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 technique for time resolved fluorescence has been reported. The global target analysis of fluorescence depolarization in model membranes has been applied to the widely employed DPH (diphenylhexatriene) probe. Time resolved fluorescence has been applied to the measurement of rate constants for reversible intramolecular two-state processes and for the effect of added quenchers, quenching reactions between ions in aqueous solution and the associated effect of temperature, and non-equilibrium effects on the kinetics of fluorescence quenching. An interesting technique for probing biosensor interfaces employs multifrequency phase and modulation of total internal reflection fluorescence. A description of a system for time resolved intracellular imaging using the fluorescence of disulfonated aluminium phthalocyanine has been published. A modified phase modulation spectro-fluorometer has been designed specifically for detection of both time resolved and steady state circular dichroism. A microfluorimeter for studying nonstationary dynamics of excimer formation in two- dimensional fluids has been used for the study of pyrene labelled lecithin in DMPC films. A pulsed laser diode excitation system has been used to good effect in the measurement of ps lifetimes in infra-red emitting dyes.

Single molecule spectroscopy is rapidly developing as a technique as discussed in the papers by Moerner and Orrit et al. Single-molecule spectroscopy with polarized light and Stark and pressure effects on such systems are reported in work carried out by Wild's group. Fluorescence microscopy of single molecules of pentacene in host crystals and polymer at low temperatures allow location of the molecule in space to be made within 0.5ps. Single photon detection of single rhodamine 6G molecules has been demonstrated in the observation of fluorescence from levitated microdroplets. Fs transient absorption micro-spectrophotometry combined with optical trapping techniques has been used for dyes in liquid droplets and also for perylene crystals'. Microdroplets can constitute excellent cavity radiators due to the possibility of internal reflection; the theory and description of this interesting behaviour has been described. A useful review has also appeared on the developing subject of time resolved fluorescence microscopy. Zinc tetraphenylporphyrin in individual droplets of tributyl phosphate has been made the subject of a detailed optical study.

A very sensitive detector for PAH (polycyclic aromatic hydrocarbons) in capillary electrophoresis using luminescence allows detection of (3-15)x10-20 mol and the response of the system has been found to be linear over a concentration range of four orders of magnitude. Sensitive fluorescence detection using laser diodes and multiplex dyes for excitation offer applications which are also likely to be useful. The analysis of multicomponent mixtures is possible by the use of Fourier transformation to luminescence data as demonstrated with mixtures of dyes and PAH. A dual linear image sensor can be used to measure transient absorption spectra in the ps time regime, a streak system design and performance are discussed for their use in time resolved spectroscopy, and a microprocessor controlled system designed for this purpose has also been described.

Another new instrument which has been reported yields an excitation-emission matrix and associated polarization measurements with high sensitivity. Synchronous luminescence has been determined by means of a battery-operated portable spectrofluorimeter, the analysis of organic rich fluid inclusions exemplified by oil analysis in cement, and in a new spectral format a lifetime synchronous system which uses phase resolved fluorescence spectroscopy gives the decay time as a function of emission wavelength. A portable dye laser spectrofluorimeter using optical fibres which can be used for examining in vivo human tissue shows promise for the detection of disease states. This could be a very significant development in the application of luminescence spectroscopy. A high repetition rate, ps time resolved, resonance Raman spectroscopy equipment has also been described. A simple phosphorimeter capable of measuring both spectra and lifetimes employing pulse excitation suitable for undergraduate teaching is likely to be useful, as is also a published design of a simple liquid N2 gas flow cryostat for variable temperature luminescence studies'.

A laser excited confocal fluorescence gel scanner suitable for the detection of labelled nucleic acids and a fibre optic confocal fluorimeter using fibre optics capable of depth resolution with a microscope is adaptable to work with living tissue.

Trace level detection by time resolved two-photon excited fluorescence involves very low absorption cross-sections and the elimination of background emission by a blank presents problems which have been assessed. Adventitious light scattering is an artefact which can affect fluorescence anisotropy measurements which are made with front face viewing and some, but not all, of the problems encountered in correcting fluorescence spectra are reviewed from a photo-biological standpoint. There is also a need for correction of the effects of unequal intensities of left and right circularly polarized light in steady state and lifetime resolved fluorescence detected CD.

Examination of the excited state behaviour of thermally stable radical ions by emission needs some care since side products often cause an interfering luminescence. Also the short lifetimes of these species restrict the possibilities for use of these materials as sensitizers.

