Enhancement of Photoluminescence Quantum Yield of Silver Clusters by Heavy Atom Effect

Many ligand-protected metal clusters exhibit phosphorescence at room temperature. However, strategies for improving their phosphorescence quantum yield, a critical parameter of performance, remain poorly developed. In contrast, fluorescent dyes are commonly modified by introducing heavy atoms, such as iodine (I), to enhance intersystem crossing in the excited state, thereby harnessing the heavy atom effect to increase phosphorescence efficiency. In this study, a pair of ligand-protected silver (Ag) clusters is successfully synthesized with internal cavities encapsulating anions (X^{z -}), namely sulfide ions (S^2-) or iodide ions (I^-), which significantly differ in atomic number each other. Single-crystal X-ray diffraction and nuclear magnetic resonance spectroscopy revealed that the resulting Ag clusters are composed of X@Ag₅₄S₂₀(thiolate)₂₀(sulfonate)_m, where (X, m) = (S, 12) or (I, 11). X-ray photoelectron spectroscopy revealed that the Ag atoms in these compounds exhibit a mixed-valence state. Furthermore, experiments on their photoluminescence revealed that a heavy central anion induced an internal heavy-atom effect similar to that observed in organic fluorescent dyes. As a result, the phosphorescence quantum yield became 16 times higher when S^2- is replaced by I^- as the central atom.