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

The ultrafast S-1((1)pi pi*)-> S-0 deactivation 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 (similar to 80%) decay to the ground state (S-0) 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 S-1((1)pi pi*)-> S-0 internal conversion process. After thiophene is excited to the S-1((1)pi pi*)-> S-0 state in the Franck-Condon region, it quickly relaxes to the minimum of the S-1((1)pi pi*) state, then overcomes a small barrier near the conical intersection (CI((1)pi pi*/(1)pi sigma*)), and eventually arrives at the minimum of one C-S bond fission (S-1((1)pi sigma*)). In the vicinity of this minimum, the conical intersection (CI(1 pi sigma*/S-0)) funnels the electron population to the ground state (S-0), completing the ultrafast S-1((1)pi pi*)-> S-0 internal conversion process. This decay mechanism matches well with previous experimental and theoretical studies.