Amorphous-to-crystalline transformation: a mechanochemical pathway to imine-linked covalent organic frameworks

Amorphous-to-crystalline transformation is of profound importance in the crystallization of covalent organic frameworks (COFs), yet its potential through solid-state mechanochemistry remains largely unexplored. Here, we introduce a mechanochemical amorphous-to-crystalline pathway to synthesize imine-linked COFs under ambient conditions. By ball milling their amorphous progenitors, nine imine-linked COFs with distinct core structures, topologies (hcb, sql, kgm, and dia), and dimensions are constructed in as little as one hour. Notably, the unique advantage of this method is highlighted by the successful synthesis of a highly crystalline, porous pyrene-based COF inaccessible by de novo mechanosynthesis. A mechanochemical "scrambling" reaction of imine-based model compounds confirms the high reversibility of the imine bonds in the solid state, which is crucial for facilitating error correction during COF reconstruction. This study underscores mechanochemistry as an effective means for amorphous-to-crystalline transformation, establishing a facile, generic, and green pathway to imine-linked COFs, including those unattainable via conventional de novo mechanosynthesis.