Abstract: A semiclassical dynamics simulation study was undertaken to determine the photophysical deactivation of the lowest excited state of two stacked thymines. Only one thymine, referred to as T, was excited by a laser pulse and the other molecule, referred to as T′, remained in the ground state. The simulation results show that charge transfer between the two thymines because of a π-stacking interaction leads to the formation of an excimer state, which includes a negative T and a positive T′. Additionally, the simulation study indicates that a steric effect of the neighboring bases inhibits the out-of-plane deformation, which is essential in accessing the conical intersection between the lowest electronic-excited state and the ground state. The steric effect eventually leads to a longer electronic-excited state lifetime for the two stacked thymines. The simulation results reveal that when the interbase distance is less than 0.3 nm the molecule T has a remarkable deformation at its C₅ and C₆ sites resulting in charge recombination. The charge recombination ultimately makes the system electronically neutral. On the other hand, the molecule T′ has a strong twist about its C₅′―C₆′ bond in the proximity of the avoided crossing by which the system decays to the ground state. Finally, the two thymine molecules in their ground states recover their planar geometries.

Key words: Thymine, Semiclassical dynamic simulation, Charge transfer, Excimer, Deactivation