Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY By R. B. CUNDALL

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The review is presented in the same format as in previous years. The emphasis of research in this period has been on the detailed study of particular systems in most of the published work, rather than many extensive accounts of the discovery of new systems or techniques.

1 General

Reports have been published on the proceedings of several conferences dealing with various aspects of photochemistry. An example of special photophysical interest is the collection of papers on excited state processes in condensed matter presented at the International Conference on Luminescence in Beijing in 1987. Another noteworthy example is the group of papers from a discussion meeting on the use of lasers in the life sciences.

Theoretical papers of direct photophysical relevance have been, as in past years, comparatively few and, as remarked in previous surveys, no particular themes command extensive coverage. A translation of a paper by Plotnikov describes results of theoretical studies on the mechanism and rules governing unimolecular processes for formation of free radicals by the unimolecular photodecomposition of organic compounds. Two papers discuss the determination of transition moment directions in molecules of low symmetry by use of polarized fluorescence. This method has been applied to the examination of dyes in polymer films, including pyranine, perylene, and diphenylhexatriene (DPH). It is highly questionable whether the DPH molecule when excited has the high cylindrical symmetry which is usually assumed when this compound is used as a fluorescent probe. Two other papers describe a symmetry based procedure for determining molecular geometry changes following electronic excitation. An extension of the Bader-Pearson concept has been applied to the examination of relaxation pathways for the molecules ethylene, acetone, butadiene, and methyleneimine. Stevens and McKeithan have made a theoretical analysis of static fluorescence quenching in the finite sink approximation and compared their predictions with experimental data. Another paper in this category examines details of the kinetics of partly diffusion controlled reactions in which long distance electronic energy transfer competes with collisional quenching. In many other papers theory is developed for the interpretation of reported experimental data. An intriguing example of wide interest is the chiral discrimination of excited states by measurement of time resolved circular polarised luminescence. The measurement of polarised fluorescence emission from probes near dielectric interfaces is relevant to lipid membrane research but widely interesting in view of possible applications to the study of surfaces generally.

A highly topical review has appeared which deals with magnetic field effects in chemical kinetics and related phenomena. Braslavsky reviews the application of photoacoustics to biology in another timely article.

Descriptions of equipment still maintain fairly extensive literature coverage. Fluorimeters even still merit description in the journals. A simple instrument with a compact inexpensive nitrogen laser source with a good performance could prove useful for teaching of photophysics. In contrast to this, is a report on the design and evaluation of an all purpose dual multichannel detector spectrometer which allows simultaneous observation of absorption and luminescence.

Ultra-low detection limits for an organic dye (IR-140) of about 4.6 x 104 molecules have been achieved by a fluorescence spectrometer using laser diode excitation. Techniques used for imaging have considerable potential for use in observing the photophysics for very small and heterogeneous samples such as biological cells. For example, Jovin and his coworkers have exploited studies of resonance energy transfer and delayed luminescence by use of digital imaging microscopy. This type of experiment offers considerable potential for research in biology and medicine.

The most important aspect of contemporary instrumental development in photophysics is concerned with methods for use in ultra-short time resolution spectroscopy. Lin and Fain have applied the theory of two dimensional spectroscopy to real time femtosecond transition state spectroscopy. This is a method which is now applicable to dense condensed phases. Ultrafast emission spectroscopy in the UV is obtained by time gated up-conversions. Measurements using full width ps pulses gives a minimum of 300 fs full width half maximum response function. From measurements of Raman to fluorescence intensity ratios it is possible to estimate ultra-short excited state lifetimes. Values have been obtained for crystal violet (1.2 ps), malachite green (0.8 ps), and β-carotene (0.15 ps). A gallium arsenide detector for ps photon counting has a wide dynamic range and is reported to be particularly useful for long wavelengths as demonstrated between 766-772 nm.

A diffuse reflectance laser photolysis study of the ps dynamics of excited states in organic microcystals has been reported for the first time. This is certain to be a fruitful field of investigation.

A picosecond flash photolysis apparatus with high absorption resolution using fibre optics in the probing pathway has been used to measure the S1 state absorption of erythrosin and rose bengal. An image intensifier coupled with a linear position-sensitive detector has been used to record absorption and emission spectra extending from the ns to ms time regimes. Johnson et al have also described a ps time-resolved spectrometer with a remarkable expanded delay range which covers nine decades of time (ps to ms). This, it is claimed, will be particularly useful for the examination of complex systems such as proteins. The first detection of singlet oxygen luminescence using pulse radiolysis with a germanium diode detector offers considerable promise.

Techniques for the analysis of fluorescence decay data continue to be refined. Beechem has produced a second generation global analysis program for recovery of...