Electron hopping through films of arenethiolate monolayer-protected gold clusters

Electron hopping in films of arenethiolate (benzylthiolate, phenylethylthiolate, phenylbutanethiolate, and cresolthiolate) monolayer-protected cluster molecules (MPCs) is investigated through measurements of solid-state electronic conductivity. Electron donor-acceptor hopping rates between the Au cores of arenethiolate MPCs exceed those of previously studied solid-state alkanethiolate MPC films, but the electronic coupling term, beta = 0.8 Angstrom(-1), is nearly the same. Rate constants range from 10(8) to 10(11) s(-1) across the series of arenethiolate MPCs; for the case of cresolthiolate, the rate corresponds to a single molecule resistance of similar to10(7) Omega/cresolthiolate ligand. The 4-8 kJ/mol activation-barrier energies for electron hopping are generally in line with the Marcus theory prediction. The low barrier energies and large rate constants arise from a combination of the low dielectric medium environment of the reactants (the MPC cores) and the partly aromatic tunneling bridges. The sharp increase in film conductivity upon substituting arenethiolate ligands for >50% of the hexanethiolates on a hexanethiolate-protected MPC suggests a percolation effect.