Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The study of photophysics is an active area of research although the survey indicates that most work is now concentrated in parts of the world where financial support for the necessarily expensive equipment for popular problems is available. Major new discoveries are not much in evidence and most published work is concerned with detailed analysis of systems selected for reasons decreed by fashion or possible technological applications. Systems relevant to biology probably provide the problems in which photophysical methods are exploited most fully.

1 General

A survey of photochemistry over forty years by Sir George Porter which concentrates on time scales of milli- to attoseconds as a means of delineating experimental effort provides an excellent start to the survey and modern techniques are exemplified by a full description of picosecond flash equipment in use at the Royal Institution.

Theoretical studies are not profilic and appear to be diverse in subject. The effects of tunnelling and the distinction from thermal activation have been analyzed by Formosinho and is almost unique in its consideration of primary processes. The proximity effect and the consequences of the pseudo-Jahn Teller has been applied to nπ* and ππ* states of heterocyclic and aromatic carbonyl compounds by Lim. Other theoretical studies deal with the application of Marcus theory to photochemical proton transfer, the concentration gradient model of fluorescence quenching, the effect of coulombic interaction on fluorescence quenching by ions, the theory of photoisomerization of impurity molecules in solids, time delayed two photon processes, and solvent effects on the absorption intensities of nπ* and ππ* transitions. The linearity of free energy relationships for excimer formation is examined in an attempt to correlate very extensive data. Razi-Naqvi has critically approached the correction of fluorescence depolarization with anisotropic rotational diffusion. Line broadening and the red shift of fluorescence as solvent relaxes around the excited state provide a probe of solvation dynamics. A theoretical model for this has been proposed.

Other papers deal with applications of different experimental techniques. Possibilities for examining nuclear spin coherence transfer in photochemical reactions have been assessed. The nature of excited states of solute molecules and their coupling with mechanical degrees of the medium can be probed by picosecond photon echo experiments. Solvation dynamics can also be elucidated by combined fluorescence and Raman studies as exemplified by results from appropriate model systems and bacteriorhodopsin. Lewis has reviewed proton transfer effects in photogenerated radical ion pairs. An article with 144 references examines the Shpol'skii effect as an analytical tool. Drickamer describes how high pressure optical absorption can be used to understand electronic interactions and excitations in molecular crystals and solutions. Orvaeskii examines the scope for selective laser initiation of a number of interesting effects. A wide field is covered and he puts forward suggestions for detecting the formation of solitons in nucleic acids. The influence of magnetic fields on fields excited singlet and triplet states in irradiated organic molecules has been discussed by Brocklehurst and his coworkers.

Development of experimental techniques largely dictates the progress of photophysics. For example, resonance Raman studies make it possible to examine the femtosecond ring opening dynamics of excited 1. 3-cyclohexadiene. The technique is shown to be particularly useful when luminescence is weak and other spectroscopic methods are inconvenient. An amplified colliding pulse mode locked laser system has been described which is capable of subpicosecond resolution and has been used to measure microscopic friction in the reorientation of trans-stilbene. A specialist issue of Photochemistry and Photobiology has dealt with the application of synchrotron radiation to spectroscopy and photobiology. The facility at the Brookhaven National Laboratory is described in detail. Improvements to widely used techniques are exemplified by description of a flash lamp probe source and a fast time resolved infrared detector for solution chemistry.

A wide ranging review of the principles and applications of fluorescence techniques in biophysical chemistry has been made by Geacintov. A survey of chemical microscopy includes section on fluorescence techniques. This indicates the development of an extremely useful methodology, especially in biology. An instrument for the simultaneous acquistion of fluorescence spectra and lifetimes from single cells or organelles with a time resolution of better than 300ps has been described. A single photon counting Fourier transform microfluorometer uses multifrequency phase shift techniques. A technique for obtaining fluorescence vivo uses an intensified photodiode array. A review of fluorescence correlation spectroscopy and photobleaching recovery shows the advantages of using a combination of the chemical specificity of fluorescence and high spatial resolution of microscopy. A fluorescence spectrometer applicable to single electrodynamically levitated microparticles has also been described.

Improvements in fluorescence techniques continue to be produced. Fast Fourier Transform analysis of luminescence decay curves using microcomputers in the case of nonexponential decay has been published. The global analysis of fluorescence decay has been extended to excited state reactions and ground state heterogeneity.

The determination of rate constants from fluorescence decay surfaces is also described in detail. A microcomputer based system for collecting and processing multidimensional fluorescence data has been developed. A time resolved multiphoton counting method has less time resolution than single photon counting but is advantageous for measuring...