Reorganization energetics for ferrocene oxidation/reduction in self-assembled monolayers on gold

Electron-transfer rates in ferrocene-containing alkanethiolate monolayers o n gold electrodes were measured as a function of temperature using fast-sca n cyclic voltammetry. Reorganization energies for the ferrocene oxidation h alf-cell reaction obtained from fits to peak potential versus log (scan rat e) data are compared with activation energies obtained from Arrhenius analy ses of the standard rate constants measured at different temperatures. The Arrhenius analysis yielded activation energies that were consistently lower than one-fourth of the reorganization energy obtained from analyses of ind ividual fast-scan voltammetry data sets at a fixed temperature. This discre pancy is interpreted in terms of a non-zero entropic contribution to the ac tivation energy. The magnitude of this entropic contribution is consistent with expectations based on the reported reaction entropy for the ferrocene half-cell reaction in water. (C) 1998 Elsevier Science S.A. All rights rese rved.