Part I

Physical Aspects of Photochemistry

By Robert B. Cundall

Photophysical Processes in Condensed Phases

BY ROBERT B. CUNDALL

Activity in the area of photophysics has certainly not diminished, although published papers on the topic tend to be more esoteric in their nature. As noted in last year's review, the nature of ultrafast processes and their role in many diverse types of photochemical systems are now being clarified. The theme of charge transfer effects remains dominant and the fullerenes still probably lead the field as far as a compound group which are at present being selected for photochemical examination.

1 General

Reviews and compilations of related papers are helpful guides to the extensive publications appearing every year. The Handbook of Photochemistry, now in its second edition, provides a valuable reference manual for the photochemist. Most aspects of ultrafast dynamics are covered in the book edited by Simon, and Zewai1 has provided an authoritative overview of femtochemistry as an introduction to issue 48 of the Journal of Physical Chemistry (1993) which provides the manuscripts of papers presented at an international conference held in Berlin. The comprehensive biennial review of luminescence which appears in Analytical Chemistry (1994) lists 477 references. Another conference proceedings edited by Wolfbeis deals with methods and applications of fluorescence spectro-scopy. Issue 50 in the Journal of Physical Chemistry (1993) which is a memorial to the late G.L. Closs has many papers of photophysical interest. A two-volume monograph describes advances in an area extending due to an improving combination of instrumental and computing techniques, namely multidimensional luminescence. The third volume in the review series edited by Lakowicz covers some highly topical biochemical applications of fluorescence spectroscopy which is very relevant to physical photochemistry. A special issue of the Journal of Biophysical Chemistry deals in sixteen papers with many aspects of time resolved optical spectroscopy over the temporal range of femto-seconds to seconds. Photophysical aspects of some condensed systems are treated in a monograph by Di Bartoli and Beckwith on the properties of excited states in solids, and also amongst the 104 papers presented at the 9th International Conference on Dynamical Process in Excited States of Solids there are a number of direct relevance to the theme of this review.

Theoretical contributions to photochemistry during the year have not been extensive and cover a diversity of topics. The theory of nonadiabatic transitions at crossings of potential surfaces is developed in a nonperturbative approach to the understanding of non-Condon effects. A purely theoretical paper analyses in detail the effect of temperature, the size of the energy gap, and distortion of the potential energy surface on the efficiency of photoinduced electron transfer. The influence of environmental polarity and molecular rigidity on the absorption and fluorescence parameters and intersystem crossing rates has been examined for an extensive selection of aromatic molecules. Smoluchowski theory has been extended by Molski to include the effect of unimolecular processes which can occur in fluorescence quenching. The effect of both static quenching and light pulse intensity on time dependent fluorescence quenching is very significant for the analysis of experimental data in time resolved systems.

The influence of fluorescence depolarization in isotropic solutions and ordered systems is assessed in the context of time resolved polarized fluorometry and the steady state optical polarization anisotropy of rod-like molecules undergoing torsional twisting has also been examined. Intramolecular rotational effects on radiationless transitions have been exemplified by a detailed examination of the specific case of pyrazine and then extended further by observing the effect of external magnetic fields on transitions in typical azines.

Theory related and essential to the utilization of experimental techniques is developed in a number of papers. These deal with the determination of dipole moments in excited states by fluorescence in electric fields', the relation of solvatochromism to dielectric constant of solvent, and femtosecond Fourier-transform spectroscopy of inter-molecular motions in weakly interacting liquids. The theory and experimental aspects of non-Markovian optical dephasing dynamics in liquids are investigated by fs transient absorption spectroscopy for the particular case of an infra-red dye in ethylene glycol. The theory for examining optical coherent transients observed with fast and ultrafast optical diphasing is discussed by Osadko. Third order polarization effects seen in fs polarization spectra can be treated by a density matrix.

A number of new experimental techniques have been described. These include a photobleaching method for measuring ultraslow reorientation near and below the glass transition for tetracene in o-terphenyl over the timescale 10-2 to 104.5 s. A laserdrop method has been used for multiple photon chemistry with a pulsed laser for following the reactions of diphenylmethyl and cumyloxyl radicals. A new fluorescence decay measuring system with single photon counting gives both time and spectral resolution simultaneously. An on-the-fly fluorescence lifetime detection system for HPLC uses Fourier transform phase modulation to detect fluorescence lifetimes of carcinogenic polycyclic aromatic hydrocarbons. A fluorescence demodulation method for determining fluorescence decays using two beating ring dye lasers has been reported and used with rhodamine 640 and rose bengal. A new signal processing scheme has been used for the phase-locked detection of different fluorescence lifetimed, error analysis for the measurement of fluorescence lifetimes in ultradilute dye solutions has been re-examined, and the mathematical technique used for recursive convolution analysis of decay data has been extended further. Data processing of experimentally determined fluorescence decays involving the global analysis of unmatched polarized fluorescence decay curves has been used with solutions of resorufin and cresyl violet in 1-propanol and a new reconvolution analysis...