Long-range electronic coupling between ferrocene and gold in alkanethiolate-based monolayers on electrodes

High-speed cyclic voltammetry was used to measure rates for ferrocene oxidation/reduction in a series of self-assembled monolayers formed by coadsorption of N-(mercaptoalkyl)ferrocenecarboxamide ((C5H5)Fe(C5H4)CONH(CH2)(n)SH where n = 7-10, and 15) with mercapto alcohol (HO(CH2)(n+1)SH where n = 7-10, and 15) on gold. Standard electron-transfer rate constants were obtained as a function of chain length and from these rate constants, a beta value (describing the exponential decay of rate with adsorbate chain length) of 1.1 methylene(-1) or 0.85 Angstrom-(1) was obtained. The rate data were also used to estimate coupling factors, \V-AB\, describing the long-range electronic coupling between the immobilized ferrocene groups and the underlying gold electrode. Electronic coupling factors varied from a low of 0.06 cm(-1) for the long-chain monolayer (18 bonds in the pathway linking ferrocene to the electrode) to a high of 6.5 cm(-1) for the short-chain monolayer (10 bonds linking ferrocene to the electrode). The latter value is in good agreement with a value of 6.2 cm(-1) previously reported by Closs and Miller for an electronic coupling factor in a donor-acceptor molecule in which the bridge is a saturated hydrocarbon (a steroid) in which there are also 10 bonds linking the donor and acceptor.