Part I Physical Aspects of Photochemistry

By Anthony Harriman

Photophysical Processes in Condensed Phases

BY ANTHONY HARRIMAN

1 Introduction

The format of this chapter follows that used in previous volumes. Coverage is given to the multifarious routes by which an electronically excited state may undergo deactivation in solution or solid phase. Additional attention is given to instrumental methods used to detect photophysical processes and to the application of photophysics in contemporary analytical chemistry. The huge literature accompanying these subjects precludes a thorough review of each important development and it is regretted that, given page restrictions, all relevant publications cannot be covered.

2 General Aspects of Photophysical Processes

A database of absorption and fluorescence spectra for some 125 photoactive compounds has been established, together with accompanying routines for calculating various photophysical events. The numerous scientific achievements of J. and F. Perrin, Vavilov, Levshin and Pringsheim have been documented and a historical overview of fluorescence analysis has been compiled. A review of luminescence techniques and instrumentation has appeared. Separate reviews have covered most areas of contemporary luminescence spectroscopy, including general theoretical aspects, photoluminescence, ionoluminescence, thermoluminescence, different forms of sonoluminescence, mechanoluminescence, bioluminescence, and chemiluminescence. Critical reviews of single bubble sonoluminescence have appeared while further attention has been given to understanding the emission properties of complex molecules in solution, crystals and thin solid films. The kinetics of fluorescence quenching, including fast bimolecular reactions, have been reviewed while an overview of the effects of complexation on emission properties has been presented. This latter study is addressed primarily towards the use of non-radiative energy transfer between lanthanides and appropriate chelating functions. Recent trends in the analytical applications of chemiluminescence have been reviewed while the technique of laser flash photolysis has been summarized.

Many important aspects of electron-transfer reactions have been reviewed and current topics of research in this field have been highlighted. The possible application of electron-transfer processes in solar energy conversion has been considered and a comprehensive theoretical description of ultrafast electron transfer has been presented. This latter review covers most areas of electron transfer in condensed phase. A comparison of through-space and through-bond electron transfer has been made while additional interest has been given to the effects of separation distance on the rates of intramolecular electron-transfer events. The effect of the conformation of the spacer group on the rates of through-bond electron transfer has been considered for σ-bonded systems in solution. The similarity of electron transfer and exciplex chemistry has been noted. The theory of electron-transfer processes has been applied to ultrafast intermolecular reactions occurring in polar solvents. Light-induced charge separation has been reviewed for small clusters, bichromophoric molecules in the gas phase, and solvent-free donor-bridge-acceptor systems. The use of emission spectral data to estimate electron-transfer barriers and rate constants has been described in some detail. A comprehensive review has considered the importance of coherence and adiabaticity in ultrafast electron transfer while the effects of vibrational dynamics on the rates of non-radiative deactivation have been considered.

The stereodynamical aspects of light-induced bimolecular reactions, as studied by way of Doppler-resolved polarized laser pump-probe techniques, have been reviewed. Recent advances in theoretical and experimental understanding of ultrafast solvation processes have been highlighted with special reference to the role of high-frequency vibrational modes. The photoinduced electron-transfer reactions of cyclopropane derivatives have been described while the time scales for electron-transfer processes occurring in J-aggregates formed at AgBr surfaces have been analysed in terms of theoretical models. Various types of artificial antenna systems have been described, the photochemical reactions of stilbenes and related heterocyclic derivatives have been reviewed, and the photophysical properties of tetrapyrrolic pigments have been summarized. Special attention has been given to the photodissociation of NO from nitrosyl metalloporphyrins.

Considerable effort continues to be expended in the study of the photophysical processes taking place in transition metal complexes, especially with regard to the possible construction of light-activated molecular-scale devices. The photo-induced energy- and electron-transfer processes occurring in rigidly-linked Ru/Os complexes have been reviewed in considerable detail. Similar attention has been given to the light-induced electron-transfer reactions taking place in metal-organic dyads and in related supramolecular assemblies. The rational design of molecular devices based on luminescent transition metal complexes has been considered while the photophysics of transition metal complexes bound to nucleic acids have been reviewed. This latter study has concentrated on trying to identify new types of specific luminescent probes for DNA. A comprehensive review deals with pressure tuning of the photochemical properties of transition metal complexes in solution.

