The Hidden Mechanism: Excited‐State Proton‐Electron Pair Transfer in Metal Nanocluster Emission

Comprehending the underlying factors that govern photoluminescence (PL) in metal nanoclusters (NCs) under physiological conditions remains a highly intriguing and unresolved challenge, particularly for their biomedical applications. In this study, we evaluate the critical role of excited‐state proton‐coupled electron transfer in the emission of metal NCs. Our findings demonstrate that hydronium ion (H3O+) binding can trigger a nonlinear, pH‐dependent excited‐state concerted electron proton transfer (CEPT) reaction. This involves simultaneous electron transfer from the Au(0) core to the Au(I)‐ATT (ATT denotes 6‐aza‐2‐thiothymidine) surface and proton transfer from H3O+ to the ATT ligand in a single step, greatly promoting vibrations and rotations of the Au(I)‐ATT surface, resulting in substantial PL quenching of Au10(ATT)6 NCs. Further analyses show that the unique CEPT dynamics are strongly influenced by the opposing effects of increased reorganization energy and a larger pre‐exponential factor on the electron transfer rate. Moreover, the proposed excited‐state CEPT process is found to be prevalent in core‐shell relaxation metal NCs, such as Au25(SR)18 (SR denotes thiolate) NCs, and serves as an important factor in limiting their PL emission. By simply controlling the pKa of the ligands, the emission performance of Au25(SR)18 can be easily regulated in physiological environments.