Homogeneous and Heterogeneous Electron Transfer Rates of the Tetrathiafulvalene‐System

Abstract

Abstract Using ESR spectroscopy and cyclic voltammetry the rate constants of homogeneous and heterogeneous electron self exchange in the system TTF/TTF were measured at different temperatures in solvents of different polarities and relaxation times (TTF = tetrathiafulvalene = 2‐(1,3‐dithiol‐2‐ylidene)‐1,3 dithiol). The rate constants were expressed by the preequilibrium encounter model for adiabatic processes. For both reactions the results are discussed in terms of Marcus's theory for the reorganization of ellipsoidal molecules in the overdamped relaxation case. From the experiments the geometric term g = 1/ r̄ − 1/ d ' was derived and compared with model calculations r̄ = mean molecular radius, d ' = formal distance between the reactants resp. reactant and image charge). Approximately interaction energies of the reactants (in the homogeneous case) and double layer correction (in the heterogeneous case) can be neglected. Preexponential and activation terms of the rate constants agree satisfactorily with the theory. g (het) is about the tenfold of g (hom). This is explained by an increased d ' caused by strong adsorption of the counterions of the supporting electrolyte on the electrode.