The technique of transient grating spectroscopy has been reviewed, with particular emphasis on its application to monitoring non-radiative deactivation. A unified theory of time-resolved fluorescence anisotropy and Stokes shift spectroscopy has appeared. A separate review has considered the chemical and photophysical events occurring from upper excited states as accessed by multiphoton absorption techniques.

3 Kinetic and Theoretical Considerations

The measurement of kinetic parameters remains a very important part of photochemistry and there have been several recent attempts to improve our understanding of the dynamics of complex processes occurring in condensed phase. A new mathematical expression has been formulated to explain the kinetic processes inherent to certain types of photochromic systems. A treatment has been advanced to account for reversible diffusion-influenced complex formation, as studied by laser flash photolysis. The line shapes of a two-level resonance fluorescence system, subject to stochastic collisions, have been analysed in terms of the Rabi frequency. A theory has been proposed to explain photo-induced nucleation in one-dimensional systems. The effect of quantum beats on the recombination kinetics of radical ion pairs has been considered. Various algorithms based on Prony's method have been proposed for the determination of individual lifetimes from dual-exponential decay curves. This non-iterative method has been applied to the real time study of quasi-distributed temperature sensors that operate by way of time-resolved fluorescence spectroscopy. A new power law dependence has been suggested for the long-time behaviour of reversible diffusion-influenced reactions. A theoretical model has been introduced to account for rapid dephasing relaxation effects. The observed non-exponential kinetics for delayed fluorescence in Langmuir-Blodgett films has been analysed in terms of a percolation model while reversible fluorescence quenching has been treated through a series on non-Markovian generalised kinetic expressions. A model has been developed to explain the non-exponential decay kinetics associated with ultrafast electron transfer in bimolecular systems. Kinetic models have been expressed to account for fluorescence quenching in sol-gel xerogel transitions, doped glasses, and inter-particle interactions. A kinetic analysis has been made of the fluorescence behaviour of rigid and non-rigid dye molecules under lasing conditions. The transport kinetics governing triplet-triplet annihilation in solid media have been described while the kinetics of fluorescence polarization in solid bichromophore molecules under intense illumination have been explored. This latter study attempts to relate the excited state lifetime to the intensity of the excitation pulse. A unified theory for the kinetics of bimolecular photoionization followed by geminate recombination has been proposed. Fluorescence quenching rate constants have been analysed under conditions where a hopping mechanism is expected to play an important role and the heats of formation of radicals in solution have been discussed in terms of a solvation model.

A critical review has assessed the value of quantum dynamical simulations for modelling ultrafast processes in polyatomic molecules. A semi-classical regime exists for the dynamics of vibrational relaxation following excitation by an ultrashort laser pulse. The molecular mechanics valence-bond method has been applied to the problem of understanding molecular structure and photo-physical properties of polyatomic species in solution. Theoretical descriptions have been proposed for energy pooling among three-centre systems, for the suppression of resonant dipole-dipole interactions, and for intramolecular energy-' and electron-transfer processes. Rate constants for electron transfer in protein matrices have been analysed in order to derive more information about the electronic factor and about the importance of nuclear tunnelling. A theory has been advanced for electron-transfer processes that involve multi-dimensional solvation dynamics. A generalized theory for superexchange interactions has been proposed and applied to exchange-coupled pairs. Theoretical studies have addressed problems related to light-induced charge separation, the electronic factor in photo-induced electron transfer, and re-organization energies accompanying electron transfer. A critical comparison has been made of the Rehm-Weller and encounter complex models for bimolecular electron-transfer reactions. A new methodology for describing distance dependence effects in radical pair reactions has been proposed. Semi-empirical studies have addressed the issue of solvation dynamics associated with light-induced charge-separated states. Other theoretical studies have examined the emission properties of charge-transfer states and exchange interactions in transition metal complexes.

The theoretical basis of sonoluminescence has been reviewed and a method has been presented to predict the absorption spectral band shape of polar dye molecules. A related treatment can be used to explain the fluorescence spectral profiles of aromatic amino acids in different states of ionization. Theoretical calibratrion curves have been compiled for typical fluorescence-based temperature sensors. Theoretical studies have also addressed various issues relating to molecules that undergo a large change in dipole moment under illumination.' Electronic energy levels have been calculated for ds-stilbene, substituted benzofurans, isoquinolinium cations, and small aromatic heterocycles. Numerous theoretical calculations have been directed towards understanding the role of torsional motion in controlling the rates of isomerization and/or non-radiative deactivation."

