photophysics of platinum and iridium organometallic...
TRANSCRIPT
PHOTOPHYSICS OF PLATINUM AND IRIDIUM ORGANOMETALLIC MATERIALS: FROM MOLECULAR WIRES TO NONLINEAR OPTICS
By
RICHARD T. FARLEY
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2007
1
© 2007 Richard T. Farley
2
To my sister
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ACKNOWLEDGMENTS
I would like to express gratitude to my advisor, Prof. Kirk S. Schanze, whose support and
unending patience have allowed me to complete my studies. His scope of knowledge on
scientific material and laboratory equipment remain impressive. The thorough explanations and
advice he has given in the lab have led me to work on things I never imagined as a chemistry
graduate student. I would also like to thank my committee members, Dr John Eyler, Dr. Valeria
Kleiman, Dr. Nicolo Omenetto, and Dr. Elliott Douglas, for their time and support.
This work could not have been completed without the many collaborators with whom I
have interacted: Ranjani Narayanan and Dr. Stephen Hagen for their stimulating discussions
regarding the instrumentation discussed in Chapter 2; Thomas Cooper and the Air Force
Research Laboratory for providing the compounds studied in Chapter 3; Qinglin Zheng and Dr.
John Gladysz for their collaboration and providing the materials studied in Chapter 4; and Dr.
Kye-Young Kim of the Schanze group for the synthesis and characterization of the compound
studied in Chapter 5. Much of our custom lab equipment would not exist without the hard work
and creativity of Joe Shalosky, Brian Smith and Todd Prox of the UF Chemistry Machine Shop.
I would also like to thank Mike Sytsma and Benjamin Walker for their programming help in the
laser lab.
I thank of all the present and former members of the Schanze research group for accepting
the oddball physical chemist as (almost) one of their own. The have been great friends and
wonderful scientists. I appreciate the guidance of Dr. Ksenija Glusac-Haskins and Dr. Thomas
Cardolaccia who patiently introduced me to the laboratory and photophysical measurements
during my first year in the group. I hold dear the friendships I have made in the group,
especially with Dr. Kye-Young Kim. Her stories, support and sharing of secrets have helped
make being in the lab much more than just research. I need to thank the various members with
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whom I have shared desk space throughout the years, and with whom I’ve become close friends:
Dr Xiaoming Zhao, Amir Faraji, and Dr. Katsu Ogawa. Amir was always there to offer a bizarre
yet intriguing opinion on the matter at hand. Special thanks go to Katsu; his unfortunate close
proximity in the lab put him at the receiving end of unending questions, whether technical,
chemical or metaphysical.
Survival would not have been possible without the support, knowledge and friendship of
Lori Clark in the Graduate Office. Her role in the administrative process from recruiting to
graduation is essential to the department, but it is her friendship and support outside of work that
have made her invaluable to me. I would like to thank her and Dr. Ben Smith for the opportunity
to help with recruiting. Hopefully they realize it was more fun than it was work.
I would also like to thank my many friends who have supported and encouraged me
throughout the last five years. Rob and Lindsay have been there from the beginning, through
classes and seminars, qualifiers and research, suffering and celebrating together. I value the
existence of MUSCO and all my friends there who have kept me sane.
My family has offered unconditional support through all my endeavors, for which I am
most grateful. Each member has shared his or her own style of encouragement, love and
understanding during my years as a graduate student. They are my inspiration to succeed in all
that I do.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES...........................................................................................................................8
LIST OF FIGURES .........................................................................................................................9
ABSTRACT...................................................................................................................................12
CHAPTER
1 INTRODUCTION ..................................................................................................................14
Wave-Particle Nature of Photons and Electrons ....................................................................14 Absorption of Light ................................................................................................................15 Nature of the Excited State .....................................................................................................17 Radiative Decay......................................................................................................................18 Radiationless Transitions........................................................................................................19 Triplet Excited States in Conjugated Organic Systems..........................................................25
Molecular Wires ..............................................................................................................25 Nonlinear Optics and Platinum Acetylides .....................................................................27
Objective of Present Study .....................................................................................................35
2 TRANSIENT ABSORPTION SPECTROSCOPY AND INSTRUMENTATION................37
Background.............................................................................................................................37 Techniques..............................................................................................................................38 Apparatus................................................................................................................................42
Hardware .........................................................................................................................42 Data Collection................................................................................................................46 Data Processing ...............................................................................................................48 Sources of Noise..............................................................................................................52 Baseline Measurements ...................................................................................................54 Results .............................................................................................................................54 Limitations of Current System ........................................................................................57
3 DELOCALIZATION IN EXCITED STATES OF DINUCLEAR PLATINUM ACETYLIDES........................................................................................................................61
Introduction.............................................................................................................................61 Experimental...........................................................................................................................65 Results.....................................................................................................................................67 Discussion...............................................................................................................................75 Conclusion ..............................................................................................................................79
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4 PHOTOPHYSICS OF DIPLATINUM POLYYNEDIYL OLIOGMERS: CHAIN LENGTH DEPENDENCE ON THE TRIPLET STATE IN SP CARBON CHAINS...........80
Introduction.............................................................................................................................80 Experimental...........................................................................................................................82 Results.....................................................................................................................................84
UV-Vis Absorption Spectra ............................................................................................84 Steady-State Photoluminescence Spectroscopy ..............................................................85 Phosphorescence Decay Kinetics: Radiative and Non-Radiative Decay Rates ..............91 Transient Absorption Spectroscopy: Triplet-Triplet Absorption of the Oligoynes........94
Discussion...............................................................................................................................96 Variation of Triplet Energy with Carbon Chain Length .................................................96 Energy Gap Law Correlation for the Oligoynes..............................................................99 Excited State Decay above the Solvent Glass Point......................................................105
Summary and Conclusion.....................................................................................................105
5 AN IRIDIUM COMPLEX THAT EXHIBITS DUAL-MECHANISM NONLINEAR ABSORPTION .....................................................................................................................107
Introduction...........................................................................................................................107 Experimental.........................................................................................................................113 Results...................................................................................................................................115 Discussion.............................................................................................................................121 Conclusion ............................................................................................................................123
6 CONCLUSIONS AND FUTURE WORK...........................................................................124
LIST OF REFERENCES.............................................................................................................128
BIOGRAPHICAL SKETCH .......................................................................................................140
7
LIST OF TABLES
Table page 3-1 Summary of spectroscopic data of dinuclear platinum-acetylide oligomers. ....................67
4-1 Emission spectral fitting parameters for PtCnPt at 100 K.................................................90
4-2 Photophysical parameters for PtCnPt complexes..............................................................94
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LIST OF FIGURES
Figure page 1-1 Potential energy curves for electronic transitions..............................................................16
1-2 Effects of energy difference and nuclear distortion on vibrational overlap.......................20
1-3 Jabloski diagram illustrating possible transitions. .............................................................23
1-4 Structures of molecules studied by Davis et al. .................................................................26
1-5 Conditions for reverse saturable absorption. .....................................................................29
1-6 Structure of platinum-ethynyl and corresponding photophysical data. .............................30
1-7 Jabloski diagram illustrating possible transitions for platinum-ethynyl. ...........................32
1-8 Structures of platinum acetylide oligomers studied by Cooper, Rogers, et al...................33
2-1 Transient absorption apparatus developed by Lindqvist. ..................................................38
2-2 CCD intensifier. .................................................................................................................41
2-3 The transient absorption apparatus. ...................................................................................43
2-4 Spectrum of xenon lamp used as the probe. ......................................................................44
2-5 Illustration of timing parameters for ICCD data collection. ..............................................47
2-6 Change in intensity of transmitted light during the lifetime of a transient species............49
2-7 Construction of a time trace and transient absorption spectra from gated multiwavelength measurements.........................................................................................51
2-8 Relative probe intensities for I0 and I with the pump beam off. ........................................53
2-9 Baseline noise of the apparatus..........................................................................................55
2-10 Transient absorption spectrum of 1.2x10-4 M anthracene solution in deoxygenated benzene. .............................................................................................................................56
2-11 Transient absorption spectrum of 1.4x10-4 M aqueous solution of Ru(bpy)32+. ................57
3-1 Platinum-acetylide polymer and model compound studied by Beljonne et al. .................62
3-2 Platinum-acetylide oligomers studied by Liu et al. ...........................................................63
3-3 Platinum acetylide oligomers studied by Glusac-Haskins et al.........................................63
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3-4 Platinum-acetylide oligomers studied by Cooper, Rogers, et al........................................64
3-5 Dinuclear platinum-acetylide oligomers............................................................................66
3-6 Absorbance spectra of dinuclear platinum-acetylide oligomers collected at room temperature in THF solution..............................................................................................68
3-7 Transient absorption spectra of dinuclear platinum-acetylides. ........................................69
3-8 Phosphorescence spectra of dinuclear platinum-acetylide oligomers. ..............................71
3-9 Variable-temperature emission spectra of P-PE2 in MTHF. ............................................72
3-10 Excitation spectra of dinuclear platinum-acetylide oligomers...........................................73
3-11 Variable-temperature excitation spectra of P-PE2 in MTHF............................................74
4-1 Structures of the PtCnPt series. .........................................................................................81
4-2 Absorption spectra of PtCnPt complexes in THF solution. ..............................................85
4-3 Photoluminescence spectra of PtCnPt complexes in 2-MTHF solvent glass at 80 K. ......86
4-4 Emission spectra of PtC8Pt in MTHF solvent (glass) over 77 – 298 K temperature range...................................................................................................................................88
4-5 Overlay of absorption, emission excitation and emission spectra (red lines) for PtCnPt complexes in MTHF solvent glass at 100 K. ........................................................89
4-6 Fit of emission spectrum of PtC6Pt using Equation 4-1 and parameters listed in Table 4-1. ...........................................................................................................................91
4-7 Temperature dependence of photoluminescence decay lifetimes for PtCnPt complexes in MTHF solution (glass).................................................................................92
4-8 Transient absorption spectra of PtCnPt complexes following pulsed excitation with a 355 nm pulse (10 ns, 5 mJ-pulse-1). ...................................................................................95
4-9 Correlation of E00 (triplet energy) vs. 1/n for π-conjugated oligomers, where n = # carbons in the chain. ..........................................................................................................97
4-10 Energy gap law correlation. .............................................................................................102
4-11 Plot of Huang-Rhys parameter (Sm) vs. the triplet energy (E00) for PtCnPt complexes and for series of ruthenium polypyridyl complexes and plot of calculated Franck-Condon factors vs. natural log of non-radiative rate for PtCnPt complexes...................104
5-1 Jabłoński diagram and corresponding representative absorption spectra illustrating two-photon absorption. ....................................................................................................108
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5-2 Jablonski diagram for a four-level system illustrating TPA with ESA. ..........................111
5-3 Structural motifs for two-photon absorbers. ....................................................................112
5-4 Structures of the ligand, L, and iridium(III) complex, ML. ............................................113
5-5 Apparatus for measuring nonlinear transmittance. ..........................................................115
5-6 Absorption of ligand L and complex ML and one-photon emission of ligand L and complex ML. ...................................................................................................................116
5-7 Transient absorption of ML in deoxygenated THF solution obtained at 400 ns increments following 355 nm excitation..........................................................................117
5-8 Two-photon induced emission of ML after pulsed excitation by 1064 nm. ...................118
5-9 Photograph of 5 mM ML solution...................................................................................119
5-10 Emission area dependence of ML on incident laser energy at 1064 nm. ........................119
5-11 Transient absorption spectra of 5 mM deoxygenated THF solution of ML following 1064 nm pulsed excitation. ..............................................................................................120
5-12 Transmittance of 1064 nm pulsed beam at various concentrations of ML in THF.........121
6-1 Variations of molecular wires synthesized by Stahl, Owen et al. ...................................127
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
PHOTOPHYSICS OF PLATINUM AND IRIDIUM ORGANOMETALLIC MATERIALS: FROM MOLECULAR WIRES TO NONLINEAR OPTICS
By
Richard T. Farley
December 2007
Chair: Kirk S. Schanze Major: Chemistry
My research presents the results of an investigation into the photophysical properties of
organometallic materials that contain platinum and iridium. Characterizing the excited-state
properties of these types of compounds is necessary for a complete understanding of the
materials which may have application in opto-electronics, as molecular wires, or as nonlinear
optical materials, as well as help satisfy the need to gain insight into the fundamental nature of
triplet excited states of conjugated organic systems.
First, in order to partially characterize the excited states of these materials, a transient
absorption apparatus was designed and constructed. Second, a series of platinum-containing
molecules with two platinum atoms and different end groups were studied to examine interrupted
conjugation through metal centers. Third, long carbon chains end-capped with platinum atoms
were analyzed to determine the relationship between chain length and nonradiative decay rates.
Lastly, an iridium complex with a known two-photon-absorbing ligand was studied to determine
if excited-state absorption could be achieved through an initial multi-photon absorption process,
and consequently exhibit nonlinear optical properties. The goal of this work was to further the
understanding of triplet excited states in organometallic systems.
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The most important conclusions of this study are as follows: (i) the triplet excited states of
small dinuclear platinum acetylides are more localized than the singlet excited states, and have
disrupted conjugation through the platinum atom; (ii) long carbon chains display correlation to
the energy gap law; (iii) the triplet excited state can be populated through two-photon absorption,
which leads to nonlinear optical effects through enhanced dual-mechanism absorption.
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CHAPTER 1 INTRODUCTION
Wave-Particle Nature of Photons and Electrons
The interaction of light with matter is a topic that has caused interest and debate since the
early 1600s. Our understanding of the atomic world has developed almost concurrently with the
understanding of light. Light was theorized to be a wave by Huygens in 1678, although this
work was largely overshadowed by Newton’s corpuscular theory that light consisted of particles.
In the early 1800s, Young and Fresnel completed the double-slit experiments that supported
Huygens’ theory by showing interference patterns from light. In 1860, Maxwell described light
with a series of equations. Using the existing laws of magnetism and electricity developed by
Coulomb, Farady, Ampere, and Gauss, Maxwell’s theory of electromagnetism defines an
electromagnetic wave that should propagate at a speed of 3x108 m/s. At this point, Newton’s
laws of classical mechanics were still the basis for the atomic structures proposed by Dalton,
Thomson and Rutherford.
At the end of the 19th century, it was widely accepted that electrons were particles, and
light was a wave. Planck’s blackbody experiment drastically changed these views at the
beginning of the 20th century. His findings concluded that blackbody radiation is limited to
discrete values; that is, the energy is quantized. Newtonian physics could not explain this
phenomenon. Soon after the blackbody experiment was completed, Einstein discovered the
photoelectric effect, crushing the idea that light is merely a wave. Although it is now accepted
that light exhibits a dual wave-particle nature, Maxwell’s wave equations generally explain most
light-related phenomena.
Because of the wave-particle duality of light, scientists soon postulated that electrons could
also exhibit the same duality. The idea that all particles, not just electrons, act as waves was
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theorized by de Broglie in 1924. Research done independently by Thomson and by Germer and
Davisson showed electron diffraction patterns and confirmed the de Broglie hypothesis for
electrons. In 1928, Schrödinger formulated mathematical equations to describe the wave nature
of electrons in atomic structure, similar to the classical treatment of sound and light waves. This
development of quantum theory has dramatically impacted our understanding of atomic and
molecular structure.
Absorption of Light
The interaction of light with matter has given us considerable insight into electronic
molecular structure. The electronic energy levels of a molecule are discrete, and can be
measured through their interactions with photons. The energy of a photon is related to the
frequency of its electromagnetic wave by the following equation:
λν hchE == (1-1)
where E is the energy of the photon, h is Planck’s constant, ν is the frequency, c is the speed of
light, and λ is the wavelength. When a molecule absorbs light (the energy of the photon), an
electron is promoted to a higher energy level. The difference in energy levels is equal to the
energy of the photon absorbed. A quantitative description of the amount of light absorbed for a
certain wavelength is given by Equation 1-2.
bCA ε= (1-2)
where A is the absorbance, ε is the molar absorptivity (units of M-1 cm-1), b is the pathlength of
absorption and C is the molar concentration of the absorbing species. The molar absorptivity is a
measure of the probability that a transition will occur and is proportional to the transition dipole
moment between the initial and final states. A related term called the absorption cross section,
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σ, represents the area around a molecule which is accessible to collide with a photon,1 and is
related to ε by Equation 1-3.
[ ] εεσ 2510825.310ln)1.0( −⋅==AN
(1-3)
where σ has units of m2 and NA is Avogadro’s number.
Although the electronic energy levels of a molecule are discrete, absorption in molecules
does not appear as a sharp line, as is the case in atomic absorption. Rather, the absorption
appears as a broader band. The explanation lies in the fact that electronic excitation is usually
accompanied by vibrational transitions. The absorption of light takes place faster (10-15 s) than
the equilibration of nuclei (10-13 s).1 This is referred to as the Franck-Condon principle (Figure
1-1). To illustrate this phenomenon, the ground and excited electronic states are represented by
A BA B
Figure 1-1. Potential energy curves for electronic transitions. (A) Transition between states of similar equilibrium geometry. (B) Transition between states of different equilibrium geometry.
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potential energy curves as a function of their relative equilibrium geometry. The equilibrium
geometry for a particular vibrational mode (or combination of modes) is where the potential is at
a minimum. The most probable electronic transition occurs in a “vertical” manner, suggesting
that when an electron is excited, it is promoted to the higher state regardless of the equilibrium
geometry. That is, if the nuclear arrangement between the equilibrium ground state and the
equilibrium excited state are very different, excitation will promote the molecule to a higher
vibrational mode of the excited state, and then the molecule will relax to the lowest vibrational
excited state. The differences in energy between the lowest vibrational ground state (ν = 0) and
some vibrational excited state (ν' > 0) will always be larger than the 0-0 transition.
Nature of the Excited State
Following absorption and population of the electronic excited state, the molecule will first
relax to the lowest vibrational level (ν = 0) through thermal (loss of heat) or collisional (with
other molecules, either solvent or solute) relaxation. For a molecule whose initial ground state
has paired electrons, the electrons will have opposite spin. This is called a singlet state. Upon
excitation, the electron does not change its spin due to spin restrictions effected by quantum
mechanics. The excited state therefore is also a singlet state. Although the process is forbidden,
certain conditions allow for the excited electron spin to flip, resulting in a triplet excited state.
This process is called intersystem crossing (ISC). Coupling of singlet and triplet vibrational
levels of the same energy is required to allow ISC to occur. As in population of the singlet
excited state, population the triplet excited state also involves vibrational relaxation to the lowest
vibrational level.
An important characteristic of excited states is that the lowest singlet excited state is
always higher in energy than the lowest triplet excited state. For singlet excited states, the
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electrons are of opposite spin, and therefore can exist in the same region of space, as allowed by
quantum mechanics. In the triplet state, the electrons have the same spin, which prohibits them
from occupying the same region of space. With a larger distance between electrons in the triplet
state, there is less Coulombic repulsion energy than in the singlet state where electrons can be
near each other.
There are several pathways which an excited state can follow once it is populated. One
deactivation pathway is radiative decay by emission of a photon. Radiative decay can occur
from the singlet or triplet excited state. Nonradiative decay is another process for an excited
state to return to the ground state in which the energy is lost through dissipation of heat. In
addition, the excited state could return to the ground state by transferring energy or an electron to
another species.
Radiative Decay
Radiative decay involves the excited state emitting a photon and returning to the ground
state. The energy of the emitted photon corresponds to the difference in energy between the two
states. Emission from a singlet excited state to a (singlet) ground state is called fluorescence.
Because the transition is between states of the same multiplicity, it is an allowed transition. The
radiative rate for fluorescence is relatively large (kF ~109 s-1). If the emission of a photon is from
a triplet excited state, the process is called phosphorescence. Because the transition involves
states of different multiplicity, the rate of emission is much lower (kP ~ 102 – 104 s-1) than that of
fluorescence. As stated earlier, the triplet state is lower in energy than the singlet state, and as a
result, phosphorescence will occur at a lower energy than fluorescence.
