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 adopted last year. The first section deals with the general aspects of photophysical properties of molecules in condensed phase, with particular emphasis being given to supramolecular systems. This is followed by a review of progress made in the theoretical description of photophysical events and of the kinetic models used to describe photophysical processes taking place in solution. The third section reviews the many different types of photophysical event that might accompany deactivation of an excited state while a separate section is concerned with possible applications for molecular photophysics. The final section describes advances made with instrumentation and data analysis. Regretably, shortage of space prohibits a full listing of all the relevant literature that has appeared during the review period.

2 General Aspects of Photophysical Processes

The complementary features of laser flash photolysis and pulse radiolysis have been reviewed while the application of electron spin polarization techniques to the measurement of molecular photophysics in solution has been highlighted. The fundamental aspects of luminescence and chemiluminescence have been considered in detail and particular attention has been given to the theory of spontaneous emission. A comprehensive review of the dynamics of the fluorescence Stokes shift has appeared and the underlying theory for radiative migration of electronic excitation energy has been considered by special reference to the fluorescence of Rhodamine 101 in ethanol. Attention has been paid to the possibility of extracting a statistical temporal signature of quantum chaos from time-resolved fluorescence decay curves while other studies have concentrated on the implications of gelatin fluorescence for photography. The technique of non-resonant two-photon fluorescence spectroscopy has been reviewed. This is a new field of laser spectroscopy, having particular relevance to the in situ study of biological molecules, that can be readily adapted for examination of two-photon anisotropy of large molecules dispersed in membranes.

The mechanism for light-induced hydrogen atom abstraction by n,π* excited states has been considered while separate studies have addressed the issue of ultrafast proton-transfer reactions. Application of femtosecond spectroscopy to solvation and electron-transfer dynamics continues to be an attractive subject. Recent attention has focussed on the dynamics of geminate recombination in charge-transfer complexes and in geminate radical pairs and of nonergodic reactions. Theoretical models have been presented to account for the selectivity of organic singlet and triplet photoreactions. Other studies have considered the implications of vibronic coupling for light-induced electron-transfer processes occurring in multicomponent supramolecular systems. Marcus theory has been applied to electrochemiluminescence and quantum beats have been reported in the recombination of radical ion pairs, as caused by hyperfine interaction in the radical anions.

Interest has been shown in the development of protecting groups that can be cleaved by photochemical electron-transfer reactions and in the use of polarized light to attain stereocontrol of reactive encounters. The possible discrimination between enantiomers by way of electron-transfer quenching of excited states continues to attract attention, as does the application of fluorescence spectroscopy to monitor chiral resolution of enantiomers. Several new systems have been developed for chiral recognition of targeted substrates. The interconversion of enantiomers has been monitored by time-resolved chiroptical luminescence spectroscopy while a theory has been proposed to account for the special case of circularly polarized fluorescence emanating from chiral nematic liquid crystals. Circularly polarized luminescence has also been reported from rigid complexes of chiral macrocyclic tetranaphthylamides.

Considerable attention has been given to the study of those systems in which light is used to engineer a conformational change in a large molecule. Such effects are important in certain biological systems and the effect of solvent on the conformational equilibrium of previtamin D has been described. Related studies have been devoted to following the α-helix-to-coil transformation for a series of photochromic polypeptides. Several artificial phototropic systems have been developed" in which light is used to drive a largescale conformational change. The mechanism and dynamics of the transformation have been followed for the photoejection of a guest from a macrocyclic host and of its subsequent reentry into the cavity. The photochemistry of many supramolecular assemblies has been considered in terms of energy or electron transfer between the reactive subunits. A strategy has been proposed for the construction of extended 2D and 3D arrays that display long-range magnetic ordering and for which there are interesting photophysical properties. During the past year there has been a tremendous upsurge in interest in the design of fluorescent sensors for the recognition of target molecules in solution and the field has been comprehensively reviewed. Because of its great biomedical significance, attention has focussed on the detection of nitric oxide by fluorescence techniques. The development of photozymes for water purification has been reviewed.

Attempts to better understand natural photosynthesis and to construct artificial models continue to be important areas of modern photochemistry. The photophysical properties of certain bacteriochlorophylls and carotenes have been reported while the mechanisms of electron and energy transfer in natural photosynthetic systems have been reviewed. Particular attention has been given to the design of artificial photosystems that mimic the manganese-containing enzyme of PS2 that is responsible for water oxidation in green plants. Most of these photosystems use a powerful oxidant to photo-oxidize a simple manganese complex but they lack the ability to store oxidizing equivalents that is...