In both systems, the population transferred to the lowest-lying triplet states could deactivate through phosphorescence, ISC to the ground state, and reversed ISC to the singlet states. According to the potential energy profiles along the reaction pathways, the ISC to the ground state is hindered in both systems, because the S 0 /T 1 crossings, which are in the vicinity of the S 1 -S 0 MECI (Figure ), are classically inaccessible following the S 1 excitations.