ELECTROLYTE AND TEMPERATURE EFFECTS ON LONG-RANGE ELECTRON-TRANSFER ACROSS SELF-ASSEMBLED MONOLAYERS

The thermodynamics and kinetics of electron transfer are examined as a function of aqueous electrolyte and temperature on self-assembled monolayers containing attached redox centers. The monolayers are formed by coadsorbing an alkanethiol with a pendant pyRu(NH3)5 redox center with an alkanethiol with a terminal carboxylic acid onto clean gold surfaces. The formal potential of the attached redox centers tracks the formal potential of a solution analog in all electrolytes (1 M Na2SO4, NaCl, NaNO3 or NaClO4) and at all temperatures (5-55-degrees-C). No change in the monolayer structure is discernible in chronoamperometry or Tafel plots over the same range of conditions. The electron transfer kinetics are independent of the identity of the anion in the electrolyte. The temperature dependence of the Tafel plots conforms qualitatively with the predictions of the Marcus model. A comparison of three voltammetric methods (cyclic voltammetry, chronoamperometry, and ac impedance spectroscopy) confirms that one standard rate constant and one reorganization energy consistently describes the electron transfer kinetics of the majority of the redox centers.