Phenolenone has been shown to be a useful reference compound for measuring the quantum yield of 1O2 (1Δg) sensitization, Φ = 0.95 [+ or -] 0.05 in 13 oxygen saturated solvents. Potassium iodide proves to be a convenient chemical actinometer for 254nm light which is blind to any effect of longer wavelengths. Using N2O saturated solutions to eliminate recombination of transients, the quantum yield is reported to be 0.22 to 0.26. The photodegradation of sulfamethoxazole is a chemical system capable of monitoring changes in environmentally significant UVB intensity; the quantum yield is approximately 0.47.

Photophysical techniques applied to problems of general interest are illustrated by the use of pyrene to investigate self association of alcohol in aqueous solution by measuring the ratio of the I1/I3 fluorescence bands and also for examining the local polarity within supercritical fluid/liquid solutions. A useful review covers the application of spectral hole burning spectroscopy techniques in the study of amorphous molecular solids and proteins. Another paper published during the year overviews the use of selective spectroscopy in solid solutions, Shpolskii matrices and LB films.

Another timely paper relevant to topical research discusses the problems involved in measuring quantum yields in heterogeneous photocatalysis.

Photocoustic calorimetry is probably not as widely used in photochemistry as it should be. Peters reviews the subject in an article covering a wide range of systems and recent developments in time resolved photoacoustic spectroscopy are also surveyed by Crippa et al. Quantum efficiency measurements made by this technique in solids have also been reported. Thermal lensing methodology has been applied to measuring photoinitiating reactions brought about by triplet states of benzophenone. The use of a fibre optic pressure sensor and nonradiative deactivation of excited states in thin films are other reports on the application of photoacoustic calorimetry.

A paper appearing during the year deals with possibilities for introducing fluorescence anisotropy as a topic in undergraduate teaching. Another paper in the series discussing aspects of industrial photochemistry deals with the warping of samples which occurs in laser flash induced polymerization.

2 Singlet State Processes

Three papers discuss the photophysics of thiophosgene, where the S2 state is significantly involved as is the case in many other sulfur compounds. Self association of this compound is also found to be significant in perfluoralkane solvents.

Other studies on aliphatic systems are a high resolution induced fluorescence of 1,4-diazobicyclo[2.2.2]octane (DABCO), a theoretical analysis of PE surface crossing in acrolein which serves as model for α,β-enones where there is an interplay possible between S0, S1, T2, and T1 states, and the effect of hydrogen bonding on formamide and N-methylformamide excited states which occur on the fs timescale.

Foote has reviewed the extensive studies which have been made on photo-physical and photochemical properties of fullerenes. A new aspect of this very active topic is the attention also now being given to the properties of the highly excited states of fullerenes. A careful examination of the fluorescence of C60 and C70 in cyclohexane at room temperature has precisely assigned the 0-0 transition which shows that there is a very small Stokes shift. Another photoluminescence study of C60 and C70 allows better assignment of the energies of the S1 states and shows the small S1-T1 state separation. These measurements have been carried out in a variety of solvent media. Ps laser studies of the photophysical properties of C70 give S1 [right arrow] Sn and T1 [right arrow] Tn, absorption data over the range 400-1150nm and report that the decay time τ(S1 [right arrow] T1) = 530 [+ or -] 40ps. The fluorescence of (5,6)-fullerene 70 in room temperature solution has been studied as a function of exciting wavelength. The lack of dependence of the spectrum and quantum yield observed shows that decomposition does not compete with rapid internal conversion of the upper states to S1. Nonlinear behaviour of absorption is apparent in the optical spectra of solid fullerene C60.

The fluorescence anisotropy decay and its behaviour in viscous medium of 2-methyltetrahydrofuran has been reported by Brocklehurst and Young. Little work has been reported during the year on the benzene systems except for a theoretical study on the internal conversion which occurs in bemene and a fs time resolved transient bleaching examination of the benzene excimer in the neat liquid. The time resolved nonpolar solvation dynamics in supercooled and low viscosity solutions of n-butylbenzene and dimethyl-s-tetrazine are determined and an examination is made of the associated Stokes shift. Fluorescence has been used to study relaxation of excited states in phenylsiloxanes and the vibrational structure of the electronic absorption spectra of 4,4'-disubstituted azobenzene derivatives has also been examined. A theoretical investigation of PE surfaces is relevant to excited hydrogen transfer occurring in the case of o-hydroxybenz-aldehyde.