Understanding UV-Induced Photochemistry Involving Isomerization and Ring-Opening in Thiophenone*

Photoinduced isomerization reactions are the driving force behind many chemical processes in nature. We report the results of a combined experimental and theoretical study of the photoinduced ring-opening and isomerization reaction in thiophenone. To visualize the ultrafast structural changes as well as the underlying potential energy landscape, the same reaction was studied by time-resolved photoelectron spectroscopy (TRPES) using a seeded free-electron laser and by Mega-electron-volt ultrafast electron diffraction (MeV-UED). Both methods reveal detailed insights into the ring opening dynamics as well as the formation of three major open-ring photoproducts along with vibrationally hot ground state thiophenone within a timescale of a few hundred femtoseconds. The experiments are complemented by ab initio electronic structure and molecular dynamics calculations of the ground and excited-state molecules. The ring opening is shown to be driven by ballistic S-C bond extension over the timescale of ~100 fs, followed by a redistribution of the energy into several other vibrational degrees of freedom and, in some cases, a subsequent dissociation of the molecule, e.g. by elimination of CO, on a time scale of hundreds of picoseconds.