Photoredox Organocatalysts with Thermally Activated Delayed Fluorescence for Visible-Light-Driven Atom Transfer Radical Polymerization

Abstract

Photoredox catalyzed atom transfer radical polymerization (photo-ATRP) has emerged as a fundamental and powerful polymerization method. Herein we report a class of facilely synthesized N-aryl phenoxazines that serves as highly efficient visible-light-driven photoredox organocatalysts. The strategy of thermally activated delayed fluorescence (TADF) was introduced with the organocatalysts by constructing an orthogonally linked D–A molecular configuration. The offset between the lowest singlet S1 and lowest triplet T1 excited states (ΔEST) is less than 0.30 eV in TADF molecules. The small ΔEST (<0.1 eV) and the long-lived excited states (>0.1 μs) of the organocatalysts were confirmed by theoretical and experimental studies. The combination of a strong electron donor and weak electron acceptors endows the molecules with high-lying HOMOs and LUMOs, which provide a sufficiently negative excited state reduction potential E0(PC•+/PC*) of ca. −1.5 V. The catalysts were applied in the photo-ATRP of methyl methacrylate using ethyl α-bromophenylacetate as the initiator under white-light irradiation. The photo-ATRP was successfully conducted with catalyst loadings as low as 30 ppm. The monomer scope was expanded to benzyl methacrylate (BnMA) and acrylate butyl methacrylate (BMA). Finally, the block polymer P(MMA-b-BnMA) was synthesized, demonstrating the high chain-end fidelity of the polymers.