The rate of separation into free ions of a geminate ion pair generated by photoinduced electron transfer between 9,lO-anthraquinone excited to the lowest triplet state and 1,2,4-trimethoxybenzenein 1,1,2,2-tetrachloroethane has been measured at different temperatures by nanosecond time-resolved resonance Raman spectroscopy (TR3). The intrinsic activation energy for the separation amounts to 0.04 eV, suggesting that the center-to-center interionic distance in the geminate ion pair is about 7.5 A. The activation barrier is due to a loss of electrostatic stabilization upon separation to a distance of about 9.5 A, where a solvent molecule or part of it can interpenetrate to increase the solvation energy. This suggests that the geminate ion pair is a loose ion pair but is not truly solvent separated.

The electron transfer reaction between 9,lO-anthraquinone (AQ) excited to the lower triplet state and 1,2,4trimethoxybenzene (TMB) in solvents of different polarity has been studied by nanosecond time-resolved resonance Raman spectroscopy. In no solvent was there evidence for the formation of a triplet exciplex. All the observed vibrations were characterized as AQ* or TMB" modes. The frequency of the band assigned to the C-C stretch of TMB" has been found to be very dependent upon the environment and this effect has been used to observe the separation of the geminate ion pair into free ions in polar solvents. The data strongly suggest that the ions are at van der Waals contact in the geminate ion pair and not separated by solvent molecules.

A study of viscosity and temperature effects on the rate of back electron transfer (BET) within an exciplex (9,10-dicyanoanthracene/N,N-dimethylaniline) with a strong charge transfer character in six non-polar solvents is reported. The extent of charge transfer has been estimated from the solvatochromic and thermochromic shifts of the fluorescence. Conformational changes are a prerequisite to the BET. In non-viscous solvents, where they are much faster than the ET step itself, the observed rate can be explained within the theory of non-adiabatic ET reactions, while in more viscous solvents, a time-dependent electronic coupling constant V has been introduced. In decalin and butylbenzene, a transition from a "solvent independent" to a "solvent controlled" non-adiabatic regime is observed.