Ab Initio Trajectory Surface-Hopping Study on Ultrafast Deactivation Process of Thiophene.

The ultrafast S1(¹ππ*)→ Sodeactivation process of thiophene in the gas phase has been simulated with the complete active space self-consistent field (CASSCF) based fewest switch surface hopping method. It was found that most of the calculated trajectories (~80%) decay to the ground state (So) with an averaged time constant of 65 ± 5 fs. This is in good agreement with the experimental value of about 80 fs. Two conical intersections were determined to be responsible for the ultrafast S1(¹ππ*)→ So internal conversion process. After thiophene is excited to the S1(¹ππ*) state in the Franck-Condon region, it quickly relaxes to the minimum of the S1(¹ππ*) state, then overcomes a small barrier near the conical intersection (CI(¹ππ*/¹πσ*)), and eventually arrives at the minimum of one C-S bond fission (S1(¹πσ*)). In the vicinity of this minimum, the conical intersection (CI(¹πσ*/So)) funnels the electron population to the ground state (So), completing the ultrafast S1(¹ππ*)→ So internal conversion process. This decay mechanism matches well with previous experimental and theoretical studies. [ABSTRACT FROM AUTHOR]