Phenothiazine-Functionalized Triazine-Based Covalent Organic Frameworks for Efficient Photocatalytic Organic Synthesis

Abstract

In this work, we present a donor–acceptor molecular engineering strategy in which three electron-donating moieties including benzene, triphenylamine, and phenothiazine are condensed with electron-deficient triazine monomers via Schiff-base reaction to furnish three crystalline photocatalysts, denoted as COF-C, COF-N, and COF-NS. Combined PXRD, FT-IR, and solid-state NMR analyses confirm the formation of all three COFs with eclipsed AA-stacked crystalline frameworks. Photophysical and photoelectrochemical measurements reveal that phenothiazine-functionalized COF-NS, bearing dual N and S heteroatoms, possesses the narrowest DFT-calculated HOMO–LUMO gap of 1.92 eV (together with an experimental optical band gap of 2.0 eV) and the broadest visible-light absorption range, thereby efficiently suppressing the recombination of photogenerated electron–hole pairs and delivering the optimal charge transport performance. The oxidative cyclization of o-phenylenediamine with benzaldehyde to 2-phenylbenzimidazole was selected as the model photocatalytic reaction. The catalytic activity follows the order COF-C < COF-N < COF-NS: under optimized conditions COF-NS affords a yield as high as 94%, accompanied by broad substrate compatibility, reliable gram-scale preparation, and excellent cycling stability. Reactive oxygen species trapping and EPR experiments identify superoxide radical anions, together with photogenerated electrons and holes, as the principal active species, from which a complete catalytic pathway was proposed. DFT calculations further demonstrate that the push–pull electronic system constructed between phenothiazine and triazine broadens the surface electrostatic potential and enhances charge separation efficiency, accounting at the electronic level for the superior performance of COF-NS. Overall, this study offers a rational strategy for the molecular design of high-performance COF photocatalysts and the green synthesis of nitrogen-containing heterocycles.