Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

By R. B. CUNDALL

BY R. B. CUNDALL

Published research on this topic is very largely concerned with the observation of effects occurring in the ultra-short time regime and their detailed interpretation. In spite of this activity it is probably the extensive work on electron transfer which is of greatest relevance to the general field of photochemistry. It is not evident from published literature that industrial applications of photochemistry, apart from the fields of polymers and polymerization, and technological use of photophysics have so far achieved some of the earlier optimistic chains.

No clear new areas of interest are emerging and the subject is now in a steady state where the various very fast reaction techniques are being used to examine competently elementary processes in specific systems.

1 General

A number of research monographs cover a variety of topics. Two new volumes edited by Rabek have relevant articles dealing with magnetic field effects, semiconductors, luminescence probes, and biological membranes amongst others. A series of useful volumes entitled Topics in Fluorescence Spectroscopy edited by Lakowicz has been commenced with two volumes containing articles which are the concern of this review. Dewey has surveyed biological and biochemical aspects of fluorescence spectroscopy in an edited monograph. Time resolved optical spectroscopy and photochemistry of organized and constrained media are two subjects of topical interest that have been reviewed.

A unique monograph by Braun, Oliveros, and Maurette on industrial photochemistry is concerned with process technology and different types of reaction. Kinetics and system design are discussed in detail.

Biochemistry provides the most extensive area of science for the application of all forms of luminescence spectroscopy. The monograph by Hemmila on the use of fluorescence in immunoassay illustrates this. Laser application in biology forms the subject of a recent SPIE conference. Many papers of photophysical interest are included in this report, notably many deal with femtosecond techniques. The two year review of fluorescence, phosphorescence, and chemiluminescence spectroscopy in Analytical Chemistry provides the usual classified, but daunting reference collection covering a two year period.

Theoretical papers cover only a limited range. The application of recently developed computational methods to the understanding of spin forbidden radiationless decay in polyatomic systems has been discussed by Yarkony. A very significant review of the methodology and computational techniques which can be applied to the molecular orbital treatment of excited states is of general interest. In a very much more restricted area, the theory of fluorescence depolarization in nonlinear molecular pairs has been analyzed by Gulen. Recognition of nonlinearity effects has become a feature of modern science. An examination of their role in photochemical systems shows the possibility of multiple steady states, oscillations, and nonlinear Beer's law behaviour. Such effects should be widespread since photochemical reactions are straightforward examples of out of equilibrium systems where nonlinearity results in formation of structured dissipative structures. Lerf and Suppan have examined hydrogen bonding in solvatochromic shifts. In mixtures of two solvents the solvent shell becomes enriched in one component producing distinguishable effects on both absorption and fluorescence. A detailed analysis of fluorescence anisotropy in systems with reversibly interacting fluorophores shows the role of rotational and translational diffusion in interactive complex formation.

The theoretical calculation of natural radiative lifetimes from absorption and fluorescence properties is well known as a useful aid to understanding the mechanism of photochemical reactions. Three methods for doing this are presented in an article by Bolton and Archer and exemplified by application to gallium arsenide and rhodamine 6 G.

An experimental test of the models used to study rotational diffusion in aminoanthraquinones using fluorescence has been put forward to compare predictions from the alternative hydrodynamic and dielectric friction theories.

Progress in photophysics depends upon the development and improvement of both existing and new techniques. Only a selection can be cited. A stroboscopic optical boxcar technique for determination of fluorescence lifetimes is described by James, Siemiarczak, and Ware. Malliaris reviews recent advances in single photon counting technology especially as applied to the study of molecular aggregates. Treatment of data from the single photon counting method is still subject to revision. A non a priori analysis of fluorescence decay surfaces has been used to investigate the effect of a quencher, iodomethane, on intermolecular formation of the pyrene excimer. A sine transform method is related to the earlier established Fourier transform technique and applied to single exponential decay signals. Another mathematical procedure which has been put forward involves iterative reconvolution based on estimation of a covariance matrix.

Experimental improvements to the single photon counting technique include a near IR spark source with high intensity at 750 nm which has been reported by Birch, Hungerford and Imhof. A microchannel plate photodetector can give simultaneous spectral and temporal resolution as shown by coupling with a ps laser system. Lytle and coworkers have described a system with very fast collection times. Examples of acquisition times as short as 10 ms and less are reported.

A system for analysis of lifetimes of weak emissions together with time resolution of spectra using a subnanosecond dye laser and gated analog detection has been designed for use in studies on nucleic acids. A subnanosecond single photon timing system with a pulsed diode laser for 660 nm has been successfully tried with compounds of photobiological interest. A critical review of the resolution limits of two colour pump/probe has been made by Alavi and Waldeck. a state of the art system for fluorescence decay studies using a CW femtosecond dye laser with ungated inverse time-correlated single photon counting system with 120 MHz pulse repetition rate has been...