ELECTRON-TRANSFER REACTIONS OF ANTHRACENE ADSORBED ON SILICA-GEL

The production and decay of the anthracene radical cation on silica ge l 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 b y 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 com peting efficiently with geminate recombination. Addition of electron d onors such as triphenylamine, N,N,N',N'-tetramethyl-1,4-phenylenediami ne, N,N-dimethylaniline, and azulene greatly accelerate the rate of ra dical ion decay via an electron transfer mechanism. Kinetic analysis r eveals that the observed decay can be described by either the dispersi ve kinetic model of Albery et al. or a fractal dimensional rate consta nt model of the type which has been used to describe triplet-triplet a nnihilation on surfaces. The bimolecular rate constants vary considera bly and do not show a simple dependence on the free energy for electro n transfer. This can be explained either on the basis of bulk diffusio n of electron donors being slow relative to the electron transfer proc ess, or by the presence of a Marcus ''inverted region'' at relatively modest negative free energy values.