Electrochemical Measurements of Electron Transfer Rates through Zirconium l,2-Ethanediylbis(phosphonate) Multilayer Films on Gold Electrodes

The rates of electron transfer between gold electrodes and solution-phase or covalently attached redox molecules were measured as a function of the thickness of an intervening metal phosphonate thin film. Multilayer films of zirconium l,2-ethanediylbis(phosphonate) (ZEDP) were prepared on gold electrodes by sequential adsorption of aqueous zirconyl chloride and 1.2-ethanediylbis(phosphonic acid) solutions. Ellipsometry shows a stepwise increase in film thickness of 8 A per ZEDP layer. A logarithmic dependence of the heterogeneous electron transfer rate constant for the oxidation of Fe(CN)₆⁴⁻ in 0.1 M NaClO₄ on the thickness of the ZEDP multilayer film was obtained from Tafel plots. The low electron tunneling constant (0.43 Å⁻¹) obtained suggests that defect sites in the ZEDP multilayer film dominate the electrochemical response. Films containing covalently bound ferrocene-terminated phosphonate groups and ZEDP spacers were studied by the same technique. Similar values of the tunneling constant were obtained, even after most of the ferrocene units were replaced by electroinactive benzylphosphonate spacers. However, potential step chronoamperometry revealed kinetic heterogeneity in the ferrocene layer even at a ferrocene mole fraction of 0.2. This indicates that there is a heterogeneous distribution of ferrocene sites in the monolayer and that electron hopping between ferrocene units may be important in mediating charge transport between the least accessible electroactive groups and the metal electrode.