Synthesis of micrometre-thick oriented 2D covalent organic framework films by a kinetic polymerization pathway

Despite advances in the field of 2D polymerization, the synthesis of high-quality, micron-thick films of oriented 2D covalent organic frameworks (COFs) remains challenging. Conventional approaches focusing on thermodynamic control of the polymerization pathway face a detrimental trade-off between orientation and thickness. In this study, we describe a straightforward method for preparing imine-linked 2D COF films with a near-perfect face-on orientation by leveraging kinetically-trapped amorphous 3D covalent adaptable network (CAN) intermediates. These off-pathway intermediates are generated as coatings through solution casting, during which the CANs spontaneously align to relax tensile stresses induced by solvent evaporation. A subsequent lift-off process, followed by an amorphous-to-crystalline transformation under solvothermal conditions, converts the 3D-oriented polymer networks into thermodynamically stable, porous, and free-standing 2D COF films. This versatile kinetic trapping strategy is suitable for a range of building blocks and network topologies, constituting a convenient synthetic tool for accessing high-quality, robust, large-area 2D COF films with a strongly aligned polycrystalline structure.