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 described and applied to cresyl violet and rhodamine 101.

Frequency modulation and phase shift techniques are now also widely used for measurement of fluorescence decay times. Synchrotron radiation has been used with ps resolution in the near UV and its application exemplified by measurements on diphenylhexatriene (DPH) in erythrocyte ghosts. Sugar examines the use of Fourier transforms in analysis of fluorescence data and establishing relationships between models and photophysical models.

Imaging technology is now being deployed extensively and interest in fluorescence lifetime imaging is developing rapidly. A system for making lifetime selective imaging using a phase sensitive camera with a model locked cavity dumped ps dye laser has been demonstrated with samples of rhodamines 6 G and B. A two dimensional fluorescence lifetime imaging system has been described which uses a grating image intensifier. Japanese workers have constructed a three dimensional space and time resolved fluorescence system. This has ps time resolution and submicrometre three dimensional resolution. It is widely applicable to inhomogeneous systems and the performance is demonstrated with films and interfaces. In contrast a versatile flash photolysis apparatus using a conventional source has been described for photoaffinity labelling experiments.

Fluorescence lifetime measurement using fibre-optic laser scanning confocal microscopy allows spatially resolved fluorescence decay profiles to be determined for dyes in solution and polymer films with rhodamine B as a test dye. Low level photon counting has been used for polarized ps fluorescence measurement of malachite green molecules with optical upconversion in a modified Michelson set up. A Pockels cell electrofluorimeter has been used to measure changes in the polarization components for DNA/dye complexes. The technical problems of laser spectroscopy and induced photochemistry in micrometre sized samples have also been critically assessed.

A few papers describe design of new spectrofluorimeter systems. Senga and Minami describe a excitation-emission matrix spectrofluorimeter using a modified xenon lamp source. Cabaniss develops the theory of variable angle synchronous fluorescence spectra in which constant wavelength or frequency differences are not maintained. It is claimed that more spectral information is obtained when scanning is carried out in this way. A design of a fluorimeter that corrects for the effects of quenching uses simultaneous measurement of intensity and lifetime. The ratio corrects for the presence of quenching effects. A system for measuring thermoluminescence has been described also during the year.

The theory, technique, and applications of time resolved CD spectroscopy to biological systems has been described and a new technique reported for measuring chirality distributions from lifetime resolution in fluorescence detected CD and CD polarized luminescence.

A number of papers deal with interpretation of data from photophysical measurements. For example, solution dynamics can be determined by line shape analysis using resonance light scattering and fs four wave mixing. Solvatochromic probe fluorescence allows study of solvation and solvation dynamics in ionic solutions. Many details of these processes can be elucidated by such methods. The estimation of pKa* of states is illustrated by a simple teaching experiment that explores acid-base properties of excited states.

The measurement of quantum yields for molecular systems capable of interconversion by both light and heat:

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has been described. This analysis is applicable, for example, to reversible cycloaddition and photochromic interconversion reactions.

The use of some β-carboline derivatives as fluorescence standards has been proposed and a chemical actinometer in 250-270 nm region described. Technical improvements in obtaining data from flash photolysis and pulse radiolysis experiments are given in two papers by Janata.

Estimation of detection limits in laser induced fluorescence in ultra-trace analysis is another topic discussed together with a detailed summary of requisite theory.

The eighteenth paper of the unique series by French workers on industrial photochemistry deals with improvements of photochemical conversion rates by use of fluorescence relays acts by simulating an optical fibre effect.

A whole issue of the journal Chemical Physics, is concerned with magnetic field effects upon photophysical and photochemical phenomena. Of the twenty papers in this compilation nineteen fall within the field of review and are not cited elsewhere in this article.

2 Singlet State Processes

Some problems in photochemistry continue to stimulate further analysis and new experiences. Such is the case with excimer formation in pyrene. The mechanism proposed by Martinho et al. has been examined for pyrene in cyclohexanol and avoids the approximations implicit in previous models such as that due to Birks. A lifetime recovery scheme, which is applied to this reversible excimer system, gives participating excited state lifetimes which are independent of any assumptions made in formulation of the mechanism. Pressure effects can aid mechanistic studies and rate constants for pyrene excimer formation have been measured by the time resolved fluorescence in six solvents at pressures up to 300 MPa. An intriguing study is the steady state and time resolved fluorescence investigation of pyrene excimer formation in supercritical carbon dioxide. Solvatochromic shifts of pyrene excimer fluorescence in polar and nonpolar solvents shows that solute-solvent interactions influence coupling of S1 and S0 states. Ps studies have furthered examination of the details of excimer formation processes in pyrene crystals.

