Ultrafast T1 generation in pyrene-4,5-dione and pyrene-4,5,9,10-tetrone

Intersystem crossing pathways associated with the ultrafast generation of the lowest triplet state (T1) of pyrene-4,5-dione (PD) and pyrene-4,5,9,10-tetrones (PT) are studied with the aid of quantum chemistry and quantum dynamics simulations. Both these molecules exhibit a rapid relaxation to dark (dipole-forbidden) S1 upon populating higher bright (dipole-allowed) singlet excited states. The S1-T3 pathway is responsible for the T3 generation via the direct spin-orbit coupling in PD and the vibrational coordinate-dependent spin-vibronic coupling in PT. A rapid T3 to T1 internal conversion is observed in these molecules. The photosensitization abilities of these molecules are discussed based on the efficiency of the T3 state formation and its decay to T1 state.