Sequential Ag doping of Au ₂₅ ⁻ atomically precise nanoclusters induces alternating positive and negative shifts of the HOMO–LUMO gap

Au₂₅(SR)₁₈ ^- is the most studied atomically precise metal nanocluster and is frequently used to explore the effects of doping on nanocluster performance in a range of applications. A quantitative accounting of the impact of dopants on its electronic structure remains elusive due to the inability to isolate clusters with exact numbers of dopants and the low resolution of experimental techniques used to probe electronic structure. We present high-resolution UV-vis spectra of Au_25-<i>n</i> Ag _<i>n</i> (SC₆H₁₃)₁₈ ^- (<i>n</i> = 0-6) recorded at low temperature in the gas phase after being precisely separated by mass spectrometry. These spectra feature substructure in the transitions among the frontier orbitals that reveal their shifts upon doping. Ag-doping blue shifts most spectral features, in accordance with theoretical predictions and observations, but the HOMO-LUMO gap shows an alternating pattern of red and blue shifts with increasing Ag substitution. We interpret these spectral shifts using the superatomic (jellium) model as resulting from perturbations to specific superatomic orbitals with nodes or nodal planes oriented with respect to the Ag dopant sites. Quantum chemical results reproduce these observations. These results show an unanticipated quantized doping effect that can be rationalized using accepted intuitive models and serve as clear quantitative tests for ongoing efforts to improve quantum chemical treatments of doped nanoclusters.