Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

1 General

Theoretical aspects of photophysics have not been the subject of a concerted effort although notable contributions have been made. Elliott has produced an authoritative review of the Jahn-Teller effect and three papers deal with the details of ultrafast non-radiative processes in molecules and the time evolution of fluorescence. Fluorescence and absorption by large anharmonic molecules have been analysed by the use of a reduced correlation formulation without having to consider individual eigenstates. An extensive review dealing with the quantum theories of solvent-effects is basic to most experimental studies of environmental influences . Amongst other topics covered are polarization effects in molecular crystals, effects of local heating from radiationless deactivation of electronically excited molecules in rigid media, fluorescence depolarization for a probe in a cylindrical phase, determination of moments of inertia of prolate luminescent molecules, the relaxation time of the orientation polarization and its influence on the luminescence intensity distribution, the effect of quenching upon rotational depolarization of fluorescence, and also a critical contribution to the theoretical analysis of concentration depolarization of fluorescence.

The design of equipment and the processing of data is crucial to development of research in photophysics and its applications. A design of an inexpensive subnanosecond fluorimeter is a noteworthy example. Another paper analyses conventional methods of obtaining fluorescence spectra and shows how sensitivity of detection can be enhanced. The precision of fluorescence measurements has been subject to a fairly detailed experimental examination. The determination of fluorescent decay kinetics still needs improvements of technique and examples are a design for a gating circuit for repetition pulsed light sources and the advantages of a pulsed semiconductor laser as a source for lifetime measurements demonstrated. The latter is smaller and less expensive than conventional lasers as well as being capable of providing picosecond pulses. Amongst papers dealing with fluorescence decay data are an application of the λ invariance test on nonexponential decays, treatment of multiexponential decay data by the method of zero determinants, and the use of Z transforms and method of spike recovery, and a comparison of different fitting techniques. The time resolution of single photon counting of fluorescence from excimer/exciplex systems has been improved by dual analysis using an iterated reconvolution program. A recently published book deals in detail with the whole subject. Advantages of phase-resolved spectroscopy for lifetime measurements have been increasingly recognised especially when lifetime selectivity is combined with wavelength selectivity. Very detailed information about a variable-frequency phase modulation fluorimeter system has been provided by Lakowicz and Maliwal. Phase resolution can enhance signals in both luminescence and Raman spectroscopy and four component determinations have been reported. The so called "colour delay error" can also be overcome and lifetimes with the accuracy of a few picoseconds can be obtained. Amongst papers which deal with improvement of picosecond pulse generation is one dealing with distributed feedback dye lasers and another on pulse on shortening by travelling wave amplified spontaneous emission Dye laser intracavity absorption can also be used as a method for monitoring local environmental change. Improvements in streak cameras also are valuable in the picosecond flash excitation experiments.

Review papers deal with picosecond laser photolysis studies of charge-transfer phenomena and Raman processes. Femtosecond optical techniques allow ultrafast photochemical reactions to be followed and the methodology exemplified by photoionization in micellar solutions.

Multidimensional luminescence measurement enormously increases the information which can be obtained and used to advantage in chemical analysis. Computing methods have also been used to develop a quantum counter comparator and calibration of quantum counters. The usefulness of room-temperature phosphorimetry is reviewed in the book by Vo-Dinh. An interesting example of the use of this technique is the design of a passive monitor for the presence of polynuclear hydrocarbons. Investigations directed towards improvement of the technique continue. Micellar enhanced analytical fluorimetry is another useful method in which the consequences of interfacial effects and molecular organization are exploited. Other reviews of specific applications of fluorescence spectroscopy include biological systems, determinations in clinical and agricultural samples, and fluoro-immunoassay

A very important application of fluorescence in biology is flow cytometry, a subject reviewed by Steinkamp. Since all aspects of fluorescence phenomena can be exploited much development is likely in this subject in the near future.

The study of transient absorption in optically dense materials by diffuse reflectance laser flash photolysis has been surveyed and the theory of absorption presented.

A specific example of the use of luminescence lifetime measurements to determine formation constants has been given for the terbium acetate complex.

A review of the now widely used technique of two-photon spectroscopy applied to aromatic molecules draws attention to the selection rules (parity, symmetry, and pairing) and likely future developments.

2 Excited Singlet-state Processes

The 1.27 µm luminescence from O2 (1Δg) allows direct measurements of relaxation of singlet oxygen to be made. Liquid mixtures are different from pure liquids in their quenching ability and this may provide a method for studying liquids and liquid mixtures. Reversible exciplex formation between O2 (1Δg) vitamin E is deduced from the experimental activation enthalpy and a model of...