Photons emitted by the excited state via fluorescence have a slightly lower energy than the
photons absorbed by the ground state. This is due to loss of absorbed energy from vibrational
relaxation in the excited state. The difference in energy between the absorbed photon and the
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emitted photon is known as the Stokes shift. The amount of the Stokes shift for a molecule is a
representation of the degree of structural (nuclear) difference between the excited and ground
electronic states. That is, if the distortion for the excited state is large, the observed Stokes shift
will be large.
As is the case with absorption spectra, emission spectra also do not appear as sharp lines.
The “vertical” transitions as described in the Franck-Condon principle illustrate the most
probable transitions from one electronic state to another, even if their equilibrium nuclear
configurations are quite varied. The transition between electronic states takes longer than the
rearrangement time of the nuclei of the molecule. Because vibrational relaxation occurs very
rapidly, emission originates from the ν = 0 vibrational level of the excited state. In contrast to
absorption, the transitions originating from vibrational energy levels will be lower in energy for
transitions from a higher electronic state to a lower electronic state.
Radiationless Transitions
All transitions between electronic states that do not involve absorption or emission of
photons are considered radiationless transitions.1 These processes, such as energy loss through
release of heat or through collisions, are referred to as internal conversion (IC) for transitions
between states of the same spin. Nonradiative transitions occur only when the potential energy
curves of two electronic states overlap (Figure 1-2). The shift will take place where the curves
intersect, and produce a vibrationally excited ground electronic state. Vibrational deactivation
through thermal or collisional loss will bring the molecule to the relaxed ground state at ν = 0.
Figure 1-2 shows the potential energy curves as a function of nuclear distance. As illustrated,
there exists an intersection between the two states depending on the energy difference between
the two states and also the nuclear equilibrium configuration differences between the two states.
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A B C
Figure 1-2. Effects of energy difference and nuclear distortion on vibrational overlap. (A) Small ∆E, large overlap; (B) Large ∆E, small distortion, low overlap; (C) Large ∆E, large distortion, large overlap.
If there is a large energy difference between the two electronic states, their potential energy
curves will not intersect, and the so-called Franck-Condon overlap will be small. A large
overlap will bring about a fast and efficient radiationless transition of the excited state. The
overlap of the two curves increases exponentially as the difference in energy levels between the
excited state and ground state decreases. Therefore, the decrease in energy difference facilitates
faster nonradiative decay between the two states.2 This effect is known as the energy gap law.
Additionally, for two systems that have the same difference in energy between the excited and
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ground states, the overlap will be greatest for the system that has a higher degree of nuclear
distortion between the states.
A radiationless transition that occurs between states of different spin is called intersystem
crossing (ISC). The transition follows the Franck-Condon principle as described above where
the overlap of the two electronic potential energy surfaces is required for vibrational coupling.
For ISC from the excited singlet state to the (lower energy) excited triplet state, the triplet state
will initially be in a vibrationally excited state, and the transition will be followed by rapid
internal conversion to the lowest vibrational level of the triplet excited electronic state. The
transition from a singlet state to a triplet state (or vice versa) is a forbidden process as defined by
selection rules because it involves states of different spin multiplicity. Consequently, the triplet
state acts as an energy reservoir because radiative decay back to the ground state is spin
forbidden.3 The spin angular momentum and orbital angular momentum of electrons in a
particular state are separate from one another. This leads to the outcome that transitions between
states of different spin are forbidden by the momentum conservation rule. This effect is more
important for molecules comprised of lighter elements. As a result, ISC yields are typically low
for organic molecules. In molecules with heavy atoms (or light molecules in the vicinity of
heavy atoms), the conditions are different. The relative orientations of spin momenta and orbital
momenta are less important. Rather, the spin and orbital angular momenta are coupled together.3
This is known as spin-orbit coupling, and accounts for the breakdown of selection rules in
singlet-triplet transitions. The change in electron spin magnetic momentum coupled with the
corresponding orbital change provides a mean of conserving total angular momentum.1 This
heavy-atom effect is responsible for increased ISC yields.
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Other types of radiationless transitions are also possible, and will be mentioned briefly.
Energy transfer from an excited state to some suitable acceptor is one type of relaxation pathway,
as described by Equation 1-3.
** ADAD +→+ (1-3)
where D is an energy donor, A is an energy acceptor, and * denotes an excited state for either.
Exchange energy transfer (collisional) and Coulombic energy transfer (dipole-dipole interaction)
are two mechanisms for energy transfer. Both require that the energy of D* is higher than that of
A, and the rate of energy transfer from D* to A is greater than the decay rate of D*. Similarly, an
excited state donor can transfer an electron to a suitable acceptor.
−+ +→+ ADAD * (1-4)
Furthermore, triplet-triplet annihilation is another type of nonradiative decay involving two
triplet excited state species:
SSTT +→+ *** (1-5)
where T* and S* are the excited triplet and excited singlet, respectively, and S is the ground-state
singlet. Triplet-triplet annihilation occurs when two molecules in the triplet state interact
(generally by collision) to create an excited singlet state and a ground singlet state. If the excited
singlet state returns to the ground state through emission of a photon, it is called delayed
fluorescence.
A Jabłoński diagram (Figure 1-3) will be useful to summarize the probable potential
transitions between the common electronic states in a molecule as discussed above.
Before introducing a brief review of organometallic materials and their photophysical
behavior, there are two characteristic properties that should be discussed: the photoluminescence
quantum yield and lifetimes of the excited states. Quantum yield of emission is simply the ratio
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ν = 0 ν = 1ν = 2ν = 3
IC S1 ISC
ν = 0 ν = 1ν = 2ν = 3
ICT1
A ICF P IC
S0
Figure 1-3. Jabloski diagram illustrating possible transitions. S0: Ground singlet state; S1: Excited singlet state; T1: Excited triplet state; A: Absorption; F: Fluorescence; P: Phosphorescence; IC: Internal conversion; ISC: Intersystem crossing.
of absorbed photons to emitted photons. A general expression for quantum yield1 is given by
Equation 1-6:
τ0ee k∗φ=φ (1-6)
where φ e is the emission quantum yield, ∗φ is the formation efficiency of the emitting state, k
is the rate constant for emission, and τ is the measured experimental lifetime. The lifetime is
defined by Equation 1-7:
0e
ie kk Σ+= 0
1τ (1-7)
where Σki is the sum of all other deactivating rate constants. More specifically, we can define
the two terms φ and τ for fluorescence and phosphorescence. For these values, is assumed to ∗φ
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be unity. For the processes indicated in Figure 1-3, the fluorescence quantum yield, φF, is
described by Equation 1-8:
SFF τk=φ
where τS is the lifetime of the singlet state, and is defined in Equation 1-9:
nrISCFS kkk ++≡τ . (1-9)
In Equation 1-9, kF is the rate of fluorescence, kISC is the rate of intersystem crossing (to the first
excited triplet state), and knr is the rate of all other nonradiative processes from the singlet state.
Similarly, and assuming the triplet excited state is populated only through excitation of the
ground state to form the singlet excited state, followed by ISC, the phosphorescence quantum
yield can be taken as Equation 1-10:
TPISCP τkφ=φ (1-10)
where φISC is the intersystem crossing quantum yield, andτT, the triplet lifetime, is defined in
Equation 1-11:
nrPT
1kk +
≡τ (1-11)
where kP is the rate of phosphorescence and knr is the rate of all other nonradiative processes
from the triplet excited state. Fluorescence lifetimes are generally short (10-11 – 10-8 s) because
they correspond to allowed transitions. In contrast, phosphorescence occurs on a much longer
timescale (10-6 – 101 s) due the forbidden nature of the triplet-to-singlet transition. As mentioned
earlier, heavy atoms may help facilitate intersystem crossing to populate the triplet excited state.
However, the heavy atom effect may also bring out spin-orbit coupling in the radiative decay
process, shortening the phosphorescence lifetime.
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Triplet Excited States in Conjugated Organic Systems
Although slow ISC prevents detection of triplet states in organic materials, triplet excited
state properties are nevertheless important in these systems. The increasing use of organic
materials in organic light emitting diodes, field-effect transistors, electronic semiconducting
materials, light harvesters, photosensitizing and nonlinear applications makes the triplet state an
interesting research area.4-8
Molecular Wires
The concept of an organic molecular electronics device was first proposed over thirty years
ago by Aviram and Ratner.9 However, it was not until the measurement of electrical conduction
through a single benzene-1,4-dithiol molecule by Reed and coworkers10 in 1997 that molecular-
scale electronics attracted serious attention. Since then, the study of prospective molecular wires
for application in electronics has grown tremendously.11-13 Conjugated organic rigid-rod type
molecules offer potential for such use due to their delocalized π structure which allows electronic
communication throughout the molecule.
Poly-(para-phenylenevinylene) (PPV) oligomers are a class of molecules that afford
electron delocalization through π-conjugation. Davis and coworkers14,15 have used π-conjugated
linkers as a bridge between electron donor and electron acceptor molecules to investigate charge
separation. These molecules, consisting of a tetracene donor and pyromellitimide acceptor
moieties connected by phenyl, stilbene, and PPV oligomers of varying length, are illustrated in
Figure 1-4. The usefulness of the bridge is to act as an electronic connector between the end-
group molecules under study. The understanding of the electronic properties of the bridge linker
is important to determine the mechanism of charge transfer between the donor and acceptor end-
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A
B
C
Figure 1-4. Structures of molecules studied by Davis et al.15 Linkers include (A) phenyl, (B) trans-stilbene, and (C) an oligo-PPV.
groups. For example, in the short linkers, charge transfer was achieved through tunneling (due to
a high energy barrier in the bridge), whereas in the longer PPV chains, there existed larger
delocalization (i.e. a lower energy barrier) in order to help facilitate the movement of charge.14,15
The work completed by Davis and coworkers has encouraged the use of computational methods,
such as density functional theory, to further study the electronic structure of PPV systems as π-
bridges for molecular wire-like applications.16
Among the most versatile active components for establishing electronic communication
are transition metal centers17-19 and unsaturated organic systems have proven to be among the
most useful bridging units.20,21 Also of interest are polymers which contain metal atoms in the
backbone.22,23 The incorporation of heavy atoms, particularly metals, into organic structures
changes several other characteristics, including redox, optical and electronic properties. As
mentioned earlier, introducing heavy atoms into organic molecules induces spin-orbit coupling
26
of the excited state. From a synthetic standpoint, the alteration of structure through ligands,
spacer units, and types of metals used can systematically vary the singlet and triplet energy
states.
Polyynes, e.g. –(C≡C)n– oligomers or polymers, are distinctive in that they provide one of
the simplest possible linear π-conjugated organic units of variable length. With the addition of
metal atoms, such as platinum, to the ends of highly conjugated organic chains, the metal centers
not only terminate the extended π system, but become integral parts of them. Furthermore, metal
atoms as end-caps increase the stability of long carbon chains.24-26 In such α,ω-dimetalla-π-
systems the communication of electronic information between the metal centers is expected to be
particularly efficient. Additionally, luminescence properties become of interest.23,27 The spin-
orbit coupling increases population of the triplet state, therefore giving access to the triplet
manifold and allowing for further investigation of the electronic state energies.
Nonlinear Optics and Platinum Acetylides
Due to the advancement of laser technology in a variety of applications, the need for
protection from intentional or erratic laser pulses to prevent damage of human eyes, optical
sensors, and other sensitive optical components has become important to a number of fields. An
optical power limiter is a device that will allow the transmittance of light at low intensities, but
will strongly attenuate the incident energy at high intensities.28,29 It is also desirable for a
material which exhibits optical power limiting behavior to have a fast response time, reasonable
processability, good linear transparency, and a high damage threshold.29-31 Many materials
currently used to protect eyes or sensors from damaging light absorb strongly at only particular
wavelengths, restricting use to certain wavelengths while also limiting visibility at low
incidences.
27
Due to their high optical nonlinearities and fast response time, organic materials are
attractive for use as optical limiters. More specifically, metallophthalocyanines and
metalloporphyrins display excellent nonlinear optical behavior due to large ISC efficiencies from
spin-orbit coupling of the heavy atoms which populate the highly absorbing triplet excited
states.32,33 Unfortunately, poor solubility has led to difficulties in processing and hindering the
development of practical devices.
Reverse saturable absorption (RSA) is one mechanism by which nonlinear absorption can
occur. RSA is a relatively slow, sequential multiphoton process in which the ground electronic
state first absorbs a photon, and then an excited state absorbs a second photon. For RSA to
occur, the absorption cross section of the excited state must be larger than the absorption cross
section of the ground state (Figure 1-5). In transition metal-containing organic compounds, the
absorbing excited state is usually the triplet state, populated through ISC facilitated by the heavy-
atom effect. Transient absorption (TA) spectroscopy is a convenient way to probe the excited-
state absorption properties (a more detailed discussion about TA spectroscopy will be given in
Chapter 2).
Platinum-acetylide complexes have been shown to have highly absorbing triplet states with
long lifetimes.34-41 The large ISC yields afforded by the platinum atom create strongly
phosphorescent triplets, making these molecules excellent candidates for studying triplet-state
properties. Structural changes in conjugation length of the organic moieties allows for spectral
tunability in absorption and emission to create optical bandgaps in the UV, visible and near-IR.
McKay and coworkers34-39 were the first to report on the nonlinear optical properties of a
platinum-acetylide compound and have since thoroughly investigated the optical properties of a
platinum-ethynyl complex (shown in Figure 1-6). The compound exhibits strong singlet
28
S1
S0
T1σgs
ISC
Tn
σes
σes > σgs
Input Intensity
Out
put I
nten
sity Linear
absorbance
RSA
Figure 1-5. Conditions for reverse saturable absorption.
absorption with the maximum at 355 nm. (Interestingly, the absorption maximum makes the
molecule convenient for optical excitation by the third harmonic of a Nd:YAG laser.) There is
emission from the compound at 410 nm, which is attributed to fluorescence, and another peak at
520 attributed to phosphorescence. The peaks were assigned mostly due to the short (330 ps)
and long (0.5 µs) lifetimes, respectively. At low temperatures, the phosphorescence lifetime
increases, up to 700 µs at 17 K, while the fluorescence lifetime is unchanged. The intensity of
phosphorescence emission increased as well at low temperatures, unlike the fluorescence. The
29
PtPBu3
PBu3
Figure 1-6. Structure of platinum-ethynyl and corresponding photophysical data. Figures adopted from McKay et al.36
unvarying fluorescence intensity and decay rate over a range of temperatures show that ISC
dominates the singlet state decay path and is independent of temperature, indicating that ISC
efficiency approaches unity even at room temperature. Excited-state absorption measurements
reveal that platinum-ethynyl demonstrates strong RSA over some wavelengths. For example, at
513 nm, the excited-state absorption cross section is 104 times larger than the ground state
absorption.
Further study of platinum-ethynyl by Staromlynska and coworkers35 leads to observation
of an alternate absorption pathway. Emission from the compound was detected when excited
where the compound has effectively no ground-state absorption at 595 nm, by either a 45 ps
pulse or 2 ns pulse. Emission spectra caused by excitation at 595 nm are identical to those
caused by excitation at 355 nm, indicating that the same state (the triplet excited state) is arrived
via two different absorption pathways: one photon absorption and two photon absorption (TPA),
30
each to the singlet excited state, followed by fast ISC to the triplet excited state. TPA will be
discussed in more detail in Chapter 5. Briefly, two-photon absorption occurs when a material
simultaneously absorbs two photons whose combined energy equals that of the energy difference
between the absorbing state and the excited state. An additional report by the same group36
shows that TPA dominates at wavelengths between 560 and 700 nm. There is no absorbance in
this wavelength region at low incident energy. The two-photon absorption coefficient is found to
be independent of excitation wavelength. For shorter wavelengths (< 560 nm), one-photon
excitation dominates. The transmission as a function of input intensity was also measured at 595
nm for 45 ps pulses and 2 ns pulses. The results demonstrate the same nonlinear behavior,
regardless of pulse length. Slightly stronger nonlinear behavior is observed for 2-ns pulses at
455 nm.
Two additional reports by McKay, Staromlynska and coworkers38,39 on platinum-ethynyl
provided several further findings and summarized the results of their work. Three main regions
for excitation are described: (i) blue (< 500 nm), where single photon excitation to the singlet
state, followed by rapid ISC to the triplet state, (ii) green (500 – 540 nm), where direct excitation
of the triplet state from the ground singlet state occurs, as spin-orbit coupling aids in the
forbidden transition, and (iii) red (540 – 700 nm), where no one-photon process occurs between
the singlet ground and either singlet or triplet excited states, but two-photon absorption
dominates to populate the singlet excited state, followed by ISC to the triplet excited state. The
triplet excited state in all three regions is highly absorbing throughout the visible. Furthermore,
the excited state absorption cross section is independent of the excitation pathway to the excited
triplet state. Absorption from the excited singlet was not studied as no direct evidence of this
31
transition has been found. The Jabłoński diagram in Figure 1-7 summarizes the transitions
occurring in platinum-ethynyl.
S1
S0
T1
ISC
Tn
1 2 3
S1
S0
T1
ISC
Tn
1 2 3
Figure 1-7. Jabloski diagram illustrating possible transitions for platinum-ethynyl. (1) Blue (< 500 nm) – single photon absorption followed by ISC. (2) Green (500 – 540 nm) – direct excitation to the triplet state. (3) Red (540-700 nm) – Two-photon absorption followed by ISC. All three regions include triplet-state absorption.
Platinum-ethynyl shows promise as a broadband optical limiter in the visible region that is
effective over a range of pulse lengths from picoseconds to hundreds of nanoseconds. Between
530 and 650 nm, incident energies of greater than 500 µJ were attenuated to ~6 µJ by a 0.08 M
solution of the compound. However the authors did note some degree of decomposition at
irradiances of > 6 J/cm2 at 532 nm and 1064 nm. Also, solutions of platinum-ethynyl showed
degradation over a period of several weeks. Such behavior is not acceptable for a practical
optical limiting device, and if stabilization is not possible, alternate approaches should be found.
32
Although the photophysics of platinum-ethynyl have been thoroughly investigated, the
lack of a series of related molecules makes any conclusions of structure-property relationships
difficult. Cooper, Rogers, and coworkers40-45 have synthesized and studied the photophysical
behavior of a series of platinum(II)-containing phenyl-ethynyl oligomers (Figure 1-8). The goal
was to determine structure-property relationships in a well-defined system through IR, UV/Vis,
photoluminescence, and transient absorption spectroscopy and kinetics of some of the
transitions.
Pt
PBu3
PBu3
Pt
PBu3
PBu3
Pt
PBu3
PBu3
Pt
PBu3
PBu3
(Ch3)3Si Pt Si(Ch3)3
PBu3
PBu3
E1
PE1
PPE
PE2
PE3
Figure 1-8. Structures of platinum acetylide oligomers studied by Cooper, Rogers, et al.40-45
The PtCC acetylenic vibrational bands for E1 were 2033 cm-1 compared to 2100 cm-1 for
the other compounds, suggesting that the PtC and CC bonds have more double-bond character in
E1 than the compounds with the larger ligands.43 UV/Vis absorption maxima systematically red-
33
shifted from 308 nm for E1 to 376 nm for PE3, indicating that the increase of ligand length
decreases the energy between the ground and excited singlet states. As with PE2 as studied by
Staromlynska, McKay et al.,35 fluorescence quantum yields were small. E1 and PE1 show
minimal emission. PE2 and PE3 have fluorescence maxima near 400 nm, while PPE shows
fluorescence emission at 375 nm. The higher emission energy is attributed to the twisting of
phenyl groups interrupting conjugation. The dihedral angle was calculated43 to be 40°, whereas
PE2 and PE3 have free rotation around the triple bond. Room temperature phosphorescence of
PPE, PE2 and PE3 is observed, with a 0-0 band preceding unresolved vibronic bands at lower
energy, due to combinations of CC single and triple bond stretching, out-of-plane CH stretching,
etc.43 Phosphorescence maxima systematically increase with increased conjugation length.
