Transient UV pump-IR probe investigation of heterocyclic ring-opening dynamics in the solution phase: the role played by n sigma* states in the photoinduced reactions of thiophenone and furanone

The heterocyclic ring-opening dynamics of thiophenone and furanone dissolved in CH3CN have been probed by ultrafast transient infrared spectroscopy. Following irradiation at 267 nm (thiophenone) or 225 nm (furanone), prompt (tau <1 ps) ring-opening is confirmed by the appearance of a characteristic antisymmetric ketene stretching feature around 2150 cm(-1). The ring-opened product molecules are formed highly vibrationally excited, and cool subsequently on a similar to 6.7 ps timescale. By monitoring the recovery of the parent (S-0) bleach, it is found that similar to 60% of the initially photoexcited thiophenone molecules reform the parent molecule, in stark contrast with the case in furanone where there is less than 10% parent bleach recovery. Complementary ab initio calculations of potential energy cuts along the S-C(=O) and O-C(=O) ring-opening coordinate reveals insights into the reaction mechanism, and the important role played by dissociative (n/pi)sigma* states in the UV-induced photochemistry of such heterocyclic systems.