Part I

Physical Aspects of Photochemistry

By Robert B. Cundall

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

Research on photophysical problems remains at much the same level as in past years. Increasingly the work reported deals with applications of photochemical techniques to resolve problems in other areas of chemistry and biology rather than the strict examination of the basic and elementary photophysical processes. No doubt this is a consequence of the progress which has been made in the subject over the past decades.

1 General

Books and reviews on the subject have not been as much in evidence as in previous years and papers of general theoretical relevance have certainly not been prolific. This reflects the trend, mentioned above, of photophysical papers being diverted towards the use of techniques for the elucidation of problems in other areas such as, for example, colloid science.

In the journal Analytical Chemistry there has been published the regular classified citation of literature on molecular fluorescence, phosphorescence, and chemiluminescence spectrometry. In 1995 many papers of photophysical interest have appeared in a one-off issue of Analytical Spectroscopy Library.

A timely review of a newer quantum mechanical approach to photochemistry involves the method of conical intersections and deals specifically with cycloadditions, polyene isomerizations, and di-π-methane rearrangements. The role of conical interactions of potential energy surfaces in photodissociative processes has been exemplified by consideration of the photochemical properties manifested by hydroxylamine. A CAS-SCF mechanistic study of the cascade through a singlet manifold has been made for the case of the photochemistry of hexa-1,5-dienes. High level quantum mechanical calculation has also been carried out on the nature of radiationless decay of the excited states of conjugated hydrocarbons. Zimmerman and Kutateladze have re-examined the nature of singlet triplet spin-orbit coupling and its manifestations in both photochemistry and diradical chemistry. This article covers one of the key effects in the subject.

An extensive and useful review has dealt with all aspects of photoreactor design and analysis of performance together with a discussion of applications for laboratory and large scale use. Modelling of the consequences of absorption and scattering effects in a flat plate photocatalytic reactor provides a systematic examination which is applicable to the increasing use of solid-liquid heterogeneous reactors in photochemistry.

Bandyopadhyay has used molecular dynamics and the many body Smoluchawski equation approach to examine the consequences of light intensity effects on diffusion influenced fluorescence quenching using a hard sphere liquid model when time dependent behaviour of luminescence is being analysed. A few other papers appearing during the year deal with the quantitative interpretation of experimental data. A method for measuring emission spectra from systems which are only partially transparent and also partially reflecting has been described and should be valuable for the many experimental systems encountered in practice. Fluorescence from highly excited electronic states can be used, as demonstrated, to determine the effective conjugation threshold for complex organic molecules in solution. This is a measurement often needed for the description of phenomena in many areas of physical chemistry. A two-step competitive solvation model has been applied to the interpretation of luminescence spectra of polycyclic aromatic hydrocarbons in binary solvent systems. An extension of earlier models which have been proposed is involved in this paper. Some further work on excimer formation and properties has been reported. Saigusa and Lim have analysed excited state dynamics in aromatic clusters. They establish that a correlation exists between exciton interactions and excimer and propose a possible role for an S exciton interaction in the process. Excimer spectra observed from aromatic hydrocarbons have been generalized and analysed and some correlations with molecular structure established.

The analysis of fluorescence decay data is still a major topic of investigation and appraisal. Recently attention has been given to the further development of fluorescence lifetime-based sensing and imaging; this relatively new technique and its application has been a subject of review. An authoritative and comprehensive survey of the properties and characteristics of single-photon timing detectors for fluorescence lifetime spectroscopy covers all aspects of this important subject.

Aspects of data treatment reported include a new method for the high speed deconvolution of multiple fluorescence decay, determination of biexponential fluorescence lifetimes using simulated annealing and simplex searching (an outline of the instrumentation used and mathematical procedures is given), the kinetics and identifiability of an intermolecular two state excited process in the presence of a fluorescence impurity provides a scheme for the recovery of the relevant rate constants, and a comparison has been made of simultaneous biexponential and compartmental analyses of fluorescence decay surfaces of intermolecular two state excited state processes.

The maximum entropy method is one of the relatively more recent and controversial techniques for the analysis of data on complex fluorescence decay systems. An examination of the influence of noise and analysis parameters on the determination of the width of distribution of lifetimes has been claimed to show that this method does not necessarily provide a good indicator of heterogeneity.

Two papers by Shaver and McGown also on the application of the maximum entropy method to frequency domain fluorescence lifetime analysis examine instrumental aspects and deal with the effects of frequency range and random noise and timing, mismatched intensity, and reference lifetime errors; and conclude that the maximum entropy technique has many advantages in practice. Fast Fourier transformation is also reported in its use for the analysis of data involving...