Three dimensional monolayers: Voltammetry of alkanethiolate-stabilized gold cluster molecules

Voltammetry of nonspecifically adsorbed and of freely diffusing monolayer protected clusters (MPCs) is presented. The MPC monolayers are mixtures of n-octanethiolates and omega-functionalized ferrocenyloctanethiolates. MPC adsorption coverages range from a few percent to roughly a full monolayer of cluster molecules. Rotated disk electrode voltammetry of the ferrocenated MPCs has two principal features: the ferrocene oxidation wave and sloping current baselines at prewave and postwave potentials. Each MPC molecule can have multiple ferrocene units; characteristics of the ferrocene wave indicate that the polyelectron-transfer oxidations occur as a rapid sequence of single electron transfers. Comparisons of different modes of polyelectron transfer suggest that the present one involves rotational diffusion. Deviation from ideal Nernstian one-electron-transfer behavior is modeled as a Gaussian distribution of E-0' values. The sloping current baselines are attributed to double layer charging of the cluster cores that is controlled by hydrodynamic mass transport. Further aspects of previously described relations governing the core-charging property are compared to and found to be consistent with experimental behavior. Finally, preliminary experiments for determining MPC diffusion coefficients by the Taylor dispersion method are described.