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–).

由光解产生的噻唑酮的三重激发态，与由亚甲基链连接的苯并噻吩-2-羧酰基苯胺环系发生可逆的三重能量转移。随后，酰基部分的电环闭合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-）。以环己二烯作为猝灭剂进行的斯特恩-沃尔默猝灭实验，提供了正向和逆向三重激发态转移的反应速率常数，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-羧酰基的三重态衰减率，通过斯特恩-沃尔默猝灭实验获得。亚甲基链连接系统中发现的高量子产率归因于K × kr与噻唑酮的三重激发态衰减（kd = 7.7 × 10^4 s^-1）的竞争。基于Φisc = 0.68，当LG- = Cl-时，直接光解的总体预期量子产率应为0.50，与在25 °C实验中测得的0.41相比。随着离去基团（LG- = Cl-、(EtO)2PO2-、PhCH2CO2-、PhS-和PhCH2S-）碱性的增加，Φ值降低。