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 × kr = 5.7 × 104 s–1 for LG– = Cl–, where kr 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 × kr being competitive with the
triplet excited state decay of the thioxanthone of kd = 7.7 × 104 s–1. On
the basis of Φisc = 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–).