However, studies on organic fluorescent dyes have shown that introducing heavy atoms, such as iodine (I), into the fluorophore enhances spin-orbit coupling (the internal heavy-atom effect), which increases the rate constant for intersystem crossing (ISC) from Sn to Tm, thereby improving the efficiency of phosphorescence. Similarly, encapsulating a heavy atom into Ag-S clusters may enhance the phosphorescence quantum yield. However, to date, there are limited reports on the internal heavy-atom effect of ligand-protected metal clusters. In this study, we successfully synthesized ligand-protected Ag-S clusters with central cavities encapsulating anions X z-, where X z-= S 2-(sulfide) or I -(iodide), which significantly differ in atomic number each other (Scheme 1). Single-crystal X-ray diffraction (SC-XRD) and proton nuclear magnetic resonance ( 1 H NMR) spectroscopy revealed that the resulting Ag clusters were composed of X@Ag54S20(thiolate)20(sulfonate)m, where (X, m) = (S, 12) or (I, 11) (X@Ag54). Furthermore, a comparison of the optical properties of the obtained pair of X@Ag54 (X = S or I) demonstrated that an internal heavy-atom effect occurred in these Ag-S clusters, similar to that in organic fluorescent dyes. Accordingly, the phosphorescence quantum yield was 16 times higher when S 2-was replaced by I -as the encapsulated atom (Scheme 1). Scheme 1. Enhancement of phosphorescence quantum yield by encapsulating S 2-or I -anions inside the central cavity of the Ag54S20(S t Bu)20(SO3 t Bu)m framework.