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–).

由光解产生的噻吨酮的三重激发态与由亚甲基链连接的苯并噻吩-2-羧基苯胺环系统发生可逆的三重能量转移。随后，苯胺基团发生的电环化环闭反应产生了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位驱除离去基团（LG-）。以环己二烯作为猝灭剂进行的斯特恩-沃尔默猝灭实验提供了正向和反向三重激发转移的速率常数，可以表示为平衡常数K = 0.058。总体而言，三重激发态反应的速率变为K × kr = 5.7 × 10^4 s^-1，其中LG- = Cl-，kr是C-3氯代苯并噻吩-2-羧基苯胺的三重衰减速率，通过斯特恩-沃尔默猝灭实验得到。亚甲基链连接系统的量子效率较高，是由于K × kr与噻吨酮的三重激发态衰减（kd = 7.7 × 10^4 s^-1）具有竞争性。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，而实验上在25 °C时为0.41。随着离去基团（LG- = Cl-、(EtO)2PO2-、PhCH2CO2-、PhS-和PhCH2S-）碱性增强，Φ值逐渐降低。