Relaxation processes from charge-transfer excited states of organic radical 1,3,5-trithia-2,4,6-triazapentalenyl crystals studied by ultrafast luminescence spectroscopy

The excited state dynamics of 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) was investigated by using femtosecond luminescence spectroscopy. The luminescence was measured in low and high temperature phases of TTTA between 1.42 and 1.2 eV. Only in the low temperature phase, we observed a broad luminescence band with a decay time constant of 1.6 ps in all energy range. This decay time constant suggests the existence of a large nonradiative relaxation path in the low temperature phase. Fast depolarization of the luminescence was also found, suggesting that an energy transfer between charge transfer excited states with different orientations are large and a few kinds of lattice distortion exist in this system.