MOLECULAR-STRUCTURE AND TEMPERATURE-DEPENDENT RADIATIVE RATES IN TWISTED INTRAMOLECULAR CHARGE-TRANSFER AND EXCIPLEX SYSTEMS

Contrary to the general rules, the radiative rates in intramolecular full CT excited states are temperature-dependent: fluorescence emission is thermally activated. This is demonstrated on the TICT states and on highly polar intramolecular exciplexes. The radiative (back) electron transfer from the equilibrium structure of these excited states is highly forbidden often due to zero or minimal orbital overlap (nodal plane effects); thus, the equilibrium structures in both classes of states correspond to practically pure CT configurations. Vibrational activation (100-600 cm-1) is predominantly responsible for the observed fluorescence and presents a new key to the structure of these excited species. Thermally activated emission is predicted for other flexible molecules with a strongly forbidden transition.