Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au25(SR)18 Clusters

Abstract

This study addresses how ligands module the structure and the electronic optical properties of a large set of the experimentally known anionic thiolate-protected gold clusters, Au25(SR)18[1−]. Starting from the experimental crystal structure, computational density functional theory calculations reveal that low-polarity R groups do not disturb the Au25S18 framework significantly, such that the inversion symmetry(Ci) of the crystalline state is retained. In the case of p-thiolphenolate ligands, p-SPhX, a major distortion of the Au25S18 framework, destroys the inversion symmetry, the distortion increasing in the order given X = H, Cl, NO2 and CO2H. For branched R groups, linking −CH3 or −NH2 groups at the two-position of the phenylethylthiolate ligand, the inversion symmetry is retained and lost, respectively; similarly, the N-acetyl-cysteine ligand also distorts the framework. These results demonstrate a systematic preference of inversion-symmetric versus nonsymmetric framework depending on the ligand-type. The more distorted structures also exhibit significantly reduced HOMO–LUMO gap values and affect the optical absorption spectra accordingly. This study correlates the distortion of the Au25S18 framework with the structure, electronic, and optical properties among the studied clusters.