Triplet excited state absorption observed for PE1 decayed within the length of the laser pulse (<
15 ns), however, the other compounds exhibited transient molar absorptivities that increase and
red shift with increased conjugation length. The excited state lifetimes also show a clear trend
that lifetime increases as conjugation length increases, attributed to less spin-orbit coupling
effect from platinum as the excited state is more delocalized on the ligand.40
The group further complemented their study of the platinum oligomer series by direct
comparison with a set of all-carbon butadiyne analogs44. They found that conjugation through
the platinum center occurs in the singlet state, but the triplet states are more complex. The
lowest triplet state shows metal-to-ligand charge transfer character as is likely confined to one
ligand, whereas the higher excited triplet shows ligand-to-metal charge transfer.44 The shortest
compounds are affected greatest by the presence of platinum. Fluorescence emission spectra for
the butadiyne compounds are very similar to the platinum-containing compounds, but quantum
yields decreased dramatically when platinum is incorporated into the structure. For the shorter
34
all-organic structures, triplet emission is very weak and blue-shifted compared to the platinum
analogs. However, phosphorescence emission bands begin to appear the same between platinum
oligomers and butadiyne as conjugation length increases.
Based on these findings, the authors have concluded several things from the sensitivity of
state energies. The transitions involving absorption by the ground singlet state and by the triplet
excited state are more sensitive to molecular size than emission from the triplet excited state to
the ground excited state. Therefore, it follows that the ground singlet and first triplet excited
states are more confined than the excited singlet and higher excited triplet states which are
delocalized over the ligand.
Objective of Present Study
Defining structure-property relationships between molecules of interest and photophysical
behavior is paramount to increased development of organic and organometallic compounds for
application in light-emitting diodes, chemical sensors, molecular electronics, photovoltaics,
optical sensors, and nonlinear optics. Platinum- and iridium-containing conjugated compounds
offer the ability to study triplet excited state behavior in a controlled and deliberate manner
through synthetic capabilities.
Fundamental aspects of characterization, such as singlet and triplet state energies and
quantum yields, are measured using traditional techniques with commercially available
spectrophotometers. In addition to commercial instrumentation, a transient absorption apparatus
was designed and constructed to examine photoexcited states of metal-containing organic
compounds. Included along with arrangement of optical components, user-friendly software was
developed to control the timing parameters of the hardware involved.
Platinum acetylide complexes are excellent systems for probing a variety of triplet-state
phenomena such as ground state absorption to the triplet state, intersystem crossing, triplet state
35
absorbance, and phosphorescence.45 While the consequence of the platinum atom on
delocalization of the excited states has been studied, the effect is not completely understood for a
variety of compounds. The photophysics of a series of dinuclear platinum-acetylide oligomers
with phenylene-ethynylene end-caps has been investigated to provide insight into conjugation
through platinum atoms and the effect on excited state energies.
The understanding of polyyne is crucial for the progress of molecular electronics. While
direct application of molecular wires is not the focus of this work, it has been shown that
characterization of excited-state properties of potential molecules is fundamental in
understanding the nature of these materials. Incorporation of heavy metal atoms as end-caps to
polyyne chains allows for the population, and thereby detection, of the triplet state. A series of
long carbon chains end-capped with platinum has been photophysically characterized to probe
the effect of chain length on excited state behavior.
The need for optical limiting materials is increasing as the use of lasers in a variety of
applications is becoming more popular. The nonlinear optical behavior of many organic and
organometallic materials provides promise for protection from potentially damaging laser pulses.
The incorporation of iridium with a highly absorbing organic ligand has created a material with
alternate absorption pathways. The ground and excited state absorption pathways have been
probed by single and multiphoton spectroscopy, and preliminary nonlinear behavior has been
detected.
36
CHAPTER 2 TRANSIENT ABSORPTION SPECTROSCOPY AND INSTRUMENTATION
Background
The study of transient chemical species generated by various approaches such as chemical
reactions or the absorption of energy is necessary for the complete understanding of these
processes. As some events occur on timescales of minutes to days, the observation of the species
involved is less hindered by time sensitivity. However, many chemical reactions produce
reactive intermediates that last only milliseconds or less before their demise. Indeed, the
absorption of light or other energy to produce excited-state species and the transfer of energy or
electrons within a molecule or between molecules are processes which generally do not allow for
the separation of very long-lived, stable species. The dynamics of such processes are also of
interest to characterize mechanisms and species involved. A detection technique on an
equivalent timescale to the event being monitored is necessary for unhindered observation. Also,
it is imperative that the species to be studied be generated quickly as to have a suitable
concentration available for detection. In this chapter, the construction and operation of a new
transient absorption apparatus for the detection of excited-state species (such as those discussed
in the following chapters) is described.
Transient absorption (TA) spectroscopy, which is also called flash photolysis or simply
“pump-probe” spectroscopy, has remained one of the most important techniques for detection
and characterization of short-lived chemical species since its development in the late 1940s.46
Indeed, the importance of this work was recognized as half of the 1967 Nobel Prize was awarded
to Norrish and Porter for their development of the flash photolysis method. Their research
focused on studying reactive intermediates generated by excitation from a flashlamp.46 It was
not until the 1960s that better time resolution could be achieved with use of lasers as an
37
excitation source.47 The apparatus design developed by Lindqvist is still the basis of modern
instrumentation (Figure 2-1). The system consists of three main components: an excitation
sample
mono. PMT
amplifier
oscilloscope
lamp
laser light
sample
mono. PMT
amplifier
oscilloscope
lamp
laser light
lamp
sample
mono. PMT
amplifier
oscilloscope
lamp
laser light
sample
mono. PMT
amplifier
oscilloscope
lamp
laser light
lamp
Figure 2-1. Transient absorption apparatus developed by Lindqvist.47
source (laser), a monitoring source (lamp), and a detector (monochromator/PMT). Computer
control and data acquisition was the next advancement, and such a system was developed in the
1970s by Small and Scaiano.48 In earlier instruments, the time resolution was limited not by the
electronics, but the duration of the excitation source. Pico- and femtosecond techniques are now
popular, allowing detection of short-lived species or events as short 10-15 s. However, the need
for nanosecond techniques remains prevalent.
Techniques
In the early stages of nanosecond timescale measurements, nitrogen and ruby lasers played
a defining role in the development of experimental ability.49 Currently, a variety of excitation
sources are employed for initiating events of the same time regime. Commercially available
excimer and Nd:YAG lasers offer the ability to excite samples at a large range of wavelengths
38
from the ultraviolet into the infrared. Tunable optical parametric oscillators (which are used in
combination with such pump sources as Nd:YAG lasers) offer even greater flexibility in
excitation wavelengths.
The probe beam for transient absorption is commonly one of two types of xenon lamps.
Xenon lamps are convenient due to their broadband spectral output and high intensities. A
continuous output lamp can be used, often paired with mechanical shutters to limit sample
exposure between acquisitions. The lamp intensity can be increased by a factor of 5-100, or
greater, over a several milliseconds by pulsing the lamp.50 This technique is sometimes
necessary for short timescales, and although it reduces the lifetime of the lamp, the signal-to-
noise is significantly improved. Alternatively, strobe flashlamps with high-voltage power
supplies are employed as the monitoring source. These lamps are designed for high-repetition
operation (to coincide with the pulsed excitation source) and can offer excellent arc stability,
making them ideal for use in such an application.
A number of detectors can be used for measuring and recording the probe beam as long as
the basic requirements of sensitivity in the desired spectral range and being responsive on the
timescale of the events being measured are satisfied. A common detector is comprised of a
monochromator and a photomultiplier tube (PMT). The PMT should be wired for a fast time
response. With the range of PMTs available, detection from the UV into the near IR is possible.
Using such a detector leads to collecting a time-resolved measurement of the lamp intensity at a
single wavelength, and then extracting the transient absorption data. A complete spectrum is
then constructed for a particular time within the decay of the transient (vide infra).
Alternatively, transient absorbance can be measured using a spectrally resolved detector
and gating the collection temporally. Such a detector could be a photodiode array. A diode
39
array can measure polychromatic light from a dispersive element such as a prism or grating, and
then the information from each diode is processed by a computer. The data collected from this
type of detector provides a complete spectrum from which a time trace can be constructed (vide
infra). Although multiwavelength detection is convenient, there are some limitations with diode
detectors. The main disadvantage of using diode arrays is their lack of sensitivity. Diode
detectors are generally less sensitive than PMT detectors because diode detectors do not have the
electronic gain that PMTs do, and therefore produce only one charge per photon. Diode arrays
are also noisier than PMT detectors.
A major advancement to simultaneous multiwavelength acquisition was the introduction of
the charge-coupled device (CCD) camera. Simply stated, a CCD is a two-dimensional matrix of
photosensors, or pixels.51 Each pixel will convert photons into charge, which is then converted
into a digital signal read and processed by a computer. Coupling with a dispersion element such
as a prism or grating is necessary to separate wavelengths along a particular axis of the CCD
chip. The relative intensities of incident photons will determine the amount of relative charge
that is stored in the CCD chip and subsequently read to a computer. A CCD is similar to a
photodiode array in that each of the individual pixels of the CCD responds independently to the
photons which come into contact with it. Generally, a silicon-based photosensor is used, and
therefore the response is not wavelength dependent. For imaging-type CCD cameras, filters are
used to discern colors between pixels on a CCD chip. In spectroscopy, however, such two-
dimensional wavelength dependence is not necessary. By coupling the CCD with a dispersion
element such as a spectrograph, a range of wavelengths (along one axis of the chip) can be
collected simultaneously.
40
While CCD cameras are effective instruments for recording images in a variety of
applications, the response times of early devices were not adequate for use in kinetic
measurements. The invention of the image intensifier has circumvented this problem.
Intensified charge-coupled devices (ICCDs) are often used in low-light conditions due to the
increased sensitivity of the CCD. More importantly for kinetic measurements, the intensifier can
be used as a fast electronic gate to increase temporal resolution. The intensifier is mounted
Applied Voltage
Applied Voltage
Photocathode
Incident Photons
MicrochannelPlate
Phosphor Screen
Fiber Optic Output Window
Input Window
Applied Voltage
Applied Voltage
Photocathode
Incident Photons
MicrochannelPlate
Phosphor Screen
Fiber Optic Output Window
Input Window
Figure 2-2. CCD intensifier. Figure adapted from www.andor.com.51
within the camera before the CCD chip, and consists of three main components: a photocathode,
a microchannel plate (MCP), and an output phosphor screen. A schematic representation of the
intensifier is shown in Figure 2-2. When incident photons collide with the photocathode,
41
electrons are ejected towards the MCP. The MCP consists of millions of parallel channels with
secondary electron emitters contained within their inner walls.52 When a sufficiently high
voltage is applied across the MCP, the ejected photoelectrons are drawn through the channels
ejecting other electrons, effectively acting as a photomultiplier. The electrons exiting the MCP
collide with the phosphor screen, creating an intensified image to be projected onto the CCD. A
fiber optic typically connects the intensifier to the CCD. By appropriately controlling the
voltage across the MCP, the intensifier becomes an effective electronic shutter capable of very
fast gate times (~10-9 s). The transient absorption spectra and time decay profiles from data
collected by ICCD detectors are constructed in a similar fashion as diode array detectors.
Apparatus
Hardware
A diagram of the final apparatus is shown in Figure 2-3. The three main components as
described earlier are the pump, the probe, and the detector. The pump beam (blue path) is
generally created by the third harmonic of a Nd:YAG laser (355 nm, Continuum Surelite II, 5 ns
fwhm). An alternate excitation source, which provides greater flexibility in excitation
wavelengths, is generated by using an optical parametric oscillator (Continuum Surelite OPO
PLUS) pumped by the laser described above. The pump beam is set to an energy of
approximately 10 mJ/pulse, and directed to the sample using conventional laser optics. The
beam is shaped prior to the sample using a cylindrical plano-concave lens (not shown in figure)
such that the beam hitting the sample spans the width of the cuvette along the optical axis of the
probe beam. An anodized aluminum sample holder masks the pump beam to a vertical thickness
of 4.0 mm, restricting the excitation to a specific region within the sample cuvette.
The probe beam (red path in Figure 2-3) is generated by a xenon flashlamp (Perkin Elmer
Flashpac LS-1130 with an FX-1150 bulb). The spectrum of the lamp as provided by the
42
Lamp L1 L2 L3 BS1 M1 OPO Nd:YAG laserIris Prism
L4 L5 L6BS2M2 Sample Spectrograph & CCD Camera
Lamp L1 L2 L3 BS1 M1 OPO Nd:YAG laserIris Prism
L4 L5 L6BS2M2 Sample Spectrograph & CCD Camera
43
Figure 2-3. The transient absorption apparatus. Lenses, mirrors, and beamsplitters are abbreviated by L, M, and BS and described in the text. The blue line (-·-) is the pump; red line (―) is the sample probe, I; and black line (---) is the reference probe, I0.
Figure 2-4. Spectrum of xenon lamp used as the probe. (Taken from 1100 series flashlamp datasheet, PerkinElmer.)
manufacturer is show in Figure 2-4. The red region of the measured spectrum is relatively much
weaker, and the intense peaks between 800 nm and 850 nm are not observed mostly due to the
coating on the lenses as well as the weak detector response at those wavelengths (vide infra). All
optics are antireflective coated 350-700 nm, 50.8 mm diameter plano-convex BK7 lenses, unless
otherwise noted. The beam is first collimated by L1 (focal length = 3.45 cm) and then refocused
by L2 (fl = 30 cm) through an adjustable iris which serves as the primary adjustment for the final
beam size. The beam is then collimated again by L3 (fl = 30 cm) and passed through a 50/50
beamsplitter. The transmitted beam is reflected first from a mirror and then a second
beamsplitter. The reflected beam from the first beamsplitter is reflected from a mirror and then
passed through a second beamsplitter. The distance between BS1 and M1 and between BS2 and
M2 is 8.5 cm, and the distance between BS1 and M2 and between M1 and BS2 is 19 cm. This
44
arrangement of beamsplitters and mirrors allows for the creation of two beams of equal intensity.
By creating two beams from a single source, one beam can be slightly offset to provide an
essentially identical reference beam. Both beams are focused through the sample using L4 (fl =
10 cm, diameter = 38.1 mm). The main probe beam (sample beam) is focused through the
portion of the sample that has been excited by the laser. The reference beam is focused
approximately 3 mm directly above the sample beam focus. If necessary, the focused beams can
be aligned relative to each other by adjusting the position of either mirror. After passing through
the sample, both beams are collected and collimated by L5 (fl = 17.5 cm) and focused onto the
entrance slits of the spectrograph by L6 (fl = 20 cm).
The detector is comprised of a spectrograph (Acton SpectroPro 150) coupled to an
intensified CCD camera (Princeton Instruments PI-Max iCCD). The spectrograph grating
provides a spectral range of ~350 – 850 nm. A higher resolution grating can alternatively be
used to give a spectral window of roughly 250 nm. However, the transient absorption spectra of
the compounds typically studied in our lab do not have the fine structure to necessitate gratings
of this resolution. The CCD camera settings are controlled primarily by commercially available
software (WinSpec/32 version 2.5.12.2, Roper Scientific) supplied by the camera manufacturer.
The CCD chip is 512 x 512 pixels, with the x-axis corresponding to wavelength and the y-axis
corresponding to vertical position on the entrance slits of the spectrograph. Two regions of
interest of on the chip have been defined to identify the sample beam and the reference beam.
The CCD camera and software collect and record the raw data, which are intensities of the probe
for the sample beam and the reference beam as a function of wavelength. This information must
be converted to ∆ absorbance values in a separate step.
45
Data Collection
There are several parameters that can be adjusted in order to obtain meaningful, quality
time-resolved transient absorption spectra. Following excitation, a sample relaxes to the ground
state, for example, through first order decay. The most important factor in a transient absorption
apparatus is timing; it is imperative for meaningful data to be collected that all components of the
apparatus perform their functions at intended and known times throughout the experiment. In
order to simplify this potentially complex problem, a four-channel pulse generator (Berkeley
Nucleonics Corporation model 555 pulse/delay generator) is used to send electronic triggers to
each of the 3 components (laser flashlamps and Q-switch, xenon flashlamp, and camera). To
increase the ease of operation and expand the flexibility of experiments and maintenance, a
LabVIEW-based virtual instrument (VI) was written in-house to provide a computer interface for
the pulse generator. The VI is used in collaboration with the WinSpec/32 software for data
collection. By using the pair of programs, all necessary timing parameters can be optimized.
An observer using a CCD detector has several choices to make after exciting a sample:
when to begin monitoring, for how long to integrate each image, how many images to take, and
over what timescale to make the measurements. A graphical representation of these parameters
is illustrated in Figure 2-5.
• t0 (time zero) – This is the time at which the laser fires. Assuming fast excitation to the singlet state (10-15 s) followed by rapid intersystem crossing (10-9 s), this is also the time at which there exists the greatest population of triplet excited molecules.
• Camera delay – This is the time between the laser fire (t0) and the onset of the first recorded transient. Camera delay is useful when there is a short timescale event, such as strong fluorescence or scattered laser light, which can cause artifacts in the TA spectrum.
• Gate width – This is the amount of time the CCD is exposed to light. The maximum amount of light in the shortest amount of time is ideal for greater time resolution and signal-to-noise. The gate width is determined by the amount of probe light reaching the detector, i.e. a more intense probe will need a shorter gate width.
46
• Camera delay increment – This is the time between the beginnings of each image acquisition. The camera delay increment should be equal to or longer than the gate width so that no overlap of signal is measured.
Inte
nsity
A
Inte
nsity
Timet0
B
Timet0
Figure 2-5. Illustration of timing parameters for ICCD data collection. (A) shows a comparatively long camera delay ( ), short gate width ( ) and long camera delay increment ( ) with respect to (B).
47
The length of the measurement will equal the number of transients collected multiplied by the
camera delay increment. Ideally, the total time should exceed the decay time of the excited state.
Also, the greater the number of transient spectra collected, the easier it will be to accurately fit
the set of spectra to a kinetic model. The parameters above, as well as some additional settings,
will greatly affect the signal-to-noise ratio of the system, and will be discussed in more detail
later in the chapter.
Data Processing
To look at how a transient signal is measured and calculated, it can help to begin with the
definition of absorbance and transmittance,
)(log)( λλ TA −= (2-1)
)()()(
0 λλλ
IIT = (2-2)
where I0(λ) and I(λ) are the intensities of radiation prior to passing through the sample and after
passing through the sample, respectively. Substituting Equation 2-2 into Equation 2-1, an
expression for absorbance in terms of intensities can be given as Equation 2-3, shown below.
−=
)()(log)(
0 λλλ
IIA (2-3)
This expression is a convenient way to convert measured intensities of light into
absorbance values. An important part of this equation is that I0 must be measured without the
sample in the optical path (either by physical placement or by using a dual-beam instrument).
