Molecular Au25(SR)18 Nanoclusters: Lifetime of Electrogenerated Species and Dielectric Properties of the Protecting Monolayer

Abstract

We used a series of molecular Au25(SCnH2n+1)18 clusters (n = 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18) to study the effect of n and the presence of electrolytes on their redox potentials. We found that the average dielectric constant of the monolayer, ε m, depends on n and is the result of contributions from ligands, solvent and electrolyte. When n increases, ε m decreases until a virtually constant value is attained for sufficiently long ligands. The electrochemically calculated HOMO-LUMO energy gap, on the other hand, does not depend on n and, therefore, ε m.1 We also studied the effect of n on the lifetime of electrogenerated species. Whereas on the voltammetric timescale the -1, 0, and +1 charge state of Au25 clusters are stable, formation of the -2 and +2 states causes fragmentation of the cluster. Voltammetric analysis indicates that the former entails reactions involving ligands in the Au12(SCnH2n+1)18 protecting shell, whereas the latter is accompanied by a slower but also more substantial fragmentation affecting the integrity of the central Au13 core. 2