Phenothiazine‐based photosensitizers for free radical polymerization under visible light in an air atmosphere

Abstract Six novel visible light photosensitizers with donor–acceptor structure (BPT, BPT‐F, BPT‐O, BPT‐Ph, BPTF‐Ph and BPTO‐Ph) were synthesized by using phenothiazine as core. The effects of the conjugated structure and different functional groups on the photophysical properties were investigated. The six photosensitizers present extremely high molar absorption coefficients and excellent solubility, enabling their efficient generation of abundant excited‐state molecules under identical visible light irradiation conditions (405 nm light‐emitting diode, same light intensity and irradiation time for all samples). The photopolymerization system comprising the photosensitizers and bis(4‐methylphenyl) iodonium hexafluorophosphate exhibited highly efficient electron transfer and radical generation so that the instantaneous radical concentration far surpassed that of dissolved oxygen, which overwhelmed the oxygen's quenching capacity. In an air atmosphere, the final C=C bond conversion rate in different monomers reached 70% under the 405 nm light‐emitting diode irradiation. This high conversion demonstrated that sufficient free radicals were generated to enable rapid polymerization and effectively overcome the oxygen inhibition effect. © 2026 Society of Chemical Industry.