Efficient photosensitization and surface ligand-controlled ultrafast electron transfer in copper nanoclusters in aqueous media.

Ultrasmall metal nanoclusters (MNCs) are emerging as efficient photosensitizers due to their discrete electronic states and prolonged excited-state lifetimes; however, a clear understanding of their ultrafast excited-state dynamics, particularly in aqueous media, remains limited for earth-abundant copper nanoclusters (CuNCs). Herein, we investigate the ligand-dependent photoinduced electron transfer (PET) dynamics and photosensitizing activity of tannic acid and cysteine-capped CuNCs (TA-CuNCs and Cys-CuNCs) in water. To probe the photosensitizing capabilities of CuNCs, we have investigated the photoreduction of methyl viologen (MV2+), a model molecular electron acceptor, by CuNCs (electron donor) in aqueous solution. Using ultrafast transient absorption spectroscopy (TAS), complemented by steady-state and time-resolved optical measurements, we have directly monitored charge carrier evolution from femtosecond to nanosecond timescales, revealing pronounced ligand-controlled differences in charge separation and recombination pathways. TA-CuNCs have exhibited slower back electron transfer and longer-lived charge-separated states compared to Cys-CuNCs, enabling more efficient interfacial electron transfer. These ultrafast spectroscopic insights confirm the photosensitizing ability of TA-CuNCs in aqueous medium, as demonstrated by their ability to drive photoinduced redox reactions, exemplified by the reduction of ferricyanide to ferrocyanide. This work establishes a direct correlation between ligand-modulated ultrafast dynamics and photosensitizing efficiency and further links it to improved photocatalytic performance in water, highlighting the critical role of surface chemistry in governing charge-transfer processes. The findings provide fundamental design principles for developing sustainable, high-performance MNCs-based photocatalysts for aqueous-phase energy and environmental applications.