both second- and third-order nonlinearities. The second common mechanismthat can contribute to the NLO response is molecular reorientation. This mechanism can

Limiting threshold

Nonlinear absorption

Linear absorption

Incident Fluence (J/cm2)

Tra

nsm

itted

Flu

ence

(J/

cm2 )

(a)

Incident Fluence (J/cm2)

Limiting threshold

T = 50% of lineartransmittance

Linear transmittance

Tra

nsm

ittan

ce

(b)

Figure 5. The response of an optical limiter. An ideal limiter limits the output energy to some specified value.Reproduced with permission from [79], Y. Chen et al., Chem. Soc. Rev. 34, 517 (2005). © 2005, Royal Society ofChemistry.


lead to nonlinear refractive indices with a picosecond response time, but it will not con-tribute to the third harmonic generation signal. In general, an orientational nonlinearity canbe larger than an electronic nonlinearity. Another important mechanism for a third-orderNLO response is optical pumping. In this case, the incident laser frequency approaches atransition frequency in the molecule. The light is absorbed causing transition to an excitedstate. The optical properties of the excited state differ substantially from those of the groundstate. The higher the population in the excited state, the larger the changes in the opticalproperties of the material and the larger the optical nonlinearities. Optical pumping involvesreal transitions to excited states. This is a big difference compared to the small perturbationsof the electronic cloud mentioned above. This mechanism is the most important mecha-nism giving rise to saturable and reverse-saturable absorption. Like the second mechanism,the response time for optical pumping is too slow to give rise to frequency mixing andharmonic generation, but it does give useful nonlinear absorption and refraction qualities.Figure 3 gives a generalized five-level model used in calculation of excited-state dynamicsof a phthalocyanine system. This mode describes the interaction of light with a molecularcompound in terms of electronic transitions in the material [59]. Within a few femtoseconds,the ground state (S0) of a molecule is excited to a higher vibronic level of the first singletstate (S1) by a laser with pulse width (�). Via radiationless relaxation processes, the lowestvibronic level of S1 is reached within about a picosecond. Several competing processes canoccur from here. These incur further radiationless relaxation, fluorescence, or intersystemcrossing from S1 to the triplet state (T1). The latter process sometimes takes place withinseveral picoseconds. The very long lifetime of about many microseconds or longer of T1reflects that the transition of T1 to S0—known as phosphorescence—is forbidden by theselection rules for electronic transitions. The long lifetime of T1 is one of the prerequisitesfor a large positive nonlinear absorption coefficient.Usually, the most important mechanisms for optical limiting are nonlinear absorption,

nonlinear refraction, and nonlinear scattering. A negative nonlinear refractive index givingrise to self-defocusing of a light beam can cause substantial amounts of the energy of theincident light to be absorbed by an exit aperture in the optical system. Materials with apositive nonlinear absorption coefficient exhibit RSA, and are characterized by a high trans-mission at normal light intensities and a decrease in transmission under high intensity orhigh fluence illumination. Conversely, the opposite situation, a process where the trans-mission increases with increasing incident intensity, is termed saturable absorption (SA).The materials possessing SA usually have a negative nonlinear absorption coefficient. Foroptical limiters that rely on RSA from rapidly photogenerated transient states, [56, 60, 61]they are almost transparent for weak light but opaque for intense light. An efficient RSAmaterial has a high ratio of excited state (T1 → T2) to ground state (S0 → S1) absorptioncross-section (�1), a rapid intersystem crossing rate (�isc �), a long internal conversionlifetime, a high intersystem crossing quantum yield ("S1→T1

∼ 1), and a long triplet lifetime(�T1

� �), where the triplet state of the material absorbs the incoming laser so effectivelythat the laser can be greatly attenuated and the sensors can be protected. Two-photonabsorption (TPA), [62] associated with the imaginary part of third-order susceptibility, mayalso account for optical limiting. Basically, in the visible region organic chromophore, RSAstend to have their primary absorption, #max��–�∗�, at 400 nm or below, to give reason-able transmission for solution or thin film measurements. Two-photon absorbers, on theother hand, would tend to depend on absorption in the 600–800 nm range to give riseto optical limiting effects in the visible region [62]. TPA chromophores whose limits arebetween 600 and 800 nm via enhanced two-photon absorption, would complement chro-mophores which limits are between 400 and 600 nm by RSA. It would thus be very desirableif one could design OL chromophores that might function in a bimechanistic fashion, forinstance:(1) RSA behavior at the high energy end of the visible, and(2) TPA behavior at the low energy end of the visible [62].

In the case when the incident light is sufficiently intense so that a significant populationaccumulates in the excited state and if the material has an excited state absorption cross-section, �ex, that is larger than the ground state cross-section, �0, the effective absorption


coefficient of the material increases. To achieve the largest nonlinear absorption, both a largeexcited-state absorption cross-section and a long excited-state lifetime are required. Whenthe lifetime of the excited state being pumped is longer than the pulse width of the inci-dent light, the changes in the absorbance and the refractive index are fluence (J cm−2)—notintensity (W cm−2)—dependent. Therefore, in materials with long upper-state lifetimes, it isthe fluence rather than the intensity that is limited. Limiting the fluence is usually desirablesince damage to optical devices is also often fluence-dependent. Some criteria necessary fora large, positive nonlinear absorption are apparent, including a large excited-state cross-section, �ex, and a large difference between the ground- and excited-state absorption cross-sections (�ex − �0). A variety of organic and organometallic materials, including porphyrins,metallophthalocyanines, fullerenes, organometallic cluster compounds and other materialshave been found to fulfill these conditions [63–67]. For porphyrins and metallophthalocya-nines, they can exhibit strong excited-state absorption, high triplet yields and a long excited-state lifetime, while their ground-state absorption is mostly confined to a few narrow regions(B- and Q-bands), allowing high transmission in the spectral window between these bands[17, 66]. The spectral bandwidth or window over which the limiter operates, and the groundstate and excited-state spectra and lifetimes, can be controlled or engineered by altering theaxial or peripheral substituents, central metal cations, and the structure of the main rings.A practical optical limiter must operate over the wide range of incident intensities that

might be encountered. The condition that �ex is greater than �0 is necessary but it is not suf-ficient for a useful OL material. The nonlinear response should possess a low threshold andremain large over a large range of fluences before the nonlinearity saturates. High saturationfluence normally requires a high concentration of the nonlinear material in the optical beam.For an organic material, this means it must be highly soluble in common organic solvents, orit must be a pure liquid or a solid film that can be prepared with good optical quality. Fromthe device point of view, the use of several successive limiters (tandem strategy) seems usefulto optimize both the overall limiting and damage thresholds in phthalocyanines [68, 69]. Itshould be noted that the response time of the nonlinear absorption and refraction in thinfilms of Pc materials may be strongly affected by intermolecular interactions [70]. The useof a heavy-metal Pc material together with an appropriate nonhomogeneous concentrationprofile along the beam path leads to greatly improved optical limiting devices [66, 71].

3. NLO AND OL RESPONSE IN SOLUTIONSAND IN THE SOLID STATES

It is known that many of the dyes, including phthalocyanines and porphyrins, used as non-linear materials tend to aggregate at high concentrations. These aggregates, which usuallyadd relaxation pathways, shorten the excited-state lifetime, and reduce the effective non-linear absorption [16], are usually depicted as a coplanar association of rings progressingfrom monomer to dimer and higher-order complexes, and are driven by enhanced Van derWaal’s attractive forces between phthalocyanine rings [72]. The tendency to aggregation ismainly dependent on the structural characteristics of the phthalocyanine compounds (e.g., inaxial-substituted phthalocyanine compounds the intermolecular interactions are weaker thanthose without such substituents. A great deal of experimental results have demonstrated thataxial substitution in Pc complexes provokes relevant changes in the electronic structure ofthe molecule by altering the �-electronic distribution due to the dipole moment of the cen-tral metal-axial ligand bond [27–35]. In terms of optical limiting effectiveness in the visiblerange, axial substitution can improve the efficiency of excited-state absorption through theinhibition of the decay of the excited state formed in a nonlinear optical regime.Most phthalocyanines are centrosymmetric materials except for monoaxially substituted

