Two-Photon Excitation Photoredox Catalysis Enabled Atom Transfer Radical Polymerization.

In recent years, metal-free photoredox-catalyzed atom transfer radical polymerization (O-ATRP) has gained wide attention because of its advantages (e.g., no metal contamination and mild reaction conditions). However, this traditional one-photon excitation catalysis has thermodynamic limits. Most photocatalysts cannot effectively reduce the initiators and drive the polymerization under visible light. Herein, we investigate the two-photon excitation-catalyzed O-ATRP, in which the catalyst can absorb two photons to accumulate energy. Compared to one-photon excitation catalysis, this method not only has distinct advantages in the controllability, reaction rate, and catalyst loading but also can chemically reduce the various initiators (e.g., aryl halides) to initiate the polymerization. Density functional theory (DFT) calculation reveals that the two-photon excitation process reached a higher energy end state with stronger reduced ability via a thermodynamically more stable intermediate. We believe that this work will provide a new strategy for photoredox-catalyzed O-ATRP.