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 3+ electro-reduction in aqueous 0.50 M KCl) exhibit characteristic exponential increase with the electrode potential (with transfer coefficient α = 0.25), and an exponential decay with the monolayer thickness (with a through-bond decay constant, βtb = 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, βts = 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.

Electron tunneling experiments involving Hg−Hg junctions incorporating two alkanethiolate monolayers are described. Formation of a symmetric junction (Hg−SC n −C n S−Hg) is accomplished by bringing in contact two small (3 × 10-3 cm2) mercury drop electrodes in a 5−20% (v/v) hexadecane solution of an alkanethiol. Formation of asymmetric junctions (Hg−SC n −C m S−Hg) and junctions containing n-alkane-3-thiopropanamide bilayers are also described. Tunneling currents in the junctions were measured for voltage biases extending to ±1.5 V. The currents decrease exponentially with the junction thickness yielding the tunneling decay constant, β = 0.89 ± 0.1 per CH2. The decay constant exhibits only a weak dependence on the voltage bias suggesting that electron tunneling follows a through-bond mechanism. Tunneling currents in the n-alkane-3-thiopropanamide bilayer junctions were larger than those in alkanethiolate junctions with the same number of atoms suggesting that introduction of an amide group increases the strength of the electronic coupling through these types of σ-bonded systems.

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Measurements of the reorganization energies (λ) and the maximum values of the electron transfer rate constants (k app max , obtained for -η g 2λ) of two redox probes at monolayer-coated electrodes are used as diagnostic parameters of the location of a probe at the monolayer/solution interface. Kinetics of the electroreduction of IrCl 6 2and FcCH 2 N(CH 3 ) 3 2+ at hexadecanethiolate-coated Hg drop electrodes were investigated in a broad range of overpotentials extending to values in excess of the reorganization energies of the two redox probes. Rate vs overpotential data were analyzed in terms of the Marcus-Gerischer formalism to yield the reorganization energies and k app max values. The former show that both probes reside initially in an aqueous environment at the alkane/solution interface. A larger value of k app max for the ferrocene probe was interpreted to indicate its closer approach to the interface. Access of the more strongly hydrated IrCl 6 2to the interface is more restricted by an interfacial water layer. Gradual expansion of the Hg drop, up to 20% of its initial surface area, has no effect on the magnitude of the reorganization energy obtained for IrCl 6 2-, proving that the iridium probe is located in the aqueous environment outside the alkane monolayer film. In contrast, a more hydrophobic ferrocene probe permeates the alkanethiolate monolayer immediately when even a small expansion of the Hg drop of ca. 2% is attempted.