However, in transient absorption experiments, the intensity of the lamp itself is never measured
directly. Instead, I0 is defined as the intensity of the light that passes through the ground state
(i.e. before optical pumping and creation of excited states). After populating the excited state,
there will be some change in intensity of the transmitted light,
48
)()()( 0 λλλ III ∆+= . (2-4)
Figure 2-6 shows a graphical representation of the change in intensity of the transmitted light
throughout the lifetime of a transient at a single wavelength. I0 is an arbitrary value determined
Inte
nsity
∆I
t0
I0
Time
Inte
nsity
∆I
t0
I0
Time
Figure 2-6. Change in intensity of transmitted light during the lifetime of a transient species.
by the probe lamp, alignment, and camera settings. The laser fire defines t0. Equation 2-4 can be
substituted into Equation 2-3 to define ∆A as
∆+−=∆
)()()(
log)(0
0
λλλ
λI
IIA , (2-5)
and simplifying to
−=∆
)()(log)(
0 λλλ
IIA . (2-6)
49
In many transient absorption apparatus, a continuous-wave monitoring source is used (such
as a PMT connected to an oscilloscope) for transient absorption spectroscopy. Therefore I0 can
be measured prior to t0, and stored for calculation later. This method would also be possible
using a pulsed monitoring source such as a CCD camera, although it would not necessarily
simplify any of the calculations. For this reason, and more importantly to try to minimize shot-
to-shot fluctuations of the flashlamp, the probe beam in the current apparatus has been split into
a sample beam and a reference beam. The probe beam passes through the region of the sample
that has been excited by the laser. The reference beam passes through an area of the sample
which has not been excited (i.e. the ground state). This allows for I0 and I to come from the
same flash of the probe (minimizing noise), as well as the simultaneous collection of each
(minimizing experiment time). In order to normalize the sample and reference beams, a simple
correction factor, k(λ), can be calculated:
)()(
)( 0
λλ
λII
k = , when pump is off. (2-7)
When measured prior to t0 (i.e. with the laser off), k(λ) will correct for any slight differences in
intensity of the two beams caused by the detector or alignment. The correction factor will be
different for each experiment, and therefore should be collected at the beginning of each
measurement. Placing the correction factor into Equation 2-6,
−=∆
)()()(log)(
0 λλλλ
IIkA , (2-8)
a relationship between raw data from the camera software and a presentable set of spectra is
easily made.
Using a combination of full spectral range detection, along with pulsed delay detection,
time-resolved spectral data can be constructed. Figure 2-7 shows how the light intensities of
50
∆Ab
sorb
ance
Wavelength
∆Ab
sorb
ance
Wavelength
Inte
nsity
Wavelength t0t1
t2t3In
tens
ity
Wavelength t0t1
t2t3
Wavelength t0t1
t2t3
∆A
bsor
banc
e
Time
∆A
bsor
banc
e
Time
Figure 2-7. Construction of a time trace and transient absorption spectra from gated multiwavelength measurements.
the sample and reference beams at a single wavelength vary throughout the excited state decay
using a pulsed monitoring source. By having this data for many wavelengths, time-resolved
transient absorbance data for a particular wavelength can be constructed as shown.
It is convenient to use temporally resolved data to characterize the nature of the excited
state of a material. Often in combination with spectral information, lifetimes can help discern
51
between simple excited-state decay, electron transfer, energy transfer or even creation of another
species. In the most general case of monitoring the relaxation of an excited (triplet) state, kinetic
information can be elucidated quite easily. As described earlier, transient absorption intensity as
a function of time can be plotted for a particular wavelength. These data can then be fitted
following first-order kinetics. That is, assuming one excited state and one ground state, the
kinetics follow that of reactive chemistry:
ktAA
tt
−=
∆∆
=
)()(
ln]A[]A[
ln 00 (2-9)
where [A]t and [A]0 are the concentrations of a species A at some time t, and the initial
conditions at time t0, and (∆A)t and (∆A)0 are the transient absorbance at some time t, and the
initial time t0. This relationship can be made due to the linear relationship between sample
concentration and absorbance. For more complicated kinetic systems, several wavelengths could
be monitored independently, or commercially available software could be used for analysis.
Sources of Noise
A majority of the noise in the current system can be ascribed to the inherent stabilities of
each of the components that make up the apparatus, which are described in further detail below.
A smaller part of the noise is attributed to the sample itself, in which photodegradation can
occur, causing potentially severe changes in the signal. This can generally be controlled by
using an appropriate excitation energy and properly mixing the solution throughout the course of
the experiment; however, some samples are more prone to photodegradation, and little can be
done to prevent it. Repetitive sampling for each transient greatly affects the signal-to-noise ratio,
and some examples are given below.
Each component has inherent fluctuations which contribute to the noise of the system. For
example, the stability of the laser power output will influence the excited-state population and
52
affect the intensity of the transient absorbance signal. The probe beam stability, while not
affecting the intensity, contributes to the shot-to-shot fluctuations that limit reproducibility and
causes baseline error. The camera has excellent stability at full-well capacity; the wavelength
range at which this happens is dependent on the output of the xenon lamp. In other words, based
on the spectral distribution of the probe, the noise is much lower between 400 and 650 nm where
the measured lamp output is strongest, and the noise is larger at the tail ends of the spectrum,
especially around 800-850 nm where the measured output is weakest. A spectrum of the xenon
probe source used, in terms of I and I0 as measured by the detector, is shown in Figure 2-8. The
discrepancy around 500 nm is due to a defect in the CCD chip (presumably from damage);
however, this has minimal effect on the transient absorption signal.
Wavelength / nm400 500 600 700 800
Rel
ativ
e In
tens
ity /
a.u.
Figure 2-8. Relative probe intensities for I0 (––) and I (----) with the pump beam off.
53
Baseline Measurements
Baseline measurements were made using the apparatus and the representative results are
displayed in Figure 2-9. In this series of tests, the primary goal was to ascertain the overall noise
of the system as a whole. Using a blank sample (i.e. an empty cuvette), spectra were collected
under the same conditions while the number of averaged images were changed. For each set, a
collection of ten “transients” was collected, each at t0, using a gate width of 10 ns. The camera
software was set to accumulate 10, 25, 100, or 500 images. As shown clearly in Figure 2-9A and
B, the extremes of the spectrum are the noisiest, due to the low output of the probe at those
wavelengths. Increasing the number of averaged images from 10 to 100 improves the limit of
detection from greater than 15 mO.D. to roughly 5 mO.D. While increasing the number of
averaged images from 100 to 500 results in slightly better reproducibility, the overall noise is not
improved greatly (Figure 2-9C and D). In order to limit the amount of laser energy to which the
sample is exposed and reduce the experiment time, as well as maintain a satisfactory baseline,
averaging 100 images per transient delay time is the most suitable for data collection.
Results
A 1.2x10-4 M solution of anthracene (Acros) was prepared in HPLC grade benzene (Sigma
Aldrich). The solution was deoxygenated by bubbling with argon for 40 minutes. The camera
was set to collect with a 10-ns gate width with a 3-µs delay between transients. Prior to
excitation, the spectrograph slits were adjusted (130 µm) so that the peak probe intensity was
near the maximum for the detector. The laser energy was set to ~10 mJ per pulse. The transient
absorbance of the anthracene solutions under these conditions is shown in Figure 2-10. The λmax
at 432 nm, as well as the intensity, matches well with previously published data.53 The lifetime
54
-15-10-505
1015
∆ Ab
sorb
ance
/ 10
-3
-15-10-505
1015
-15-10-505
1015
Wavelength / nm400 500 600 700 800
-15-10-505
1015
A
B
C
D
Figure 2-9. Baseline noise of the apparatus. Conditions: 10 ns gate width, 10 transients, camera delay and delay increment 0 ns, number of averages (A) 10, (B) 25, (C) 100 and (D) 500.
55
Wavelength / nm
400 450 500 550 600 650 700
∆ Ab
sorb
ance
-0.1
0.0
0.1
0.2
0.3
0.4
Figure 2-10. Transient absorption spectrum of 1.2x10-4 M anthracene solution in deoxygenated benzene. Camera delay 50 ns, delay increment 3 µs, 100 images averaged.
of the triplet was extracted and shown to be 9.1 µs, which corresponds with the literature.54
Lifetimes were calculated by using global analysis of the spectral-kinetics data using the
SPECFIT/32 software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info).
A 1.4x10-4 M aqueous solution of tris(2,2'-bipyridine)ruthenium(II) chloride (Ru(bpy)32+)
(Aldrich) was also prepared and deoxygenated by argon purging for 40 minutes. The camera
delay was set to 10 ns, and a small bleaching peak at 710 nm can be seen from the second order
of the spectrograph grating. A 250-ns delay between transients was used. The transient
absorbance is shown in Figure 2-11. The signal is quite noisy between 420 nm and 470 nm, due
56
Wavelength / nm
400 500 600 700 800
∆ Ab
sorb
ance
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
Figure 2-11. Transient absorption spectrum of 1.4x10-4 M aqueous solution of Ru(bpy)32+. The
solution was deoxygenated by bubbling with argon for 40 minutes. Camera delay 10 ns, delay increment 250 ns, 100 images averaged. The small peak at 710 nm is from scattered excitation light.
to the large absorbance of Ru(bpy)32+ in that region (therefore giving a very weak I0). However,
the spectrum and excited-state lifetime of 724 ns are in good agreement with the literature.55,56
Limitations of Current System
While the current system described provides the ability to study the excited state properties
of many materials, there are some limitations. These restrictions arise from several sources,
namely the spectral range of the components used, including the pump, probe, detector and
optics used therein. Another source of limitation comes from the temporal nature of the
57
electronics and hardware used in the apparatus. Lastly, the optical geometry is such that
particular attention needs to be made while preparing samples to be studied.
The excitation source used can limit the types of chromophores available for study. The
primary excitation source is the third harmonic of a Nd:YAG laser at 355 nm. This output can
also be used to pump an OPO that produces wavelengths from 420 nm to 1200 nm. Currently,
excitation cannot be produced at wavelengths shorter than 355 nm and between 355 nm and 420
nm. Another excitation source, such as different laser or OPO system, could be incorporated into
the apparatus to provide a larger range of excitation wavelengths. However, the types of
materials studied in our lab generally provide a fairly broad absorption throughout the UV and
blue region (excitation by 355 nm) or further into the red (excitation via the OPO).
The detector range is limited by a combination of the probe output wavelength,
spectrograph range and the chip used in the CCD camera. As shown earlier in Figure 2-4, the
xenon flashlamp is not a true white light source. Other probe sources may offer a wider or more
specific wavelength range suitable for different chromophores. A variety of CCD cameras and
corresponding spectrographs are available from commercial manufacturers to expand detection
wavelengths.
Currently, the apparatus has been optimized for use with solution samples which can be
contained in a 1-cm x 1-cm square cuvette. For most cases, sample concentrations must be
adjusted such that intermolecular interactions, such as self-absorption or triplet-triplet
annihilation, do not disrupt the signal. The sample concentrations are then adjusted such that the
absorbance is optimal for excitation and detection (that is, A ~0.8 at the excitation wavelength).
If highly absorbing or concentration-dependent samples are to be studied, various sample cells
could be used, including small or large pathlength cuvettes. As shown in Figure 2-3 earlier, the
58
sample is placed in a right-angle geometry with respect to the pump and the probe. Therefore,
the studying of thin films poses a problem because the beams are not colinear. Films are also
damaged easily under the excitation conditions used for solution samples.
The kinetic resolution is limited almost entirely by the electronics of the system. The
repetition rate of each measurement is limited by the occurrence of t0, generated at 10 Hz by the
Nd:YAG laser. The pulse generator used has a limit near 4 ns. Similarly, the camera can not
record images with a gate width less than 5 ns, although the resolution is reported at 40 ps.
Neither of these timings is limited by the 10 Hz repetition rate of the laser. Even so, events on a
timescale of less than 100 ns are not able to be monitored with a high degree of accuracy. This
does not pose a problem when studying triplet states, which are generally long-lived.
There are several other concerns that affect ease of use and presentation of data. The first
issue involves the nature of data collection, as described above. Because of the type of the
information recorded (I0 and I), and that those data must be processed after the experiment, there
is no ability for real-time monitoring of the transient absorption signal. An experimental run is
necessary to determine if the sample exhibits transient absorption and under what time
conditions; parameters can not be adjusted on the fly. Therefore, if the user has little knowledge
of the sample behavior, several runs may be necessary before the appropriate parameters can be
determined. Secondly, for highly emissive species, fluorescence from the sample can cause
bleaching in the transient absorption signal. The fluorescence will sometimes obscure the
absorbance of the excited state. This problem can be remedied by collecting data with the pump
on and the probe off, resulting in what is essentially an emission spectrum of the sample. If this
is done at each time delay, the emission can be corrected simply by subtraction for each time
59
measurement of the transient. A LabVIEW VI and Matlab application are under development to
address this problem.
60
CHAPTER 3 DELOCALIZATION IN EXCITED STATES OF DINUCLEAR PLATINUM ACETYLIDES
Introduction
The interesting properties of materials with extended π-conjugated systems have made
such materials potential candidates for use in a variety of applications, including nonlinear
optics,57 light emitting diodes,6,58 and conductance switches.59 The incorporation of transition
metals into the organic structure changes the redox, electronic, and optical properties of the
molecule. The organized combination of metals and organic moieties through deliberate
synthesis allows for tunability of these properties.
Photophysics of transition-metal acetylide complexes have garnered considerable interest
throughout the last decade.60-66 Platinum acetylides are square-planar molecules having the
general formula trans-Pt(PR3)2(C≡CR')2. Typical R groups are methyl, ethyl, butyl, or phenyl,
which give the compound crystalline properties, aid in solubility, or form glasses41,67 or gels.68
The R' group is normally an aromatic substituent, such as phenyl, thienyl, or fluorenyl. There
has been a wide range of structures synthesized, including oligomers,69 polymers,70 and
dendrimers.71 The spin-orbit coupling that is a result of the heavy platinum atom allows for
formation of triplet states in high yield. Hence, platinum-acetylides afford the ability to study
spin-forbidden phenomena such as singlet-to-triplet-state absorption (S0→T1), intersystem
crossing of the singlet excited state to the triplet excited state (S1→T1), and triplet-state emission
(phosphorescence, T1→S0). Population of the triplet state also provides an opportunity to probe
triplet-state absorption (T1→Tn).
A key point in the advancement of use of these types of materials is to make distinct
relationships between chemical structure and spectroscopic properties. The fundamental nature
of this type of structure-property relationship is also of importance to the general understanding
61
of interaction of light with matter and the information that can be elicited from experiments.
One such issue is the relation between structure and the delocalization of the singlet and triplet
excitons. Beljonne and coworkers72 have carried out an experimental and theoretical
investigation on a platinum-containing acetylide polymer, the structure of which is shown in
Figure 3-1. Using various spectroscopic techniques, such as absorption, emission, and excited-
state absorption, along with quantum chemical calculations, the spatial extent of singlet and
triplet excited states were determined. Transitions including S0→S1, S0→T1, and T1→Tn have
Pt ClPtCl
PBu3
PBu3
PBu3
PBu3
Pt ClPtCl
PBu3
PBu3
PBu3
PBu3n
Pt ClPtCl
PBu3
PBu3
PBu3
PBu3
Pt ClPtCl
PBu3
PBu3
PBu3
PBu3n
Figure 3-1. Platinum-acetylide polymer and model compound studied by Beljonne et al.72
shown chain-length dependencies on these excitation energies. The authors suggest that the
lowest triplet excited state, T1, is localized on one phenylene ring, while the S1 excited state is
confined to two repeat units.72 However, the higher Tn excited state is less restricted, and spread
out over at least three repeat units. The use of model compounds (Figure 3-1) through Huang-
Rhys fitting of phosphorescence emission spectra and theoretical modeling also indicates
considerable distortion between the lowest triplet excited state and the singlet ground state.72
Our group has more recently studied delocalization in singlet and triplet states of platinum-
acetylide oligomers,69 the structures of which are shown in Figure 3-2. It was found that the
ground-state absorption and fluorescence have significant dependence on the length of the
oligomer. As the chain length increases, the ground-state absorption and singlet emission red-
shift systematically. However, phosphorescence maxima are less affected by chain length, as the
62
Pt
PBu3
PBu3
H
n
Figure 3-2. Platinum-acetylide oligomers studied by Liu et al.69 Oligomer length varies from n = 1-5, 7.
peaks are only slightly red-shifted with respect to longer oligomer length.69 The findings of this
report further support the idea that the triplet state is more localized than the singlet state.
Furthermore, another investigation by our group73 involving the probing of delocalization was
Figure 3-3. Platinum acetylide oligomers studied by Glusac-Haskins et al.73
63
completed on platinum-acetylide stilbene compounds, the structures of which are shown in
Figure 3-3. Similar results in the absorption spectra indicate delocalization over the entire π-
conjugated system. That is, the absorption maxima are affected by the conjugation length of the
molecule. Also, π-conjugation is more efficient when the ligands are substituted trans to
platinum, indicating delocalization through the d orbitals of the platinum atom. Moreover, the
triplet-state properties of the ligand and platinum-containing molecules are notably similar,
suggesting that there is no significant delocalization through the platinum atom in the triplet state
and that the state is localized on the ligand.
The photophysics of a short platinum-acetylide oligomer, PE2, was described in Chapter 1,
and a more complete series studied by the Air Force Research Laboratory40,42-45,74,75 was also
introduced. In addition to the symmetrical molecules shown in Figure 1-8, the group also
synthesized mono-substituted45 and asymmetrically-substituted75 platinum-acetylide complexes
and all-organic butadiynes.44 The complete series of compounds is shown in Figure 3-4. As
bPta
PBu3
PBu3
PE1PE2PE3
PE1-2PE2-3PE1-3
half-PEn
L1
L2
L3
a = b = L1 a = b = L2a = b = L3
a = L1, b = L2a = L2, b = L3a = L1, b = L3
a = Cl, b = Ln
PE1-BDPE2-BDPE3-BD
m = 0 m = 1m = 2
m m
Figure 3-4. Platinum-acetylide oligomers studied by Cooper, Rogers, et al.40,42-45,74,75
64
mentioned in Chapter 1, the symmetrically-substituted complexes show similar fluorescence
spectra, but absorption and triplet emission spectra red-shift with increased conjugation length.40
The mono-substituted complexes also exhibit greater dependence on oligomer length for ground-
state absorption and triplet-state emission than for singlet-state emission.45 Also, the
phosphorescence spectra for the mono- and di-substituted oligomers are nearly identical,45
further indicating localization of the triplet state. Transient absorption spectra match for each
oligomer pair as well. The photophysics of the asymmetrical platinum-acetylide complexes75
reveal more support on the confinement of the triplet excited state and delocalization of the
singlet excited state. It was observed that the singlet exciton is delocalized through the platinum
atom, but phosphorescence comes from the lowest energy (largest) ligand.
Most of the work to date has considered platinum acetylide oligomers with a single
platinum atom and various ligands, or oligomers and polymers with identical repeat units. The
study of delocalization in excited states is therefore limited. A set of dinuclear platinum-
acetylides with various short spacers and end-capped substitution of either chloride or a
conjugated phenyl-ethynyl-type substituent are presented in Figure 3-5. The photophysics of the
series have been investigated and are presented in the following sections. The effects of the end
group, platinum atom, and spacer are considered, and some conclusions are made regarding the
delocalization of singlet and triplet states.
Experimental
Steady-state absorption measurements were recorded on a Varian Cary 100 dual-beam
spectrophotometer. Corrected steady-state emission measurements were conducted on a either a
FluoroMax-3 or SPEX Fluorolog-3 spectrophotometer. All sample solutions were prepared in
tetrahydrofuran (THF) or 2-methyltetrahydrofuran (MTHF). Solvents were distilled over CaH2
65
immediately prior to use. Room-temperature measurements were carried out in 1-cm square
quartz cuvettes. Solutions were deoxygenated by purging with argon for 30 minutes and sample
P: R = ClP-PE2: R = PE2
BP: R = ClBP-PE2: R = PE2
PEP: R = ClPEP-PE2: R = PE2
PE2:
Pt Pt RR
Pt RPtR
PtPt RR
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
P: R = ClP-PE2: R = PE2
BP: R = ClBP-PE2: R = PE2
PEP: R = ClPEP-PE2: R = PE2
PE2:
Pt Pt RR
Pt RPtR
PtPt RR
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
Figure 3-5. Dinuclear platinum-acetylide oligomers.
concentrations were made such that the solutions were optically dilute (Amax < 0.20). Low-
temperature emission measurements were conducted in 1-cm i.d. borosilicate glass tubes in a
liquid nitrogen-cooled Oxford Instruments OptistatDN cryostat connected to an Omega
CYC3200 autotuning temperature controller. Samples were degassed by three consecutive
freeze-pump-thaw cycles on a high vacuum (10-5 torr) line. Transient absorption measurements
were conducted on the apparatus described in Chapter 2. Samples were contained in a cell with a
total volume of 10 mL and the contents were continuously circulated through the pump-probe
region of the cell. Solutions were deoxygenated by purging with argon for 40 minutes and
concentrations adjusted so that A355 ~ 0.8. Excitation was generated using the third harmonic
output of a Nd:YAG laser (355 nm, Continuum Surelite). Typical excitation energies were 8 mJ-
66
pulse-1, corresponding to a fluence of ~20 mJ-cm-2. Transient absorption spectra were generated
by using software developed in-house following the calculations described in Chapter 2. Kinetic
data were calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32
software package (Bio-logic SAS, Grenoble, France, www.bio-logic.info).
