ELECTRON-TRANSFER BETWEEN AMPHIPHILIC FERROCENES AND ELECTRODES IN CATIONIC MICELLAR SOLUTION

Standard heterogeneous electron-transfer rate constants (k') for ferrocene (Fc) esters attached to the 2- and 5-carbon atoms (I and II) away from nitrogen on the hydrocarbon chain of dodecyltrimethylammonium ions were similar to each other and to ferrocene (Fc) in homogeneous organic solvent. However, in micellar solutions of cetyltrimethylammonium bromide (CTAB), differences were found for k' in the order Fc > I > II. Activation free energies from temperature dependences of k' and microenvironmental polarities from spectroscopic studies were similar for I and II in CTAB solutions, suggesting similar reorganization energies. The distance dependence of k' in micellar solution was consistent with predictions of Marcus theory using a through-space model with average distances of electron transfer estimated from molecular dynamics or a through-bond model assuming bonding of head groups to the electrode. Results suggest that CTAB adsorbed to the electrode helps to orient the electroactive surfactants with alkylammonium head groups down on the Pt surface prior to electron transfer. Electron transfer to Pt from Fc in cationic micellar solutions occurs over a shorter distance than for I.