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 two component fluorescence lifetimes.

The advantages of Fourier transform spectroscopy for photoluminescence spectroscopy in the near infrared region are described and possible experimental pitfalls have been discussed in some detail. Reciprocal space analysis of emission and excitation spectra with the Fourier transformation method can be used to determine energy distribution also.

A method of computer-assisted analysis of rotational diffusion fluorescence is demonstrated in the case of 3,9-dibromoperylene in glycerol and used for discriminating between alternative models. The global analysis of polarized fluorescence decay curves of systems which have a residual anisotropy at long times has also been extended by a very active Belgian group.

On-line fluorescence lifetime determinations have been used in conjunction with capillary electrophoresis in the near IR region by means of single-photon counting.

Wavelength resolved fluorescence detection has been used together with capillary electrophoresis to provide high sensitivity detection capable of providing complete spectra for the separated components. The application of Monte Carlo integration to three-dimensional fluorescence spectrometry gives something of the order of a hundredfold increase in sensitivity as compared with that from data obtained at a single pH.

Another interesting analytical technique is the use of a fast scanning synchronous luminescence spectrometer of a design which is based on the use of acoustic optical filters with no moving parts operating on a µs timescale. A method for the determination of orientational order parameters of uniaxial films with a commercial 90° angle spectrometer has also been developed. The effects of varying absorbance on the determination of synchronous spectra have been overcome in a new design of fluorimeter which uses fibre optics. A simple luminescence decay and flash photolysis equipment using an inexpensive laser system is particularly suitable for instructional use.

Fluorescence microscope spectroscopy and the observation of fluorescence from single molecules are probably the most interesting luminescence techniques that have been rapidly developing in the last few years. A critical review of the detection of single molecules in liquids examines the balance of sample throughput against the signal-to-noise ratio which is the factor which ultimately determines the intrinsic sensitivity limit. The set-up which has been employed for the resolution of spectral transitions in single molecules and the time dependence of the emission using fluorescence microscopy has been described by Wild and co-workers. Micro-volume time resolved fluorescence spectroscopy has been reported using a confocal synchrotron radiation microscope. Microdroplets have practical advantages over bulk samples for ultrasensitive detection and observation of single molecule photophysics as exemplified by a report dealing with the behaviour of rhodamine molecules. Droplets are microcavities and the fluorescence of oriented molecules in cavities 3.5 to 25 pm diameter shows that observed decay rates depend upon the inverse of the cavity radius. Femtosecond transient absorption microscopy of benzil confined within single beads of porous glass also shows an effect of pore size as well as that arising from environmental polarity. Single molecule imaging by fluorescence microscopy has been achieved using very sophisticated methodology which involves inserting the microscope objective in superfluid helium and using video projection. Molecular identification can be made from the fluorescence patterns developed from single molecules by applying information theory. It appears that 350 photons can serve to distinguish between dye molecules providing that the lifetimes are different. Three-dimensional pH microprobing has been demonstrated by optical manipulation by laser trapping of fluorescently labelled polyacrylamide particles. This is a technique that should be useful for probing the structure of biological cells.

The cooling of a dye solution can be brought about by the occurrence of anti-Stokes fluorescence. This can be used to provide the basis of a thermal lensing method for the determination of fluorescence quantum yields. Lees has developed a procedure for measuring quantum efficiencies in multicomponent systems where there are overlapping inner filter absorbencies. The transient grating method has been developed to provide a method for measuring enthalpy and reaction volume changes for the particular case of the photochemical dissociation of diphenylcyclopropenone in several media.

The opportunity to control the yield of photophysical and photochemical processes by excitation with selectively delayed ultrashoot phase-locked light pulses has been demonstrated by a study of the S2 [right arrow] S1 internal conversion process in pyrazine.

There are some interesting reports on the nature of solvation processes. The determination of the solvation time correlation functions from the time dependence of Stokes shifts has been described.

Preferential solvation can be conveniently studied by the use of fluorescence lifetimes as exemplified by acridine in water-alcohol mixtures. The rotational behaviour of fluorescent probe molecules near the critical point of cyclohexane-methanol binary mixtures gives evidence for selective solvation effects. Finally steady state and time resolved measurements have been made on dye solutions in solid tetraalkylammonium salts. This provides information on behaviour in these unique highly polar environments.

