Exciplexes, Electron Donor-Acceptor Complexes, and Related Charge-transfer Phenomena

Exciplexes, Electron Donor-Acceptor Complexes, and Related Charge-transfer Phenomena.—A wide-ranging review of the role of exciplexes in photochemical reactions has appeared.1 5 

A theory of diffusion-limited recombination of donor-acceptor pairs, based on the probability of DA pair recombinations according to the relationship o 0 exp — r/ I, where w0 is constant, r is the separation between defects, and is the half Bohr radius of the more diffuse wavefunction,1 has been developed. 

The viscosity dependence of emission from the charge-transfer complexes of tetracyanoethylene with benzene and a number of alkylbenzenes has been studied. At high viscosities the fluorescence spectra of alkylbenzenes have a double-band character and the two sub-bands change in different ways with changing viscosity. These observations and the dependence of the excitation spectrum on observation wavelength are discussed in terms of different orientational isomers of the ground-state charge-transfer complexes.1 7 

The fluorescence from polycyclic aromatic hydrocarbons is quenched by fumaronitrile, and a new long-wavelength exciplex emission is observed in nonpolar or weakly polar solvents. The ionization potentials of the aromatic donors show a good correlation with the of the exciplex emission. Furthermore, on direct irradiation into the charge-transfer absorption bands (high concentration), the same exciplex emission is observed.1 8 Fluorescence from exciplexes in the sjvn-tetracyanobenzene-p-xylene1 0 and 

10-dicyanoanthracene and alkyl-substituted naphthalene systems170 has been studied. Photoconductivity measurements on benzoquinone and tetramethylene-phenylenediamine171 have been carried out, and triplet-triplet absorption, phosphorescence, and e.s.r. spectra obtained for some jym-tricyanobenzene-alkylbenzene systems.172 

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