Insights into the Interface Between the Electrolytic Solution and the Gold Core in Molecular Au 25 Clusters

We used a large series of Au 25 (SC n H 2 n +1 ) 18 0 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 the redox potentials. The electrochemical results were analyzed in the framework of concentric‐capacitor models that were previously proposed to describe larger monolayer‐protected clusters. We found that the average dielectric constant of the monolayer ( ϵ m ) is significantly larger than that of the ligand chains. The effective value of ϵ m depends on n and is the result of contributions from ligands, solvent, and electrolyte. As n increased, ϵ m decreased until a virtually constant value was attained for sufficiently long ligands. The electrochemically calculated HOMO–LUMO energy gap, conversely, does not depend on n and, therefore, ϵ m . This energy gap is, within error, the same as that obtained from the optical spectra of the same clusters.