Reversible Triplet Excitation Transfer in a Trimethylene-Linked Thioxanthone and Benzothiophene-2-Carboxanilide that...

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^-).