Comparison between heterogeneous and homogeneous electron transfer in p-phenylenediamine systems

Abstract

The one-electron oxidation of selected p-phenylenediamines to form the corresponding radical cations has been studied at a Pt electrode in dimethylformamide and acetonitrile solutions containing 0.1 mol dm–3 NaClO4. The standard redox potentials, the diffusion coefficients and the standard rate constants have been evaluated in the range between room temperature and the melting point of the respective solvent using cyclic voltammetry. It has been shown that the dynamics of solvent reorientation affects the heterogeneous electron-transfer rate, as opposed to the homogeneous process. The free energies of activation are much greater in the heterogeneous than in the homogeneous case. Both results are explained by the different shape of the activated complexes, consisting of one or two ellipsoidal molecules.