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 A_(2)B_(2) 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.