Ketazine-Linked Crystalline Porous Covalent Organic Frameworks

Covalent organic frameworks (COFs) are a class of crystalline porous materials with well-defined pi arrays and ordered channels, which can be predesigned with a topology diagram and prepared via a polycondensation reaction. Over the past decade, various types of pi building units with different functional groups have been developed. Among them, aldehyde is one of the most widely used monomers that form COFs with azine, hydrazine, imine, squaranine, and C=C linkages. In contrast, its closest analogue, i.e., ketone, despite its broad structural diversity, has not yet been investigated for the design and synthesis of COFs. Herein we report the first examples of ketazine-linked COFs by developing ketones as monomers to enable polycondensation with hydrazine under solvothermal conditions. We observed that a careful screening of reaction conditions including solvent, catalyst, concentration, reaction temperature, and reaction time leads to the finding of optimal polymerization systems to produce highly crystalline and porous ketazine-linked COFs. Surprisingly, the ketazine linkage enables pi conjugation between knot and linker sites and renders the resultant materials able to emit a strong blue fluorescence, highlighting the pi electronic features of this new family of COFs. Our findings of ketones as monomers and ketazine as linkage bring unprecedented structures, functions, and applications to the field of COFs.