Synthesis of Single-Crystal Two-Dimensional Covalent Organic Frameworks and Uncovering Their Hidden Structural Features by Three-Dimensional Electron Diffraction

Two-dimensional covalent organic frameworks (2D COFs) are formed by the polycondensation of geometrically specific monomers to grow covalently connected 2D polygonal polymers over the a-b plane and supramolecular polymerization and/or crystallization of 2D sheets along the c direction to constitute layer architectures. Despite various efforts, the synthesis of single-crystal 2D COFs remains a challenging goal. Here, we report the synthesis of single-crystal 2D COFs, by taking the representative imine-linked TPB-DMTP-COF as an example, to reveal the key synthetic parameters that control the crystallization of 2D COFs. We systematically tune the synthetic conditions including the glassware setup, the degas method, the solvent, the temperature, the modulator, and the reaction time and observed that all these parameters greatly affect the polymerization and crystallization processes, controlling the crystal quality. We found that a homogeneous system with all components dissolved and the presence of a suitable modulator at a temperature of 50-70 degrees C allows the growth of TPB-DMTP-COF single crystals as isolated individual rods, with tunable diameters of 200 nm to 3 mu m and a length of 1-20 mu m. The single-crystal structure was characterized by three-dimensional electron diffraction (3DED), which revealed two conformations of trans and cis for the linker in the 2D polymer sheets, which stack in an antiparallel mode to shape the frameworks with double-sized unit cells. These results uncover these hidden structural features which have been overlooked in polycrystalline and single-crystal studies and provide new insights into the synthesis of high-quality single crystals of 2D COFs.