Ligand effects on the structure and the electronic optical properties of anionic Au₂₅(SR)₁₈ clusters

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(C_i) of the crystalline state is retained. In the case of p-thiolphenolate ligands, p-SPhX, a major distortion of the Au ₂₅S₁₈ framework, destroys the inversion symmetry, the distortion increasing in the order given X = H, Cl, NO₂ and CO ₂H. 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.