Viologen-Based Cationic Radical Porous Organic Polymers for Visible-Light-Driven Photocatalytic Oxidation

Rational design of radical porous organic polymers with redox functions for metal-free heterogeneous photocatalysis is a promising research topic. In this work, we reported a class of viologen-based cationic radical porous organic polymers (VCR-POPs), which were facilely constructed by the one-pot Knoevenagel condensation reaction between 1,3,5-tris(p-formylphenyl)benzene and an acetonitrile-functionalized viologen-based ionic monomer (VIL-CN). Interestingly, the stable viologen cationic radical was found in the obtained VCR-POPs, probably undergoing in situ single-electron reduction of the dicationic monomer VIL-CN during the base catalyst Cs₂CO₃-involved Knoevenagel condensation reaction. The radical character of the typical porous organic polymer VCR-POP-1 was confirmed by the electron paramagnetic resonance spectrum and X-ray photoelectron spectroscopy, and the chemical structures and porous textural properties were fully characterized. The optical and electrochemical properties of VCR-POP-1 manifest the semiconductor nature, excellent light-harvesting ability, and better charge separation and transfer efficiency of this well-designed cationic radical polymer compared with the ionic monomer VIL-CN. Therefore, the polymer VCR-POP-1 with viologen-based cationic radicals and π-conjugated structures could be regarded as a highly efficient metal-free heterogeneous photocatalyst for visible-light-driven oxidative coupling of amines in air by virtue of the dominant reactive oxygen species of singlet oxygen.