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 +1SH 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.

A treatment of linear sweep voltammetry for redox-active groups irreversibly immobilized on electrodes is presented with use of the Marcus theory of electrode kinetics to relate rate constants to overpotential. The present treatment extends an earlier treatment of the same problem (Laviron, E. J. Electroanal. Chem. 1979,101,19) that used the Butler-Volmer theory to relate rate constants to overpotential. The behavior predicted in the present treatment matches that of the earlier treatment for very high reorganization energies; however, for reorganization energies below about 2.0 eV, voltammograms are predicted to be broader and peak potentials are in most cases predicted to shift further from E °' than in the earlier treatment. These effects are most pronounced at high overpotentials and at high sweep rates. Voltammetric data acquired over a wide range of sweep rates for ferrocene oxidation/reduction in selfassembled monolayers of N-(15-mercaptopentadecyl)ferrocenecarboxamide coadsorbed with 16-mercaptohexadecanol onto gold were analyzed using the present treatment. Predictions of broader voltammograms and greater shifts in peak potential with increasing sweep rate were fully realized in the experimental data. A protocol based on fitting peak potential vs log (sweep rate) data to predictions from theory is suggested as the preferred means of analyzing voltammetric data. Fitting was accomplished using a simplex algorithm with the heterogeneous electron-transfer self-exchange rate constant, km and the reorganization energy, X, as fitting parameters. Values of k0 = 7.0 s-1 and X = 0.87 eV, and k0 = 6.6 s-1 and X = 0.87 eV, were obtained from fits to two independent voltammetric data sets for two separate ferrocene-containing monolayers. These k0 and X values are in excellent agreement with those in a recent report (Chidsey, C. E. D. Science 1991, 251, 919) on a related system studied by potential-step amperometry.An important research problem in modern electrochemistry is the study of long-range electron transfer, specifically that between a metal electrode and a redox-active group held at a fixed distance from the electrode surface. This seemingly simple process has relevance to the flow of electrons in biological superstructures such as redox proteins and photosynthetic reaction centers.1-3 It is also likely to be a primary underpinning of the future science and technology of molecular electronics.* I.**4'5 In this regard, molecular self-assembly chem-

Electron-transfer rates between ferrocene and gold were measured for two series of ferrocene-based alkanethiolate monolayers on gold electrodes. In one series, the bridge linking the ferrocene group to gold consisted of a simple alkane chain, and in the other, the bridge was modified such that the two methylene linkages immediately adjacent to the ferrocene group were replaced by a carboxamide linkage. Monolayers in both series were studied using a novel ac voltammetry method to measure the standard electron-transfer rate constants for oxidation/reduction of the ferrocene groups. Rate constants were nearly the same for pairs of molecules in the two series when the number of bonds in the direct pathway linking ferrocene to gold was the same, regardless of whether the group immediately adjacent to ferrocene was a pair of methylene units or a carboxamide unit. This observation suggests that the contribution of the carboxamide group to the overall bridge-mediated electronic coupling is not greatly different from that of a pair of methylene groups. An unusual “even−odd effect” in the diminution of rate with chain length for both monolayer series was also noted. Several factors that could cause such an alternation in rate are proposed, including an unusual quantum interference effect in the bridge-mediated coupling that has been predicted theoretically.

An electrochemical current rectifier based on a ferrocene-monolayer-modified electrode in contact with an electrolyte solution containing sodium hexacyanoferrate(II) (sodium ferrocyanide) was constructed and evaluated. Rectification was accomplished via mediated ferrocyanide oxidation by electrogenerated ferricenium in monolayers of (10-mercapto-N-decyl)ferrocenecarboxamide with dodecanethiol and (7-mercapto-N-heptyl)-ferrocenecarboxamide with nonanethiol on gold electrodes. The reverse reaction of ferricyanide reduction by immobilized ferrocene is thermodynamically disfavored, thereby providing the basis for rectification. A model proposed to describe the rectification process is reminiscent of earlier treatments of electron-transfer mediation at electrodes modified with redox polymers, with the important difference that, in the present system, long-range electron transfer across the monolayer replaces electron hopping through the polymer layer as a possible rate-limiting step. Steady-state current-voltage curves corresponding to mediated ferrocyanide oxidation were recorded and analyzed using the proposed model to give values for k 0 , the standard rate constant for electrochemical oxidation/reduction of immobilized ferrocene groups, and k cross , the rate constant for reaction of ferricenium ions in the monolayer with ferrocyanide ions in solution. Mean k 0 values of 2700 ( 1000 s -1 for the (10-mercapto-N-decyl)ferrocenecarboxamide/dodecanethiol system and 90 000 ( 60 000 s -1 for the (7-mercapto-N-heptyl)ferrocenecarboxamide/nonanethiol system obtained by analysis of steady-state current-voltage data are to be compared with values of 1400 and 51 000 s -1 for the same systems obtained by the fast-scan cyclic voltammetry method. The k cross value of 1.1 × 10 8 M -1 s -1 obtained from analysis of steady-state current-voltage data for the (10-mercapto-N-decyl)ferrocenecarboxamide/dodecanethiol system is in good agreement with a value calculated using the Marcus expression with literature values for the electron self-exchange rate constants for ferrocene/ferricenium and ferrocyanide/ferricyanide, indicating that the reactivity of ferricenium ions in the monolayer is similar to that expected for ferricenium in free solution.