Pc complexes like PcTiO, PcVO, PcAlCl, PcInCl, or PcGaCl. Third-order optical nonlinear-ities are therefore of main interest in Pc materials, as they can be large with sub-picosecondresponse times. NLO properties of porphyrins and phthalocyanines mostly correspond tothe effect of RSA. Such an effect can be altered via the insertion of proper metals inthe center of the conjugated macrocycle and through the functionalization of the variousparts constituting the conjugated macrocycle-metal complex [73]. A new concept based on


molecular engineering for the design and synthetic preparation of supramolecular reversesaturable absorbers (SupraRSA) for strong optical limiting at concerned wavelengths or ina concerned wavelength range was proposed by Sun and his coworkers [74]. As an example,the optical limiting properties of the dyad carotene-porphyrin as a candidate for SupraRSAhave been investigated in detail [75]. Comparatively, solution nonlinear-spectroscopy dom-inated research in the NLO field with phthalocyanines and/or porphyrins in recent times,owing mainly to investigations of the many dramatic and profound new phthalocyaninesand/or porphyrins that have been synthesized [27–35, 76–78]. Several excellent reviews onthe NLO properties of phthalocyanines and porphyrins in solutions, and on the structure-property relationship of these materials, which was only partially accomplished in the past,have recently been published in peer-review journals [67, 68, 73, 79–83]. It should be notedthat in many cases comparison between NLO processes, wavelength regimes, measurementtechniques, and investigators is tenuous [68]. This is a basic problem in all nonlinear studies.A practical NLO device built from porphyrins and phthalocyanines would inevitably

involve incorporating the porphyrin or phthalocyanine units into some form of a multi-layered solid-state entity. Polymerizing the porphyrins and/or phthalocyanines, or embeddingthem as inclusions in a polymer host to form a composite material, would allow tradi-tional methods such as spin casting to be employed to produce suitable films for thesesolid-state applications. Fewer studies, however, have considered the practical applicationof such compounds which would almost inevitably involve the solid-state casting of suchcompounds in some manner [44, 84–86]. The use of guest-host systems, where phthalo-cyanines or porphyrins assume the role of guest inclusions in an otherwise homogeneoushost polymer film is a feasible method toward applying the phthalocyanine or porphyrinmoiety “en route” for real-world optical-limiting applications. O’Flaherty and Blau haveproduced and discussed high quality, relatively smooth, polymer-phthalocyanine compositefilms with surface roughness—or on the order of only nanometers—as passive solid-stateoptical limiters [87, 88]. Figure 6 gives an example for the thickness measurement of thePc-polymethylmethacrylate (PMMA) films. A simple approximate nonlinear absorption coef-ficient based on molecular orbital analysis has been developed, which allows one the useof two “yardstick” parameters to quantify the nonlinear optical response. Additionally, thisnonlinear absorption coefficient is folded with the distribution of energy in the exciting pulseto model more closely the experiment, and it was shown that by omission of this spatial reso-lution one may underestimate the numerical magnitudes of the nonlinear optical parameters.The linear optical absorption spectra of the five films in the visible to near-IR regions

are depicted in Figure 7. Over this excitation range the pristine polymer case in the samemanner as the composite films exhibited a flat response and has been omitted from thefigure. In all cases, it can be seen that the Q-band is significantly broader from that gener-ally observed in solutions of the same molecules. This is due to intermolecular aggregationeffects not unexpected, as the sample is cast in the solid state. These effects are considerablylower than one would observe from drop-cast films. Despite the aggregation, the signatureQ-band was observed in all cases and the “window” region spanning the green wavelengths(∼500–600 nm) is clearly evident. The linear absorption coefficients measured in solutionsand in the solid states at 532 nm are presented in Table 1. The solution and solid-state linearand nonlinear optical responses are very different. For each composite film, its nonlinear

(a) Overall thickness: 44.42 µm (b) Overall thickness: 4.31×10–3 µm

Figure 6. Thickness measurements of (a) tBu4PcInCl-PMMA film (including gold coating) and (b) gold coating ona substrate.


Figure 7. UV/Vis absorption spectra of the films of PMMA-phthalocyanine host-guest composites.

absorption coefficient is two orders of magnitude higher that value exhibited by the corre-sponding phthalocyanine in solution, as listed in Table 1. The Fsat values are noticeably lowerthan the solution values. It should be noted that transferring the results of solution studiesto solid state systems may not be straightforward. A future study repeated with all promisingphthalocyanine molecules for NLO (including OL), embedded in multi-layer polymer filmswould be extremely beneficial and would yield important information for the fabrication ofsolid state practical NLO (OL) devices.Optical limiting with phthalocyanines was first reported for the chloroaluminum phthalo-

cyanine (PcAlCl) [89] and subsequently many other phthalocyanine compounds have alsobeen investigated as passive optical limiting materials. Perry et al. [44] studied a seriesof phthalocyanine compounds, proposed a practical optical limiting device structure, andfabricated this using tetrasubstituted chloroindium phthalocyanine (tBu4PcInCl). Undernanosecond irradiation, this device was able to attenuate laser pulses by factors up to 540.Furthermore, Henari et al. reported the nonlinear optical properties of a series of phthalo-cyaninatotitanium (IV) oxide with different peripheral groups (RxPcTiO) [90]. The opticallimiting properties of an octasubstituted lead phthalocyanine, [C12H25O]8PcPb, has also beenstudied by Qu et al. [40]. The nanosecond optical-limiting properties of nickel phthalo-cyanines, [91] gallium- and indium phthalocyanines, and naphthalocyanines [27–35] havebeen reported. Shirk et al. reported the effects of axial substitution at the central metal

Table 1. Optical coefficients for the materials measured. In the first column, (S) implies experiments in solution(0.5 g/L in toluene) while (F) implies experiments in solid-state. In the remaining columns, L is the optical pathlength, 0 is the linear absorption coefficient, � is the ratio of excited to ground-state absorption cross-sections, $I

are third-order nonlinear absorption coefficients, and Fsat is the saturation energy density. Values for 0� �, and Fsatwere determined at 532 nm excitation.

Materials I [�m] 0 [cm−1] � %�ex/�0& $I [cm W−1] Fsat [J cm−2] Ref.

(PhS)4PcZn (S) 1000 1'49 13'0± 0'4 �3'9± 0'6�× 10−8 5'7± 0'2 [20, 87]tBu4PcZn (S) 1000 1'64 11'2 ± 0'6 �2'8± 0'5�× 10−8 8'8± 0'6 [20, 87]%tBu4PcIn&2O (S) 1000 1'13 12'4± 0'3 �4'6± 0'9�× 10−8 7'6± 0'3 [28, 80]tBu4PcInCl (S) 1000 0'53 27'4± 0'6 �4'4± 0'9�× 10−8 24'2 ± 0'8 [28, 32]PMMA(PhS)4PcZn (F) 36.4 207'5 7'5± 0'3 �6'2 ± 1'0�× 10−6 3'0± 0'1 [20, 87]PMMAtBu4PcZn (F) 34.5 207'1 6'7± 0'2 �5'7± 0'9�× 10−6 3'6± 0'3 [20, 87]PMMA[tBu4PcIn]2O (F) 41.9 151'5 4'0± 0'1 �8'0± 1'7�× 10−6 1'8± 0'1 [20, 88]PMMAtBu4PcInCl (F) 34.7 167'3 4'1± 0'1 �7'6± 1'6�× 10−6 2'0± 0'1 [20, 88]PMMAtBu4(Me2N)PcVO (F) 7.6 301 10'0± 0'1 �3'2 ± 0'6�× 10−6 11'5± 0'5 [88]


atom on the optical-limiting performance of indium phthalocyanines [16]. They found thatthe optical-limiting properties of these materials were robust relative to structural changesin the axial position and, consequently, the changing of axial substituents from chloro top-trifluoromethylphenyl (p-TMP) results in somewhat of an enhancement of the optical-limiting response. Tian et al. [92] reported the nonlinear properties of non-aggregated zincand vanadium phthalocyanines, where they found that the zinc phthalocyanine had a largermacroscopic optical nonlinearity than the vanadium phthalocyanine. The optical nonlinear-ities of PcYb2 were reported by Mendonca et al. [93]; PcLu2, PcNd2, and PcEu2 by Wenet al. [94], and of tin phthalocyanine thin films by Yamashita et al. [95]. Previous reportsinto device modelling [84, 96] suggests that heavy-atom Pc materials in optimized optical-limiter designs that ensure uniform saturation could produce attenuations of 104 magnitudefor devices with 70% linear transmission.Numerous papers have reviewed various aspects of this topic, [68, 80, 97–100] and the

reader is referred to these papers, for both complementary and additional information to thisreview, including in-depth examinations of the relationship between the OL response andthe chemical structure for both phthalocyanines and related compounds. Table 2 presentsoptical-limiting parameters for a selection of phthalocyanines.

Table 2. Nonlinear optical and optical-limiting parameters for a selection of phthalocyanines in solution. T impliestoluene and C implies chloroform; p-TMP, tmed, p-CPO, SDPO, and Nc are p-trifluoromethylphenyl, N ,N ,N ′,N ′-tetramethylethylenediamine, p-chlorophenoxy, 4,4′-sulfonyldiphenoxy, and naphthalocyanine, respectively.

