Isomeric Effects of Au₂₈(S‑c‑C₆H₁₁)₂₀ Nanoclusters on Photoluminescence: Roles of Electron-Vibration Coupling and Higher Triplet State

The exploration of near-infrared photoluminescence (PL) from atomically precise nanoclusters is currently a prominent area of interest owing to its importance in both fundamental research and diverse applications. In this work, we investigate the near-infrared (NIR) photoluminescence mechanisms of two structural isomers of atomically precise gold nanoclusters of 28 atoms protected by cyclohexanethiolate (CHT) ligands, i.e., Au_28i(CHT)₂₀ and Au_28ii(CHT)₂₀. Based on their structures, analysis of ³O₂ (triplet oxygen) quenching of the nanocluster triplet states, temperature-dependent photophysical studies, and theoretical calculations, we have elucidated the intricate processes governing the photoluminescence of these isomeric nanoclusters. For Au_28i(CHT)₂₀, its emission characteristics are identified as phosphorescence plus thermally activated delayed fluorescence (TADF) with a PL quantum yield (PLQY) of 0.3% in dichloromethane under ambient conditions. In contrast, the Au_28ii(CHT)₂₀ isomer exhibits exclusive phosphorescence with a PLQY of 3.7% in dichloromethane under ambient conditions. Theoretical simulations reveal a larger singlet (S₁)–triplet (T₁) gap in Au_28ii than that in Au_28i, and the higher T₂ state plays a critical role in both isomers’ photophysical processes. The insights derived from this investigation not only contribute to a more profound comprehension of the fundamental principles underlying the photoluminescence of atomically precise gold nanoclusters but also provide avenues for tailoring their optical properties for diverse applications.