Surface ligand networking promotes intersystem crossing in the Au₁₈(SR)₁₄ nanocluster

Understanding the relationships between the structure and optical properties of ligand-protected, atomically precise metal nanoclusters (NCs) is of paramount importance for exploring their applications in photonics, biomedicine and quantum technology. Here, two Au₁₈(SR)₁₄ NCs protected by 2,4-dimethylbenzenethiolate (DMBT) and cyclohexanethiolate (CHT), respectively, are studied using time-resolved absorption and emission spectroscopies. Although the two NCs exhibit similar photoluminescence (PL) quantum yields (QY ∼ 0.1%) at room temperature, their excited state dynamics are very different, which are modulated by the interactions between the Au core and the ligands, as well as the networking interactions among aromatic ligands. Specifically, Au₁₈(CHT)₁₄ exhibits a single exponential decay of its singlet excited state (time constant <i>τ</i> = 17 ns) with almost no triplet population. In contrast, there is a triplet population of more than 15% for Au₁₈(DMBT)₁₄, and an intersystem crossing (ISC) process of ∼4 ns is identified. Temperature-dependent PL measurements of Au₁₈(DMBT)₁₄ show three radiative processes, including prompt fluorescence, thermally activated delayed fluorescence and phosphorescence. The nonradiative process is partially suppressed at low temperatures, leading to enhanced photoluminescence (QY up to 9.0%) and exclusive phosphorescence was observed below 120 K. The obtained insights into the excited state energy flow and PL dynamics will benefit future design of luminescent NCs for optoelectronic applications.