Fsat 0

Compounds � %�ex/�0& [J cm−2] $eff [cmW−1] Im{��3�eff } [esu] [cm−1] Sol. Ref.

tBu4PcInCl 27'4± 0'6 24'2 ± 0'8 �4'4± 0'9�× 10−8 �1'6± 0'3�× 10−11 0.53 T [28][tBu4PcIn]2O 12'4± 0'3 7'6± 0'3 �4'6± 0'9�× 10−8 �1'5± 0'3�× 10−11 1.13 T [28][tBu4PcIn]2'2tmed 12'5± 0'4 9'8± 0'5 �2'5± 0'5�× 10−8 0.93 T [29, 32]tBu4PcIn(p-TMP) 14'8± 0'3 7'0± 0'2 �4'0± 0'8�× 10−8 0.93 T [32]tBu4PcGaCl 13'5± 0'4 27'0± 1'0 �3'2 ± 0'6�× 10−8 �1'2 ± 0'2�× 10−11 1.10 T [32][tBu4PcGa]2O 11'3± 0'1 13'5± 1'0 �3'5± 0'7�× 10−8 �1'3± 0'2�× 10−11 1.60 T [28]tBu4PcGa(p-TMP) 13'6± 0'4 8'4± 0'4 �2'9± 0'5�× 10−8 �1'1± 0'2�× 10−11 0.91 T [32]tBu4PcGa(p-CPO) 9'5± 0'4 16'3± 0'9 �0'9± 0'2�× 10−11 1.37 T [79, 115][tBu4PcGa]2'SDPO 4.8 ± 0.6 7.5 ± 0.8 (0.7 ± 0.1)×10−11 3.13 T [79, 115][tBu4PcGa]2' 10'4± 0'3 8'9± 0'4 �3'5± 0'7�× 10−8 1.30 T [79]

2dioxane(C6H13)8PcPd 5'9± 0'1 2'1± 0'1 �9'6± 1'9�× 10−8 �3'6± 0'7�× 10−11 2.60 T [79](C6H13)8PcInCl 16'1± 0'3 10'1± 0'5 �3'2 ± 0'6�× 10−8 �1'2 ± 0'2�× 10−11 0.93 T [20](C6H13)8PcZn 11'4± 0'3 7'1± 0'3 �4'0± 0'8�× 10−8 �1'5± 0'3�× 10−11 1.17 T [20](C6H13)8PcPb 16'1± 0'3 9'8± 0'3 �2'9± 0'6�× 10−8 �1'1± 0'2�× 10−11 0.83 T [20](C6H13)8PcNi 2'4± 0'2 18± 3 �1'6± 0'3�× 10−8 �5'9± 1'1�× 10−13 1.05 T [20](C6H13)8PcH2 14'5± 0'3 16'8± 0'6 �1'8± 0'3�× 10−8 �6'6± 1'3�× 10−12 0.94 T [20](C10H21)8PcCo 2'2 ± 0'7 95± 70 �1'6± 0'3�× 10−8 �5'9± 1'1�× 10−14 0.83 T [20](C10H21)8PcH2 14'4± 0'5 20'2 ± 1'0 �1'5± 0'3�× 10−8 �5'8± 1'1�× 10−12 0.83 T [20](C10H21)8PcNi 2'1± 0'1 13'3± 2'0 �1'5± 0'3�× 10−8 �5'5± 1'1�× 10−13 0.94 T [20](C10H21)8PcZn 11'7± 0'3 13'6± 0'5 �2'4± 0'4�× 10−8 �9'1± 1'8�× 10−12 1.17 T [20][(OC4H9)6(C 4'9± 0'3 32 ± 3 �7'2 ± 1'4�× 10−9 �2'7± 0'5�× 10−12 1.80 T [20]

CH)]PcCo[tBu3(C 8'9± 0'3 13'9± 0'7 �3'5± 0'7�× 10−8 �1'3± 0'2�× 10−11 1.95 T [20]

CH)]PcZn[tBu3(C 3'3± 0'8 76± 30 �1'4± 0'3�× 10−9 �5'1± 1'0�× 10−13 1.76 T [20]

CH)]PcCoPcCo[tBu3C 11'0± 0'1 9'5± 0'2 �3'5± 0'7�× 10−8 �1'3± 0'2�× 10−11 1.60 T [20]

CtBu3]PcCoPcCo[tBu3C 1'8± 0'1 1'3± 0'1 �5'6± 1'1�× 10−9 �2'1± 0'4�× 10−12 1.22 T [20]

CtBu3]PcZnPcZn[tBu3C 3'0± 0'1 1'9± 0'1 �1'2 ± 0'2�× 10−8 �4'6± 0'8�× 10−12 1.06 T [20]

CtBu3]PcZnPcZn[tBu3C 5'4± 0'2 3'9± 0'3 �2'3± 0'5�× 10−8 �8'7± 1'6�× 10−12 1.19 T [20]

CtBu3]PcZntBu4NcGaCl 4'8± 0'2 5'6± 0'2 �5'8± 1'1�× 10−8 �2'2 ± 0'4�× 10−11 4.40 C [30]tBu4NcGa (p-TMP) 9'2 ± 0'8 8'5± 0'9 �2'6± 0'5�× 10−8 �9'7± 1'9�× 10−12 2.30 C [30][tBu4NcGa]2O 4'1± 0'1 3'9± 0'2 �4'8± 0'9�× 10−8 �1'8± 0'3�× 10−11 5.00 C [30]


As previously discussed for phthalocyanines, the theory of optical limiting is based onreverse-saturable absorption. The expansion of the �-electron system has long been inves-tigated for potential OL properties, have resulted in such materials as naphthalocyanines.From a synthetic view point, the preparation of fused binuclear planar phthalocyanines[101–104] is another interesting approach since it leads to condensed Pc-based systems withdistinct electronic properties with respect to those of the separated Pc units [28, 30, 32, 34,67, 80, 81, 105–115]. Furthermore, wavelength tuning of organic compounds to extend theRSA band has been investigated by various groups [95, 116, 117]. To achieve this, compoundssuch as anthralocyanines [118] were synthesized, which resulted in a shift of the Q-band tolonger wavelengths. However, chemical stability and solubility issues led to the fabricationof more exotic structures such as porphyrin dimers [116, 117]. These novel materials displaychemical and photo stability characteristics as well as a noted red shift in the absorptionspectra (Q-band of ∼1000–1200 nm). McEwan et al. [116] showed that, by using meso-bromoand meso-iodo substituents, noted triplet yields are achievable, thus allowing investigationsinto excited triplet-state dynamics.Nalwa et al. [117] examined a series of vanadylphthalocyanine compounds (VOX(R)4) in

both Pc and Nc form with varying terminal alkyl-chain groups attached. For the compoundVONc(COO5H11)4, he concluded that it was possibly both the change of ring size and ligandthat ultimately affects the packing of the molecules and thus causes variations in the inter-molecular interactions and the optical nonlinearity. Tutt et al. [56] studied the enhancementof the dynamic range through the use of multiple devices in series. Said et al. [119] demon-strated a high dynamic range and a low activation threshold for optical limiting with suchdevices. This two-component limiter comprised of a liquid optical limiter, which exhibitsRSA, was placed in front of a solid-state two-photon/nonlinear refractive material. When aPcCl derivative was used as the RSA material and the solid material was ZnSe, the dynamicrange was increased by about a factor of 5 and the limiting threshold was reduced by one-half over that obtained using ZnSe alone. However, introducing a liquid NcSi derivative asthe optical limiter increased the dynamic range by 30, but the threshold was twice as large.It has already been reported that the introduction of bromine atoms at the pyrrolic positions

drastically increases the intersystem crossing rate [120]. Some authors have measured thereverse saturable absorption behavior of mesotetraalkynylporphyrins [121–126]. Specifically,a systematic survey of groups III and IV metal complexes of porphyrins was been reportedrecently [126]. The authors found an increase in the �ex/�0 when moving toward the heaviermembers of each group. Thus, the In(III), Tl(III), and Pb(II) complexes show the highest

Figure 8. Typical open aperture z-scans for a (PhS)4PcZn molecular solution and a nanoparticle dispersed sample.


Table 3. Optical limiting results at 532 nm. The reference (M) implies molecules in solution, while (NP) impliesnanoparticles dispersed in deionized water.

� Fsat $eff EPulse 0

Samples [�ex/�0] [J cm−2] [cm W−1] [mJ] [cm−1] Ref.

