Eosin Y Catalyzed Photochemical Synthesis of Arylated Phenothiazones

In the presence of
Eosin Y (EY), the synthesis of
substituted phenothiazones was carried out efficiently using various
substituted 2-aminothiophenol, diazonium salts, and 1,4-napthaquinones
(1,4-NQ) at room temperature (RT) (condition: green LED
of 525 nm, 44 W; reaction time: 8 h, isolated yield: 68–90%).
A fluorescence quenching experiment and density functional theory
(DFT) calculations suggested that the triplet photoexcited state of
EY (EY*; τT = 320 ± 10 ns) converts to EY+• via oxidative quenching by ArN2+ (−1.11
V vs SCE for EY* to EY+•) initially. Thiyl and aryl
radicals were captured as TEMPO adducts in high-resolution mass spectroscopy
(HRMS). The reaction was not inhibited by the addition of a singlet
oxygen quencher such as 1,4-diazobicyclo [2.2.2] octane (DABCO), which
suggests that singlet oxygen is not participated.