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–).

经光解产生的噻唑酮三重激发态与通过亚甲基连接的苯并噻吩-2-甲叉基苯甲酰胺环系发生可逆的三重态能量转移。随后，苯甲酰胺官能团经历的电环化闭环反应产生一种假定的两性离子中间体，该中间体能够从苯并噻吩环的C-3位置排出离去基团（LG-）。以环己二烯为猝灭剂的斯特恩-沃尔默猝灭实验提供了正向和反向三重态激发转移的反应速率常数，该常数可表示为平衡常数K = 0.058。总体而言，三重激发态反应的速率成为K × kr = 5.7 × 10^4 s^-1，其中LG- = Cl-，kr是C-3氯代苯并噻吩-2-甲叉基苯甲酰胺的三重态衰减速率，通过斯特恩-沃尔默猝灭实验得到。对于亚甲基连接的系统，发现其高量子产率是由于K × kr与噻唑酮的三重态衰减速率kd = 7.7 × 10^4 s^-1具有竞争力。基于Φisc = 0.68，对于LG- = Cl-，直接光解的整体预期量子产率应为0.50，而在25 °C的实验中为0.41。离去基团（LG- = Cl-、(EtO)2PO2-、PhCH2CO2-、PhS-和PhCH2S-）的碱性增强会导致Φ值降低。