Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The study of photophysics is an active area of research although the survey indicates that most work is now concentrated in parts of the world where financial support for the necessarily expensive equipment for popular problems is available. Major new discoveries are not much in evidence and most published work is concerned with detailed analysis of systems selected for reasons decreed by fashion or possible technological applications. Systems relevant to biology probably provide the problems in which photophysical methods are exploited most fully.

1 General

A survey of photochemistry over forty years by Sir George Porter which concentrates on time scales of milli- to attoseconds as a means of delineating experimental effort provides an excellent start to the survey and modern techniques are exemplified by a full description of picosecond flash equipment in use at the Royal Institution.

Theoretical studies are not profilic and appear to be diverse in subject. The effects of tunnelling and the distinction from thermal activation have been analyzed by Formosinho and is almost unique in its consideration of primary processes. The proximity effect and the consequences of the pseudo-Jahn Teller has been applied to nπ* and ππ* states of heterocyclic and aromatic carbonyl compounds by Lim. Other theoretical studies deal with the application of Marcus theory to photochemical proton transfer, the concentration gradient model of fluorescence quenching, the effect of coulombic interaction on fluorescence quenching by ions, the theory of photoisomerization of impurity molecules in solids, time delayed two photon processes, and solvent effects on the absorption intensities of nπ* and ππ* transitions. The linearity of free energy relationships for excimer formation is examined in an attempt to correlate very extensive data. Razi-Naqvi has critically approached the correction of fluorescence depolarization with anisotropic rotational diffusion. Line broadening and the red shift of fluorescence as solvent relaxes around the excited state provide a probe of solvation dynamics. A theoretical model for this has been proposed.

Other papers deal with applications of different experimental techniques. Possibilities for examining nuclear spin coherence transfer in photochemical reactions have been assessed. The nature of excited states of solute molecules and their coupling with mechanical degrees of the medium can be probed by picosecond photon echo experiments. Solvation dynamics can also be elucidated by combined fluorescence and Raman studies as exemplified by results from appropriate model systems and bacteriorhodopsin. Lewis has reviewed proton transfer effects in photogenerated radical ion pairs. An article with 144 references examines the Shpol'skii effect as an analytical tool. Drickamer describes how high pressure optical absorption can be used to understand electronic interactions and excitations in molecular crystals and solutions. Orvaeskii examines the scope for selective laser initiation of a number of interesting effects. A wide field is covered and he puts forward suggestions for detecting the formation of solitons in nucleic acids. The influence of magnetic fields on fields excited singlet and triplet states in irradiated organic molecules has been discussed by Brocklehurst and his coworkers.

Development of experimental techniques largely dictates the progress of photophysics. For example, resonance Raman studies make it possible to examine the femtosecond ring opening dynamics of excited 1. 3-cyclohexadiene. The technique is shown to be particularly useful when luminescence is weak and other spectroscopic methods are inconvenient. An amplified colliding pulse mode locked laser system has been described which is capable of subpicosecond resolution and has been used to measure microscopic friction in the reorientation of trans-stilbene. A specialist issue of Photochemistry and Photobiology has dealt with the application of synchrotron radiation to spectroscopy and photobiology. The facility at the Brookhaven National Laboratory is described in detail. Improvements to widely used techniques are exemplified by description of a flash lamp probe source and a fast time resolved infrared detector for solution chemistry.

A wide ranging review of the principles and applications of fluorescence techniques in biophysical chemistry has been made by Geacintov. A survey of chemical microscopy includes section on fluorescence techniques. This indicates the development of an extremely useful methodology, especially in biology. An instrument for the simultaneous acquistion of fluorescence spectra and lifetimes from single cells or organelles with a time resolution of better than 300ps has been described. A single photon counting Fourier transform microfluorometer uses multifrequency phase shift techniques. A technique for obtaining fluorescence vivo uses an intensified photodiode array. A review of fluorescence correlation spectroscopy and photobleaching recovery shows the advantages of using a combination of the chemical specificity of fluorescence and high spatial resolution of microscopy. A fluorescence spectrometer applicable to single electrodynamically levitated microparticles has also been described.

Improvements in fluorescence techniques continue to be produced. Fast Fourier Transform analysis of luminescence decay curves using microcomputers in the case of nonexponential decay has been published. The global analysis of fluorescence decay has been extended to excited state reactions and ground state heterogeneity.

