Time-Resolved Laser-Induced Ring-Opening and Dissociation Dynamics in Bromothiophene Isomers.

Photoinduced molecular ring-opening reactions play a critical role in many natural chemical processes; however, there are pending questions regarding the fundamental mechanisms that govern these transformations. Furthermore, the understanding of ring-opening reactions has an important impact on optoelectronic and molecular control applications. These chemical reactions are driven by nonadiabatic coupling between electronic states through conical intersections in their potential energy surfaces, and their excited state dynamics are still a matter of debates. Here, we use a combination of on-the-fly ab initio molecular dynamics simulations and a powerful imaging ultrafast UV pump-IR probe spectroscopy technique to examine in detail the dissociation pathways followed by 2- and 3-bromothiophene. Using time-dependent momentum imaging, we identify clearly three dominant fragmentation channels following UV photon absorption: C-Br bond dissociation and cleavage of either C-S bond, which induce structural changes through molecular ring-opening prior to IR-induced ionization. We also obtain their fragmentation times, which span 600 to 1300 fs. Our findings elucidate both the initial excited states that initiate these dynamics and the nature of the electronic states reached by the nonadiabatic population transfer, a process essential to the observed ring-opening dynamics. Investigations of both 2- and 3-bromothiophene highlight the isomer dependence of UV absorption and reveal differences unknown before. Our results unambiguously support the role of ultrafast internal conversion from the dominant electronic states initiating ring-opening dynamics and identify the conical intersections that determine ring opening. Our work provides findings that advance the understanding of excited-state dynamics in ring molecules.