To confirm the presence of an internal heavy-atom effect, the PL decay curves of X@Ag54 in deuterated toluene were evaluated using the time-correlated single-photon counting method (Figure ). The obtained decay curves were fitted with multiple exponential decays to determine the average photoluminescence lifetimes (τPL) of S@Ag54 and I@Ag54, which were 106.2 and 652.9 ns, respectively (Tables and). From this fitting, the radiative rate constants (kr) of I@Ag54 and S@Ag54 were estimated to be 1.9×10 3 and 4.8×10 4 s -1 , respectively (Table ). Therefore, when X changed from S 2-to I -, kr increased by approximately 25 times. This trend is similar to the enhancement of phosphorescent parameters through the heavy atom effect in conventional fluorescent dyes. Therefore, it can be interpreted that replacing S 2-with heavier I -as X in X@Ag54 accelerated ISC via the heavy atom effect, which increases the population of the bright triplet state and kr, thereby improving the ΦPL of X@Ag54.