Eosin Y Catalyzed Photochemical Synthesis of Arylated Phenothiazones

In the presence of Eosin Y (<b>EY</b>), the synthesis of substituted phenothiazones was carried out efficiently using various substituted 2-aminothiophenol, diazonium salts, and 1,4-napthaquinones (<b>1,4-NQ</b>) 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.