Controversy is still being sustained over the nature of excimer fluorescence from 1,3-di(1-pyrenyl)propane. Zachariasse et al. state that there is no evidence, contrary to the opinion of other workers, for lifetime distributions from ps laser experiments with ns time resolution, and they consider that the maximum entropy method for data analysis used in earlier work is unsatisfactory since they maintain that it yields distributions even for discrete two species exponential data. Further work is to be anticipated to resolve controversy on this subject.

Another currently topical problem is derivation of excited state data on buckminsterfullerene and its derivatives. Fluorescence emission study of solutions of C60 and C70 in toluene show excitation at 525 nm gives rise to emission between 600 and 850 nm with lifetime of 1.17 ± 0.02 ns for C60 and 0.66 ± 0.02 for C70. A theoretical study of low lying excited states of C60 has examined the properties of derived anions up to the hexanion.

A unique report provides data on the measurement of radiative lifetimes of species that arise from confined excitation in sigma conjugated silane oligomers. An analysis of the decay of fluorescence anisotropy of 2,4,6,8-decatetraene in viscous hydrocarbon solutions shows the off axis orientiation of the transition dipole moment. cis- and trans-Decalin excited at 161 nm have both been used to sensitize benzene fluorescence. From the quenching of decalin conformer emission the measured efficiencies of exciting benzene are 0.44 and 0.33 respectively for the cis and trans-isomers. The low conversion efficiencies indicate that excited state population of benzene during the quenching encounter is of limited efficiency.

Fluorescence spectra of radical intermediates in the photolysis of phenylsilane and phenylmethylsilanes have been obtained at 77 K.

A fs study shows the vibrational motion in excited pyrrole molecules as well as fs optical Kerr effects in the neat liquid and ether solution. Other studies on nitrogen ring substituted compounds include the effects of conformation on the luminescence of 2,2'-bipyridine and 2,2'-biquinoline, fluorescence of 2,2'-bipyridine in aqueous solution, the influence of hydrogen bonded dimers on the luminescence of 2-pyridone, ππ* excited states and hydrogen bond controlled internal conversion in pyridones, and the electronic absorption and fluorescence of dipyridamole.

all-trans-1,3,5-Diphenylhexatriene is another example of a system that continues to produce new and interesting information. Solvent effects on radiative and nonradiative transitions have been subjects of further work. A very significant report on this system is the new evidence adduced for ground state s-cis conformers in the fluorescence spectra of DPH. Temperature and wavelength dependence effects show 6-25% of the cis form for which the calculated AH difference is 14.2 ± 1.6 kJ mole-1. The cis-form is apparent in red shifting of the fluorescence emission.

A number of studies have been carried out with a variety of carotene compounds. The molecular dynamics of β-carotene in solution have been measured by the sub-ps transient optical Kerr effect. The all-trans isomer after absorption decays to the S1 state with a lifetime of 8 ps. Relaxation of vibrational energy is also reported in the ground state. Ps fluorescence and coherent anti-Stokes Raman spectroscopy have also been used to study the electronic relaxation in these systems. Analysis of solvent effects on relaxation processes in carotenoids shows dual emission characteristic of fluorescence from both the S1 and S2 states of carbonyl containing compounds. Vibrational relaxation of carotenoids in vivo and in vitro using ps time resolution and anti-Stokes resonance Raman spectroscopy shows vibrationally excited 1Ag levels are involved. Absorption spectral shifts of carotenoids relate the 1Ag [right arrow] 1Bu transition to the polarizabilty of the solvent medium. Polyene 21Ag and 11Bu states and their involvement in the photochemistry of previtamin D3 have been investigated by observing the effects of exciting wavelength variation and the correlation between fluorescence and chemical properties.

Fs vibrational relaxation studies of azulene using incoherent light in ethylene glycol shows that relaxation time depends on the wavelength of excitation. Values are 50 fs at 617 nm and 300 fs at 648 nm.