Electrochemistry of Molecule-like Au25 Nanoclusters Protected by Hexanethiolate

Near-monodisperse fractions of hexanethiol-protected gold nanoclusters (Au(25) and Au(144)) have been obtained by using a variation of the two-phase Brust method based on constant low temperature (4 degrees C) and control of the stirring conditions during the synthesis procedure. The size and dispersity of the Au-25(SC6)(18) and Au-144(SC6)(59) nanoclusters are determined by electrochemical means. The two first oxidation steps for the molecule-like Au-25(SC6)(18) clusters are quasi-reversible, and the electron transfer rate constants and diffusion coefficients have been determined by cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical behavior of these nanoclusters in different solvents can be explained by using the model described by Girault et al. (J. Phys. Chem. B 2006, 110, 21460; 2005, 109, 23925.) that considers the contribution of the bulk solvent relative permittivity on the charging energy values. Electrolyte ions and solvent molecule penetration must be taken into account to explain the deviation of the charging energy value in solvents such as toluene, whose relative permittivity is close to that of the protecting monolayer. The HOMO-LUMO energy gap measured by electrochemical and optical spectroscopy means does not show any influence of the solvent nature as observed for Au-25(PhC2S)(18).