Light-Induced Conformational Switching of 2-Formyl Benzothiazole in Low Temperature Inert Gas Matrices

Benzothiazole-a versatile heterocyclic moiety omnipresent in a range of applications-can ring-open under photochemical conditions owing to its photolabile C-S bond. Conversely, an attachment of aldehyde functionality into a benzothiazole moiety can potentially lead to light-induced conformational change. To identify the photochemical outcome among these competitive channels, we investigated the photochemistry of 2-formyl benzothiazole 2FBT in an inert gas matrix under cryogenic conditions (4 K). Only the global minimum of 2FBT ((s)-trans conformer) was isolated during deposition, which undergoes conformational changes to (s)-cis in both argon and nitrogen matrices upon irradiation at λ=254 nm. Only under prolonged irradiation, traces of ring-opening products were observed. Ground and excited state potential energy surface computations at CAM-B3LYP/6-311++G(d, p) level of theory revealed two adiabatic mechanistic pathways involving C-C bond rotation and C-S bond cleavage. The formation of (s)-cis (4.4 kcal/mol higher energy conformer) by light, no further conformational changes after 23% (s)-cis formation, and the absence of reversibility hinted at the possibility of the channel involving C-S bond cleavage. Computationally, the involvement of an intermediate in the excited state and relatively lower energy barriers further support that the C-S cleavage is the preferred channel.