Accelerated intersystem crossing enhances NIR emission in Au(SR) nanoclusters by surface ligand engineering.

Surface ligand modification is an effective strategy for enhancing the photoluminescence (PL) of atomically precise gold nanoclusters. Nevertheless, the underlying mechanism of PL enhancement, particularly the role of triplet states remains vague. In this work, we picture the formation and relaxation of triplet states in a series of four Au(SR) nanoclusters with different -R groups by probing their excited state dynamics. Electronic pump-probe spectroscopy reveals that the nanoclusters with fewer methyl groups on the ligand exhibit a faster intersystem crossing (ISC) pathway, hence boosting the triplet-state-related emission. Additionally, the metal core of the four nanoclusters exhibits similar low-frequency vibrations, suggesting that the variation in non-radiative relaxation is primarily mediated by ligand vibrations, rather than the metal core vibrations. These findings establish that ligand-induced PL enhancement is driven by both accelerated ISC and reduced vibrational dissipation. This work sheds light on the mechanism by which ligand engineering enhances PL in gold nanoclusters, highlighting the critical role of triplet state dynamics in tailoring their emission properties, which hold promise in applications such as sensing, bioimaging, optoelectronics, and solar energy conversion.