(PhS)4PcH2(M) 2'1± 0'2 9'4± 1'8 �1'0± 0'2�× 10−10 0.27 0.065 [78a](PhS)4PcH2(NP) 1'7± 0'5 48± 45 �1'6± 0'3�× 10−11 0.34 0.071 [78a](PhS)4PcZn (M) 4'0± 1'0 20'0± 0'2 �4'0± 0'8�× 10−10 0.20 0.135 [20](PhS)4PcZn (NP) 11'3± 0'6 18'5± 1'4 �2'1± 0'4�× 10−9 0.30 0.146 [20]tBu4PcZn (M) 5'1± 0'4 14'2 ± 1'5 �4'9± 1'0�× 10−10 0.16 0.100 [20]tBu4PcZn (NP) 14'6± 2'1 17'5± 3'6 �1'6± 0'3�× 10−9 0.20 0.095 [20]tBu4PcCu(M) 4'4± 0'7 6± 1'4 �3'5± 0'7�× 10−10 0.11 0.082 [78a]tBu4PcCu(NP) 9'6± 0'1 100± 40 �1'5± 0'3�× 10−10 0.24 0.084 [78a]

cross section ratio for macrocyclic dyes; this fact indicates that the heavy atom effect is againone of the most significant factors influencing limiting behavior. More recently, the nonlinearoptical transmission of meso-tetrakis{4-[2-(trimethylsilyl)ethynyl]-phenyl}porphyrins (Zn(II),Ni(II), Ga(III), In(III), and Sn(IV) complexes) have been determined [127]. The amount ofexcited-state absorption is increased with regard to the cases of previously reported TPPs[128] by modifications at the para positions of meso-phenyl rings.Other interesting dyads with the predicted large excited-state absorption cross-section

in their charge-separated state are those comprising both a porphyrin and fullerene moi-eties. Specifically, a donor–acceptor system composed of a porphyrin and a fullerene linkedthrough an o-phenylene bridge, has been prepared and measured for its optical limiting abil-ity [129, 130]. The OL performance reported for the dyad is poorer than that of the modelcompounds, probably due to ultrashort lifetimes of the charge separated species.Subphthalocyanines have also shown nonlinear absorption properties. Trineopentyloxy-

subphthalocyanine spin-coating films have been measured via the Z-scan technique. Reversesaturable absorption at 532 nm and two-photon absorption at 1064 nm was observed[131]. The optical limiting properties of a series of axially substituted tetrapyrazino-tetraazaporphyrinato, and other Pc-related compounds have also been described in theRefs. [113, 132–134].There exist other methods with which to modify the phthalocyanine system at the nano-

scale by creating novel aggregated systems without the need for chemical or structural alter-ations, except to exhibit interesting responses to external stimuli such as optical excitation.This technique involves the fabrication of aqueous nanoparticle dispersions of phthalocya-nines via a novel method that combines a reprecipitation method and microwave irradiation.Nitschke et al. reported the first study of the NLO properties of nanoparticles fabricatedfrom phthalocyanines [20, 78]. As a result, a significant improvement in the NLO response ofthe nanoparticles over solutions of the same phthalocyanines was obtainable. A sampleof typical open aperture Z-scan results with normalized transmission plotted as a functionof sample position z is depicted in Figure 8 for the molecular and nanoparticle (PhS)4PcZnsample for incident pulse energies of ≈0.16 and 0.3 mJ, respectively. The solution of thissample exhibited the nonlinear absorption coefficient ($I� ≈ �4'0 ± 0'8� × 10−10 cm W−1

while its associated nanoparticle exhibited a $I value approximately 5.3 times larger at�2'1 ± 0'4� × 10−9 cm W−1. This indicates that this method of phthalocyanine nanoparticlepreparation has significant impact and potential for exploitation in the field of optical limit-ing. Attempts to embed these nanoparticles in polymer films would be of great interest to theOL field. Table 3 presents linear and nonlinear optical properties of two PcZn compoundsin molecular solution and nanoparticle form.

4. PHOTOINDUCED ELECTRON TRANSFER PROCESSPhotoinduced Electron Transfer (PET) is a simple process in which an electron is transferredfrom an electron-donating species (D) to an electron-accepting species (A), producing theradical cation of the donor (D•+) and the radical anion of the acceptor (A•−), when oneof these species is photoexcited. Another process that can follow electron transfer is backelectron transfer (BET) leading to the initial state of the reactants. A critical factor in the


electron transfer process lies in the successful matching of donor and acceptor with suit-able electrochemical and photophysical properties for the occurrence of such an exothermicelectron transfer (ET) [135–138]. Knowledge of the excited-state energies of the chromo-spheres and the redox potentials of electron donor and electron acceptor is thus an essentialrequirement for investigating electron transfer processes. The intermolecular electron trans-fer events can be monitored by observing the radical ions by means of nanosecond transientabsorption spectra in the visible and near-IR regions with which the mechanism and kineticsof the electron transfer processes can be characterized.Phthalocyanines, porphyrins, and fullerenes are well-known building blocks in the area

of organic molecular optoelectronic and photonic materials science due to their unusualstructures and outstanding electric, conductive, and photophysical properties. Porphyrins andphthalocyanines as sensitizers and electron donors have been extensively investigated withthe purpose of generating photocurrent, in addition to their unique photophysical and photo-chemical properties [139–142]. As electron-acceptors, benzoquinones and methyl viologenswere used to generate photocurrent and hydrogen evolution in the presence of catalysis[143–145]. Covalently connected porphyrin-quinone dyads and triads have been synthesizedto realize long lifetimes of the charge-separated states to mimic the photosynthesis process[146–153]. Indeed, porphyrin (phthalocyanine)-fullerene systems have recently become anactive area of research [154–179]. The outstanding electron-accepting properties, larger exci-ton diffusion length, and low reorganization energy of C60 makes this molecule an idealcomponent with which to construct an efficient electron transfer system. A more practi-cal aspect of C60 and C70 concerns the optical absorption spectra of their �-radical anionssuch as C•−

60 and C•−70 , which show narrow bands in the near IR region around 1080 and

1380 nm, respectively, serving as diagnostic probes for their identification [180, 181]. Theseisolated absorptions allow an accurate analysis of inter- and intramolecular ET dynamics ofC60 and C70, even in the presence of porphyrins, phthalocyanines, naphthalocyanines, andchlorophylls, which have wide absorptions in the visible region.

4.1. Porphyrin-Fullerene Systems

The electronic excited states of porphyrins survive long enough in the singlet and triplet statesto offer a high possibility with which to interact with molecules before deactivation [182–184].The stability of mono- and di-cation porphyrin �-radicals makes these systems especiallyinteresting for photoionization processes. The simplest way to prepare the intermolecularsystem is to mix the electron donors (e.g., porphyrins, chlorophylls, phthalocyanines, andnaphthalocyanines, with electron acceptors such as fullerenes in a suitable solvent. In 1994,Guldi et al. [185] reported the first evidence of intermolecular electron transfer process ofmetalloporphyrin and fullerene systems by applying pulse radiolysis techniques. �-Radicalanions of several metalloporphyrins (tetraphenyl- and tetrapyridylporphyrins) reduced C60with rate constants of (1–3) × 109 M−1 s−1; whereas, metalloporphyrins which are reduced atthe metal center (NiII, CuII, CrII) reacted with C60 somewhat more slowly, with rate constantsof (0.7–2.3) × 108 M−1 s−1. SbVOEP•− (octaethylporphyrin) did not reduce C60 (k < 107 M−1

s−1), and SnIV(Ph)3(Py)P•− reacted in an equilibrium process (K = 14). Nojiri et al. [186]reported the photoinduced electron transfer process of zinc tetraphenylporphyrin (ZnTPP)with fullerenes (C60/C70) in polar solvents via both triplet states of porphyrin and fullerenemoieties by observing the transient absorption bands in the near-IR region. The transientabsorption bands of the fullerene radical anion (C•−

60 ) at 1080 nm and the C70 radical anion(C•−

70 ) at 1380 nm in the near-IR region gave evidence of the electron transfer process.

Phv−−→ 1P∗ isc−−−→ 3P∗ C60−−−→ C•−

60 + P•+ (10)

C60hv−−→ 1C∗

60isc−−−→ 3C∗

60P−−−→ C•−

60 + P•+ (11)

Fujisawa et al. [187, 188] demonstrated with the aid of time-resolved electron paramagneticresonance (EPR) that photoexcitation of porphyrins (MOEP and MTPP; M = H2, Zn) inthe presence of C60 in toluene can lead to triplet-energy transfer. The authors also reported


on a study of photoinduced electron and energy transfer from photoexcited porphyrins toC60 in benzonitrile. Martino and van Willigen [189] studied the energy and electron transferfrom porphyrins [magnesium tetraphenylporphyrin (MgTPP) and free-base octaethylpor-phyrin (OEP)] in the triplet excited-state to C60 in toluene and benzonitrile with a Fouriertransformation EPR (FT-EPR). In toluene, the authors observed the energy transfer processfrom 3MgTPP∗ (3OEP∗) to C60 yielded triplet C60. In a polar solvent, photoexcitation ofMgTPP/C60 produced FT-EPR spectra with signal contributions from MgTPP•+ and C•−

60 .In 2000, El-Khouly et al. [190] reported the ET process between metal octaethylporphyrins

(MOEP, where M = H2 and Zn) and fullerenes (C60/C70) (Fig. 9) via the triplet excitedstates of porphyrin (3P∗) and fullerene (3C∗

