Wavelength-Dependent Photochemistry of 2-Formyl Thiazole in Solid Argon: High-Energy Conformational Trapping vs Ring-Opening Channels

Abstract We report the photochemistry of 2-formyl thiazole 2FT in inert gas matrices at 4 K, complemented by computations. The present study aims to unravel the dominant photochemical channel among ring-opening of thiazole and light-induced conformational changes (LICC) associated with the formyl group. 2FT underwent LICC from (s)-trans to (s)-cis under 365 nm UV light irradiation. However, at 254 nm, 2FT led to LICC and photoproducts via ring-opening and fragmentation channels under short- and prolonged irradiation conditions, respectively. To gain mechanistic insights, we computed ground- and excited-state potential energy surfaces at the CAM-B3LYP/6–311++G(d,p) level of theory. Apparently, an adiabatic low-barrier (13.9 kcal/mol) rotational pathway in S1 triggers the LICC; conversely, the C–S cleavage pathway (17.7 kcal/mol barrier in S1) becomes accessible under prolonged irradiation conditions. The primary photofragmentation pathway of 2FT involves the formation of CO and the photoproducts ethynethiol, HCN, and other ring-opened species. Additionally, alpha cleavage and C–S ring scission channels led to several intriguing sulfur- and nitrogen-rich photoproducts. Overall, the investigations delineate the wavelength dependence from the computed absorption profiles of the two conformers. Furthermore, the importance of the excited-state barrier in the competition between irreversible high-energy conformational trapping and photofragmentation in 2FT has been unraveled.