Probing Photochemically and Thermally Induced Isomerization Reactions in α-Pyrone

The isomerization dynamics of α-pyrone dissolved in CH₃CN have been probed by femtosecond 267 nm pump/broadband infrared (IR) probe spectroscopy. A novel experimental setup allowed the populations of the parent molecule and ring-opened photoproducts to be monitored over pump/probe time delays ranging between 2 ps and 100 μs within a single experiment, and at 5 different temperatures between 0 and 40 °C. The photochemically prepared α-pyrone(S₁) molecules decay rapidly (0 potential energy surface, with an initial quantum yield for parent molecule re-formation of 60%. Probing the antisymmetric ketene stretch region (2100-2150 cm^-1) confirms the presence of at least two ring-opened photoproducts, which are assumed to have an E-configuration with respect to the central C=C double bond. These ketenes are observed to undergo two distinct, thermally driven, isomerization processes which occur on the nanosecond and microsecond time scales, respectively. The former reaction is ascribed to thermalization of the initially prepared E-isomer populations, while the slower (microsecond) process involves rotation around the central C=C double bond leading to formation of Z-isomers. Subsequent rapid Z → Z isomerizations (occurring on a nanosecond time scale) result in ring-closure and a second, longer time recovery of parent molecule population. By determining rates as a function of the sample temperature, barrier heights of 0.23(3) eV and 0.43(2) eV are obtained for the E → E and E → Z transformations, respectively.