Crystalline Covalent Organic Frameworks with Tailored Linkages for Photocatalytic H₂ Evolution

Crystalline covalent organic frameworks (COFs) are porous polymeric semiconductors with network topologies, which are built from the integration of selected organic blocks with covalent bond linkages. They have shown great promise for artificial photosynthesis, owing to broad light harvesting, high crystallinity, and high carrier mobility. This Minireview introduces state-of-the-art COF photocatalysts based on different linkages and discusses the origin of photocatalytic activities for hydrogen evolution. Three typical COF photocatalysts, with linkages including imine (-C=N-), β-ketoenamine (O=C-C=C-NH-), and vinylene (-C=C-), are discussed with a particular focus on the advancements in synthetic methodologies and structural design, as well as photoelectronic properties that are relevant to photocatalytic performance. The Minireview is expected to elucidate their structure-property relationships and the way to design photoactive COFs with enhanced performances.