Nanoparticle-Mediated Synthesis of Single-Crystal Covalent Organic Frameworks of Uniform Size and Shape

The synthesis of highly crystalline covalent organic frameworks (COFs) has recently been achieved, but strategies to obtain monodispersed single crystals of uniform size and shape remain elusive. Here, we show that simply adding silica nanoparticles (SiNPs) to a conventional solvothermal synthesis directs imine-based COFs (e.g., COF-300) to grow into monodispersed, polyhedral single-crystal microparticles. Our experiments suggest dual functions of these nanoparticles. They promote burst nucleation of COFs via monomer adsorption and, then, by adhering to the crystal surface, prevent crystal aggregation and nonclassical crystallization pathways. This nanoparticle-mediated strategy is compatible with common small-molecule modulation to tune COF morphologies and shapes while maintaining low size dispersity. The method also extends to other three-dimensional (3D) imine COFs, such as LZU-111 and LZU-79. Notably, the obtained microcrystals show excellent colloidal stability and can assemble into superstructures reflecting their anisotropic shape. This work paves the way for COF-based crystalline materials and applications where size and shape uniformity are essential.