The triplet excited state of thioxanthone produced by photolysis undergoes reversible triplet energy transfer with a trimethylene-linked benzothiophene-2-carboxanilide ring system. The ensuing electrocyclic ring closure of the anilide moiety produces a putative zwitterionic intermediate that is capable of expelling leaving groups (LG) from the C-3 position of the benzothiophene ring. Stern-Volmer quenching studies with cyclohexadiene as quencher furnish the rate constants for the triplet excitation transfer in the forward and reverse directions, which can be expressed as an equilibrium constant K = 0.058. Overall, the rate of the triplet excited state reaction becomes K × k = 5.7 × 10 s for LG = Cl, where k is the triplet decay rate of the C-3 chloro-substituted benzothiophene-2-carboxanilide, found through Stern-Volmer quenching. The high quantum efficiencies found for the trimethylene-linked systems are due to K × k being competitive with the triplet excited state decay of the thioxanthone of k = 7.7 × 10 s. On the basis of Φ = 0.68, the overall expected quantum yield for direct photolysis should be 0.50 for LG = Cl as compared to 0.41 at 25 °C experimentally. Φ decreases with increasing basicity of the leaving group (LG = Cl, (EtO)PO, PhCHCO, PhS, and PhCHS).
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