CHAPTER 1
Photophysical Processes in Condensed Phases
BY R. B. CUNDALL
The stream of papers dealing with condensed phase photophysics has not diminished and may in fact be increasing. The ultrafast processes attract most interest and there is a tendency now to relate these to overall reaction mechanisms rather than with the elucidation of the details of physicochemical effects. This is especially the case in the still very active study of photochemically induced electron transfer processes.
Fashion dictates that the buckminister fullerenes are probably the most actively researched group of compounds in the area and even reduces interest in the triplet state of benzophenone to a lesser role.
It is possible to speculate as to whether a final frontier has been reached with femtosecond time resolved spectroscopy. Is the role of pioneer in photophysics to be displaced by the collector of more refined data on well identified processes?
1. General
A number of extensive reviews of various topics have appeared during the year. Comprehensive surveys extending over about 600 pages under the editorship of Ramamurthy and Turro appeared in Chemical Reviews. The articles of interest to our subject will be referred to in the text. Other surveys appear in Progress in Photochemistry and Photophysics, Volumes 5 and 6, edited by Rabek. Volume 17 of Advances in Photochemistry includes two relevant reviews on the bimolecular reactivity of singlet oxygen and the photochemistry and photophysics of onium salts. A review of molecular photophysics with emphasis on effects which occur on the ps and fs time scales by Beddard is also very timely. Other authoritative reviews cover the dynamics and structure of aromatic molecular van der Haals complexes by Topp and the photochemistry and photophysics of organosilicon compounds. The recent wide ranging research in electron tunnelling effects has been surveyed and the role of such reactions in chemistry and biology described in detail.
Monographs recently published include one on the theoretical aspects of molecular photochemistry by Michl and Bonacic-Koutecky, an updated edition of Guilbault's well-known text on practical fluorescence which has emphasised environmental effects, and a timely manual on the variety of luminescence techniques which find application in chemical and biochemical analysis.
The unique series of papers on various aspects of industrial photochemistry from French workers continues with articles on three dimensional machining of polymers with lasers and effects of deformation and the techniques available for photochemical purification of water. The latter subject is an aspect of applied photochemistry which has been comprehensively reviewed by Legrini et al.
Non-linearity is a fashionable topic in all areas of science and nonlinear effects on photochemical conversion of species A into B arising from cooperative influence of photon flux, concentration of substrate, photokinetic factors, and quantum fields are analysed. Another paper on the same class of system discusses the method for determining absorption coefficients, reaction rate constants and thermodynamic data in the system and the analytical model derived was tested by simulated data and applied to photochromic transformation of dihydroindolizines.
Theoretical papers which have appeared during the year deal with a variety of topics. An analysis of the distribution of Franck-Condon transition components which occur in chemical bond formation processes in condensed systems complements observations made by ultra short time spectroscopy. Nonclassical Franck-Condon processes in nonvertical transitions are effects which are resolved in the wings of fs and ps absorption spectra. A survey of the analysis of femtosecond pump and probe spectra of nonstationary states in terms of third order non-linear polarization response to laser light pulses has been given by Pollard and Mathies. Selection rules for bimolecular photoabsorption processes in molecules are delineated by Andrews and Bittner.
Other studies on molecular processes include the time dependence of relaxation parameters in non-Debye solvents, a theory for which is applied to time resolved fluorescence data in the literature. A determination of electronic-transition energies for unperturbed molecules from solvent shift measurements is used to assign π-π* transitions for porphyrins, arenes, and polar molecules. The nature of transient effects on the mechanisms of the activationless electronic relaxation in solution is examined by Jayannavar in a generalization of earlier published work. Another extension of early theory is given in a general treatment of solvent effects in electron transfer at comparatively high temperature. The influence of solvent relaxation effects is particularly emphasized in this paper.
The analysis of Stern-Volmer quenching kinetics still requires active research. Laws and Contino have analyzed nonlinear aspects of this effect. A generalization of kinetic analysis to situations where decay is nonexponential and scavenging or quenching is time dependent and the condition where there is excitation hopping through a Markovian random walk of excitation and dipole-dipole energy transfer are examples of detailed study applicable to the refinement of available experimental data. An analysis of transient effects on the quenching of an excited porphine by methylviologen in aqueous solution is a specific example of new theory applied to the text of experimental data.