4 Photophysical Processes in Liquid or Solid Media

A tremendous number of publications continue to appear that report photophysical properties of individual molecules measured under a wide variety of conditions. Many reports follow standard lines and present little or no new properties – merely being concerned with examining a new derivative of a well-known family. Page restrictions preclude coverage of such work and require that attention be focussed on emerging trends in photophysics.

4.1 Detection of Single Molecules – The spatial resolution needed to isolate and detect fluorescence from single molecules has been reviewed and a theory for single molecule detection has been advanced that accounts for a simple two-state system. A Monte Carlo simulation of single molecule fluorescence has been presented. Fluorescence from single molecules embedded in host crystals has been described for several different systems and the essential experimental methodology needed to monitor single molecule fluorescence has been improved and revised. Spatial photoselection of single molecules on surfaces has now been reported. The importance of the triplet excited state for single molecule detection has been stressed and the effects of photobleaching have been described. Spontaneous emission from a single molecule has been considered in terms of a Monte Carlo approach while the reasons for fluorescence intensity fluctuations associated with single molecule detection have been explored.

4.2 Radiative and Non-radiative Decay Processes – The dynamical response of a irans-polyacetylene chain to excitation with an ultrashort laser pulse has been interpreted in terms of a charged soliton having a lifetime less than 250 fs. Related studies have explored the photophysical properties of diphenylacetylene and diphenylbutadiyne, with emphasis being placed on the importance of the SB2-SB1 energy gap. The photophysics of substituted polyenes and diphenylhexatriene have also been studied by ultrafast spectroscopy. The photophysical and lasing properties of several types of laser dye have been studied in polar solvents.' Vibrational relaxation within the Si level of azulene has been monitored and the photophysics of aromatic thioketones supported on cellulose have been probed by laser flash photolysis. The photophysical properties of several derivatives of tyrosine have been recorded in solution and interpreted in terms of theoretical conformational studies. There is considerable interest in developing new fluorescent reagents based on the benzofuran nucleus and the photophysical properties of several analogues have been reported. Photophysical data have been described for 9,10-anthraquinone-2-sulfonate, 4-chlorotoluene, ethidium bromide in micellar media, angelins and thioangelins, and 2-aminooxanthone. Excimer emission has been reported for pyrene-derived cyclophanes and for microcrystalline polycyclic aromatic hydrocarbons dispersed in water.

Metalloporphyrins and related compounds continue to attract attention as photosensitizers and as building blocks for construction of artificial light-harvesting arrays. The photophysical properties of several structurally modified porphyrins have been measured with a view to identifying new sensitizers for use in photodynamic therapy. The triplet state properties of porphyrins adsorbed onto the outer surface of vesicles have been described and the fluorescence spectral properties of some amphiphilic porphyrins have been recorded. Similar studies have been carried out with halogenated tetraarylporphyrins. The acretion of individual porphyrin units into larger arrays has been achieved in several cases and the photophysical properties of the final assembly have been described.

Environmental effects, especially changes in solvent polarity, on the photophysical properties of dyes have been described. Likewise, solvation dynamics have been measured for dyes that undergo a substantial increase in dipole moment following excitation." Light-induced intramolecular proton transfer is an important route for non-radiative deactivation of an excited state and has been studied extensively in recent times." Double proton transfer has been reported for [2,2'-bipyridyl]-3,3'-diol and for 7-azaindole.

The photophysical properties of singlet molecular oxygen, O2(1Δg), have been recorded in a variety of solvents and the importance of charge-transfer interactions on the rate of radiative decay has been noted. An attempt has been made to analyse these solvent effects and also to account for solvent-induced variations in the efficiency of O2(1Δg) production with phthalocyanine-based sensitizers. In this latter system, the energy gap between O2(1Δg) and the lowest-lying triplet state localized on the sensitizer is very small, allowing accurate assignment of the triplet energy of free-base phthalocyanine.

The availability of ultrafast laser spectroscopic techniques has resulted in an upsurge in the study of higher-energy excited states. Thus, weak fluorescence has been observed from the S2 level of anthracene crystals using a two-step excitation approach. Ultrafast relaxation from higher-lying excited states has been recorded for Coumarin 481 in cyclohexane, for certain phenazine derivatives, and for azulene. The latter case is very well known but a thorough investigation of this system has now been completed. Fluorescence from an organic radical cation has been detected in liquid solution at room temperature.