60/3C∗

70) by applying 532 and 610 nm of laser light,respectively. The absorption spectrum of a mixture of fullerenes (C60 and C70) and MOEP isa superimposition of the corresponding components, indicating that no interaction occurredbetween fullerenes and octaethylporphyrin in their ground states. Upon photoexcitation ofZnOEP in polar solvents, the transient absorption spectra exhibited a fast decay of tripletstate of porphyrins (3ZnOEP∗) accompanied by the formation of a radical cation of por-phyrin (ZnOEP•+) at 640 nm and a radical anion of C60 (C•−

60 ) at 1080 nm. In the case of C70as an electron acceptor, similar electron transfer behavior was also observed by recordingthe absorption bands at 750, 630, and 1380 nm that were assigned to 3ZnOEP∗, ZnOEP•+,and C•−

70 , respectively [191–193]. The .Get values for electron transfer via 3ZnOEP∗ and3C∗

60 /3C∗

70 in polar solvents were far from negative, anticipating that the rate constant of theelectron process is close to kdiff [194].To reveal the effect of metal ions in the porphyrin cavity on the efficiency of electron

transfer of octaethylporphyrin with fullerenes, El-Khouly et al. reported the ET process ofMOEP (M = H2, PdII, NiII, CoII, V = O, MgII, ZnII, and CuII) with fullerenes [195–197].Basically, the steady-state absorption bands of MOEP appear at longer wavelengths thanthose of metal tetraphenylporphyrin (MTPP). The insertion of metal atoms into the cavity ofoctaethylporphyrin usually strongly changes the visible absorption spectra (i.e., the Q-bandsof H2OEP (498, 532, 567, and 521 nm), PdOEP (512, 546 nm), NiOEP (517, 552 nm),CuOEP (526, 562 nm), (V = O)OEP (534, 572 nm), and MgOEP (544 and 579 nm).The ET quantum yield (/et) values via 3C∗

70 varied with the central metal according tothe following order: PdOEP > MgOEP > ZnOEP > (V = O)OEP > CoOEP > NiOEP >CuOEP. The change in the donor abilities of the MOEPs may be explained mainly by theirEox values. The observed /et values are less than unity, suggesting that there are somedeactivation routes (e.g., collisional quenching and/or an encounter complex). The possibilityof energy transfer from 3C∗

70 to MOEP in benzonitrile is quite low, because the rise of3MOEP∗ was not observed. Thus, the deactivation process may be attributed to collisionalquenching. In addition, it was also observed that the /et values gradually increase withdecreasing Eox(D/D+), except for (V = O)OEP (Table 4). The values of the electron transferrate constants (ket) increase with decreasing free energy changes (.G0

et) along the curvecalculated by the semiempirical Rehm-Weller plot [137, 138].The rate constants of a back electron transfer process (kbet) were evaluated by record-

ing the decay of C•−70 in long-time scale (200 �s). The decay time profile fit with second-

order kinetics suggesting that a bimolecular electron transfer process from C•−70 to MOEP•+

takes place. On employing the reported molar extinction coefficient of radical ion of C•−70

Metal octaethylporphyrin(MOEP)

N N

NNM

Et

Et Et

Et

Et Et

Et

Et

C60 C70

Figure 9. Structures of C60, C70, and MOEP.


Table 4. Oxidation potentials (Eox), free-energy change (.Get), and kinetic parameters (kq, /et, and ket) for ETprocess from MOEP via 3C∗

70 in benzonitrile; kbet: Back electron transfer rate constant between C•−70 and MOEP•+.

.Get kq ket kbet/1 kbet

Systems (kJ mol−1) Eox (V) /et (M−1 s−1) (M−1 s−1) (M−1s−1) (M−1 s−1)

3C∗70/PdOEP −66.5 0.44 0.74 2'2 × 109 1'6× 109 8'0× 105 3'2 × 109

3C∗70/MgOEP −57.7 0.53 0.52 2'4× 109 1'2 × 109 1'2 × 106 4'7× 109

3C∗70/ZnOEP −48.1 0.63 0.40 2'9× 109 1'1× 109 2'2 × 106 9'0× 109

3C∗70/NiOEP −47.3 0.64 0.32 2'7× 109 8'7× 108 1'6× 106 6'5× 109

3C∗70/CoOEP −43.5 0.68 0.39 2'2 × 109 8'5× 108 1'0× 106 4'0× 109

3C∗70/CuOEP −27.2 0.85 0.21 2'0× 109 4'3× 108 2'0× 106 8'0× 109

3C∗70/(V=O)OEP −17.2 0.96 0.40 1'8× 109 7'2 × 108 1'2 × 106 4'6× 109

Source: Reprinted with permission from [195], M. E. El-Khouly et al., Phys. Chem. Chem. Phys. 4, 3322 (2002). © 2002, RoyalSociety of Chemistry.

(4000 M−1 cm−1 at 1380 nm), [195–197] kbet values were obtained (see Table 4). The kbetvalues seem to be close to the diffusion-controlled limit in benzonitrile. Since the concen-trations of the anion radicals are much lower than the reactants, the observed decay rates ofthe backward process are far slower than that of the forward process, even though kbet � ket.

C•−60 + P•+ BET−−−−→ C60 + P (12)

In the presence of MOEP in toluene, no ET process was observed by laser excitation ofC70. The energy transfer process from 3C∗

70 to MOEP in toluene was also not observed; thisobservation is reasonable because ET �3C∗

70 = 1'54 eV) is slightly lower than ET �3MOEP∗ =1.60–1.90 eV). Thus, the formation of the triplet exciplex 3[C�2−�

70 · · ·MOEP�2+�]∗ would beexpected to be dominant in non-polar solvents. The formation of such triplet exciplexesin non-polar solvents has been reported during the quenching of the triplet states of met-alloporphyrins by various quinones in toluene [198]. The authors observed the hole shiftprocess from MOEP•+ to a second donor (having a lower oxidation potential). The transientabsorption spectra observed after laser excitation of MgOEP in the presence of C70 is shownin Figure 10. The absorption bands of MgOEP•+, 3C∗

70 and C•−70 appear at 640, 980, and

1380 nm, respectively. In the presence of N , N -diphenyl-N -(1,2,3,4-tetrahydroquinoline-6-yl-methylene)hydrazine (DTQH), the rapid decay of MgOEP•+ was observed, with a con-comitant rise of DTQH•+ in the 800–1300 nm region. Scheme 2 summarizes the wholeprocess where the photosensitized electron transfer occurs at first from 3MgOEP∗ to C70,yielding C•−

70 and MgOEP•+, followed by the hole-shift from MgOEP•+ to DTQH, yieldingDTQH•+. Such a hole shift is energetically possible since the oxidation potential of the hole-shift reagent DTQH (Eox = 0'32 V vs SCE) is lower than that of MgOEP (Eox = 0'53 V

0.16

0.12

0.08

0.04

0.00

Abs

orba

nce

140012001000800600

Wavelength/nm

0.12

0.08

0.04

0.00

∆Abs

86420–2Time/µs

ab0.5 µs

5.0 µs

MgOEP•+

3C70

C70•–

*

Figure 10. Transient absorption spectra obtained by 532 nm laser light of MgOEP in the presence of C60 in polarsolvents. Inset: Absorption-time profiles at (a) 980 nm and (b) 1380 nm. Reproduced with permission from [195],M. E. El-Khouly et al., Phys. Chem. Chem. Phys. 4, 3322 (2002). © 2002, Royal Society of Chemistry.


1P*

P·+ + C60/C70

3P*

P + 1O2

C60/C70

P + C60/C70O2

kischν

ket

Φet

1–Φet

P kbet

.– .–

Scheme 1. Routes for ET process occurring by the photo-excitation of P in the presence of C60/C70 in polar solvents.The symbol “P” represents porphyrins, chlorophylls, phthalocyanines, and naphthalocyanines.

versus SCE). The rate constant of the final back-electron transfer process (kfbet) wasevaluated as 6'1 × 108 M−1 s−1 by following the long time decay profiles of DTQH•+

and C•−70 .

