An In Situ Film-to-Film Transformation Approach toward Highly Crystalline Covalent Organic Framework Films

Fabrication of highly crystalline covalent organic framework (COF) films with tunable thickness and good substrate adaptability remains a substantial challenge. Here, we have developed an effective approach for fabricating crystalline COF films based on elaborately designed bifunctional A2B2 monomers. Typically, amorphous drop-casted monomer [e.g., 1,4-bis(4-formylphenyl)-2,5-bis((4-aminophenyl)ethynyl)) benzene (BFBAEB)] films were directly transformed into corresponding highly crystalline BFBAEB-COF film upon in situ vapor-assisted self-polycondensation in high yields (93-97%). The thickness of the BFBAEB-COF film could be modulated readily by varying the monomer concentration. These crystalline COF films could be grown on various substrates, including silicon, indium-doped tin oxide (ITO), glass, and gold.Moreover, such an in situ film-tofilm transformation approach has been demonstrated as versatile and applicable to different A2B2 monomers. This work provides a novel pathway toward homogeneous and highly crystalline COF films, representing a key step forward to explore the application of COFs.