Unique Ultrafast Visible Luminescence in Monolayer-Protected Au₂₅ Clusters

The luminescence of quantum-sized metal clusters has enthralled the scientific community in recent years. In this study, ultrafast luminescence dynamics of hexanethiol (C₆S)- and glutathione (GS)-protected Au₂₅ clusters are investigated with time-resolved luminescence spectroscopy. The focus of the present investigation is to understand the dynamics of higher excited states and also the relaxation of core Au states to Au semi-ring states in Au₂₅L₁₈ (“L” is the protecting ligand) clusters. Comparative luminescence measurements on larger monolayer-protected gold clusters (2.2 nm Au(C₆S) and 2.2 nm Au(GS)) and gold nanoparticles (3 nm Au(C6S) and 13 nm Au(citrate-stabilized)) are also carried out. The investigated Au₂₅L₁₈ clusters have shown a low quantum-yield visible photoluminescence in addition to near-infrared luminescence, which is used as a probe to follow the dynamics of core Au states. The luminescence decay traces of Au₂₅L₁₈ clusters have shown unique ultrafast growth and decay kinetics that are absent in the larger monolayer-protected gold clusters. The growth time constants are independent of the passivating ligand, suggesting that the luminescence arises out of the Au₂₅ core states. The decay of the luminescence is dependent on the passivating monolayer and is ascribed to the relaxation of the core Au states to S−Au−S−Au−S semi-ring states. However, the excited-state dynamics in Au₂₅L₁₈ clusters is not a typical two-state relaxation from core to semi-ring states, but rather proceeds through a manifold of electronic states as the luminescence traces show wavelength-dependent growth and decay kinetics. Also, femtosecond time-resolved luminescence measurements of Au₂₅L₁₈ have proved that the higher excited states in monolayer-protected Au₂₅ clusters decay with a finite lifetime (200 fs up to a few picoseconds) that can be utilized for applications in solar energy harvesting and catalysis.