This is the report of a DOE-sponsored workshop organized to discuss the status of our understanding of charge-transfer processes on the nanoscale and to identify research and other needs for progress in nanoscience and nanotechnology. The current status of basic electron-transfer research, both theoretical and experimental, is addressed, with emphasis on the distance-dependent measurements, and we have attempted to integrate terminology and notation of solution electron-transfer kinetics with that of conductance analysis. The interface between molecules or nanoparticles and bulk metals is examined, and new research tools that advance description and understanding of the interface are presented. The present state-of-the-art in molecular electronics efforts is summarized along with future research needs. Finally, novel strategies that exploit nanoscale architectures are presented for enhancing the efficiences of energy conversion based on photochemistry, catalysis, and electrocatalysis principles.

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Electron-transfer rates and electronic coupling factors for ferrocene groups attached to gold electrodes via oligo(phenylethynyl) “molecular wire” bridges of variable length and structure are reported. Attachment to gold was achieved via thiol groups at the end of the bridge opposite the ferrocene. Bridge structures were designed to promote strong coupling between gold and ferrocene, thereby promoting rapid electron transport over long distances. The effects of bridge length and of substituents on the phenyl rings in the bridge were addressed. Bridges containing between three and six phenylethynyl units were studied, and a “beta” value of 0.36 Å-1 describing the exponential distance dependence of bridge-mediated electron-transfer rates was obtained. The effect on the rates of adding two propoxy groups onto one of the phenyl rings in the bridge was examined and found to be minimal. The standard electron-transfer rate constant of 350 s-1 obtained for the adsorbate with the longest bridge (six phenylethynyl units, corresponding to an electron-transfer distance of approximately 43 Å) corresponds to an electronic coupling factor between ferrocene and gold of approximately 0.7 cm-1. The extrapolated rate constants at very short distances were nearly the same for the conjugated bridge series and for a related monolayer series in which ferrocene groups were linked to gold via aliphatic bridges. The extrapolated rate constants at short distance also agree with a calculated rate constant for the limiting case of adiabatic electron transfer at an electrode.

The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO(2)) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expected on the basis of aqueous solvent dynamics). This limit is independent of both the identity of the redox couple and the nature of the linkage of the couple to the bridge, and it is definitely different (smaller) from the limit derived from an equivalent analysis of the rate constant (versus temperature) data for the interfacial electron-transfer reaction through oligophenylenevinylene bridges between gold electrodes and ferrocene. There are a number of possible explanations for this behavior including, for example, the possible effects of bridge conformational flexibility upon the electron-transfer kinetics. Nevertheless, conventional ideas regarding electronic coupling through alkane bridges and solvent dynamics are insufficient to explain the results reported here.