The influence of different substituents on porphyrin on the rates of the electron transferprocess has been systematically investigated. El-Khouly et al. [199] observed the electrontransfer of systems composed of free tetraphenylporphyrin (H2TPP), tetra(p-hydroxy-phenyl)porphyrin (H2THPP), tetra(p-aminophenyl)porphyrin (H2TAPP), and tetra(p-meth-oxyphenyl)porphyrin (H2TMPP) as electron donors (Fig. 11) with fullerenes (C60/C70) aselectron acceptors. The electronic absorption bands of H2TMPP, H2THPP, and H2TAPPare shifted to longer wavelengths compared with those of H2TPP. Such red shifts canbe interpreted according to Gouterman’s four-orbital model [200]. By photoexcitation ofH2TPP in deaerated benzonitrile using a 550 nm laser, the absorption bands of the tripletstate of H2TPP (3H2TPP∗) are located at 450 and 780 nm. In the presence of C60, thegeneration of C•−

60 was observed via a build-up of the absorption at 1080 nm at 10 �sthat parallels a concomitant decay of 3H2TPP∗. The radical cation of H2TPP•+ at 650 nmwas difficult to observe as a result of its overlapping with the depletion and emissionof H2TPP. Interestingly, in the case of the H2TAPP/C60 system, the transient spectra ofH2TAPP/C60 system show the broad absorption bands of H2TAPP•+ in the range of 600–1400 nm. Similarly, H2THPP•+ and H2TMPP•+ show broad absorption bands in the Vis-NIR region. The quenching rate-constant (kq) values of 3H2TAPP∗/C60, 3H2THPP∗/C60,and 3H2TMPP∗/C60 are in the range of (1.1–1.3) × 109 M−1 s−1—about 3 times largerthan 3H2TPP∗/C60 (3'3 × 108 M−1 s−1). A possible rationale for this finding involvesthe lower oxidation potential (Eox) values of substituted porphyrins H2TMPP (0.98 V),H2THPP (0.75 V), and H2TAPP (0.48 V) compared to H2TPP (1.05 V) versus Fc/Fc+

[201, 202].In long-time scale measurements, it was clearly observed that C•−

60 begins to decay slowlyafter reaching its maximal absorbance. The decay-time profile was fitted with second-orderkinetics, suggesting that a bimolecular back-electron transfer process from C•−

60 to H2TAPP•+

takes place after these radical ions are solvated separately into free radical ions in polarsolvents. The rate constant of a bimolecular back-electron transfer process �kbet� value (2'0×1010 M−1 s−1) of C•−

60 /H2TPP•+ is much faster than that of substituted systems C•−60 /H2TAPP•+

(2'4 × 109 M−1 s−1), C•−60 /H2THPP•+ (3'5 × 109 M−1 s−1), and C•−

60 /H2TMPP•+ (5'5 × 109

M−1 s−1), suggesting the delocalization of the hole in the substituted porphyrins, whichis supported by the appearance of the longer wavelength absorption band of H2TAPP•+,H2THPP•+, and H2TMPP•+.

P.+

3P*DTQH.+

DTQH

P

HTET

hν

N

HHC

HC N

DTQH

C60/C70

C60/C70

.– .–

Scheme 2. Routes for electron-transfer/hole-shift cycle start with photoexcitation of P in the presence of C60/C70

and DTQH in Ar-saturated benzonitrile.


NH N

HNN

R

R

R

R

R = H: H2TPPNH2: H2TAPPOH: H2THPPOCH3: H2TMPP

Figure 11. Structures of tetrasubstituted porphyrins.

4.2. Chlorophyll-Fullerene Systems

Chlorophylls (close cousins of metalloporphyrins) play key roles in absorbing light energyover a wide spectral range and converting that light energy into a highly directional transferof electrons. Green plants employ chlorophylls (Chls)—magnesium-chlorins—as the chro-mophores to harvest light. The intense absorptions in the visible and near-IR spectral regionsof Chls make them potentially important in developing photosynthetic light-harvesting. Oneof the specific features of Chls is related to the quenching of their photoexcited states bycompounds with high electron affinity via ET process [203–205]. Of particular interest hasbeen the study of donor–acceptor systems containing Chl-a, which can mimic the photoin-duced ET process of natural photosynthesis. It has been demonstrated through the flashphotolysis and ESR techniques that various quinones can be used to quench the triplet statesof Chls. By ESR measurements, the signal of semiquinone (Q•−) was observed. Also, byapplying laser flash photolysis measurements, intermediates Q•− and Chls•+ were observedthrough the light excitation of Chls. The main problem frequently faced during flash pho-tolysis measurements is the overlap of the absorptions of the intermediates, which leads todifficulties in the interpretation of the mechanisms and quantitative analysis of the rates andyields of the electron-transfer processes. The absorption region of the Chls strongly overlapswith the absorption band of Q•− at 435 nm. In addition, the absorption bands of the 3Chls∗

at 480–500 nm masks most of the absorption band of the Chls•+ in the visible region. Incontrast with quinones, the transient absorptions of C•−

60 and C•−70 in the near-IR region make

it easy to study quantitatively the elemental steps in the PET process.The photoinduced electron transfer between Chl-a and fullerene using nanosecond laser

photolysis was reported by El-Khouly et al. [206]. By the excitation of Chl-a/Chl-b, the elec-tron transfer from 3Chl-a∗/3Chl-b∗ to C60/C70 was observed by recording the characteristicpeaks of C•−

60 and C•−70 in the near-IR region. The cation radical of Chl-a was not clearly

detected due to its overlap with Chl-a in triplet state. The electron transfer rate-constants(ket) via 3Chl-a∗ are slightly larger than the corresponding values from 3Chl-b∗. The presenceof the electron-withdrawing group (–CHO) decreases the electron-donor ability of Chl-bcompared to Chl-a, with its electron-donating methyl group. The high electron-donor abil-ities of Chl-a and Chl-b are quite similar to that of MgOEP and ZnOEP, in spite of theirlong chain, electron-withdrawing substituents.Barazzouk et al. [207] reported the photoinduced electron transfer between Chl-a and gold

nanoparticles due to the unique optical and electronic properties of the gold nanoparticles.Results show the ET occurs from excited Chl-a to gold nanoparticles. The difference betweenthe oxidation potential of Chl-a and the Fermi level of Au provides the necessary drivingforce for such an electron-transfer process. Nanosecond laser flash photolysis experimentsof Chl-a in the presence of gold nanoparticles and C60 have indicated that Au nanoparticles,besides accepting electrons, can also mediate electrons to another acceptor thus aiding incharge separation. The higher photocurrent generation efficiency (IPCE) of the photochemi-cal cell using an OTE/TiO2/Au/Chl-a electrode compared to that without the gold film is dueto the ability of Au nanoparticles in accepting and shuttling the electrons from an excitedChl-a to TiO2, leading to an enhancement in charge separation.


4.3. Phthalocyanine/Naphthalocyanine-Fullerene Systems

Several studies have been performed to examine the photophysical properties as well as thepotential for an ET process from metal phthalocyanines (MPcs) and/or naphthalocyanines(Ncs, Figure 12) to electron acceptor molecules. It has been reported that the photosensitiv-ity of ZnPc in a polymeric binder is increased by the addition of C60 [208]. From photoemis-sion experiments, C60 and C70 have proven to be appropriate electron-accepting materialswhen they are brought into contact with Pc in solids [209, 210]. The selective excitation ofC60/C70 is possible because MPcs do not show any absorption at 532 nm at all. Nojiri et al.[196] studied the intermolecular ET between C60/C70 and MPcs (M = H2, Zn) via 3C∗

60/3C∗

70by applying a 532 nm nanosecond laser light in polar solvents. Excitation of C60/C70 in ben-zonitrile gives rise to the rapid formation of C•−

60 /C•−70 at 1080 nm/1380 nm, and ZnPc•+ at

840 nm, with concomitant decay of 3C∗60/

3C∗70 at 740 nm/980 nm.

The electron transfer process from 3MPc∗ to C60/C70 was also studied by applying a 670 nmlaser, which excites predominantly MPc in the presence of an excess of C60/C70 [197]. Theobtained transient absorption spectra of ZnPc in benzonitrile exhibited absorption bands at490 and 840 nm, corresponding to 3ZnPc∗ and ZnPc•+, respectively. This indicated the occur-rence of photoionization via 3ZnPc∗ with a quantum yield (/ion) less than 0.1 in benzonitrile.In the case of 3H2Pc∗, such a photoionization process did not occur—even in polar solventssuch as benzonitrile. In the presence of C60/C70, an ET takes place from 3ZnPc∗ to C60 with/et � 0'8 with only a small contribution from photoionization. It was found that the second-order quenching rate constant (kq) value of the 3ZnPc∗-C60 system (8'7× 108 M−1 s−1) is 300times higher than that for the 3H2Pc∗-C60 system (2'9× 106 M−1 s−1�. Moreover, the electrontransfer efficiency of 3ZnPc∗ to C60 (/et = 0'77) is 10 times higher that of 3H2Pc∗ to C60(/et = 0'07). In order to confirm whether such a large difference in electron donor abilitybetween 3ZnPc∗ and 3H2Pc∗ was specific to the spherical fullerene molecule, the authorsexamined the ET from 3ZnPc∗ or 3H2Pc∗ to benzoquinone (BQ) as a representative of flatsmall molecules. The results revealed that the higher electron donor ability is a general char-acteristic of 3ZnPc∗ and 3H2Pc∗, but not a specific characteristic of the fullerene acceptor.In 2003, Luo et al. reported the ET process in the system of tBu4PcTiO and C60/C70 to

examine the effect of metal ions on the ET process [211]. The B- and Q-bands of tBu4PcTiOare centered at about 350 and 700 nm, respectively. The fluorescence time profile shows asingle exponential decay giving a lifetime of 5.1 ns. No dynamic quenching of the excitedsinglet state of tBu4PcTiO was observed on addition of C60 or C70—even in polar solvents.The transient absorption spectra observed by the selective excitation of tBu4PcTiO with laserlight at 355 nm in Ar-saturated toluene exhibited a rapid decay of an absorption band at1400 nm, with a rate of ca. 1 × 108 s−1), which may be ascribed to the S1-Sn transition oftBu4PcTiO. With the decay band at 1400 nm, the growth of a band at 1300 nm was observed.Since the species with absorption at 1300 nm has a long lifetime (� = 67 �s), this specieswas attributed to 3tBu4PcTiO∗. By the selective excitation of C70 and C60 in a non-polarsolvent, the energy transfer process from 3C∗

