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 [AunSm]+ 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.