Long-Lived Triplet Excited-State Bichromophoric Iridium Photocatalysts for Controlled Photo-Mediated Atom-Transfer Radical Polymerization

The "energy reservoir" tactic is used for the first time to regulate the photochemical properties, including excited lifetime and photoluminescence quantum yield of photocatalysts (PCs) for controlled photo-mediated atom transfer radical polymerization (ATRP). Polymerizations are efficiently conducted using long-lived bichromophoric complexes Ir(ppy-s-PPO) and Ir(ppy-s-DBP) as PCs at the ppm level (<10 ppm). The relationships between PC properties and the catalytic performance are analyzed and discussed. Critically, the prolongation of excited lifetime is beneficial for promoting electron transfer during the activation/initiation step and increasing the concentrations of deactivators. Therefore, faster reversible deactivation and a better activation-deactivation equilibrium were achieved. In comparison to Ir(tBuppy)(3) with a short excited lifetime, Ir(ppy-s-PPO) and Ir(ppy-s-DBP) offered the advantage to synthesize poly(methyl methacrylate) with high molecular weights at low initiator loadings. Furthermore, featuring both a long lifetime and an appropriate quantum yield, Ir(ppy-s-PPO) had better control than Ir(tBuppy)(3) over the polymerizations of other monomers, including benzyl methacrylate, isobutyl acrylate, and styrene. It is an effective method to mediate liveness in polymerization by regulating the excited lifetime and quantum yield of PCs. This study may shed light on the design strategy of high-performance photocatalysts with broad utility for photopolymerization reactions.