Figure shows the PL spectra of X@Ag54 in toluene under an Ar atmosphere. To obtain strong PL, the excitation wavelength was set to 405 nm. Both cluster solutions exhibited a broad PL peak at approximately 610 nm (Figure ). Recent studies have shown that the PL of metal clusters consisting of a large number of metal atoms entails photoexcitation to generate a dark excited singlet state, which subsequently undergoes intersystem crossing (ISC) to a bright excited triplet state, resulting in phosphorescence. Because X@Ag54 also comprised a large number of metal atoms, the observed emission of X@Ag54 is likely phosphorescence. In fact, the estimated radiative lifetime of X@Ag54 was on the order of tens to hundreds of microseconds (Table ), which is significantly longer than that of fluorescent materials (≤ 1 µs). The photoluminescence quantum yield (ΦPL) of X@Ag54 was evaluated using a relative method, revealing that the ΦPL of I@Ag54 was approximately 16 times greater than that of S@Ag54 (Table ). Because there was no significant difference in the symmetry of the geometries of the two X@Ag54 clusters, the difference in ΦPL did not arise from symmetry breaking. Figure 5. (A) CID-mass spectra of (a) S@Ag54 and (b) I@Ag54 as a function of collision energy. CE indicates the collision energy. (B) Survival yield obtained from CID profiles of X@Ag54 (Figure ). Details for calculating the survival yield are provided in the Supporting Information (Section S1.8). (C) CID process of X@Ag54. However, 1) the two X@Ag54 clusters differs in the type of anion present in their internal cavities, and 2) ΦPL increases with increasing atomic number of encapsulated anion. Introducing heavy halogens (heavy atoms) into aromatic dyes, such as acene-based compounds, enhances ISC from the excited singlet to the excited triplet state owing to the heavy atom effect, which increases the radiative rate constant (kr) and, consequently, ΦPL. In the present system, a similar effect likely occurred for I@Ag54 to exhibit photoluminescence with high ΦPL.