Acylhydrazone-Linked Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are crystalline porous materials featuring ordered π arrays and well-defined channels, whose structures can be rationally designed through topology-guided polymerization. Over the last two decades, the development of diverse π-building units bearing various reactive groups has enabled a wide range of COF architectures. Particularly, aldehydes are the most extensively employed monomers for constructing COFs ranging from azine to imine, squaraine, hydrazone, and olefinic linkages. However, ketones, a handy analogue to aldehydes with even richer diversity, remain to be well explored as monomers for the design and synthesis of COFs. Here we report acylhydrazone-linked COFs by exploring ketones as building units to condense with hydrazides under solvothermal conditions. We observed that a careful screening of reaction conditions including solvent, catalyst, temperature, and time enables the finding of optimal systems to produce highly crystalline and porous acylhydrazone-linked COFs with designable micro- and mesoporosity. Surprisingly, the acylhydrazone linkage plays a decisive role in both structural formation and functional evolution. For the pores, the acylhydrazone linkage strengthens pore confinement, enhances water adsorption, and increases adsorption heat. Concurrently, for the skeletons, it enhances thermal and chemical stabilities, promotes π-delocalization, enables tunable light emission, decreases bandgaps, and improves photoconductivity. These findings introduce a new paradigm for designing COFs with acylhydrazone linkages to create unparalleled structures, properties, and applications.