Multiple electron tunneling paths across self-assembled monolayers alkanethiols with attached ruthenium(II/III) redox centers

Alkanethiol monolayers with pendant redox centers are deposited on gold electrolytes by self-assembly. The monolayers are composed of both an electroactive thiol, HS(CH2)(n)C(O)NHCH(2)pyRu(NH3)(5)(2+/3+), with 10-15 methylene groups, and a diluent thiol, HS(CH2)mCoOH, also with 10-15 methylene groups. The monolayers are classified as ''matched'' (n = m), ''exposed'' (n = 15, m = 10-14), and ''buried'' (n = 10, m = 11-15) according to the relative position of the redox center. Cyclic voltammograms in aqueous Na2SO4 indicate that the monolayers are close-packed with the redox centers residing in the aqueous phase in all but the most buried cases. Measurements of electron transfer kinetics by several methods (cyclic voltammetry, ac impedance spectroscopy, chronoamperometry) yield an internally consistent set of kinetic parameters, the standard rate constant k degrees, and the reorganization energy lambda of the redox centers. The reorganization energies are in good agreement with the theoretically predicted value of 1.0 eV for the pyRu(NH3)(5) redox centers. Plots of ln(k degrees) vs m are linear in all three cases. The slopes of the linear regression fit provide tunneling parameters (beta, where k degrees approximate to e(-beta m)) of 0.97 +/- 0.03 (matched cases), 0.83 +/- 0.03 (exposed cases) and 0.16 +/- 0.02 (buried cases) per methylene. This pattern of beta's is interpreted in terms of electronic coupling between the redox center and the electrode via both the redox thiol and the proximate diluent thiols, with the coupling via the diluent thiols dominating in the exposed cases.