Results
In Figure 3-6, the absorption maxima for all compounds in THF solution are shown to be
within 15 nm of each other. BP has the most blue-shifted maximum at 346 nm, while P-PE2,
BP-PE2 and PEP-PE2 have maxima at 358 nm. The absorption of PEP-PE2 is slightly broader
than the others, suggesting there may be a secondary band red-shifted from the maximum. The
absorption spectra for P and PEP are nearly identical with two closely-spaced bands near their
absorption maxima at 351 nm. For the PE2-end-capped molecules, the molar absorptivity in
benzene is an order of magnitude larger than the chloride-end-capped molecules, as given in
Table 3-1. (No data for P was obtained, although presumably its molar absorptivity is
considerably lower than that of P-PE2.)
Table 3-1. Summary of spectroscopic data of dinuclear platinum-acetylide oligomers. λmax, abs
a
nm ε b
M-1 cm-1 λmax, em
c nm
λmax, ex c
nm λmax, em
d
nm λmax, ex
d nm
λmax, TA a
nm τT
e µs
P 351 – f 558 372 545 387 608 28.3P-PE2 360 97 100 532 365 524 389 576 42.7BP 346 12 900 556 351 541 366 569 27.1BP-PE2 358 123 000 559 363 543 388 590 94.8 533 359 522 381 PEP 351 13 900 554 373 552 386 611 63.0PEP-PE2 358 117 000 559 375 550 396 629 82.9 533 358 523 381 a THF solution. b Benzene solution. c THF solution, room temperature. d MTHF solution, 80 K. e Extracted from TA data. f Not measured.
67
A
B
C
D
Abs
orba
nce
E
F
Wavelength / nm300 350 400 450 500
Figure 3-6. Absorbance spectra of dinuclear platinum-acetylide oligomers collected at room temperature in THF solution. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2.
68
Fluorescence is observed for P at room temperature, although the maximum at 395 nm is
weak compared to the phosphorescence. Some broad vibronic structure is seen, though three
bands are shown and it does not quite mirror the absorption. Weak fluorescence is also shown
for BP, and although broader than P, it exhibits similar structure at the same wavelengths. The
photoluminescence of the remaining compounds is dominated by phosphorescence, even at room
temperature.
Transient absorption data are shown in Figure 3-7. Intense, broad signals throughout the
visible are seen from the chloride-end-capped compounds with strong peaks at 608 nm, 596 nm,
P P-PE2
Nor
mal
ized
∆ A
bsor
banc
e
BP BP-PE2
Wavelength / nm400 500 600 700 800
PEP PEP-PE2
Figure 3-7. Transient absorption spectra of dinuclear platinum-acetylides. Samples prepared in deoxygenated THF solution and spectra collected 20 ns after excitation by 355 nm pulse (10 ns, 8 mJ pulse-1). The horizontal dotted line represents ∆A = 0.
69
and 611 nm for P, BP and PEP, respectively. The PE2-end-capped molecules exhibit much
broader TA signals throughout the visible. For the BP and PEP series, the PE2-substituted
molecules have about a 20 nm red-shift from their analogs. P-PE2 is actually blue-shifted 32 nm
from the P peak. The PE2 analogs red-shift following the order P-PE2 < BP-PE2 < PEP-PE2.
Triplet-state lifetimes calculated from transient absorption decay are listed in Table 3-1. The
relative error in measurement of lifetimes approaches almost 20%; however, it is clear that the
lifetimes are considerably longer for the larger compound of each pair.
Low-temperature and room-temperature phosphorescence spectra were collected and are
shown in Figure 3-8. The room-temperature phosphorescence for P, P-PE2, BP, and PEP show
a single peak with an unresolved vibronic progression. BP-PE2 and PEP-PE2 show similar
structure, with the addition of a smaller peak blue-shifted from the main emission band. The
smaller peaks of the two PE2-substituted compounds correspond in wavelength with the main
peak of P-PE2 at 532 nm. The main peaks of the chloride-end-capped compounds all have
phosphorescence maxima near 557 nm. At 80 K, the emission spectra of all of the compounds
are blue-shifted and the vibronic progressions become slightly more resolved. The resolution
increase is most notable for P and PEP, although more structure is observed for P-PE2 and
PEP-PE2 as well. BP and BP-PE2 show very little fine structure at either temperature.
Representative variable-temperature emission is shown by P-PE2 in Figure 3-9. As the
temperature is increased from 80 K, the emission intensity decreases steadily. Upon the glass
transition temperature of the solvent, the peak red-shifts 7 nm, and also increases intensity. After
further heating of the sample from 120 K, the intensity decreases and vibronic structure is lost.
Similar behavior is seen from the entire series.
70
0.00.20.40.60.81.01.2
0.00.20.40.60.81.0
Wavelength / nm300 400 500 600 700
0.00.20.40.60.81.00.00.20.40.60.81.0
Nor
mal
ized
Inte
nsity
/ a.
u.
0.00.20.40.60.81.0
0.00.20.40.60.81.0
A
B
C
D
E
F
Figure 3-8. Phosphorescence spectra of dinuclear platinum-acetylide oligomers. Room-termperature (––) spectra collected in THF solution and 80 K spectra (----) in MTHF glass. Excitation wavelength set at absorption maximum for each compound. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2.
71
Wavelength / nm500 550 600 650
Inte
nsity
/ a.
u.80K 90K 100K 110K 120K 130K 140K 160K 180K 200K 220K 240K 260K
Figure 3-9. Variable-temperature emission spectra of P-PE2 in MTHF. Excitation wavelength = 360 nm.
Excitation spectra for the set of compounds are shown in Figure 3-10. Room-temperature
excitation spectra collected at the maxima of emission are displayed as featureless bands shifted
a few nanometers lower in energy than the absorbance maxima. However, P and PEP exhibit a
blue-shifted shoulder that corresponds with the absorption spectra. Excitation spectra collected
at the smaller higher-energy peak in BP-PE2 and PEP-PE2 have blue-shifted maxima which
match the absorbance maxima. At 80 K, the excitation spectra are more structured, and
significantly (~20 nm) red-shifted from the room-temperature data. For the chloride end-capped
series, the maxima are shifted approximately 15 nm to the red compared to
72
0.00.20.40.60.81.01.2
551558
0.00.20.40.60.81.0 524
532
Wavelength / nm300 320 340 360 380 400 420 440
0.00.20.40.60.81.0 550
523560533
0.00.20.40.60.81.0 550
554
Nor
mal
ized
Inte
nsity
/ a.
u.
0.00.20.40.60.81.0 536
556
0.00.20.40.60.81.0 544
522559532
A
B
C
D
E
F
Figure 3-10. Excitation spectra of dinuclear platinum-acetylide oligomers. Room-temperature spectra (––) collected in deoxygenated THF and low-temperature spectra (––) collected at 80 K in MTHF glass at emission wavelengths denoted in the figure. The dotted lines correspond to the higher-energy bands in the PE2-end-capped oligomers. (A) P, (B) P-PE2, (C) BP, (D) BP-PE2, (E) PEP, and (F) PEP-PE2.
73
room-temperature excitation, whereas the PE2 end-capped molecules show a larger shift of about
24 nm. The excitation spectra of the higher-energy peak for BP-PE2 and PEP-PE2 have
maxima blue-shifted from the excitation peak measured for the main emission band.
Representative variable-temperature excitation spectra for P-PE2 are shown in Figure 3-11. At
80 K, the maximum peak is at 389 nm. As the temperature increases, the peak decreases and
blue-shifts. As seen in the emission spectra, the vibronic structure is lost as the solvent glass is
warmed. The excitation maximum shifts a total of 24 nm. A comparable trend throughout the
series is observed as the temperature is increased from 80 K to room temperature.
Wavelength / nm300 320 340 360 380 400 420
Inte
nsity
/ a.
u.
80K 90K 100K 110K 120K 130K 140K 160K 180K 200K 220K 240K 260K
Figure 3-11. Variable-temperature excitation spectra of P-PE2 in MTHF. Emission monitored at 532 nm.
74
Discussion
The properties of π-conjugated materials attract interest due to the potential applications of
these types of compounds. By introducing platinum atoms into the π-conjugated structure, the
triplet-state properties are more easily investigated due to population through spin-orbit coupling
of the heavy atom. The results of previous work have uncovered more questions as to the nature
of the singlet and triplet excited states and the role of platinum in the chromophore. The
photophysical characterization of a series of dinuclear platinum-acetylide oligomers that consist
of different spacer units and a well-studied end-cap will further the understanding of this type of
molecule.
The ground-state absorption of platinum-acetylide complexes originates from a ππ*
transition with some metal-to-ligand charge transfer (MLCT) character.44 Theoretical
calculations of the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) have contributed to the understanding of electronic transitions in this
type of compound.63,74 A combination of the platinum d orbital and the aryl π orbitals
contributes to the HOMO, while the LUMO consists of only π* orbitals and no contribution from
the platinum d orbital. Without quantum chemical methods to study the geometry and orbital
configurations of this series, limited conclusions can be made. The central phenyl rings in BP
and BP-PE2 are most likely twisted, which disrupts the conjugation. Cooper and coworkers43
have calculated the dihedral angle in the related PPE to be 40°. Therefore, it is not possible to
determine trends based purely on conjugation length as the exact conjugation length is not
known. However, direct comparison between the chloride and PE2 end-capped molecules in
each pair is more legitimate. The absorption maxima of the PE2 end-capped molecules are red-
shifted 7-12 nm from the chloride analogs. The asymmetric PE2-3, which has a similar
75
absorption maximum to the PE2 end-capped oligomers, has a longer conjugation length if
comparing only the “ligand-metal-spacer” as the chromophore. The increase in conjugation
length between P and PEP predicts a greater red-shift than what is observed. However, the
second peak at 375 nm observed in the absorbance of PEP occurs near the absorbance maximum
of PE3. Interestingly, the main absorption band occurs at same wavelength as the asymmetrical
PE2-3. This suggests that there maybe conformational twisting near the platinum that interrupts
the conjugation to a “ligand-metal-spacer” chromophore, but also there exists delocalization
through the platinum. The PE2 end-cap contributes heavily to the oscillator strength, increasing
the value of the chloride end-capped molecules by an order of magnitude. The values are higher
than the reported value75 for the related PE2 (ε = 89 000 M-1 cm-1). The increased molar
absorptivity and absorption maxima suggest that there is delocalization through both platinum
atoms in the singlet state. Due to the small size of the spacers, it is inconclusive at this point as
to the origin of absorbance and relationship of the platinum in the HOMO and LUMO.
The trends observed for the transient absorption spectra are different for the chloride- and
PE2-end-capped series. P and PEP have similar TA spectra, and BP has a peak 40 nm higher in
energy. However, the maxima of the broad TA spectra of the PE2-end-capped oligomers
increase in the order P-PE2 < BP-PE2 < PEP-PE2. The trend in the latter can be attributed to
the delocalization through the platinum atoms into the conjugated end-caps. It has been shown
for the related platinum-acetylide oligomers that the Tn excited state is delocalized through the
metal d orbitals. Additionally, as stated earlier, theoretical calculations have shown that the
triplet exciton in platinum-acetylide polymers is delocalized onto at least three repeat units.72 In
contrast to their study on films, the experimental triplet absorption maximum of P is 608 nm for
the solution sample, nearly 20 nm red-shifted from their film sample. The PE2 end-capped
76
oligomers also show much broader TA signals. The Tn state is likely delocalized throughout the
molecule, as suggested by previous reports of similar molecules.45
The lack of fluorescence in the emission spectra of the series, even at room temperature,
indicates that intersystem crossing via spin-orbit coupling dominates decay of the singlet excited
state. In other platinum-acetylide oligomers, the reported ISC quantum yields approach
unity.35,36 Only P shows reasonable fluorescence emission at room temperature; the maximum
value occurs at a lower energy (395 nm) than related oligomers that contain only a single phenyl
group per platinum atom. PE1, which has a single phenyl-ethynyl group on each side of the
platinum atom has a fluorescence maximum at 364 nm, whereas maximum emission near 395
comes from PE2 (385 nm) or PE3 (400 nm).44 It has been shown that the excited singlet states
have delocalized character. It would seem reasonable then that there is a strong contribution
from the metal orbitals in the singlet state, extending the conjugation from the phenyl π orbitals
through the platinum d orbitals. The red-shift in the expected fluorescence is attributed to
delocalization of the singlet excited state into the d orbitals of both platinum atoms.
As stated earlier, definitive trends on conjugation length cannot be determined by structure
alone. Low-temperature emission spectra for all compounds, excluding P-PE2, have
phosphorescence maxima within 5 nm of 546 nm. P-PE2 phosphorescence is at higher energy
of 524 nm. Similarly, the other PE2 end-capped oligomers, BP-PE2 and PEP-PE2, exhibit a
higher energy peak at the same wavelength. In the asymmetrical series of platinum complexes,75
phosphorescence was observed by emission from the lowest energy (largest) ligand. Only in
PE2-3 was very weak emission from the PE2 ligand observed at 77 K. In the dinuclear platinum
series, this higher energy peak in the dinuclear platinum compounds is ascribed to triplet
localization on the PE2 ligand. In P-PE2, the entire triplet is localized on the PE2 end-cap, and
77
no emission is observed from the spacer group. However, in BP-PE2 and PEP-PE2, the triplet
can be localized on the end-cap or the central spacer. An explanation for this resides in
intramolecular triplet energy transfer between the end-cap chromophore and the spacer
chromophore through Dexter coupling.75 While emission is expected to come from only the
lowest energy ligand according to Kasha’s rule, our group has previously reported a contrasting
study76 on a platinum acetylide polymer containing both phenyl and thienyl monomer units.
Emission from the polymer is observed from thiophene units as well as the higher-energy phenyl
groups.
At low temperatures, the vibronic structure of the emission is more defined, showing
several peaks in the progression. P and PEP have the clearest progression of small peaks, while
the BP pair is fairly broad with two large bands. The lower resolution of vibronic structure in P-
PE2 and PEP-PE2 can be attributed to a greater amount of vibrational modes stemming from
the large end-cap, which results in overlap of the emission bands. Also, at low temperatures the
vibrational modes are more restricted by the solvent glass, which helps resolve the progression.
At room temperature, more vibrational modes are accessed, leading to broadening of the bands.
This effect is also responsible for the red-shift of the phosphorescence spectra as the sample goes
from glass to solution.
Similarly, the excitation spectra are much more resolved at low temperature. P and P-PEP
show the greatest amount of structure, while the rest of the series show very broad shoulders at
higher energy from the peaks. The red-shift of the spectra is also attributed to the restricted
access of vibrational modes when the sample is frozen in the glass. However, the shift in
phosphorescence energy is much smaller than the change in excitation energy. The dramatic
78
red-shift and strong broadening of the excitation spectra further support the delocalization of the
singlet state, and the localization of the triplet state.
Conclusion
In this chapter, a series of dinuclear platinum-acetylide oligomers has been characterized
by using photophysical methods. The molecules in the series consist of small aryl spacer groups
spanning two platinum atoms. The oligomer chain is end-capped with either chloride or a
phenyl-ethynyl π-conjugated group. It has been shown that the ground-state and triplet-state
absorbance are affected by the addition of the PE2 end-cap, whereas the triplet emission energies
are less dependent on end-cap substitution. In addition, phosphorescence is observed from the
spacer group as well as the higher energy end-cap. The results lead to conclusion that the singlet
S0 ground state, singlet S1 excited state and triplet Tn excited state are delocalized throughout the
molecule. The T1 states, however, are localized on the ligand and spacer groups.
Phosphorescence originates from both the spacer group and the PE2 end-cap, indicating that
there is no significant delocalization through the platinum atom in the triplet state.
79
CHAPTER 4 PHOTOPHYSICS OF DIPLATINUM POLYYNEDIYL OLIOGMERS:
CHAIN LENGTH DEPENDENCE ON THE TRIPLET STATE IN SP CARBON CHAINS
Introduction
Over the past few decades there has been increasing interest in the study of organic and
organometallic oligomers that feature extended carbon chains, e.g., R-(C≡C)n-R and LyM-
(C≡C)n-MLy.25,62,77-88 The extended π-systems and rigid linear structure characteristic of these
carbon chain oligomers make them potentially useful as molecular wires for transport of charge
(polarons) or excitons on the nanoscale.89 On a fundamental level, oligoynes are unique in that
they provide one of the simplest possible linear π-conjugated organic units of variable length.
Such systems are particularly amenable for experimental and theoretical studies focused on
issues related to charge and exciton structure and delocalization in linear π-conjugated organic
systems.90-94
While there have been a number of experimental and theoretical reports concerning the
properties of linear carbon chains and related structures, most of this work focused on vibrational
and optical absorption spectroscopy.81,87,92-94 Although several studies have reported
fluorescence emission from carbon chain structures, there are only a few reports concerning the
phosphorescence of oligoynes.93,94 In an effort to increase the stability of linear carbon chain
compounds, as well as extend the study of their photophysics to the triplet state, transition metals
have been incorporated as end-caps to the oligoyne chains.62,88 Notable examples of this work
include a spectroscopic study of 2- and 4-carbon chains spanning two gold atoms reported by
Che and co-workers,95 and a spectroscopic study of a series of platinum(II)-terpyridyl-capped
carbon chains reported by Yam and co-workers.96 Both of these studies provide evidence that
the heavy metal end-groups induce efficient population of a 3π,π* state that is localized on the
80
carbon chain. The triplet is clearly distinguished by its characteristic phosphorescence emission,
which appears as a well-defined vibronic progression with spacing of ca. 2100 cm-1.
One of our collaborators, the Gladysz group at Friedrich-Alexander-Universität Erlangen-
Nürnberg, in Erlangen, Germany, has developed synthetic methods that allow routine
construction of variable length carbon chains capped on both ends by transition metal units. In
particular, they have recently reported the synthesis and structural characterization of a broad
series of linear carbon chain oligomers of the type Ar-Pt(P2)-(C≡C)n-Pt(P2)-Ar, where Ar = aryl,
P = a phosphine ligand and n = 2 – 14.85,86 The structural, optical and electronic properties of
these oligomers have been probed by x-ray crystallography, UV-visible absorption and density
functional theory (DFT) calculations.86,90 While the optical and theoretical investigations
provide considerable insight concerning electronic structure and π-conjugation in the carbon
chains, little work has been done previously to probe the structure and dynamics of the long-
lived excited states (e.g., singlet and/or triplet excitons) in the oligoyne systems.
Pt Pt
Pt
Pt
Pt
Pt
Pt Pt
PtC6Pt
PtC8Pt
PtC10Pt
PtC12Pt
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Pptol3
Figure 4-1. Structures of the PtCnPt series.
81
In this chapter a detailed study of the photophysics of the series of platinum end-capped
oligoynes, PtCnPt (n = 6 – 12, Figure 4-1) is presented. This study has focused on the properties
of the 3π,π* state localized on the carbon chains that span the two platinum centers. The triplet
state is produced in relatively high yield in these oligoynes due to the strong spin-orbit coupling
induced by the platinum centers. The spectroscopy and dynamics of the triplet state are probed
using variable temperature luminescence, transient absorption, and time-resolved emission
spectroscopy. The results provide clear evidence for an energy gap law dependence of the non-
radiative decay rate of the triplet state.
