Exploiting Charge‐Transfer Triplet Excited States Enables Oxygen‐Tolerant Photoinduced <scp>ATRP</scp> with sub‐ppm Photocatalyst Loading

Comprehensive Summary Development of highly efficient organic photocatalysts (OPCs) is imperative in the advancement of photoinduced atom transfer radical polymerization (photoATRP). Herein, we reported a class of OPCs consisting of (difluoro)quinoxaline acceptor units, and diphenylamine donor unit. Ultrafast femtosecond transient absorption (fs‐TA) spectroscopy revealed that the photoexcited OPCs formed charge‐transfer triplet ( 3 CT) states via intersystem crossing. Well‐controlled photoATRP was developed by using sub‐ppm (parts per million molar ratio relatives to monomer) level of these OPCs. Furthermore, the photoATRP in sealed vials were tolerant towards a substantial amount of molecular oxygen, exhibiting accelerated polymerization rate after an inhibition period. Mechanistic study elucidated that two kinds of reactive oxygen species, superoxide (O 2 ·− ) and singlet oxygen ( 1 O 2 ), were concurrently generated by photoinduced electron transfer and energy transfer, respectively, from the 3 CT state of OPCs. The oxygen in the reaction system was consumed via the reaction of oxidize solvent by 1 O 2 .