Salam has applied esoteric quantum electro-dynamical theory to an understanding of the complex phenomenon of induced circularly polarized luminescence. Evanescent fields of propagating plasmon surface polaritons are effects that can be probed by fluorescence and Raman spectroscopy. Subpicosecond oscillatory effects due to impulsive stimulated Raman scattering have been shown to interfere with the interpretation of spectral data obtained from transient absorption experiments at very short-time resolution. Ps spectroscopy of excited states using nonlinear photoemission provides a general method for measuring lifetimes of excited states in the condensed phase. The method is exemplified by experiments carried out on tetracene in which the excited state has a lifetime of 20 ps. Pulse transform transient absorption spectroscopy provides a new method for obtaining ps transient absorption spectra as exemplified by measurements reported on the ground state beaching of malachite green. Thermal lens spectrometry provides another time resolved method for the study of radiationless deactivation processes and a fresh theoretical approach has been put forward for the interpretation of polarization selective transient grating experiments. An example of thermal lens techniques for measuring excited state populations is that reported for C60 fullerene in benzene. In experiments using laser excitation, different photochemical effects can arise from sequential double, triple, and even other low multiple photon processes. The chemistry involved in systems illuminated with intensities of laser light between 1023 and 1028 photons cm-2 s-1 has been surveyed for organic molecules in solution. This is a useful article for assisting in the understanding of photochemical effects which occur when high intensity light is used.
Time resolved fluorescence decay techniques are still being very actively developed and extended. Ameloot has provided a review of the Laplace deconvolution of fluorescence decay surfaces and is also coauthor of another related article on techniques for the compartmental analysis of such surfaces for characterization of excited state processes. Knutson in the same publication critically reviews the various alternative possibilities for analysis of fluorescence data. The theory for global analysis and identification of two-state excited state processes has also been presented in some detail.
Experimental aspects of time resolved fluorometry which have been published include an assessment of the fluorometric data processing strategy when using ns waveform digitizers in the photon counting mode, the use of optimized gated detection for lifetime measurement over a wide range of single exponential decays, and the design of a single photon timing instrument that covers a broad temporal range in the reversed timing configuration has been described. The broad range of decay times accommodated by the latter system involved the use of fibre optic delays. The question of number of photons which are necessary for satisfactory measurement of fluorescence lifetimes by single photon counting is examined by Köllner and Wolfrum. The effect of background noise on the least squares estimation of fluorescence decay parameters and the estimation of bias and precision of these quantities from sparse decay are related topics which have also been reported. Auto-reconvolution is a new method for the treatment and analysis of time resolved emission anisotropy data which, using synchrotron radiation for excitation, has been applied to the study of properties of fluor labelled polymers. Rotational depolarization of time resolved fluorescence data in low viscosity solvents of prolate molecules with short lifetimes 0.1-1 ns over the viscosity range 0.1 to 1.0 cp has been interpreted in terms of a new phenomenological model.
Technical innovations for lifetime data analysis using simple searching and simulated annealing is useful for systems with biexponential decay times as exemplified by measurements with dansylated amino acids and proteins and the application of Fourier transform of the time interval probe technique which works well when there is only a small detected signal, have also been reported.
A phase fluorometer using two continuously modulated laser diodes operating at 670 and 791 nm for excitation is used for measurement of fluorescence lifetimes in the range 220 ps to 1.7 ns range for cyanine dyes which emit up to 840 nm. The same experimental method has been used for measuring the distance dependence of quenching of the fluorescence of the scintillator POPOP by CBr4. The values of the quenching rate constants depend exponentially on the fluorophore-quencher separation distance.
Detection of very low concentrations or fluorescence from very small volumes are currently topical subjects. Second harmonic detection of spatially filtered two photon excited fluorescence gives very low detection times. The use of photon counting for single fluorescent molecules is considerably aided by the presence of multiple fluorophores on the molecule being studied. The report of a design of a single photon avalanche diode for detection of single molecules with sub ns response time with good quantum efficiency (>50%) and a timing filter of 168 ps exemplifies improvements which have been achieved in technique. Photon bunching in the fluorescence from single molecules provides a new method for probing fundamental processes such as intersystem crossing. Pentacene dispersed in crystals of p-terphenyl has been studied in this way. Photon burst detection of single molecules fluorescing in the near infra red has been studied by pulsed laser excitation and time gated detection. Sub ns lifetimes can be measured, however the sensitivity of the method is limited by the presence of luminescent impurities.
