Photophysical Processes in Condensed Phases

In spite of the need to keep this chapter within the size limitation set by the Chemical Society, it is hoped that the standard has been maintained. However, in order to make best use of the available space it has been decided to omit coverage of those publications which will receive some attention in other Chapters. The areas in which this change will be most noticeable are (i) publications covering spectroscopic as well as kinetic studies (refer to Part I, Chapter 1) (ii) publications covering more synthetic aspects of organic photochemistry but with some quantitative information (refer to Part III). 

In an article which is critical of many generally accepted molecular fluorescence parameters of aromatic molecules (and by inference the parameters for other systems), Birks emphasizes the precautions necessary to eliminate errors due to self-absorption secondary fluorescence and/or self-quenching.1 The points are made that reliable data for rf and Of are available for only a few compounds, e.g. diphenylanthracene (DPA), perylene, quinine bisulphate, and acridone, and that these provide suitable standards. The value of Of (DPA) is now set at 0.83. The importance of solvent effects on Of and t( of DPA is stressed in a publication which reports Tf for DPA in cyclohexane and benzene.2 The value of 6.95 0.04 ns for benzene solution is in good agreement with the earlier work of Birks and Dyson3 and Ware and Baldwin 4 7 (7.35 0.05 ns). The value obtained for cyclohexane solution, 7.58 0.04 ns, although in poor agreement with earlier results, is probably the most acceptable. The absolute fluorescence quantum yield of quinine bisulphate has also been redetermined (Of = 0.56).8 

A re-investigation of the fluorescence yields and fluorescence lifetimes of benzene in polar and non-polar solvents and an analysis of the quantum yields of benzvalene formation under a variety of conditions has led to the conclusions that benzvalene is formed from the vibrationally excited state of benzene and that the population of non-totally symmetric vibrational levels plays a key role in benzvalene formation.3-8 The quenching of benzene fluorescence by dissolved 

The absorption, fluorescence, and phosphorescence spectra and luminescence lifetimes of the o-, m-, and p-cyanoanisoles (Table 1) are consistent with absorption in the first band leading to the population of a single vn state in the case of the ortho- and meta-isomers and the population of two tttt states in the case of the para-isomer.10 The isomeric fluorobenzonitriles have also been examined (Table l).11 The large value of / for the para-isomer is the direct consequence of the forbiddenness of the 1Lb-xA transition of this isomer, the large value of A isc, and possibly also the result of a large value for the radiative rate constant 

A single-photon counting technique has been used in the measurement of the luminescence quantum yields of chlorobenzene and benzyl chloride at 77 in a range of matrices [Of(CeH5Cl) = (1 0.5) x 10-1, ( 6 1) = (2 1) x 

The year under review has shown continual progress in elucidating the detailed behaviour of excited singlet and triplet states. This has been largely due to the improvement of experimental equipment especially in the very short time domains. The need for a complete understanding of photophysical processes in any application of excited-state properties is now fully accepted, particularly in analytical applications of luminescence and biochemistry. 

The number of theoretical papers attempting to provide a unified explanation of experimental data appears to have declined. This may be due to the fact that excited states are more difficult to describe theoretically because electron correlation plays a more important role. Schweig and Thiel show that the MNDOC treatment is superior to other methods although application to photochemical problems requires caution. One of the relatively few papers dealing with radiationless transitions is that of Sarai and Kakitani in which the effect of a large nuclear rearrangement is investigated. This would appear to be an important advance on most of the theories published hitherto. 

In the condensed phase the effect of the environment is of great importance and an interesting review has been provided by Kasha et al. Myers and Birge have derived a simple expression for the effect of solvent on the oscillator strength of a solute, which involves the refractive index of the solvent and another factor depending on the molecular shape and orientation of the transition moment. The theory successfully predicts the effect of solvent polarizability on the oscillator strength of the n n transition of j3-carotene and the n - n transition in pyrazine. 

During the year under review there has been more emphasis on the improvement of experimental technique than theory. Bunce has analysed the determination of quantum yields for reactions, particularly those in which excimers may be involved. The measurement of fluorescence quantum yields is not generally 

Dellinger, and C. Brown. Bioluminescence and Chemiluminescence, Basic Chemistry and Analytical Applications, ed. M. de Luca and W. D. McElroy, AcademiePress, New York, 1981, p. 3. A. B. Myers and R. R. Birge, J. Chem. Phyx., 1980. 73, 5314. 

Many important aspects of electron-transfer reactions have been reviewed 

The technique of transient grating spectroscopy has been reviewed, with particular emphasis on its application to monitoring non-radiative deactivation. A unified theory of time-resolved fluorescence anisotropy and Stokes shift spectroscopy has appeared. A separate review has considered the chemical and photophysical events occurring from upper excited states as accessed by multiphoton absorption techniques.  

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