Electron Hopping Dynamics in Monolayer-Protected Au Cluster Network Polymer Films by Rotated Disk Electrode Voltammetry

Electrons are transported within polymeric films of alkanethiolate monolayer-protected Au clusters (MPCs) by electron hopping (self-exchange) between the metal cores. The surrounding monolayers, the molecular linkers that generate the network polymer film, or both, presumably serve as tunneling bridges in the electron transfers. This paper introduces a steady-state electrochemical method for measuring electron hopping rates in solvent-wetted and swollen, ionically conductive MPC films. The films are network polymer films of nanoparticles, coated on a rotated disk electrode that is contacted by a solution of a redox species (decamethylferrocene, Cp*Fe). Controlling the electrode potential such that the film mediates oxidation of the redox probe can force control of the overall current onto the rate of electron hopping within the film, which is characterized as the apparent electron diffusion coefficient D_E. D_E is translated into an apparent electron hopping rate k_ET by a cubic lattice model. The experiment is applied to MPC network polymer films linked by α,ω-alkanedithiolates and by metal ion−carboxylate connections. We evaluate the dependencies of apparent hopping rate on Cp*Fe concentration, film thickness, electrode potential relative to the Cp*Fe formal potential, film-swelling solvent, and temperature. The apparent hopping rates are in the 10⁴−10⁵ s^-1 range, which is slower than those for the same kind of MPC films, but in a dry (nonswollen) state measured by electronic conductivities.