Nanosecond Laser Flash Photolysis Study of the Photochemistry of 2-Alkoxy Thioxanthones

Laser flash photolysis studies (λ = 355 nm) on the characterization and reactivity for the triplet excited state of 2-methoxythioxanthone, 2-benzyloxythioxanthone, and 2-(n-propoxy) thioxanthone revealed that the transients generated in their excitation, in acetonitrile, have absorption bands with maxima at 310 and 620 nm. These transients decayed by a mixed 1^st and 2^nd order kinetics, with a lifetime of 5.3 μs for 2-methoxythioxanthone and benzyloxythioxanthone and 3.9 μs for 2-(n-propoxy) thioxanthone. In methanol, ethanol, and 2-propanol, a new absorption at 430-460 nm was observed, which was attributed to their corresponding ketyl radical (the second-order quenching rate constant (k_q) is ca. 10⁵ L mol^-1 s^-1). The energy transfer rate constants for trans-stilbene, 1-methylnaphthalene, and 1,3-cyclohexadiene were diffusion-controlled, indicating a triplet energy for these alkoxy thioxanthones higher than 61 kcal mol^-1. The quenching rate constant observed for 1,4-cyclohexadiene (ca.10⁹ L mol^-1 s^-1) suggests that the lowest energy triplet excited state for these thioxanthones must have nπ* character. For phenols and indole (k_q ca. 10⁹ L mol^-1 s^-1), the reaction mechanism must involve the initial formation of a triplet exciplex, followed by a coupled electron/proton transfer. For triethylamine and 1,4-diazabicyclo[2.2.2]octane (k_q ca. 10⁹ L mol^-1 s^-1) it was observed the formation of the radical anion of the alkoxy thioxanthones formed from electron transfer (λ = 410 nm).

Keywords:
laser flash photolysis; thioxanthones; energy transfer; hydrogen transfer; electron transfer