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...