70 and 3C∗60 to tBu4PcTiO was confirmed with

rate constants 3'3× 109 and 2'0× 109 M−1 s−1, respectively. In a polar benzonitrile solvent,the ET process from tBu4PcTiO to 3C∗

60 was confirmed by observing the decay of 3C∗60 at

750 nm with a concomitant rise of tBu4PcTiO•+ at 880 nm and C•−60 at 1080 nm. Similarly,

the ET process was also confirmed from tBu4PcTiO to 3C∗70. The k1st value of the rise of C•−

60

N

N

N

N

N

N

N

NM

tBu tBu tBu

tButBu

tBu

tBu tBuM = H; Zn; TiO

N

N

N

N

N

N

N

NZn

Figure 12. Structures of tBu4PcH2, tBu4PcZn, tBu4PcTiO, and tBu4NcZn.


was found to be smaller than the k1st value for the decay of 3C∗60, which might suggest some

intermediate process between the decay and rise, such as triplet exciplexes (Scheme 3). The/et value via 3C∗

60 was evaluated to be 0.2. Such a low (/et) value for a tBu4PcTiO/3C∗60

system suggests the presence of a deactivation process of 3C∗60 without ET (i.e., collisional

quenching, compared to the ZnPc/3C∗60 system).

Ito and his coworkers have also investigated the ET process of tBu4NcZn and C60/C70systems via 3tBu4NcZn∗ in order to probe the structural effect on the ET process [212].The steady-state absorption spectra of tBu4NcZn appears at 780 nm, which is at a longerwavelength compared to ZnTPP, ZnOEP, and ZnPc. Since C60 and C70 have no appreciableabsorption intensity at 650 nm, tBu4NcZn can be selectively excited by a 670 nm laser light.No interaction between C60/C70 and tBu4NcZn in their ground states was observed. Thetransient absorption spectra of tBu4NcZn in a polar solvent (Fig. 13) exhibited the intenseabsorption bands of 3tBu4NcZn∗ at 600 and 770 nm, accompanied by the weak growthof tBu4NcZn•+ at 970 nm, suggesting a photoionization process via 3tBu4NcZn∗. In thepresence of C60, the ET process from 3tBu4NcZn∗ to C60 was confirmed by the growth of theabsorption bands of tBu4NcZn•+ and C•−

60 , followed by a concurrent decay of the absorptionband of 3tBu4NcZn∗ at 600 and 770 nm. These findings indicate that the ET process occursfrom 3tBu4NcZn∗ to C60/C70 with a small contribution from photoionization in polar solvents.Most recently, Chen et al. successfully synthesized the axially substituted titanium Pc-C60

dyad (Fig. 14) using a convenient method that improves on the traditional asymmetricalphthalocyanine routine to covalently link phthalocyanines with other functional molecules[213]. Fluorescence quenching was observed in benzonitrile, suggesting that intramolecu-lar charge-separation might take place between the phthalocyanine donor and the fullereneacceptor, which was supported by the negative free-energy change calculated by the electro-chemical data. The charge separation process was initially confirmed by applying a 532 nmnanosecond laser photolysis. A quick rise-decay of the absorption band at 790 and 1020 nmwas observed. These two bands might be assigned to the formation of PcTi•+ and C•−

60 ,respectively.

4.4. Porphyrin/Naphthalocyanine-Fullerene Supramolecular Systems

The porphyrin macrocycle is capable of binding a variety of transition metals within itscentral cavity, leaving the positions axial to the plane of the porphyrin ring available forbinding with a variety of ligands. Fullerene-porphyrin supramolecular systems have receivedmuch attention from physicists, chemists, and materials scientists in recent years due to theirpotential application in the development of high-performance photovoltaic devices [213–220]. These systems are composed of porphyrin and fullerene derivatives functionalized insuch a way that the two entities are able to diffuse together and reversibly bind in solution.The modes of binding most often employed include �–� interactions, electrostatic attraction,hydrogen bonding, and axial ligation via a nitrogen-based ligand to the metal center of themetalloporphyrin. The self-assembled donor–acceptor systems offer several advantages overintermolecular systems. First, the relative orientation of the donor and acceptor can becontrolled, in some cases. This is quite important since ET rates are dependent upon orbitaloverlap and distance between the donor and acceptor moieties. Second, intermolecular ETis a diffusion-controlled process, while in supramolecular systems this process is not onlypartially governed by diffusion rates, but also by binding strength and concentration. Also, inintermolecular systems, the entity that is excited usually has enough time to undergo the ISCprocess from the singlet-excited state to the triplet-excited state before colliding with a donoror acceptor. Therefore, most intermolecular systems undergo ET via the triplet-excited state.

[(C60/C70)δ–…tBu4PcTiOδ+)] tBu4PcTiO + 3C60/C70* *

tBu4PcTiO + C60/C70

tBu4PcTiO.+ + C60/C70·– ·–

kdisskass

kcq

Scheme 3. Routes for the electron-transfer process occurring by the photoexcitation of C60/C70 in the presence oftBu4PcTiO in benzonitrile.


–0.6

–0.4

–0.2

0.0

0.2

0.4

0.6

Abs

orba

nce

12001000800600400

Wavelength/nm

0.6

0.4

0.2

0.0

∆Abs

151050Time/µs

1 µs

10 µs

a b

c

C60

ZnPc•+

3ZnPc*

•–

Figure 13. Transient absorption spectra obtained by a 532 nm laser light of tBu4NcZn in the presence of C60 in apolar solvent. Inset: Absorption-time profiles at (a) 500 nm, (b) 840 nm, and (c) 1080 nm.

However, for self-assembled systems in which the conditions have been properly adjustedso that a sufficient amount (>99%) of complexed donor–acceptors are present in solution,the excited species usually do not have enough time to undergo the ISC process. Therefore,most self-assembled systems undergo ET from the short-lived singlet-excited state. Third,since the binding of the donor–acceptor complex is reversible in nature, after ET occurs,the individual charge-separated species (D•+ and A•−) can diffuse away from each other,creating long-lived, solvent-separated ion pairs (SSIP) in a sufficiently polar medium; thus,increasing the lifetime of the CS state.Up to now, many research groups have studied systems composed of C60 functional-

ized with a coordinating ligand capable of axially ligating to the mater center of metaltetraphenylporphyrin (MTPP; M = Zn, Ru) [214–224]. D’Souza and coworkers havereported a system composed of a pyridine-appended C60 axially ligated to ZnTPP through

HOMO

LUMO

Figure 14. The HOMO/LUMOs and optimized structure of a PcTi-C60 dyad calculated by HF/3-21G level using aGaussian-98 pack. Reprinted with permission from [213], Y. Chen et al., Org. Lett. 7, 1613 (2005). © 2005, AmericanChemical Society.


the pyridine (Py) nitrogen [225]. Upon complexation of the pyridine-appended C60 to ZnTPP,the optical absorption spectrum experiences a red shift of the Soret band. D’Souza et al.also prepared and studied a novel triad system composed of a zinc porphyrin appendedwith hydrogen-bonding groups such as either a carboxylic acid or an amide group (ZnTPP∼COOH or ZnTPP∼NH2) and a C60 molecule appended with a pyridine group and a N ,N -dimethylaniline (DMA) group acting as a secondary electron donor (C60 ∼ DMA) (Fig. 15).The symbol “∼” refers to a covalent bond. The triad system is self-assembled via “two-point”binding where the pyridine group on the C60 axially ligates to the central metal of the zincporphyrin while the nitrogen of the fullerpyrrolidine ring hydrogen bonds with the hydrogenbonding group attached to ZnTPP, such as ZnTPP∼COOH or ZnTPP∼NH2.The electron transfer process of a supramolecular system (Fig. 16) composed of a C60

unit appended with two pyridine units (C60 ∼ Py2) that axially ligate to two ZnTPP thusforming a self-assembled triad have been observed [226]. UV-visible absorption data wereused via the Scatchard method to determine the binding constant value (K value) for thesystem. This value was determined to be 1'45 × 104 M−1, which is considerably large ascompared to the dyad counterpart analogue. Both the steady-state fluorescence and time-resolved transient absorption experimental results in non-coordinating solvents revealed thatthe charge separation takes place from the singlet-excited state of ZnTPP. The absorptionspectra observed in coordinating solvents such as benzonitrile were quite different. Bandswere detected at 770 and 840 nm, corresponding to 3C∗