Experimental
Steady-state absorption measurements were recorded on a Varian Cary 100 dual-beam
spectrophotometer. Corrected steady-state emission measurements were conducted on a SPEX
Fluorolog-3 spectrophotometer. All sample solutions were prepared in tetrahydrofuran (THF) or
2-methyltetrahydrofuran (MTHF). Solvents were distilled over CaH2 immediately prior to use.
Room-temperature measurements were carried out in 1-cm square quartz cuvettes. Solutions
were deoxygenated by purging with argon for 30 minutes and sample concentrations were made
such that the solutions were optically dilute (Amax < 0.20). Low-temperature emission
measurements were conducted in 1-cm i.d. borosilicate glass tubes in a liquid nitrogen-cooled
Oxford Instruments OptistatDN cryostat connected to an Omega CYC3200 autotuning
temperature controller. Samples were degassed by three consecutive freeze-pump-thaw cycles
on a high vacuum (10-5 torr) line.
Photoluminescence quantum yields were determined by relative actinometry, with
Ru(bpy)32+ as an actinometer (φP = 0.055 in water). Low temperature quantum yields were
calculated by appropriately scaling the integrated emission area with temperature.97
82
The phosphorescence spectra for PtC6Pt, PtC8Pt, and PtC10Pt at 100 K were fitted using a
single-mode Franck-Condon expression,97,98
( )∑=
∆+−
−
−=
5
0
2
21,0
mm00
m
m
3
00
mm00
m
m
2ln4exp!
)(ν
ν
νωνν
νωνν ηη ES
EEI (4-1)
Where I(ν ) is the relative emission intensity at energy ν , E00 is the energy of the zero-
zero transition, νm is the quantum number of the average medium-frequency vibrational mode,
ħωm is the average medium-frequency acceptor mode coupled to the triplet-excited state to
ground state transition, Sm is the Huang-Rhys factor, and ∆ν 0,1/2 is the half-width of the
individual vibronic bands.
Time-resolved emission measurements were carried out on a home-built apparatus
consisting of a Continuum Surelite series Nd:YAG laser as the excitation source (λ = 355 nm, 10
ns fwhm, < 1mJ/pulse) and detection measured with a Princeton Instruments PI-MAX intensified
CCD camera detector coupled to an Acton SpectraPro 150 spectrograph. Lifetimes were
calculated by using global analysis of the spectral-kinetics data using the SPECFIT/32 software
package (Bio-logic SAS, Grenoble, France, www.bio-logic.info).
Transient absorption measurements were conducted on a previously described home-built
apparatus.99 Samples were contained in a cell with a total volume 10 mL and the contents were
continuously circulated through the pump-probe region of the cell. Solutions were deoxygenated
by argon purging for 40 minutes and concentrations adjusted so that A355 ~0.8. Excitation was
generated using the third harmonic output of a Nd:YAG laser (355 nm, Spectra Physics GCR-
14). Typical excitation energies were 5 mJ-pulse-1, corresponding to a fluence of ~20 mJ-cm-2.
Transient absorption spectra and decay lifetimes were generated by using software developed in-
house.
83
Results
UV-Vis Absorption Spectra
The absorption spectra for the series of PtCnPt oligomers in CH2Cl2 have recently been
reported.86 However, to facilitate comparison with the photophysical data presented herein, the
absorption spectra of the series in were measured THF and the results are presented in Figure 4-
2. The spectra in THF solution are nearly identical to those reported in CH2Cl2;86 however, the
molar absorptivity values are slightly larger in THF. With the exception of PtC6Pt, the
absorption spectra of the oligoynes feature two primary transitions which each appear as a well-
defined vibronic progression that shift to lower energy with increasing carbon chain length. The
low energy transition (inset of Figure 4-2) is comparatively weak (ε < 5,000 M-1cm-1), whereas
the higher energy transition is considerably more intense, with the molar absorptivity increasing
with carbon chain length. On the basis of DFT computations, both of these bands are assigned as
arising from π−π* transitions between orbitals that are concentrated on the -(C≡C)n- chains.90
As noted above, the absorption of PtC6Pt does not follow the same pattern seen for the
longer oligomers. The absorption of PtC6Pt is considerably weaker compared to the other
complexes, and it features a broad absorption in the 320 – 360 nm region, with a second weaker
band with λmax = 381 nm. While the previously reported DFT calculations suggest that there
should not be a difference in the nature of the low energy transitions for PtC6Pt compared to the
other complexes, it is possible that the absorption of this oligomer is more complex due to
mixing of the π and π* levels of the C6 chain with orbitals on the -Pt(P2)(Ar) end groups.
84
Wavelength / nm300 350 400 450 500
ε / 1
05 M
-1 c
m-1
0
1
2
3
4
5
350 400 450 500 550
x 50
Figure 4-2. Absorption spectra of PtCnPt complexes in THF solution. In order of increasing molar absorptivity: n = 6, 8, 10 and 12.
Steady-State Photoluminescence Spectroscopy
Photoluminescence spectra for all of the PtCnPt complexes were studied in MTHF
solution at temperatures ranging from 80 – 300 K over the wavelength interval 400 – 800 nm.
The low energy limit in the spectra is determined by the fall-off in the photomultiplier detector
response. As shown in Figure 4-3, at 80 K in the MTHF glass all of the oligoynes display a well-
defined photoluminescence that can be assigned to phosphorescence from a 3π,π* excited state.
Note that little or no fluorescence emission is observed in the spectra. (The spectra shown in
Figure 4-3 were obtained with excitation at the absorption maximum for each complex; however,
85
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Inte
nsity
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength / nm400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
A
B
C
D
0-0
0-1
0-2
0-3
0-0
0-0
0-0
0-1
0-1
0-2
Figure 4-3. Photoluminescence spectra of PtCnPt complexes in 2-MTHF solvent glass at 80 K. (A) n = 6, λex = 335 nm. (B) n = 8, λex = 335 nm. (C) n = 10, λex = 350 nm. (D) n = 12, λex = 368 nm.
86
the observed emission spectra did not vary with excitation wavelength.) For the C6, C8 and C10
oligomers, the phosphorescence appears as a narrow 0-0 band followed by a series of vibronic
sub-bands at lower energy separated by ca. 2100 cm-1. The 0-0 bands of the phosphorescence
red-shift systematically with increasing carbon chain length, and the λmax value for the
phosphorescence origin for each complex is listed in Table 4-1. The vibronic progression in
each of the spectra arises due to coupling of the triplet excitation to the C≡C stretch of the carbon
chain. For PtC6Pt the 0-1, 0-2 and 0-3 sub-bands are resolved, for PtC8Pt the 0-1 and 0-2 sub-
bands are seen, and for PtC10Pt only the 0-1 sub-band appears in the accessible spectral range.
The phosphorescence of PtC12Pt is weaker than for the other oligomers, and it is so red-shifted
that it is only possible to observe the origin of the phosphorescence.
With increasing temperature, the phosphorescence spectra do not change in energy or
bandshape; however, the emission intensity steadily decreases. An example of this is shown in
Figure 4-4. The emission from all of the complexes is much weaker at ambient temperature (300
K), and for PtC6Pt and PtC12Pt the phosphorescence is too weak to detect at 300 K. The room-
temperature phosphorescence quantum yields were measured for PtC8Pt and PtC10Pt (0.003 and
0.0012, respectively). Using these values, the phosphorescence quantum yields at 100 K were
estimated by scaling the room-temperature quantum yields by the integrated emission area
(which increases with decreasing temperature). These extrapolated low temperature quantum
yields are 0.018 and 0.0053 for PtC8Pt and PtC10Pt, respectively. Because there was no
phosphorescence at ambient temperature for C6 and C12, the low temperature emission quantum
yields could not be determined.
87
Wavelength / nm500 550 600 650 700
Inte
nsity
/ a.
u.
0.0
0.2
0.4
0.6
0.8
1.0
1.277 K 100 K120 K140 K165 K180 K200 K 230 K265 K298 K
Figure 4-4. Emission spectra of PtC8Pt in MTHF solvent (glass) over 77 – 298 K temperature range. Excitation wavelength 335 nm. Spectra decrease in intensity (quantum efficiency) with increasing temperature.
Excitation spectra obtained for each of the complexes while monitoring the
phosphorescence at 80 K are shown in Figure 4-5. In general, the excitation spectra are closely
similar to the absorption spectra, except the bands are better resolved due to the low temperature
glass.
In order to provide more information concerning the nature of the triplet state that gives
rise to the phosphorescence, the low temperature (100 K) emission spectra of PtC6Pt, PtC8Pt
and PtC10Pt were fitted by using a Franck-Condon bandshape analysis according to Equation 4-
1.97,98 The spectrum of PtC12Pt could not be fitted since only the origin is observed. The fits
88
Wavelength / nm300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Nor
mal
ized
Inte
nsity
A
B
C
D
Figure 4-5. Overlay of absorption (black lines), emission excitation (green lines) and emission spectra (red lines) for PtCnPt complexes in MTHF solvent glass at 100 K. Excitation spectra monitored at peak of 0-0 emission line. (A) n = 6, (B) n = 8, (C) n = 10, (D) n = 12.
89
provide estimates for the 0-0 energy (E00, the triplet energy), the frequency of the dominant
vibrational mode coupled to the excitation (ħω), the bandwidth (∆ν 1/2), and the Huang-Rhys
parameter (Sm) which is a quantitative measure of the geometric distortion between the ground
and triplet excited states. A listing of the parameters recovered from the fits is shown in Table 4-
1, and an example of the excellent quality of the fit of the experimental spectra is shown in
Figure 4-6.
Table 4-1. Emission spectral fitting parameters for PtCnPt at 100 K. λmax, em E00 / cm-1 ħω / cm-1
∆ν 0,1/2 / cm-1 Sm ∆EST / eV C6 497 20121 2150 190 1.05 0.53 C8 580 17241 2120 200 0.90 0.71 C10 657 15221 2060 250 0.78 0.75 C12 727 13755 2020 a 260 a 0.71b 0.68 a Estimated values. b Estimated from linear plot of Sm vs E00.
Several features are of interest with respect to the parameters recovered from the fits.
First, as noted above, E00 decreases systematically with increasing carbon chain length, reflecting
the decrease in triplet energy with increasing length of the π-conjugated electron system.
Second, as noted above, it is evident that the triplet excitation is coupled to only one high
frequency vibration that corresponds to the stretching mode. Interestingly, the frequency of the
mode decreases slightly with increasing oligomer length, consistent with previous theoretical
studies which indicate that the oligoyne stretching mode decreases in frequency with length.92
Third, it is quite evident that the Huang-Rhys parameter decreases with increasing oligomer
length (vide infra). This is consistent with the triplet exciton becoming more delocalized as the
number of carbons in the sp chain increases.
90
Energy / cm-1
14000 16000 18000 20000 22000
Nor
mal
ized
Inte
nsity
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Figure 4-6. Fit of emission spectrum of PtC6Pt using Equation 4-1 and parameters listed in Table 4-1. Dots are experimental data and red solid line is calculated fit.
Phosphorescence Decay Kinetics: Radiative and Non-Radiative Decay Rates
Variable-temperature time-resolved emission spectra were measured for the PtCnPt series
in degassed MTHF solution (glass) over the 80 – 300 K temperature range and the decay
lifetimes recovered from fits of the emission decays are plotted vs. temperature in Figure 4-7.
Several features are of interest with respect to the temperature-dependent lifetimes. First, for all
of the complexes the emission lifetimes decrease with increasing temperature. This lifetime
decrease is especially large in the temperature region corresponding to the glass-to-fluid
transition of MTHF (120 – 140 K). Specifically, for PtC6Pt, the lifetime decreases by nearly a
factor of 20 between 110 and 150 K. For the other complexes, the decrease in lifetime is less but
91
Temperature / K50 100 150 200 250 300
Life
time
/ µs
0
5
10
15
20
25
30
C6
C10C12
C8
Figure 4-7. Temperature dependence of photoluminescence decay lifetimes for PtCnPt complexes in MTHF solution (glass). Carbon chain length (n) indicated in plot.
nevertheless it is evident. Second, below the glass point of the solvent (e.g., T = 100 K) the
emission lifetime increases with decreasing carbon chain length. For example, at 100 K, the
emission lifetime increases by approximately a factor of 3 between PtC12Pt, PtC10Pt and PtC8Pt
and then it increases by a further factor of 5 for PtC6Pt. As discussed in more detail below, the
overall trend of increasing triplet lifetime with chain length arises due to the energy gap law.
By using the emission quantum yield and lifetime data at low temperature it is possible to
estimate the radiative and non-radiative decay rate constants (kr and knr, respectively). The triplet
92
lifetime (τT), phosphorescence quantum yield (φP), and intersystem crossing efficiency (φISC) are
related to kr and knr by the following expressions:
( )nrrT
1τkk +
= (4-2)
TrISCP τkφ=φ (4-3)
Equations 4-2 and 4-3 can be rearranged to expressions that allow knr and kr to be
computed directly from the experimental parameters:
TISC
Pnr τ
11
φφ
−=k (4-4)
TISC
Pr τ
1
φφ
=k (4-5)
In order to apply these expressions to compute kr and knr, values of τT, φP and φISC are
needed, but we note that φP is only known with certainty for two of the complexes, and φISC is
not known. However, given the lack of fluorescence from any of the complexes and the
observation of relatively strong triplet-triplet absorption, it is likely that φISC is significantly
larger than 0.1, and for the shorter oligomers it is likely close to unity. Thus, it is safe to
conclude that φP << φISC and Equation 4-4 reduces to Equation 4-6. We further rearrange
Equation 4-5 to the usable form Equation 4-7 which emphasizes that there is some uncertainty in
the computed radiative rate constants.
Tnr τ
1=k (4-6)
T
PrISC τ
φ=φ k (4-7)
93
The values of knr and φISCkr computed from the experimental data at 100 K are listed in
Table 4-2. Several features are of note with respect to this data. First, for PtC8Pt and PtC10Pt,
knr exceeds φISCkr by nearly 100-fold, which indicates that the oligoynes decay predominantly via
non-radiative pathway(s), even in the low temperature glass. Second, it is evident that there is a
systematic increase in knr with increasing carbon chain length. As noted above, this trend is a
manifestation of the energy gap law. Finally, while there is limited data, it is also interesting that
φISCkr decreases from PtC8Pt and PtC10Pt; this effect probably arises because spin-orbit
coupling induced by the Pt centers decreases with increasing chain length.
Table 4-2. Photophysical parameters for PtCnPt complexes. τT / 10-6 s φP φP
a φISCkr / 103 s-1 b knr / 105 s-1 b τ / µs c C6 26.85 d - - 0.372 e 1.20 C8 5.24 0.003 0.018 3.44 1.87 1.41 C10 2.92 0.0012 0.0053 1.82 3.41 1.20 C12 0.54 d - - 18.5 e 1.47 a Adjusted for 100 K. b Measured at 100 K. c Extracted from room temperature transient absorption. d Not observed. e knr = 1/τ, est.
Transient Absorption Spectroscopy: Triplet-Triplet Absorption of the Oligoynes
In order to provide additional data concerning the properties of the triplet state of the
oligoynes, nanosecond transient absorption spectra were measured at room temperature in
deoxygenated THF solution. As shown in Figure 4-8, near-UV excitation of all of the complexes
produces strongly absorbing transients which decay on a timescale of a few µs (computed decay
lifetimes are listed in Table 4-2). The transient which gives rise to the absorption is the triplet
excited state. In each case the transient difference-absorption spectra are characterized by
bleaching of the near-UV ground state absorption bands, combined with moderately intense
94
300 400 500 600 700 800
-0.04
-0.02
0.00
0.02
0.00
0.05
0.10
0.15
0.20
∆ A
bsor
banc
e
-0.40
-0.20
0.00
0.20
Wavelength / nm300 400 500 600 700 800
-0.20
0.00
0.20
A
B
C
D
Figure 4-8. Transient absorption spectra of PtCnPt complexes following pulsed excitation with a 355 nm pulse (10 ns, 5 mJ-pulse-1). (A) n = 6, (B) n = 8, (C) n = 10, and (D) n = 12. For (A) – (C) the first spectrum was obtained 20 ns following excitation, and subsequent spectra at 400-ns delay increments. For (D) the first spectrum was obtained at 20 ns following excitation, and subsequent spectra at 80-ns delay increments.
95
triplet-triplet absorption in the visible region. Interestingly, for each complex, the triplet-triplet
absorption appears to arise in two distinct bands; one stronger band which is in the near-UV and
blue of the visible, and a second weaker band that is centered in the mid-visible. For PtC12Pt the
visible band is apparently split into vibronic bands. While assignment of these transitions will
require further (theoretical) study, it is evident from the transient absorption study that
intersystem crossing is efficient in all of the oligoynes. Furthermore, it is clear that the triplet-
triplet absorption of the carbon chains is strongly allowed.
Discussion
Variation of Triplet Energy with Carbon Chain Length
A key result of this investigation is that it provides the 0-0 triplet energy as a function of
oligoyne chain length for the PtCnPt complexes. In order to analyze the chain-dependent triplet
energy data, the correlation in Figure 4-9 has been constructed, which shows a plot of E00 vs. 1/n,
where n is the number of carbon atoms in the oligoyne chain. Interestingly, this plot features an
excellent linear correlation and it provides an intercept of 7670 cm-1 (0.95 eV) which
corresponds to the triplet energy in the of the –(C≡C)n– polymer. It is noteworthy that there is
substantial variation of the triplet energy across the PtCnPt series, a fact which implies that the
triplet state is delocalized across the entire π-conjugated system defined by the carbon chain for
all members of the PtCnPt series. There is no sign of leveling off of the triplet energy with
increasing chain length, suggesting that for chains longer than twelve carbons the triplet energy
will continue to decrease (i.e., the effective “conjugation length” for the triplet has not been
reached in this series).
In a recent study, Nagano and co-workers reported the evolution of the absorption,
fluorescence and phosphorescence of the series of diphenyl-end capped oligoynes Ph–(C≡C)n–Ph
96
(n = 1 – 6).94,100 Using their reported phosphorescence 0-0 band energies, we have included their
data in Figure 4-9 for comparison with that of the PtCnPt series. Interestingly, the correlation of
E00 vs. 1/n defined by the Ph–(C≡C)n–Ph series is curved. For the shorter oligoynes, the trend
defined by the Ph–(C≡C)n–Ph series deviates to lower energy relative to the correlation for the
PtCnPt series; however, for longer carbon chain length (n = 4 and 6) the data for Ph–(C≡C)n–Ph
series nearly converges to the correlation defined by the PtCnPt series. The curvature in the
1/n0.00 0.05 0.10 0.15 0.20 0.25 0.30
E00
/ cm
-1
5000
10000
15000
20000
25000
30000
Figure 4-9. Correlation of E00 (triplet energy) vs. 1/n for π-conjugated oligomers, where n = # carbons in the chain. (●) : PtCnPt complexes. (x) : Ph-(C≡C)x-Ph oligomers ((from Nagano et al.94). ( ) : α-thiophene oligomers (from de Melo et al.101). The dashed line is best fit to the PtCnPt data with R2 = 0.997.
97
correlation for the Ph–(C≡C)n–Ph series likely results from the fact that for short oligoyne chain
length, the triplet is delocalized significantly into the phenylene end-groups. This hypothesis is
supported by the observation of significant vibronic coupling to phenyl-based modes in the
phosphorescence spectra of Ph–C≡C–Ph and Ph–(C≡C)2–Ph.94 At longer chain length, the effect
of the phenylene end groups on the triplet energy in the Ph–(C≡C)n–Ph series becomes less
pronounced, and consequently the two correlations converge. This comparison suggests that
correlation of E00 vs. 1/n for the PtCnPt series more accurately reflects the trend in triplet state
energy for the “pure” –(C≡C)n– carbon chain (i.e., without interference from the end-groups).
