Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The stream of papers dealing with condensed phase photophysics has not diminished and may in fact be increasing. The ultrafast processes attract most interest and there is a tendency now to relate these to overall reaction mechanisms rather than with the elucidation of the details of physicochemical effects. This is especially the case in the still very active study of photochemically induced electron transfer processes.

Fashion dictates that the buckminister fullerenes are probably the most actively researched group of compounds in the area and even reduces interest in the triplet state of benzophenone to a lesser role.

It is possible to speculate as to whether a final frontier has been reached with femtosecond time resolved spectroscopy. Is the role of pioneer in photophysics to be displaced by the collector of more refined data on well identified processes?

1. General

A number of extensive reviews of various topics have appeared during the year. Comprehensive surveys extending over about 600 pages under the editorship of Ramamurthy and Turro appeared in Chemical Reviews. The articles of interest to our subject will be referred to in the text. Other surveys appear in Progress in Photochemistry and Photophysics, Volumes 5 and 6, edited by Rabek. Volume 17 of Advances in Photochemistry includes two relevant reviews on the bimolecular reactivity of singlet oxygen and the photochemistry and photophysics of onium salts. A review of molecular photophysics with emphasis on effects which occur on the ps and fs time scales by Beddard is also very timely. Other authoritative reviews cover the dynamics and structure of aromatic molecular van der Haals complexes by Topp and the photochemistry and photophysics of organosilicon compounds. The recent wide ranging research in electron tunnelling effects has been surveyed and the role of such reactions in chemistry and biology described in detail.

Monographs recently published include one on the theoretical aspects of molecular photochemistry by Michl and Bonacic-Koutecky, an updated edition of Guilbault's well-known text on practical fluorescence which has emphasised environmental effects, and a timely manual on the variety of luminescence techniques which find application in chemical and biochemical analysis.

The unique series of papers on various aspects of industrial photochemistry from French workers continues with articles on three dimensional machining of polymers with lasers and effects of deformation and the techniques available for photochemical purification of water. The latter subject is an aspect of applied photochemistry which has been comprehensively reviewed by Legrini et al.

Non-linearity is a fashionable topic in all areas of science and nonlinear effects on photochemical conversion of species A into B arising from cooperative influence of photon flux, concentration of substrate, photokinetic factors, and quantum fields are analysed. Another paper on the same class of system discusses the method for determining absorption coefficients, reaction rate constants and thermodynamic data in the system and the analytical model derived was tested by simulated data and applied to photochromic transformation of dihydroindolizines.

Theoretical papers which have appeared during the year deal with a variety of topics. An analysis of the distribution of Franck-Condon transition components which occur in chemical bond formation processes in condensed systems complements observations made by ultra short time spectroscopy. Nonclassical Franck-Condon processes in nonvertical transitions are effects which are resolved in the wings of fs and ps absorption spectra. A survey of the analysis of femtosecond pump and probe spectra of nonstationary states in terms of third order non-linear polarization response to laser light pulses has been given by Pollard and Mathies. Selection rules for bimolecular photoabsorption processes in molecules are delineated by Andrews and Bittner.

Other studies on molecular processes include the time dependence of relaxation parameters in non-Debye solvents, a theory for which is applied to time resolved fluorescence data in the literature. A determination of electronic-transition energies for unperturbed molecules from solvent shift measurements is used to assign π-π* transitions for porphyrins, arenes, and polar molecules. The nature of transient effects on the mechanisms of the activationless electronic relaxation in solution is examined by Jayannavar in a generalization of earlier published work. Another extension of early theory is given in a general treatment of solvent effects in electron transfer at comparatively high temperature. The influence of solvent relaxation effects is particularly emphasized in this paper.

The analysis of Stern-Volmer quenching kinetics still requires active research. Laws and Contino have analyzed nonlinear aspects of this effect. A generalization of kinetic analysis to situations where decay is nonexponential and scavenging or quenching is time dependent and the condition where there is excitation hopping through a Markovian random walk of excitation and dipole-dipole energy transfer are examples of detailed study applicable to the refinement of available experimental data. An analysis of transient effects on the quenching of an excited porphine by methylviologen in aqueous solution is a specific example of new theory applied to the text of experimental data.

Salam has applied esoteric quantum electro-dynamical theory to an understanding of the complex phenomenon of induced circularly polarized luminescence. Evanescent fields of propagating plasmon surface polaritons are effects that can be probed by fluorescence and Raman spectroscopy. Subpicosecond oscillatory effects due to impulsive stimulated Raman scattering have been shown to interfere with the interpretation of spectral data obtained from transient absorption experiments at very short-time resolution. Ps spectroscopy of excited states using nonlinear photoemission provides a general method for measuring lifetimes of excited states in the condensed phase. The method is exemplified by experiments carried out on tetracene in which the excited state has a lifetime of 20 ps. Pulse transform transient absorption spectroscopy provides a new method for obtaining ps transient...