Photochemistry of thioxanthen-9-one-10,10-dioxide: A remarkably reactive triplet excited state

The steady state photolysis of thioxanthen-9-one-10,10-dioxide (1) in dichloromethane does not result in any product formation. In hydrogen donor solvents, such as 2-propanol, toluene or cyclohexane, the formation of the corresponding pinacol was readily observed. The phosphorescence spectrum of 1 in EPA, at 77 K, reveals a triplet energy of 66.3 kcal mol−1. Laser flash photolysis irradiation of a degassed acetonitrile solution of 1 led to the formation of its triplet excited state (λmax = 375 and 520 nm, τ = 11 μs) that was quenched by oxygen, β-carotene and 1,3-cycloexadiene at a diffusion controlled rate constant. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, methanol, ethanol, 2-propanol, cyclohexane, substituted phenols and amines were determined, ranging from 7.1 × 106 (for methanol) to 3.1 × 1010 L mol−1 s−1 (for triethylamine). From the quenching rate constants observed for the triplet excited state and from the steady state photolysis, it was concluded that the remarkable reactivity of 1 is due to the n,π* character of the carbonyl chromophore.