Through-Bond and Chain-to-Chain Coupling. Two Pathways in Electron Tunneling through Liquid Alkanethiol Monolayers on Mercury Electrodes

Formation, structure, and properties of alkanethiolate monolayers on micrometrically driven hanging mercury drop electrodes were investigated electrochemically. Alkanethiols with the chain length from C8 to C18 were shown to form densely packed (ca. 20.3 Å2/molecule for C12SH), perpendicularly oriented monolayers on mercury in a process involving two electron oxidation of Hg to form mercuric thiolate, in agreement with earlier literature reports for a number of thiols. Electron tunneling rates across these films (due to Ru(NH3)6 electro-reduction in aqueous 0.50 M KCl) exhibit characteristic exponential increase with the electrode potential (with transfer coefficient R ) 0.25), and an exponential decay with the monolayer thickness (with a through-bond decay constant, âtb ) 1.14 per methylene group or 0.91 Å-1). Slow stepwise expansion of the mercury drop electrodes coated with alkanethiolates (C9-C14 only) results in an only small increase of the tunneling current maintaining the pin-hole free structure of the monolayers. Capacitance measurements showed that the film thickness changes inversely proportionally with the electrode surface area. The increase of the tunneling current recorded in the drop expansion experiments was accounted for by postulating existence of an additional tunneling pathway involving chain-to-chain coupling. Data analysis in view of this parallel pathways model yielded a through-space decay constant, âts ) 1.31 Å-1. Ab initio computations of the electronic coupling matrix element (based on Koopmans’ theorem approximation) and its distance dependence across a number of perpendicularly orientated n-alkanes yielded a decay constant of 1.25 Å-1 in excellent agreement with the measurements.