Abstract
Fluorescent Au25 nanoclusters recently have drawn considerable research interest due to their unique properties and potential applications. Despite significant advances in their synthesis methods and application development, the origin of the fluorescence and underlying mechanism still remain unclear. In this work we investigate the fluorescence dynamics in BSA-protected Au25 nanoclusters by time-resolved photoluminescence and transient absorption techniques covering picosecond to microsecond time scales. We demonstrate here that the red fluorescence consists of both prompt fluorescence and thermally activated delayed fluorescence, and the latter is more dominant. A small energy gap of 30 meV between the triplet and the singlet states was determined from our temperature-dependent time-resolved fluorescence measurement. Moreover, we elucidate that the absorption band at 2.34 eV corresponds to the HOMO–LUMO transition in this system due to the interaction between Au25 NCs and BSA. We also show that an effective relaxation pathway exists from the higher excited state to the LUMO.
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