Enabling Solution Processible COFs through Suppression of Precipitation during Solvothermal Synthesis

Abstract

Covalent organic frameworks (COFs) are crystalline, nanoporous materials of interest for various applications. However, current COF synthetic routes lead to insoluble aggregates which hamper processing and prohibit their use in many applications. Here, we report a novel COF synthesis method that produces a stable, homogeneous suspension of crystalline COF nanoparticles. Our approach involves the use of a polar solvent, di-acid catalyst, and slow reagent mixing procedure at elevated temperatures which all together enable access to crystalline COF nanoparticle suspension that does not aggregate or precipitate when kept at elevated temperatures. On cooling, the suspension undergoes a thermoreversible gelation transition to produce crystalline and highly porous COF materials. We demonstrate that this method enables the preparation of COF monoliths, membranes, and films using conventional solution processing techniques. We show that the modified synthesis approach is compatible with various COF chemistries, including both large- and small-pore imine COFs, hydrazone-linked COFs, and COFs with rhombic and hexagonal topology, and in each case, we demonstrate that the final product has excellent crystallinity and porosity. The final materials contain both micro- and macropores, and the total porosity can be tuned through variation of sample annealing. Dynamic light scattering measurements reveal the presence of COF nanoparticles that grow with time at room temperature, transitioning from a homogeneous suspension to a gel. Finally, we prepare imine COF membranes and measure their rejection of polyethylene glycol (PEG) polymers and oligomers, and these measurements exhibit size-dependent rejection of PEG solutes. This work demonstrates a versatile processing strategy to create crystalline and porous COF materials using solution processing techniques and will greatly advance the development of COFs for various applications.