The non-radiative rate constant (knr) of I@Ag54 was approximately 1/6 of that of S@Ag54 (Table ). Because I -has a larger ionic radius than S 2-, it fills a greater volume of the cavity than S 2-, and thus I@Ag54 seems to have a more rigid framework than S@Ag54. In addition, the bond enthalpies of Ag-I and Ag-S bonds are 234 and 217 kJ mol -1 , respectively, indicating that the Ag-I bond has a slightly higher bond energy than the Ag-S bond. These factors seems to accelerate the vibrational relaxation of I@Ag54 compared with that of S@Ag54, thereby reducing the non-radiative rate constant of I@Ag54 to approximately 1/6 of that of S@Ag54.