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 Au-25 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 Au-25 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.