Single crystal covalent organic frameworks

Covalent organic frameworks (COFs) are crystalline porous polymers that provide a versatile platform for designing dimension-specific organic architectures via topology diagrams and constructing diverse materials with structure-originated unique functions. Their synthesis relies on the organization of organic building units through coupled physicochemical processes, including covalent polymerization, supramolecular polymerization, nucleation and crystallization. As a result, the polymerization system develops a highly complex reaction landscape, characterized by the coexistence of oligomers and polymers of diverse sizes and shapes, concurrent covalent bond formations and noncovalent interactions, and competing thermodynamic and kinetic pathways that respond sensitively and distinctively to conditions. This intrinsic complexity limits precise structural control in polycrystalline COFs and poses great challenges for the synthesis of single crystal COFs, which remain scarce in terms of structural diversity, scalability and accessibility. In this review, we inspect current strategies and methodologies for the design and synthesis of single crystal COFs, scrutinize polymerization, nucleation and crystallization mechanisms to identify key control parameters, correlate structure-property relationships and compare their properties and functions with polycrystalline COFs to elucidate the impact of structural integrity on performance. We envision the key challenges and outline future major directions. By building a full picture, this review provides a conceptual framework and roadmap for advancing single crystal COFs toward fundamental innovation and practical implementation.