Electron transfer reactions of anthracene adsorbed on silica gel

The production and decay of the anthracene radical cation on silica gel has been studied using nanosecond time-resolved diffuse reflectance laser flash photolysis. The production of the radical cation has been shown to be via a multiphoton process both by a laser dose study and by millisecond flashlamp experiments. The decay kinetics of the radical cation conform well to an analysis based on geminate recombination at loadings of less than 2 mu mol g(-1). At higher loadings, deviations from these kinetics are observed caused by bulk electron diffusion competing efficiently with geminate recombination. Addition of electron donors such as triphenylamine, N,N,N',N'-tetramethyl-1,4-phenylenediamine, N,N-dimethylaniline, and azulene greatly accelerate the rate of radical ion decay via an electron transfer mechanism. Kinetic analysis reveals that the observed decay can be described by either the dispersive kinetic model of Albery et al. or a fractal dimensional rate constant model of the type which has been used to describe triplet-triplet annihilation on surfaces. The bimolecular rate constants vary considerably and do not show a simple dependence on the free energy for electron transfer. This can be explained either on the basis of bulk diffusion of electron donors being slow relative to the electron transfer process, or by the presence of a Marcus ''inverted region'' at relatively modest negative free energy values.