The determination of rate constants from fluorescence decay surfaces is also described in detail. A microcomputer based system for collecting and processing multidimensional fluorescence data has been developed. A time resolved multiphoton counting method has less time resolution than single photon counting but is advantageous for measuring time constants on the µs range. A useful extension of pulse fluorometry is the multiplexed photon instrument for routine measurement of time resolved fluorescence anistropy described by Birch et al.

Lakowicz, Laczko and Gryczynski detail the design of a 2GHz frequency domain fluorometer and present evidence of the performance which shows a capability for measuring correlation times as short as 15 ps. The technique is well suited to the resolution of complex emissions largely due to the application of phase sensitive detection at multiple modulation frequences. Correction for contaminant fluorescence, a frequent problem in biochemistry, has also been described and phase resolved spectroscopy can also eliminate scatter background in synchronous excitation spectrofluorometry. Quenching effects can also be avoided by phase resolution. Time resolution with optical fibre probes has been demonstrated by measurements on rhodamine-8 and rose bengal. A spectrometer which uses rapid frequency scanning over the range 250-1000MHz in less than 10 ms is described in another paper. Three papers by McGown and collaborators discuss the analysis of multicomponent by phase resolved techniques. Circular dichroism can be measured by multidimensional fluorescence by means of equipment which has been described.

Observation of fluorescence from thin liquid films held in metal loops offers useful possibilitites for the study of films and membranes. Fluorescence line narrowing in the fluorescence spectra of polycyclic compounds on filter paper at 4K has analytical advantages as illustrated for benzo[a]pyrene. Fluorescence is sensitive and selective for analytical determinations by HPLC and a laser videofluorometer system with laser excitation has been used for real time characterisation. Polymer modification of fibre optic chemical sensors provides a means of enhancing and generating suitable fluorescence signals for pH measurement.

Rear viewing of luminescence quantum counters in spectrofluorimeters is preferred in a critical comparison of the front and rear viewing configurations. The effects of changes in the absorbing medium with conversion and the problems caused for quantum yield measurements have been examined by Bunce. Solid state actinometers based on the photo-rearrangement of aromatic azoxycompounds in poly(methyl methacrylate)is another development. Removal of oxygen is critical for many photochemical measurements and an automated deoxygenation procedure for sample solutions described. Ionic derivatives of pyrene in γ-cyclodextrin show one band in the fluorescence is quenched by oxygen while another is unaffected. This shows great promise for O2 determination although the development has not yet made it applicable to aqueous solutions. A theoretical procedure for parameter selection in constant energy synchronous luminescence spectrometry maintains interest in this useful technique.

Wilkinson has surveyed recent studies with the technically valuable method of diffuse reflectance flash photolysis and Peters demonstrates the use of time resolved photoacoustic calorimetry over the range of 10 ns to 50µs. Time resolved total internal reflection fluorescence spectroscopy shows promise becoming a powerful technique for studying problems in surface physics as preliminary work has demonstrated. An automated procedure using ternary liquid gradient chromatography can be used for determining Stern-Volmer quenching constants. An elegant design for a triple cell dispenses with the need for a cumbersome Dewar system in photophysical measurements. Below 20K, xanthione emits luminescence from two optically resolved sublevels which are in thermal equilibrum and the relative emissions from the two states function as a molecular luminescence cryothermometer.

A very well referenced review deals with all aspects of photothermal effects in chemical analysis. Thermal lensing, transverse photothermal diffraction, photothermal refraction, and photothermal diffraction are all effects which depend on development of refractive index distribution differences. A microcomputer based time resolved electroluminescence system is an example of another different type of useful equipment.

2 Singlet State Processes

Glycoaldehyde is regarded as the simplest example of a sugar and in aqueous solution fluorescence and photochemistry reveals that both free and hydrogen bonded nπ* singlet states are invo1ved. Pulse radiolysis shows excited state aggregates and exciplex species exist in the presence of polar molecules in liquid p-dioxane. An inverse effect of deuteration on internal conversion from higher electronic states has also been studied. α-Particle and γ-ray induced solute fluorescence in cyclohexane and benzene mixtures shows charge transfer and excitation effects. Electronic excitation of tetranitromethane at 248 nm is followed by dissociation to give NO2 and (NO2)3 C-. Koehler has studied solvent effects on the fluorescence properties of aniline. Picosecond fluorescence anisotropy shows aniline rotates almost as fast in solution as in the gas phase. N, N-Dimethylaniline is slower in its rotations than aniline in hydrocarbons but is faster in alcohols due to reduced hydrogen bonding. Water quenches the anomalous emission of the dual emission from N,N-dimenthylamino-benzonitrile in specially dry acetonitrile. Solvent and temperature effects on the decay of p-N, N-(dialkylamino)benzyl-idenene malonitriles require the existence of two ground state conformers and the consequent cis-trans isomerization.