Masuhara has reviewed laser microchemistry in small volumes such as those which are encountered in microcapsules and dispersed liquid droplets. Molecular dynamics can be studied in such samples and also in the liquid phase near to a solid interface. Digital microscopy can be used for fluorescence quantitation. Particle fluorescence can be examined in biology, digital microscopy and flow cytometry constitute applications for such techniques.
A study of the laser excited synchronous luminescence of tetracene illustrates the marked increase in compound selectivity which this method can provide. A paper by Tucker and Acree on the effect of excitation wavelength on the emission spectrum show that this provides a means of examining selective fluorescence quenching. This paper also draws renewed attention to the importance of inner filter effects in fluorescence spectroscopy and how these artefacts can be allowed for and eliminated. An apparatus using a fibre optic probe for collection of two photon excited fluorescence spectra in low temperature glassy solvents and application of a Mueller matrix approach to fluorescence spectroscopy with particular application to circularly polarized emission are other examples of useful advances in experimental techniques.
Other apparatus for measuring time dependent properties involves use of photon counting and optical fibres which provide precise guidance of light for detection of fluorescence, a time resolved fluorometer based on a ns digital oscilloscope and a diode pumped solid state laser, and a spectrofluorometer based on acoustic optical tunable filter for rapid scanning which is useful in the analysis of multicomponent samples. The latter has no moving parts and can scan over a range of 150 nm in 312 ps. A digital phosphorimeter with frequency domain signal processing has been used for oxygen sensing by measurement of the effect on phosphorescent decay times.
Photoacoustic techniques are useful supplements to the more widely used spectroscopic methods. A comprehensive review of time resolved photothermal and photoacoustic methods applied to processes in solution has been published by Braslavsky and Heibel. More specifically some of the more complex effects arising in the analysis of signals in photoacoustic spectroscopy have been critically analysed and recent advances in both theory and experimental design described. The last study provides illustrative data on the triplet energies and lifetimes of 2-cyclopentenone and 1,1-bis-(p-bromophenyl)ethene. A portable photoacoustic and fluorescence photometer for measurements related to photosynthesis that can be carried out and operated in the field seems valuable for photobiology.
The theory and method for micro and nanosecond timescale resolution of biomolecular dynamics by polarized luminescence has been described for use with proteins and application to lipid bilayers in blood platelets. Anisotropy decay has been used to measure electronic dephasing, which is demonstrated to be a useful experimental inroad into the examination of molecular quantum effects. An interesting new procedure is magic angle lifetime measurement by an evanescent wave fluorometer. This is another example of research by luminescence techniques on this topical field.
A comparison of the values of solution and solid matrix luminescence parameters for a number of systems shows that fluorescence quantum efficiencies are greater for excited states in solid matrices and they are also somewhat enhanced at low temperatures. Phosphorescence efficiencies are also reported for identical conditions in this work. Phase modulation fluorometry has been used in the optical sensing of pH and PCO2 by energy transfer effects. pH affects the decay time of an excitation donor, whilst that of the acceptor is affected by CO2.
A new sensitive chemical actinometer has been proposed for time resolved and continuous photochemistry which involves DCM styrene dye. It depends upon analysis of the extent of cis-isomerization which takes place at wavelengths longer than 410 nm. The quantum yield of the system is solvent dependent, for example [empty set](CH3OH) = (2.22 [+ or -] 0.05)10-2. Another paper in the same area comments on the use of Aberchrome™ 540 in chemical actinometry. For excitation between 310 and 370 nm a side reaction occurs which leads to underestimation of light intensity with repeated use and bleaching, the solutions should therefore only be used once, not repeatedly.
A compilation of experiments suitable for teaching of photochemistry to undergraduates has been published in two separate sources. This should prove very useful to teachers of the subject.
Two interesting papers survey areas of applied photophysics which are of commercial significance. Law reviews recent trends and developments in research on organic photoconductive materials and their application to xerography etc. Synthesis and evaluation is the main theme of this article. Information storage using photophysical properties of supramolecular chains and progress towards the application of these in a molecular computer are described by Wild et al.
