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, β = 0.8 Å -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 ∼10 7 Ω/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.