A Revealing Insight into Gold Cluster Photocatalysts: Visible versus (Vacuum) Ultraviolet Light

[Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ (Au25) supported on TiO₂ (P25) exhibited distinct photocatalytic behaviors in the oxidation of amines using visible or ultraviolet light. The activity under visible light (455 nm) was superior to that under ultraviolet light. To gain insight into the origin of this difference, we investigated the photoreaction pathways of Au25 isolated in the gas phase upon irradiation with a pulsed laser with wavelengths of 455, 193, and 154 nm. High-resolution mass spectrometry revealed photon energy-dependent pathways for Au25: dissociation of the PPh₃ ligands and PPh₃AuCl units at 455 nm, dissociation into small [AuS]⁺ ions (n = 3-20; m = 0-4) at 193 nm, and ionization affording the triply charged state at 154 nm. These results were substantiated by density functional theory simulations. On the basis of these results, we proposed that the inferior photocatalytic activity of Au25/P25 under ultraviolet light is mainly due to the poor photostability of Au25.