OBSERVATION OF TEMPERATURE INDEPENDENT HETEROGENEOUS ELECTRON-TRANSFER REACTIONS IN THE INVERTED MARCUS REGION

Measurements have been performed of the temperature dependence of highly exoergic interfacial electron transfer processes from the conduction band of nanocrystalline titanium dioxide to the cation radicals of three different sensitizers, i.e alizarin, a merocyanine and a coumarin dye. Although these reactions fall deep into the inverted Marcus region, they show practically no activation energy. The results are interpreted in terms of a quantum mechanical model for non-adiabatic electron transfer including the treatment of high frequency molecular vibrations of the adsorbed sensitizer. This analysis provides apart from the electronic matrix coupling elements separate values for the internal and solvent reorganization energies. The behavior of the alizarin/TiO2 chelate is unusual in as much as the solvent reorganization does not appear to contribute significantly to the interfacial redox process.