ELECTRON-TRANSFER KINETICS IN ORGANIZED THIOL MONOLAYERS WITH ATTACHED PENTAAMMINE(PYRIDINE)RUTHENIUM REDOX CENTERS

Thiols with pendant redox centers (HS(CH2)nCONHCH2pyRu(NH3)52+, n = 10, 11, 15) adsorb from acetonitrile solutions onto gold electrodes to form electroactive monolayers. Mixed monolayers can be formed when the electroactive thiols are co-adsorbed with alkanethiols (HS(CH2)nCH3, n = 11, 15) and omega-mercaptoalkanecarboxylic acids (HS(CH2)nCOOH, n = 10, 11, 15); the diluent thiol in each case is slightly shorter than the electroactive thiol. The pyRu(NH3)52+/3+ redox centers are stable in pH 4 aqueous Na2SO4 electrolyte and have a formal potential near 0.0 V vs SCE. At sufficiently slow scan rates, cyclic voltammograms of the electroactive monolayers are nearly ideal (peak splitting = 0 mV and peak half-width = 90-100 mV) for all combinations of electroactive thiol and diluent thiol and at all coverages of the electroactive thiols. The kinetics of electron transfer in the electroactive monolayers are examined by cyclic voltammetry and chronoamperometry. Evidence is given for the existence of a population of "fast" redox centers which can mediate charge transfer to the monolayer; however, rates of direct electron transfer between the electrode and the redox centers can be obtained. Experimental Tafel plots exhibit symmetric slopes in the cathodic and anodic branches, in contradiction to the prediction of through-space tunneling. The Tafel plots are fitted to Marcus theory to obtain the solvent reorganization parameter for the redox centers. The solvent reorganization parameter varies from 0.45 to 0.7 eV, with the parameter increasing with increasing chain length. Standard rate constants obtained from intercepts of the Tafel plots are primarily determined by the chain length and are independent of the terminal group in the diluent thiol. The standard rate constants tend to be larger for the anodic branch than for the cathodic branch, which implies slight differences in monolayer conformation for the two oxidation states of the redox centers. The standard rate constants for the mixed monolayers decay exponentially with increasing chain length. The slope of the In k-degrees vs n plot is -1.06 (+/- 0.04) per CH2. Through-bond tunneling is proposed as the mechanism of electron transfer.