Importantly, this comparison also shows that the triplet energy for the –(C≡C)n– polymer which
is estimated by extrapolation of the correlation for the PtCnPt series is likely a much better
estimate than would be obtained from the Ph-(C≡C)n-Ph series.
A number of recent studies have explored the variation of the singlet and triplet energies
with oligomer length for π-conjugated oligomers such as oligothiophene,101-104 oligo(phenylene
vinylene)105,106 and oligo(fluorene).107 Of particular relevance to the present work are studies of
thiophene and substituted thiophene oligomers, where very clear correlations of excited state
energy with oligomer length emerge.101-103,107 Figure 4-9 also includes E00 values for the triplet
state in the series of α−oligothiophene oligomers (for n = 2 – 7) reported by de Melo and co-
workers.101 In the figure the oligothiophene triplet energies are plotted vs. the 1/n, where n is the
number of carbon atoms in the π-conjugated chain (there are 4 carbons per thiophene repeat
unit). Interestingly, it is clear that the trend of E00 vs. 1/n for the oligothiophene series runs
nearly parallel to the correlation defined by the PtCnPt series. This agreement indicates that the
degree of triplet delocalization is similar in oligoynes compared to the oligothiophenes. Note,
98
however, that the extrapolated value of the triplet energy for polythiophene is ca. 2500 cm-1
higher than that of the –(C≡C)n– polymer.
Since significant fluorescence was not observed for the PtCnPt series, it is only possible to
estimate the S1 energy based on the onset of the absorption bands. Using this as an
approximation for the S1 state energy, we have computed estimated singlet-triplet splitting
(∆EST) for the PtCnPt series and the values are listed in Table 4-1. Interestingly, for the three
longer oligoynes the ∆EST cluster around a value of 0.7 eV, which is in good accord with the
value observed for a variety of other long π-conjugated systems, including poly(thiophene) and
poly(phenylene vinylene).60
Energy Gap Law Correlation for the Oligoynes
The energy gap law was first suggested by Robinson and Frosch to explain the systematic
variation of triplet state non-radiative decay rates in polycyclic aromatic hydrocarbons.108
Shortly thereafter, Siebrand developed a quantitative theory for the energy gap law.109 The
energy gap law predicts that the rate of non-radiative decay of an excited state will increase as
the excited state energy decreases. The effect arises because of the relationship between the
Franck-Condon factors for vibrational overlap of the excited- and ground-electronic states and
the energy gap (and geometric distortion) between the potential surfaces for the two electronic
states (see below, Equation 4-8). The most comprehensive study of the energy gap law carried
out to date lies in the work of Meyer and co-workers who showed the existence of a quantitative
relationship between excited state energy and the non-radiative rate for the metal-to-ligand
charge transfer state in a series of Ru(II) and Os(II) polypyridine complexes.110-112 More
recently, Köhler and co-workers showed that the energy gap law holds for triplet decay in a
series of platinum-acetylide oligomers and polymers.113
99
A focus of the present investigation is to explore whether the energy gap law holds for
decay of the triplet excited state in the series of platinum end-capped oligoynes. This study is of
interest due to the remarkable simplicity of the carbon chains: they consist of a linear, π-
conjugated electronic system in which the excitation is coupled to a single, very well-defined
high frequency vibrational mode. In short, the oligoynes seem to provide an ideal platform to
examine the correlation between spectroscopy, triplet energy and non-radiative decay rate.
The following expressions provide the complete relationship between the non-radiative
decay rate and parameters accessible from spectroscopy,98,110
( ) ( )m
0001
00m2
m21,020monr cm 1000
ln5.02ln16
1lnlnω
γωωνγβ
ηηη EESk −
−
∆++−= − (4-8)
= −1
2o cm 1000
2πωβ kkC
(4-9)
1lnmm
000 −
=
SE
ωγ
η (4-10)
where Sm, E00, ∆ν 0,1/2, and ħωm, are the same terms defined in Equation 4-1, βo is the
vibronically induced electronic coupling term, Ck is the vibronic coupling matrix element, and ωk
is the frequency of the promoting vibrational mode. These equations can be simplified in order
to facilitate analysis of experimental data. The assumption that all terms on the right hand side
of Equation 4-8 except the last one are independent of changes in E00 leads to Equation 4-11.112
This equation predicts a linear relationship between the log of the non-radiative decay rate and
E00, with a slope equal to γo/ħω (this is the mathematical statement of the energy gap law). Also,
since ln βo is the only term in Equation 4-8 that cannot be computed using parameters derived
from a Franck-Condon fit of an emission spectrum, this equation can be re-cast as Equation 4-12
100
where ln[FCF(calcd)] is an abbreviation for “calculated Franck-Condon factors”. Note that
ln[FCF(calcd)] is computed by substituting the parameters recovered from the emission spectral
fits (Table 4-1) into the last 4 terms on the right hand side of Equation 4-8. The latter expression
implies that a plot of ln[FCF(calcd)] vs. ln knr will be a linear correlation with a slope equal to
unity.
00m
0nrln Eak
−=
ωγ
η (4-11)
onr lnln]FCF(calcd)ln[ β−= k (4-12)
Figure 4-10 illustrates plots of ln knr vs. E00 for the PtCnPt series (filled circles, values at
100 K, see Table 4-2). Also included in the correlation are data from the series of platinum
acetylide oligomers reported by Köhler.113 Several features are evident from this presentation.
First, the non-radiative decay rates for the PtCnPt series clearly follows the correlation predicted
by the energy gap law. This result clearly shows that in the frozen solvent glass non-radiative
decay of the triplet state in the oligoynes is controlled by the extent of vibronic coupling of the
exciton to the high frequency mode (the –C≡C– stretch), which varies exponentially on the
excited state energy. The slope of the energy gap correlation for the PtCnPt series is almost the
same as that defined by the platinum-acetylide oligomers reported by Köhler and co-workers.113
This correspondence in the energy gap law dependence likely arises because in both of the
platinum-containing systems the –C≡C– stretching mode is likely the dominant acceptor mode
coupled to non-radiative decay. The correlation for the PtCnPt series has a larger intercept
value, which reflects the fact that non-radiative decay is faster at equal excited-state energy in the
two series. This larger “intrinsic” knr (a in Equation 4-11) for the PtCnPt series may be due to
the fact that in the platinum-acetylides studied by Köhler medium frequency modes (aromatic
101
E00 / cm-1
10000 12000 14000 16000 18000 20000 22000
ln k
nr
6
8
10
12
14
16
18
Figure 4-10. Energy gap law correlation, see text for details. (●) : PtCnPt complexes. The solid line is best fit with R2 = 0.951. (x) : Platinum-acetylide oligomers (from Wilson et al.113) The dashed line is best fit with R2 = 0.946.
ring C-C stretching) are also coupled to the triplet exciton. This would effectively decrease the
Frank-Condon factors for non-radiative decay (note that ωk appears in the βo term in Equation 4-
8).
Also of interest is the correlation of the Huang-Rhys parameter (Sm, calculated from the fits
of the emission spectra of the PtCnPt oligomers for n = 6, 8 and 10, values in Table 4-1) with the
triplet energy shown in Figure 4-11A. As can be seen from this correlation, Sm decreases with
E00, with a very good linear correlation being defined by the series. (By extrapolating the linear
correlation defined for PtC6Pt, PtC8Pt and PtC10Pt the Sm value for PtC12Pt has been
102
estimated). The decrease in Sm with increasing carbon chain length is consistent with the triplet
excited state becoming more delocalized, which has the effect of decreasing the electron-
vibrational coupling. A similar correlation for Sm vs. E00 was reported by Meyer and co-workers
for a metal-to-ligand charge transfer states in a series of Ru-polypyridine complexes, and their
data is shown for comparison in Figure 4-11A.112 Despite the significant difference in the nature
of the excited states for the Ru-polypyridine and PtCnPt complexes (metal-to-ligand charge
transfer and 3π,π*, respectively), the correlations are remarkably similar.
Finally, as noted above, it is possible to compute the Franck-Condon factors (FCF) for
non-radiative decay using the parameters obtained from the emission fits. As suggested by
Equation 4-12, the calculated FCF values should vary linearly with ln knr, with a slope of unity
and a single adjustable parameter which corresponds to ln βo.98,112 Figure 4-11B shows the
correlation of ln FCF and ln knr, where the ln FCF values for the PtCnPt complexes were
computed using the parameters recovered from the spectral fits (Table 4-1) and Equation 4-8.
Since it was not possible to fit the emission spectrum for PtC12Pt, the Sm needed to compute ln
FCF for this complex was extrapolated using the observed E00 value in the correlation shown in
Figure 4-11A. The dashed line in Figure 4-11B has a slope of unity and an intercept of -24.7,
which affords a value of βo = 5.3 x 1010 cm-1. Given the relatively large range in E00 and knr
values for the series (6000 cm-1 and a factor of 50) the correlation of the calculated Franck-
Condon factors and the non-radiative decay rates is impressive. The quality of the correlation
underscores the quality of the energy gap law correlation for the PtCnPt series, and that in the
frozen solvent glass non-radiative decay is controlled by coupling to the –C≡C– mode of the
carbon chain.
103
E00 / cm-1
12000 14000 16000 18000 20000 22000
Sm
0.4
0.6
0.8
1.0
1.2
1.4
1.6
ln knr
10 11 12 13 14 15
ln F
CF(
calc
d)
-15
-14
-13
-12
-11
-10
-9
A
B
Figure 4-11. (A) Plot of Huang-Rhys parameter (Sm) vs. the triplet energy (E00) for PtCnPt complexes (●) and for series of ruthenium polypyridyl complexes (▼). Data for the ruthenium complexes are from Barqawi et al.112 Lines are best fit to data, R2 = 0.999 for PtCnPt complexes and R2 = 0.963 for ruthenium polypyridyl complexes. (B) Plot of calculated Franck-Condon factors vs. natural log of non-radiative rate for PtCnPt complexes. The line has a slope of unity. In both plots, the open circle data point ( ) is for PtC12Pt. The Sm value for PtC12Pt in plot (A) is extrapolated using the best fit line (R2 = 0.960) and the experimental E00 value and the ln FCF(calcd) value for this complex in plot (B) is computed using the extrapolated Sm value.
104
Excited State Decay above the Solvent Glass Point
As noted above, the energy gap correlation for the PtCnPt series holds only at
temperatures below the solvent glass point. It is evident that in fluid solution an additional non-
radiative decay pathway is active that dominates decay of the triplet state in the complexes.
Activation of this pathway as the glass melts is very evident in PtC6Pt; for this complex the
lifetime decreases by nearly a factor of 100 as the temperature is increased from 100 – 160 K.
Given that this decay pathway is activated by melting of the solvent matrix, it is reasonable to
assume that it is related to the increase in the ability of the carbon chain to undergo fluxional
motion. Previous theoretical and experimental studies of –(C≡C)n– oligomers indicate that in
some cases the optimum geometry in the excited state involves a transoid conformation in which
the triple bond length is significantly increased (i.e., the carbons become sp2 hybrids).94,114
Although there is insufficient experimental data from the present photophysical studies to prove
that a bending mode of the –(C≡C)n– chain is the origin of the accelerated triplet decay in fluid
solution, it is believed that it is a very reasonable hypothesis. Future DFT calculations which
probe the geometric energy landscape for the triplet state of the PtCnPt series may provide
insight into this issue.
Summary and Conclusion
A detailed photophysical investigation of a homologous series of carbon chain oligomers
that are end-capped with Pt-acetylide units has been carried out. The photophysics of the
complexes is dominated by a 3π,π* that is concentrated on the –(C≡C)n– chain, and the triplet
state is produced in comparatively high yield due to the strong spin-orbit coupling induced by the
heavy metal platinum centers. At low temperature (T < 130 K) each of the complexes exhibits a
unique, highly structured phosphorescence that is characterized by a series of narrow (< 200 cm-1
105
fwhm) vibronic bands separated by ca. 2100 cm-1. The appearance of the spectrum signals that
the triplet state is coupled to a single, high frequency vibrational mode that corresponds to the
stretch of the –(C≡C)n– chain. Analysis of the phosphorescence spectra using a single-mode
Franck-Condon bandshape analysis indicates that the 0-0 energy of the triplet state decreases by
ca. 6000 cm-1 as the length of the carbon chain increases from 6 to 12 atoms, and the electron-
vibrational coupling constant (Sm) decreases with increasing chain length. At room temperature
the triplet state can be observed by transient absorption spectroscopy, and each of the complexes
exhibits moderately intense triplet-triplet absorption throughout the visible region (400 – 700
nm).
Phosphorescence lifetime studies were carried out on the series to study the decay kinetics
of the triplet state. At low temperature in a frozen solvent glass, the triplet lifetime
systematically increases with decreasing length of the carbon chain, with the lifetimes ranging
from 27 µs (PtC6Pt) to 500 ns (PtC12Pt). Given that the phosphorescence quantum yields are
comparatively low, the lifetimes are controlled by the rate of non-radiative decay, and the
observed lifetime variation at low temperature is attributed to the energy gap law. Consistent
with this hypothesis, the non-radiative decay rates of the complexes at T = 100 K are shown to
exhibit a quantitative energy gap law correlation.
The properties of the triplet state in the PtCnPt series are believed to be minimally
perturbed by the presence of the platinum complex end-capping units. Consequently the results
of this study provide the best available quantitative insight concerning the structure and
dynamics of the triplet state in carbon chains of extended length.
106
CHAPTER 5 AN IRIDIUM COMPLEX THAT EXHIBITS
DUAL-MECHANISM NONLINEAR ABSORPTION
Introduction
Reverse saturable absorption (RSA) as a sequential, multi-step process involving two
photons as a mechanism for optical limiting was described in detail in chapter 1. When a
material has an excited state absorption cross section larger than the ground state absorption
cross section at a particular wavelength, it is said to be a reverse saturable absorber. RSA is a
slow (~10-10 – 10-8 s) process requiring first the ground state absorption of a photon to populate
the excited state, followed by excited state absorption of a second photon. While this has been
shown as an effective method of nonlinear absorption, there are several limiting factors. Most
notably is that RSA is generally a slow process requiring long-lived excited states (usually triplet
excited states populated by ground state absorption to the singlet excited state followed by
intersystem crossing). Due to the multi-step nature of the absorbing mechanism, RSA is an
ineffective pathway for limiting laser pulses below the nanosecond time regime.
Another mechanism for nonlinear absorption involves the instantaneous absorption of two
photons. Consequently, two-photon absorption (TPA) is mostly effective for optical limiting
only at very short timescales (< 100 ps).115 A second benefit of using TPA as an optical limiting
mechanism is that the material can be completely transparent at the active wavelength, whereas a
RSA material must have some ground state absorption.
One method of extending the time regime of an optical limiter is to combine absorption via
several mechanisms. By coupling slow timescale reverse saturable absorption—or more
generally, excited state absorption—with fast timescale two-photon absorption, a broad temporal
response to optical limiting can be achieved. The compound characterized in this chapter
exhibits such absorption pathways.
107
108
The theoretical prediction of two-photon absorption was first described by Goeppert-
Mayer in 1931.116 However, two-photon absorption processes were predicted thirty years before
they could be experimentally verified. It took until the advent of the laser for the first
observation of TPA in the lab by Kaiser and Garrett in 1961.117
Figure 5-1 shows a Jabłoński diagram with corresponding representative absorbance
spectra illustrating two-photon absorption. As shown, a material can absorb a single photon
corresponding to an energy, E, which will promote an electron to a higher state. Alternatively,
certain materials can simultaneously absorb two photons, whose energies sum to equal the
Figure 5-1. Jabłoński diagram and corresponding representative absorption spectra illustrating two-photon absorption.
energy between the two states. One- and two-photon excitation will not necessarily promote
electrons to the same state. That is, one- and two- photon excitation follow different selection
rules.118 Selection rules for two-photon excitation will not be explored here, as the results
presented later will be given in a more qualitative presentation. One-photon absorbance spectra
Wavelength
Abs
orba
nce
2λλ
One-photon absorption
Two-photon absorption
Energy
hν
½ hν
½ hν
for a material theoretically should be different from two-photon absorbance spectra. While the
amount of data on two photon absorption spectra is increasing, it is still a difficult measurement.
Measured values for two-photon absorption coefficients can vary significantly based upon
solvent, laser intensity, pulse duration of the laser and other experimental conditions.119
In order to understand the concept of nonlinear absorption through a multi-photon process,
it is necessary to first understand the linear, one-photon absorption process. From Beer’s law,
one can consider a sample of some thickness along the z-axis. For incident light propagating
along the z-axis through a sample, the light changes according to Equation 5-1.
IdzdI α−= (5-1)
where α is the absorption coefficient. For a one-photon process, no other terms apply.
However, for two-photon absorption, the absorption coefficient is defined by Equation 5-2.
Iβαα += 0 (5-2)
where α0 is the one-photon absorption coefficient and β is the two-photon absorption coefficient.
Substituting α in Equation 5-1,
20 II
dzdI βα −−= (5-3)
an expression is obtained illustrating the (linear) one- and (nonlinear) two-photon absorption
dependence on incident intensity.120
To further the nonlinear optical properties of a two-photon absorbing material, a second
absorbing mechanism can take place. After populating the excited state of a material, the
electron can either return to the ground state, or absorb an additional photon. The initial
absorbing state can be excited by either one- or two-photon absorption, as the resulting excited
109
state is independent of the excitation pathway. Excited state absorption (ESA) following TPA
leads to enhanced nonlinear optical properties.
If excited-state absorption occurs, a third term applies in Equation 5-3, causing the change
in intensity through the sample to become
ITIIdzdI
TT 12
0 σβα −−−= (5-4)
where σTT is the absorption cross-section of the T1 triplet excited state, and T1 is the population
of the first triplet excited state. Equation 5-4 holds only within the lifetime of the triplet state.
For many compounds, and in particular the compound discussed in this chapter, the
absorbing excited state will be the triplet state. A four-level Jabłoński diagram illustrating the
transitions involved in TPA followed by ESA is shown in Figure 5-2. The process is initiated by
simultaneous absorption of two photons via TPA by the ground state promoting an electron to
the S1 excited state. Intersystem crossing then populates the first triplet excited state, T1. The
last step involves the absorption of a photon to populate some higher triplet state, Tn.
(Intersystem crossing is not necessary for enhanced TPA; the S1 state could alternatively be an
absorbing species, although this mechanism will not be the focus of this chapter.) Rate equations
for each state can be written as shown in Equations 5-5, 5-6, 5-7 and 5-8. The following rate
equations are described assuming only the transitions illustrated in Figure 5-2 will occur.
11
1120
2dd
TS
TSIht
Sττν
β++−= (5-5)
1ISC121
12d
dS SkSIht S
−−=τν
β (5-6)
1
111ISC
1
dd
TT
nTT TTIh
TSktT
nττν
σ−+−= (5-7)
110
nT
nTTn TI
hT
tT
τνσ
+= 1
dd
(5-8)
β
hν
hν
hνS1
S0
T1
Tn
ISC σTT
Energy
Figure 5-2. Jablonski diagram for a four-level system illustrating TPA with ESA. The excited triplet state is populated by two-photon absorption of the ground state followed by intersystem crossing. The excited triplet state then absorbs an additional photon.
where S0, S1, T1 and Tn are the populations of the ground and first excited singlet states and first
and some higher triplet excited states, respectively, τ is the lifetime of the labeled state, and kISC
is the intersystem crossing rate. Integrating the equations to find an expression for T1, and
substituting into Equation 5-4, an expression for the intensity at the end of the laser pulse is
given:
32
2I
hI
dzdI TT
νβτσ
β −−= (5-9)
where τ is the duration of the laser pulse. Several assumptions are made for Equation 5-9 to be
valid:121 (i) one-photon absorbance at the two-photon absorbing wavelength is negligible for the
S0 state (ii) the lifetime of the T1 state is longer than the length of the laser pulse, (iii) intersystem
111
crossing efficiency approaches unity and is shorter than the length of the laser pulse, and (iv) the
population of the S0 ground state and is not sufficiently depleted, i.e. S1, T1, Tn << S0. From
Equation 5-9, it is shown that reverse saturable absorption occurs when σTT > 0.121 Therefore, a
material which exhibits excited-state absorption at the two-photon absorbing wavelength is a
reverse saturable absorber.