60 and 3ZnTPP∗. These bands werefound to decay faster than what was observed in o-dichlorobenzene (o-DCB). Also, a slowrise was observed in the 900–1100 nm region, corresponding to the formation of C•−

60 . Theseresults show that, in coordinating solvents, energy transfer from 3ZnTPP∗ to C60 was thepredominant process.In 2003, El-Khouly et al. prepared a new C60-naphthalocyanine supramolecular system

(Fig. 17) in which an imidazole-appended C60 (C60Im) axially coordinates via the imida-zole nitrogen to the central metal of a tBu4NcZn molecule in non-coordinating solvents(e.g., toluene and o-dichlorobenzene) [212]. Upon addition of C60Im to a toluene solutioncontaining tBu4NcZn, the absorption band at 767 nm exhibited diminished intensity, andisosbestic points appeared at 675, 717, 752, and 791 nm. The K value for the dyad in atoluene solution was determined to be 6'2× 104 M−1 by the Scatchard method. This value isan order of magnitude larger than that of the counterpart ZnTPP dyad system. Steady-state

N

NO

C

N

NN

N

Zn

O

O

H

H

N

H3C

H3C

Figure 15. Structure of ZnTPP ∼ COOH : Py C60 ∼ DMA.


N

N

N

N

N

N

Zn

H

N

N

N

NZn

N

Figure 16. Structure of the ZnTPP2-C60 triad.

fluorescence experiments were performed on the self-assembled dyad system. Upon addi-tion of C60Im to a solution containing tBu4NcZn (using toluene or o-dichlorobenzene as asolvent), the emission bands of tBu4NcZn at 781 and 812(sh) nm were gradually quenchedto about 30% of their original intensity. The excited-state lifetime of pure tBu4NcZn isabout 2.42 ns. The excited state of the C60Im:tBu4NcZn) complex formed in toluene decayedbi-exponentially, having both fast and slow components. The fast component had a life-time of approximately 71 ps, while the slow component had a lifetime of approximately2.14 ns—which is close to the lifetime of free tBu4NcZn. The short-lived nature of theexcited state of tBu4NcZn in the complex suggests quenching via ET from the singlet-excitedstate of tBu4NcZn (1tBu4NcZn∗). The rate constant of the charge separation process (kCS)value was determined to be 1'4 × 1010 s−1, with an efficiency (/CS) of 0.97 for the process(toluene).

N

N

N

N

N

NN

N

Zn

NH3C

N

N

tBu

tBu tBu

tBu

Figure 17. Structure of tBu4NcZn:C60PhIm. Reproduced with permission from [212], M. E. El-Khouly et al., Chem.Phys. Chem. 4, 474 (2003). © 2003, Wiley-VCH Verlag GmbH & Co KG.


To determine the nature of the excited-state photochemical reactions in this time domain,picosecond transient absorption spectra were measured. Upon excitation of the dyad with a388 nm laser light, new bands appeared at 710 and 985 nm in the time region of 10–200 ps.These bands were attributed to the formation of tBu4NcZn•+. The band expected at 1000 nmrepresenting C60Im•− was not observed, probably due to masking by the intense band at985 nm. The kCS value for the dyad was evaluated to be 1'4× 1010 s−1, which is in agreementwith the value determined by fluorescence lifetime measurements, while the kCR value wasevaluated to be 8'5 × 108 s−1. Nanosecond transient absorption spectra were also obtainedfor the self-assembled dyad. Upon excitation of the dyad (C60Im:tBu4NcZn) with a 532 nmlaser light in o-DCB, intense absorption bands were observed between 600–700 nm after100 ns, corresponding to 3tBu4NcZn∗ and 3C60PhIm∗. After 10 ns, absorption bands wereobserved in the region of 960–1000 nm, corresponding to the formation of the radical ion pair(tBu4NcZn•+:C60Im•−). These absorption bands show quick rise-decay behavior, indicatinga rapid charge recombination.In coordinating solvents such as benzonitrile, the transient absorption spectra were quite

different. Upon excitation of the dyad with a 650 nm laser light, the spectra exhibited bandsat 600 and 750 nm, which were attributed to an appreciable population of 3tBu4NcZn∗

and 3C60Im∗. These absorption bands of the triplet state decay, accompanied by the riseof absorption bands in the 960–1000 nm region, which corresponds to the formation ofthe radical ion pairs, tBu4NcZn•+ and C60Im•−. These data suggest that in a coordinatingsolvent, the ET process takes place from 3tBu4NcZn∗ to C60Im. The bimolecular ET rateconstant (ket) was evaluated to be 1'3× 108 M−1 s−1, while the back ET rate constant (kbet)was determined to be 3'6× 109 M−1 s−1.

5. CONCLUDING REMARKSPhthalocyanines and porphyrins offer a high architectural flexibility in structure, which facil-itates the tailoring of their physical, optoelectronic, and chemical parameters over a verybroad range. The spectral bandwidth—or window—over which the NLO device operates,and the ground state and excited-state spectra and lifetimes, can be molecularly engineeredby altering the axial or peripheral substituents, central metal cations, and structure of themain rings. Unfortunately, the establishment of the relationship between the fundamen-tal structural parameters and nonlinear optical properties of materials has only been par-tially accomplished over the past two decades, which apparently renders futile the effortson improvement of the nonlinear absorptive responses of the known phthalocyanines andporphyrin-based materials through molecular structural modifications, and the moleculardesign of new nonlinear optical functional materials. On other hand, in many cases compari-son between NLO processes, wavelength regimes, measurement techniques, and investigatorsis tenuous [68]. This has been a basic problem in all nonlinear studies. Therefore, one shouldthink about how to standardize the various different measurement methods and analysis pro-cedures for the identification of NLO materials in the near future. For development of thematerials required for high-performance NLO devices—except for the further improvementof the NLO response of the known functional materials via molecular modification—thedesign, synthesis, and evaluation of SupraRSA, such as the phthalocyanine (porphyrin) andfullerene-based donor–acceptor dyads, triads, and/or supramolecular systems—and prepara-tion of nanoscale NLO materials—are also a rapidly growing new field and are expected tobecome very important in future.The intermolecular ET processes from electron donors (porphyrin, chlorophyll, phthalo-

cyanine, and naphthalocyanine, as well as their derivatives) to electron acceptors (e.g., C60and C70) studied by laser flash photolysis techniques in polar and non-polar solvents haverevealed many interesting features. In polar solvents, the ET process takes place via thetriplet excited states of the excited acceptor or excited donor yielding a solvated radicalion pair. The back ET rates are found to be generally slow as a result of the bimolecularencounter of solvated radical pairs. The structure-photochemical reactivity probed in theintermolecularly interacting systems reveal a dependence on the nature of the porphyrin/phthalocyanine macrocycle, the metal ion present in the porphyrin/phthalocyanine cavity,


the electron donor substituents on the macrocycle periphery, and the polarity of the solventmedium.The supramolecular approach of building new systems based on porphyrin and phthalocya-

nine is beginning to provide well-characterized donor–acceptor systems which could eventu-ally be used for the development of solar energy harvesting and optoelectronic devices. It iswell known that porphyrins, phthalocyanines, and fullerenes are excellent building blocks ofsupramolecular systems for the study of photoinduced charge separation reactions via time-resolved ultrafast spectroscopic techniques. The nature of the link between the donor andacceptor entities, the solvent, and the metal ions in the porphyrin cavity influences the over-all photochemical reactivity. Studies of self-assembled supramolecular dyads, triads, tetrads,and others are only in the beginning stage; future studies are anticipated to involve morecomplex systems targeted for better charge stabilization as well as the ability to performspecific light driven photochemical processes.

ACKNOWLEDGMENTSWe are grateful for the financial support of the National Natural Science Foundation ofChina (No. 20546002), the East China University of Science and Technology, the China/Ireland Science and Technology Collaboration Research Foundation (No. CI-2004-06), SRFfor ROCS, NCET, and STCSM (No. 05XD14004), respectively. Y. Chen wishes to thankthe Alexander von Humboldt Foundation of Germany and Japan Science and TechnologyAgency for awarding him a research fellowship. M. E. El-Khouly would also like to thankthe COE Foundation for awarding him a fellowship.

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phthalocyanines and related compounds: nonlinear optical response and photoinduced electron transfer...

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