Reversible Triplet Excitation Transfer in a Trimethylene-Linked Thioxanthone and Benzothiophene-2-Carboxanilide that Photochemically Expels Leaving Group Anions

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_r = 5.7 × 10⁴ s^–1 for LG^– = Cl^–, where k_r 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_r being competitive with the triplet excited state decay of the thioxanthone of k_d = 7.7 × 10⁴ s^–1. On the basis of Φ_isc = 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)₂PO₂^–, PhCH₂CO₂^–, PhS^–, and PhCH₂S^–).