Two-photon absorbing materials commonly have donor (D) or acceptor (A) groups linked
by a π-conjugated bridge.122-124 Different types of architectures are shown in Figure 5-3. More
complex motifs, including branched structures, with D-π-A-π-D type conjugation are also
possible, allowing for increased TPA cross sections. There have been numerous reports
concerning TPA or RSA in organic and organometallic molecules, but currently there is not
much in the literature about chromophores that exhibit both TPA and RSA.125-127 Our group is
interested in the design, synthesis and characterization of materials that exhibit “dual-mode”
Donor Donorπ
Acceptor Acceptorπ
Donor Acceptorπ
Type I
Type II
Type III
Donor DonorπDonor Donorπ
Acceptor AcceptorπAcceptor Acceptorπ
Donor AcceptorπDonor Acceptorπ
Type I
Type II
Type III
Figure 5-3. Structural motifs for two-photon absorbers.
112
TPA/RSA nonlinear absorption. In particular, oligomers that contain a π-conjugated
chromophore with a large TPA cross section that is strongly coupled to a transition metal
chromophore that promotes intersystem crossing to a long-lived triplet excited state with a large
triplet-triplet absorption cross section.
The photophysical characterization of such a chromophore will be described in this
chapter. The material of interest, the Ir(III) complex ML, is shown in Figure 5-4. This complex
incorporates a heavy metal center into a D-π-D type π-conjugated oligomer L. π-Conjugated
molecular structures containing donors or acceptors at both ends of a delocalized, π-electron
system exhibit large TPA cross sections.124,128 In addition, iridium has the largest spin-orbit
coupling constant of all the transition metals, and consequently it is able to facilitate intersystem
crossing to afford a large triplet yield.129
N NNHex2Hex2N
N NNHex2Hex2N
M
L
ML
M = Ir(ppy)2+[PF6
-]; ppy = 2-phenylpyridine
Figure 5-4. Structures of the ligand, L, and iridium(III) complex, ML.
Experimental
All photophysical measurements were carried out in 1 cm x 1 cm quartz cuvettes, unless
otherwise noted. All sample solutions were prepared in dry THF and deoxygenated by bubbling
113
with argon unless otherwise noted. UV-visible absorption spectra were recorded on a Varian
Cary 100 dual-beam spectrophotometer. For emission measurements, sample concentrations
were adjusted to produce optically dilute solutions (Amax < 0.2). Corrected steady-state emission
measurements were performed on a SPEX F-112 fluorescence spectrometer equipped with a
Hamamatsu R928 PMT for visible measurement. Emission quantum yields were measured
relative to Ru(bpy)3Cl2 in air-saturated H2O, where φ = 0.0379.130 Transient absorption
measurements were carried out with solutions having an absorbance of 0.8-1.0 at 355 nm.
Transient absorption spectra were obtained on an instrument that has previously been described99
that uses the third harmonic of a Nd:YAG laser (Spectra Physics GCR-14, λ = 355 nm, 10 ns
fwhm, 10 mJ pulse-1, 20 mJ-cm-2 fluence) as the excitation source.
Two-photon-induced emission spectra were recorded on an apparatus that relies on a
Continuum Surelite series Nd:YAG laser as the excitation source. The fundamental output of the
laser (λ = 1064 nm, 10 ns fwhm) was passed through a telescope providing a final beam diameter
of 0.7 cm. This beam was focused with a 10-cm focal length lens (Newport Optics) and the
sample was positioned at the focal plane of the lens. The luminescence was detected by an
intensified CCD detector (Princeton Instruments, PI-MAX) coupled to an Acton SpectraPro 150
spectrograph. The sample was contained in a 5 mm i.d. round borosilicate tube. Nonlinear
transmittance was measured using the same apparatus (shown in Figure 5-5), with samples
contained in a 10-mm pathlength quartz optical cell that was positioned at the focal plane.
Nuetral density filters were used to attenuate the beam fluence. A 10% beamsplitter was used to
direct part of the incident energy to the reference detector, D1, (Ophir analog power meter). The
transmitted beam was passed through a 10-cm focal length lens (L1) before reaching the sample.
114
A second lens (L2) collected the beam before reaching the detector, D2 (Scientech P09 energy
meter with a Vector S310 digital display).
Transient absorption arising from two-photon excitation was carried out using an
unfocused beam from the Continuum Surelite laser as the excitation source, a Hamamatsu xenon
flash lamp, and the previously mentioned spectrograph and CCD detector.
Fluence-dependence emission intensity arising from two-photon excitation was measured
using an unfocused 0.8 cm diameter beam from the fundamental of a QuantaRay GCR series
Nd:YAG laser as the excitation source (λ = 1064 nm, 10 ns fwhm). Detection was provided by a
Hamamatsu R928 PMT coupled to an Oriel 1/8 m monochromator set at the emission
wavelength maximum.
L1 L2NDF BSNd:YAG laser Sample
D1
D2L1 L2NDF BSNd:YAG laser Sample
D1
D2
Figure 5-5. Apparatus for measuring nonlinear transmittance. NDF: neutral density filter; BS: beamsplitter; L: lens; D: detector.
Results
One-photon ground-state absorption and photoluminescence spectra for ligand L and Ir
complex ML in THF solution are shown in Figure 5-6. Ligand L exhibits absorption bands at
115
280 and 400 nm, arising from short- and long-axis π-π* transitions, respectively. Ir complex
ML features a broad absorption band at 500 nm, arising from a combination of the long-axis
polarized π-π* transition and Ir→ligand metal-to-ligand charge transfer (MLCT) transition.
Intense fluorescence from ligand L is observed with a band maximum at 520 nm. The emission
of ML is dominated by a broad featureless band having an emission maximum at 730 nm. The
quantum yield at this wavelength is 0.003, with an emission lifetime of 513 ns.
ε / 1
04 M
-1 c
m-1
0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Wavelength / nm
300 400 500 600 7000
2
4
6
8
300 400 500 600 700 800
Nor
mal
ized
Inte
nsity
0.0
0.2
0.4
0.6
0.8
1.0
A
D
C
B
ε / 1
04 M
-1 c
m-1
0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Wavelength / nm
300 400 500 600 7000
2
4
6
8
300 400 500 600 700 800
Nor
mal
ized
Inte
nsity
0.0
0.2
0.4
0.6
0.8
1.0
A
D
C
B
Figure 5-6. Absorption of (A) ligand L and (B) complex ML and one-photon emission of (C) ligand L and (D) complex ML.
116
Transient absorption spectra of ML in THF solution obtained under one-photon excitation
conditions (5 ns pulse, 355 nm) are show in Figure 5-7. The TA spectrum features a ground-
state bleaching centered near 500 nm combined with a strong absorption band that extends
throughout the visible and into the near infrared region. The absorption wavelength maximum of
the transient is approximately 875 nm with a ∆ε value of 6.1 x 104 M-1 cm-1. The transient
absorption decays with a lifetime of 1.2 µs.
Wavelength / nm400 600 800 1000 1200 1400
∆ Ab
sorb
ance
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Figure 5-7. Transient absorption of ML in deoxygenated THF solution obtained at 400 ns increments following 355 nm excitation.
117
Given that the π-π* ground state absorption in complex ML is at 500 nm, it seemed
reasonable that two-photon absorption would occur in the near-infrared at 1064 nm, i.e. the
fundamental of a Q-switched Nd:YAG laser. The two-photon upconverted photoluminescence
spectrum of ML produced by excited with the 1064 nm beam is shown in Figure 5-8. The
emission band shape and wavelength maximum are the same as those observed under one-
photon absorption conditions. The upconverted luminescence can easily be seen by eye as
shown in Figure 5-9. A plot of the integrated emission area as a function of infrared excitation
intensity is shown in Figure 5-10, and a fit shows that the emission is dependent on the square of
Wavelength / nm400 500 600 700 800
Inte
nsity
/ a.
u.
0
500
1000
1500
2000
2500
Figure 5-8. Two-photon induced emission of ML after pulsed excitation by 1064 nm.
118
A B
Figure 5-9. Photograph of 5 mM ML solution. (A) Under ambient conditions and (B) showing visible emission after excitation by 1064 nm laser light.
Laser fluence / mJ cm-20.0 0.5 1.0 1.5
Em
issi
on a
rea
/ a.u
.
Figure 5-10. Emission area dependence of ML on incident laser energy at 1064 nm.
119
the of the excitation intensity. According to Equation 5-3, this indicates a two-photon absorbing
mechanism.
Transient absorption spectra collected after excitation by 1064 nm were also observed
(Figure 5-11). The band shape of the transient absorption spectrum measured under two-photon
excitation conditions is similar to that obtained under one-photon excitation. The two-photon
excited transient absorption spectrum was obtained on a different instrument than the one-photon
excited spectrum shown in Figure 5-6(D). The detector used for the two-photon excited studies
is not sensitive beyond 850 nm, and therefore, it is not possible to observe the maximum of the
triplet-triplet absorption that occurs near 875 nm.
Wavelength / nm500 600 700 800 900
∆ A
bsor
banc
e
-0.01
0.00
0.01
0.02
0.03
Figure 5-11. Transient absorption spectra of 5 mM deoxygenated THF solution of ML following 1064 nm pulsed excitation. Excitation fluence in order of decreasing transient absorbance signal: 0.35 J cm-2; 0.25 J cm-2; 0.20 J cm-2; 0.12 J cm-2; and baseline (no laser).
120
Nonlinear transmittance measurements of ML were carried out at various concentrations
using 1064 nm, 5 ns pulses. Figure 5-12 shows the laser fluence dependence of the transmittance
of ML at a range of concentrations. While the blank THF solution responds linearly to the
incident energy, it is evident that the THF solutions of the Ir complex ML exhibit nonlinear
absorption, the extent of which increases with the concentration of ML. Notably, for a 20 mM
solution of ML, the transmitted energy is significantly reduced above an input energy of 0.8 mJ.
E in / J cm-20.0 0.5 1.0 1.5 2.0
E ou
t / J
cm
-2
0.0
0.2
0.4
0.6
0.8
1.0
1.2 THF only2.5 mM5.0 mM10 mM20 mM
Figure 5-12. Transmittance of 1064 nm pulsed beam at various concentrations of ML in THF.
Discussion
The absorption assignment for the Ir complex ML is slightly more complex than for the
ligand L. The related complex Ir(ppy)2(bpy)+ (where bpy = 2,2'-bypyridine), in which the visible
121
absorption is exclusively due to an MLCT transition, has previously been reported.131,132 The
molar absorptivity of ~104 M-1 cm-1, as reported by Glusac and co-workers,131 which is much
smaller than that of ML, indicates that the visible absorption of ML is dominated by the
intraligand π-π* transition. The absorption maximum for ML is red-shifted by approximately
100 nm compared to ligand L. The red shift arises due to the effect of the cationic Ir(III) center
on the π-electron system of the conjugated bypyridine ligand system. In particular, the metal
reduces the LUMO energy of the conjugated ligand, in part due to its positive charge, and in part
due to the effect of coordination which forces the bipyridine unit into a planar conformation.133
The emission of ML arises from an excited state having mixed character, consisting of
intraligand 3π,π* and 3MLCT configurations.131,134,135 This assignment is supported by the fact
that the emission is strongly Stokes-shifted from the absorption, and it is comparatively long-
lived.
An important question concerns the nature of the excited state that is responsible for the
strong transient absorption. As noted earlier, on the basis of previous investigations of d6
transition metal complexes with π-conjugated ligands,131,134,135 the long-lived excited state of
ML is tentatively assigned to a state having mixed 3MLCT-3π,π* character. This assignment is
based on previous studies of Ir(III) and Ru(II) complexes with π-conjugated diimine ligands of
varying conjugation length.131,134 In this work, it has been demonstrated that as the energies of
the 3MLCT and 3π,π* states converge, the lowest excited state exhibits mixed 3MLCT-3π,π*
character. In ML, the 3MLCT-3π,π* are within 0.25 eV relative to one another,131,134 and
therefore, the condition is correct for configuration mixing of the two states.
The emission band shape and wavelength maximum arising from absorption at 1064 nm
are the same as those observed under one-photon excitation conditions. The observation of
122
emission at 700 nm under 1064 nm excitation clearly indicates that ML can be excited by two-
photon absorption. That is, excitation at a particular wavelength would not cause emission at a
higher-energy wavelength unless multiphoton absorption is the mechanism by which the excited
state is populated.
Although instrument limitations restricted detection of the full transient absorption
spectrum under 1064 excitation conditions, the portion of the observed spectrum matches that of
the spectrum obtained after excitation by 355 nm. The fact that strong transient absorption is
seen for the triplet state under two-photon excitation conditions further indicates that a relatively
large triplet excited-state population can be produced via two-photon absorption, even with a
nanosecond laser.
Conclusion
In this chapter, the photophysical characterization of a two-photon absorbing iridium
complex has been described. The ligand in the complex provides a large absorption cross-
section for one- and two-photon excitation. The long-lived triplet excited state affords the
opportunity for absorption of an additional photon. This combination leads to enhanced
nonlinear optical absorption.
The photophysics of this complex have shown that the triplet state can be populated
independently by one-photon and two-photon absorption. Incident-energy-dependent emission
intensity illustrates the two-photon absorbing nature of the complex. The iridium atom
contributes to strong spin-orbit coupling of the ligand to reach the triplet excited state through
intersystem crossing from initial excitation. This is demonstrated by one- and two-photon
induced phosphorescence, as well as transient absorption. In addition, the nonlinear optical
properties are clearly shown by the trend seen with incident energy dependence of the
transmittance.
123
CHAPTER 6 CONCLUSIONS AND FUTURE WORK
In previous chapters, various aspects of the photophysical properties of some platinum-
and iridium-containing organometallic compounds have been presented. Additionally, a
transient absorption apparatus was designed and constructed to probe the excited state
photophysics in supplement to techniques possible with commercial instrumentation. The
platinum oligomers and iridium complex were designed to gain some insight into the
photophysical behavior of these novel materials, but also to gain knowledge about the triplet
state in general as it is less studied than the singlet state. The triplet state properties of all-
organic systems are less understood as access into the triplet manifold is not easily facilitated.
The studies presented in previous chapters may therefore help the understanding of the
photophysical properties of triplet excited states in organic and metal-organic conjugated
systems.
The transient absorption apparatus was designed with the primary goal of incorporating an
intensified CCD camera detector into the system. Along with a white-light probe source, a CCD
detector allows for the simultaneous collection of all wavelengths in the visible spectrum. The
benefits of using a CCD camera as opposed to another detector such as a photomultiplier tube
include reduced sample exposure to the excitation source and reduced experiment time. The
inclusion of an intensifier as part of the CCD system allows for gate times as short as 10 ns,
therefore making it possible to extract kinetics from species with lifetimes as short at ~100 ns.
Data conversion software was also developed to present the time-resolved transient absorption
spectra in a conventional manner. While the excitation and probe source, as well as the detector,
can be replaced or modified to expand the range of molecules that are possible to study, a major
limitation of the apparatus is defined by the nature of the sample. Specifically, if a material
124
exhibits strong photoluminescence, it can be collected by the detector. This may result in a
negative signal that can obscure the transient absorption. Consequently, the software used to
control the hardware and calculate the spectra are being modified to create an option for
emission correction.
The photophysical characterization of a series of dinuclear platinum oligomers reveals
several things concerning the delocalization of singlet and triplet states in these types of
molecules. Most significantly, the phosphorescence of the PE2 end-capped oligomers show
distinct emission from the spacer and the end-cap, which indicates limited delocalization of the
triplet state through the platinum centers. Temperature-dependent emission spectra also support
the idea of localization of the triplet state. In contrast, variable temperature excitation indicate
less temperature dependence of the singlet ground and excited states, suggesting more
delocalization of these states compared to the triplet excited state. It is not known if the emission
occurs simultaneously from the spacer and end-cap, or if there is energy transfer from one
chromophore to another during the lifetime of the triplet state. Time-resolved emission studies
could provide better understanding on the nature of the excited state. More definitive trends
could be assigned by characterizing a larger series of dinuclear platinum oligomers. The spacer
and end-caps should be modified systematically in order to gain better understanding of the role
of platinum in the triplet state.
In a second study, a series of molecules in which carbon chains of different lengths span
two platinum atoms was characterized by photoluminescence, lifetime and quantum yield
measurements. The low-temperature emission spectra reveal intense vibrational coupling to the
−C≡C− stretching mode. The emission spectra were subjected to Franck-Condon analysis, and a
linear relationship between emission energy and chain length (from 6 to 12 carbon atoms) was
125
revealed. The electron-vibrational coupling constant decreases with increasing chain length.
Triplet lifetimes are governed by the nonradiative decay rate, and it was found that the triplet
excited states at low temperature (i.e. in a solvent glass) are governed by the energy gap law. It
is believed that the triplet properties are minimally perturbed by the presence of the platinum
atom in the end-cap, and therefore the results of this study provide quantitative insight
concerning the structure and dynamics of the triplet state in long carbon chains.
Finally, the photophysical characterization of a two-photon absorbing iridium complex has
been described. A large absorption cross-section for one- and two-photon excitation is provided
by the ligand, and the iridium atom provides access to the triplet manifold. The long-lived triplet
excited state affords the opportunity for absorption of an additional photon. This combination
leads to enhanced nonlinear optical absorption. Incident-energy-dependent emission intensity
illustrates the multi-photon absorbing nature of the complex. In addition, the nonlinear optical
properties are clearly shown by the trend seen with incident energy dependence of the
transmittance.
In order to gain further understanding of triplet state behavior in these types of systems, it
is necessary to characterize a broad range of related molecules. In a sythentic sense, the
possibilities are nearly endless. Gladysz and coworkers136-138 have already synthesized related
molecules, and some examples are shown in Figure 6-1. These compounds would be helpful in
understanding intermolecular interactions. For example, the series shown in Figure 6-1A could
be thought of as an insulated molecular wire, with the carbon chain as the wire surround by the
alkyl chain. Depending on conformation, the alkyl chain could wrap around the center chain and
perhaps protect it from intermolecular interactions. In contrast, the carbon chains in the dimers
126
shown in Figure 6-1B are forced to be near each other, facilitating the study of aggregate-like
interactions.
A
B
A
B
Figure 6-1. Variations of molecular wires synthesized by Stahl, Owen et al.136-138 (A) A shielded molecular wire. (B) A wire dimer.
As with the platinum oligomers, further knowledge would increased through
characterization of a broader range of molecules. However, the photophysics of the molecule
presented in Chapter 5 could be more thoroughly characterized. For example, using a variable-
wavelength excitation source, such as an optically parametric oscillator (OPO), a greater
understanding of the multiphoton absorption properties would be gathered. The multi-photon
absorption properties could be further examined on a temporal scale, that is, through excitation
by a short (ps) laser pulse. Such information is essential in the development of materials for use
as broad-band optical limiters.
127
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BIOGRAPHICAL SKETCH
Richard T. Farley was born in Rochester, New York, and grew up in the suburbs of
Orlando, Florida. He spent a lively childhood with his grandparents, parents and sister. Richard
started his education in chemistry at the University of Central Florida in 1998. He took a strong
interest in analytical and organic chemistry. It was not until he began his graduate studies at the
University of Florida that Richard discovered his curiosity for the fundamental nature of physical
chemistry. He took advantage of the opportunity to merge physical and instrumental work in a
primarily organic atmosphere, working under the guidance of Prof. Kirk S. Schanze to pursue a
doctoral degree in physical chemistry.
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