The kinetics and thermodynamics of intramolecular excimer formation in naphthyl ethers have been reported. The relative quantum yields of S2 [right arrow] S1 and S2 [right arrow] S1 and S2 [right arrow] S0 fluorescence from azulene have been remeasured to resolve the discrepancy in previous measurements. Fluorescence emission from anthracene molecules absorbed on surfaces has been stimulated by collision with N2 molecules in their excited singlet state and information on the orientation of hydrocarbon molecules obtained. Torsional potentials of substituted anthracenes have been deduced in jet cooled experiments. A detailed study has been made of the photochemical behaviour of 9-vinylanthracene in methanol where photodimerization is more significant than polymerization. The effect of hydrostatic pressures shows that the excimer state of 9,10-dichloroanthracene is suppressed at about 7 GPa. A complete determination of transition moment directions from fluorescence spectroscopy as well as IR and UV linear dichroism of 1,2-benzanthracene in polymer sheets or liquid crystals has been reported. The time resolved fluorescence anisotropy of perylene in glycerol, whose behaviour is now well understood, provides a good standard for comparing and evaluating experimental techniques. Weak complexes are found between pyrene and polar solvent molecules as shown by monitoring the S1 [left arrow] S0 absorption spectra of pyrene. Kinetic parameters for intramolecular excimer formation from meso- and racemic diastereoisomers of 2, 4-di(2-pyrenyl)pentane provides strong evidence for rapid interconversion of the conformers and also for meso- and racemic bis[1-(2-pyrenyl)ethyl) ether. The excimer-formation kinetics of methyl 4-(1-pyrenyl)butyrate has been studied also. The association of pyrene with cyclodextrins has been measured by a method depending on the vibronic band intensity. Vacuum deposited films of ω-(1-pyrenyl)alkanoic acids undergo aggregation but form no excimers and exhibit nonlinear photochemical effects under laser irradiation. Solvent effects on the radiative lifetime of ovalene show the effects of reduction of the S1-S2 energy gap on this quantity. Ground state processes are important in effects on the photophysics and photochemistry of of lepidopterene. Solvent also influences the photophysics of 9, 10 diphenylanthracene in perfluorohexane by enhancing the S1 [right arrow] T2 transaction. Direct measurement gives 8.4 ± 0.6ps for the lifetime of the S1, state of all-trans-β-carotene in toluene. Solvation dynamics and vibrational relaxation from the resonance Raman and fluorescence line shapes of tetradesmethyl- β-carotene give a solvation correlation times of 111fs and vibrational relaxation of 233fs at 230K.

Alford and Palmer have shown that the fluorescence of all-trans-1,6-diphenyl-hexadiene (DPH) in non-polar solvents involves a dual emission due to thermal repopulation of S2(1Bu) state from the S1 (21Ag) state which proceeds at a rate comparable with S1, emission. This has been confirmed by studies on DPH and diphenyl-octatetraene (DPO) by Itoh and Kohler. These results are of interest in view of the very extensive applications of DPH as a biological probe. A short but useful review on the details of intramolecular excimer formation in biochromophoric molcules linked by a short flexible chain is timely. An interesting interaction occurs between pairs of 1,1-di-α-naphthylamine molecules to give so-called "bicemer" species.

Heterocyclic molecules continue to provide ample scope for investigation. Dibucaine, a quinoline analogue involves neutral species as well as monocations and dications in 1nπ* and 3ππ* states. Excited states of dihydrophenazine doped in molecular crystals have also been characterised. The photophysical properties of 1,4,5,6-tetraazaphenanthrene are determined by the close proximity of 1nπ* and 3ππ* singlet states. Tautomerism and fluorescence of lumazine and two methyl derivatives have been studied in aqueous solution. The effects of hydrogen bonding on the photophysical behaviour of indoquinoxaline, and its methylated derivatives, show that internal conversion from the S1 (ππ*) states is brought about by proton donation. The effect of transient quenching on the excimer kinetics of 2,5-diphenyloxazole has been studied by time-resolved fluorescence.

Other very detailed excited state studies have been made on homobenzenoid hydrocarbons and the flexible bimanes.

The lasing and gain properties of a giant dipole molecule, 1-[p-(N,N-dimethylamino)phenyl]-4-(p-nitrophenyl)-1,3-butadene have been investigated. The dipole moment increases from zero to more than 300 in the S1 state.