Three photon (1064 mm) induced up conversion of fluorescence from PRODAN (6-propionyl-2-dimethylaminonaphthalene) shows the potential of such molecular systems for use in use in optical power limiting applications.

The detection of molecular self-assembly in solution by a combination of fluorescence and absorption has been exemplified with carminic acid (which has pendant glycosyl moieties) in glucose solution.

The Belousov-Zhabotinskii reaction is the model system for the demonstration of non-linear effects that arise in chemistry and other areas of science. Two studies of photophysical relevance on this reaction examine inner filter and quenching effects on oscillating photoluminescence observed in the cerium catalysed reaction and also on nucleation and wave propagation effects in a photosensitive gel reactor.

2 Singlet State Processes

Two collections of papers cover luminescence and excited state studies on the fullerenes and deal particularly with behaviour in the singlet excited state. Palit and Mittal review work on the photophysical and photochemical properties of the fullerenes especially dealing with the interaction with aromatic solvents like benzene and toluene. Spectroscopic studies of fullerenes are tending to become more detailed. For example, there has been a report on the vibronic analysis of the fluorescence from the lowest excited singlet state of C60 in hydrocarbon glass at 1.2 K. Time resolved photoluminescence measurements of C60 and C70 in toluene solution and the fluence dependence shows the role of the various competing relaxation processes and excitonic recombination. Femtosecond pump-probe experiments show the roles of excitons and photopolymerization in C60 and C70, and in C60 solutions relaxation to the T1state occurs in 900 ps in contrast to the behaviour in the solid where this process occurs in 1 ps; similar behaviour occurs with C70. Two emission bands with different decays in C60 films show that energy relaxation occurs through processes with either free or trapped excitons. The photophysical behaviour of single crystal C60 is interpreted by an excimer mechanism which involves coupling of the excited state to its nearest ground state neighbour. The effect of high pressure on the luminescence of C70 up to 100 kbar shows the occurrence of a spectral shift to lower energy, behaviour which is similar to that shown by olefins rather than that exhibited by aromatics.

Room temperature photoluminescence of the fullerene derivative C60 (NH(CHCOOMe)2) shows it has, in this case, an S1 energy of 171 kJ mol-1 and a lifetime of 1.2 ± 0.3 ns.

The quenching of the fluorescence of azoalkanes by CH2Cl2 and CHCl3 is an unexpectedly efficient (k ≈ 108 M- s-) process and is suggested to involve hydrogen bonding assisted deactivation.

Benzene is a molecule which continues to reveal further fundamental data. Saik and Lipsky have investigated the 1Bu [right arrow] A1g fluorescence excitation spectrum of liquid benzene (and also in solution) from 250 to 150 nm. The results are compared with those for the vapour and the consequences of involvement of the so-called 'channel 3' process. A theoretical analysis through a conical intersections model has been applied to the S1 and S0 photochemistry of benzene. This work shows benzvalene formation is small since most deactivation pathways lead directly and preferentially to the S0 state.

Fluorescence solvatochromic shifts have been investigated for 4,4'-diamino-biphenyl and a detailed study has been made of excited relaxation channels for liquid crystalline cyanobiphenyls and related model ring-bridged compounds. The photophysics of phenyl pyridines and bipyridines in different media have also been examined.

Irradiation of 2,5-dimethylhexa-1,3,5-trienes in argon matrices gives rise to an enhanced population of less stable rotamers. The lower excited states of arylethenes still provide fruitful research after many years of intensive study. Papers have appeared on the S1 and S2 states of arylbutatrienes and the biexponential fluorescence decay of diphenylbutadiene rotational conformers after extreme red-edge excitation shows thè presence of two emitting species in this system. In the case of all-trans-1,6-diphenylhexa-1,3,5-triene in the ps regime the initially formed 1 1Bu1u state is found to relax into a mixture of 1 1Bu1u and 2 1Bu1g states within 10 ps with no evidence of any phenyl-vinyl group distortion over the examined timescale.