Taming Irreversibility in sp ² -Carbon-Conjugated COFs from Polycrystalline Powders to Single Crystals and Thin Films

Conspectus Two-dimensional sp 2 -carbon–conjugated covalent organic frameworks (sp 2 c-COFs), constructed from robust C = C linkages, represent a distinctive class of crystalline, layer-stacked conjugated polymer frameworks. The morphology and crystallinity of sp 2 c-COFs critically govern their fundamental physicochemical properties and application performance. However, the intrinsically low reversibility of C = C bond formation during framework growth limits error correction, making it challenging to simultaneously achieve high crystallinity, large domain sizes, and continuous film morphologies in sp 2 c-COFs. In this Account, we summarize recent progress in the synthesis and applications of sp 2 c-COFs, focusing on the expansion of synthetic methods for C = C bond formation and strategies to control the diverse morphologies, including polycrystalline powders, single crystals, and thin films. We first discuss the molecular design strategies for aldol-type polycondensation, using a series of functionalized monomers such as benzobisoxazole, benzobisthiazole, imidazole, and pyrylium cations to promote efficient C = C bond formation and enable the synthesis of previously inaccessible crystalline frameworks. We then highlight the development of new synthetic methods, including the imine-to-olefin transformation and Mannich-elimination reaction, which have allowed the construction of single-crystal sp 2 c-COFs for the first time. These approaches take advantage of the dynamic reversibility of imine linkages, which facilitate structural preorganization and address the crystallization challenges inherent to fully conjugated frameworks. Additionally, we explore techniques for fabricating sp 2 c-COF thin films, including copper-surface-mediated Knoevenagel polycondensation, monolayer-assisted surface-initiated Schiff-base-mediated aldol polycondensation and amphiphilic-pyridinium-assisted aldol polycondensation. These methods enable the growth of highly crystalline, free-standing films and facilitate the functionalization of diverse substrates such as silicon wafers, polyacrylonitrile films, and glass, resulting in enhanced interfacial adhesion and improved device performance. Finally, we discuss how morphology governs the applications of sp 2 c-COFs, ranging from photocatalysis in polycrystalline powders to the elucidation of intrinsic solid-state properties in single crystals, as well as interfacial energy harvesting and uranium extraction in thin films. These developments underscore the versatility of sp 2 c-COFs as a platform for investigating structure–property relationships in robust π-conjugated porous organic materials and pave the way for their integration into next-generation technologies.