Furthermore, we have attempted to find out the underlying reason for the broad emission by investigating the energetics of the self-trapped excitons (STE) emission pathway. For a ready comparison, we have studied our Sn-based (R-isomer) system and the corresponding Pb counterpart, where such broad emission is found to be absent. STE emission energy has been calculated, following the approach of our previous studies, by considering the energy difference between the energies of the triplet and singlet states (at the equilibrium geometry of triplet state) as schematically described by point 3 and point 4, respectively in the Figure . We have found an excellent agreement of emission energy of 1.70 eV compared to that of experiment, where the emission peak appears around 720-740 nm (1.72-1.67 eV). Furthermore, as shown in Figure , the Kohn-Sham Furthermore, the calculated triplet and singlet relaxation energies, as depicted in Figure , for the Sn-system are 277 and 565 meV, respectively, compared to that of 88 and 56 meV for Pb-system.