Stepwise Conjugation Extension of Covalent Organic Frameworks via Multicomponent Linkage Conversion for Optimized Photocatalytic Molecular Oxygen Activation

Conjugated covalent organic frameworks (COFs) have emerged as a promising class of semiconducting heterogeneous photocatalysts. Precise control of the conjugation within their framework is pivotal for tuning electronic structures and optimizing catalytic performance, yet conventional approaches for linker extension face significant synthetic limitations. We report in this work a multicomponent reaction strategy that enables stepwise tuning of the conjugation of COFs through linkage conversion, where imine linkages are sequentially transformed into tetrahydrophenanthridine and further into phenanthridine units. The progressively extended conjugation results in a lower bandgap and reduced exciton binding energy, thereby promoting light harvesting and subsequent free charge carrier generation and utilization. Among the COFs bearing different linkages, the phenanthridine-linked COF exhibits superior efficiency in photocatalytic activation of molecular oxygen for a variety of chemical transformations, including hydrogen peroxide generation, thioether oxidation, and benzimidazole synthesis under mild conditions.