Role of Donor and Acceptor Substituents on the Nonlinear Optical Properties of Gold Nanoclusters

In recent years, a large number of monolayer-protected clusters (MPCs) have been studied by means of single crystal structure characterization. A central aspect of research on MPCs is the correlation of their interesting optical properties with structural features and the formulation of a theoretical framework that allows interpretation of their unique properties. For this, superatom and jellium models have been proven successful. Little attention, however, has been paid to the influence of the protecting ligands. Here, we investigate the effect of changes in [Au-25(SR)(18-x)(SR')(x)](-), where SR' represents a para-substituted thiophenolate derivative (SPh-4-X). We computed the first hyperpolarizabilities, screening a broad range of substituents X. For [Au-25(SR)(17)(SR')(1)](-) clusters, significant first hyperpolarizabilities were calculated, spanning 2 orders of magnitude depending on X. A strong dependence on the electron-donating/withdrawing properties of the substituent was found for para-substituted thiophenol ligands. Protonation of amine substituents leads to a change from donor to acceptor substitution, leading to a record setting contrast for nonlinear optical proton sensing. Furthermore, push/pull systems were considered where both an acceptor and a donor ligand were positioned at opposite ends of the cluster. This induces significant increase of the first hyperpolarizability through charge transfer. Overall, our results indicate that the right choice of ligand can significantly impact the (nonlinear) optical properties of MPCs. This adds a new component to the cluster chemist's toolbox.