Direct Observation of Spherulitic Covalent Triazine Frameworks Grown in Hot Superacid Matrices

As emerging porous crystalline polymers, 2D covalent organic frameworks present exclusively in macroscopic forms of polycrystalline powders, single crystals, or thin films. Here, spherulitic 2D covalent triazine frameworks (CTFs) grown in hot superacid matrices are grown, which exhibit a typical Maltese cross pattern of light extinction. A structural evolution from small-angle branched nanofibrils to a core-shell hemispherical geometry is real-time recorded by in situ microscopies and spectroscopies. Time-resolved imaging analysis on spherulitic growth unveils a nonclassical crystallization process, which involves a secondary nucleation-elongation mechanism coupled with amorphous-to-crystalline transition. Furthermore, fluorine doping into CTF enables acceleration of 2D template polymerization and shell formation to develop size-tunable spherulites, with the optimal size up to ≈48 µm in diameter. This work provides unique insights into 2D polymerization and crystallization processes via in situ characterization and demonstrates an unprecedented form of CTF crystallites, which are potentially used as ion sensors and storage.