Building Block Design for Minimizing Defects in the Construction of Two-Dimensional Covalent Organic Frameworks.

Polymerization of monomers into two-dimensional covalent organic frameworks with precise porous structures exhibits desired catalytic, gas separation, and optoelectronic properties. However, the defects arising from covalent bonding in a polymerization process always result in amorphous films with small crystalline domains or polycrystalline powders. It is still a tremendous challenge to synthesize high-quality crystalline products, even single crystals with a large size over the micrometer scale. In this work, we propose a general strategy of building block design to reduce the defects during growth of two-dimensional covalent organic frameworks. We demonstrate that the building block with a hexagonal pore unit, i.e., a hexamer, could greatly decrease defects by directional uniform growth in polymerization, while monomer, dimer, and trimer building blocks form more defects due to linear growth. Our work provides a new strategy to construct superlarge single crystals in practical applications by combining building block design and growing dynamics control.