Facile and scalable synthesis of high-quality three-dimensional imine-linked covalent organic frameworks <i>via</i> crystalline intermediate transformation

Covalent organic frameworks (COFs), an emerging class of porous crystalline materials, have potential applications ranging from separation to catalysis. However, the harsh conditions required by the classical amorphous transformation route limit the scalability of COF synthesis, especially for three-dimensional covalent organic frameworks (3D COFs). Here, we propose a novel crystalline intermediate (CIM) transformation method that circumvents the stage of amorphous phase generation, enabling the facile and scalable synthesis of imine-linked 3D COFs. In contrast with the classical route of 3D COF synthesis, the CIM transformation process requires no deoxygenation or high-temperature treatment and offers gram-scalable production with efficient and controllable structure interpenetration. The transformation mechanism from a CIM to a 3D COF was investigated in detail, showing a direct crystal-to-crystal pathway. The structure of a CIM nanocrystal, determined by scanning three-dimensional electron diffraction (3DED), reveals a tightly packed diamond-like structure. Furthermore, this strategy was also successfully applied to the synthesis of other three imine-linked 3D COFs that leads to the discovery of two new COFs, demonstrating its broad applicability.