Crystalline Covalent Organic Frameworks with Tailored Linkages for Photocatalytic H2 Evolution.

Crystalline covalent organic frameworks (COFs) are a new class of porous polymeric semiconductors with network topologies, which are built from the integration of selected organic blocks via covalent bond linkage. They have shown great promise for artificial photosynthesis, owing to broad light harvesting, high crystallinity and high carrier mobility. In this minireview, we introduce state-of-the-art COF photocatalysts based on different linkages and discuss the origin of photocatalytic activities for hydrogen evolution. Three typical COF photocatalysts with special linkages including imine (-C=N-), β-ketoenamine (O=C-C=C-NH-) and vinylene (-C=C-) are mainly discussed, with a particular focus on the advancements in synthetic methodologies and structural design, and 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 and enhance their performances.