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 ((C 5 H 5 )Fe(C 5 H 4 )CONH(CH 2 ) n SH where n = 7−10, and 15) with mercapto alcohol (HO(CH 2 ) 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 β value (describing the exponential decay of rate with adsorbate chain length) of 1.1 methylene -1 or 0.85 Å -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 also10 bonds linking the donor and acceptor.