Acid Exfoliation of Imine-linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

Covalent organic frameworks (COFs) are highly modular, porous, crystalline polymers of interest for charge storage devices, nanofiltration membranes, optoelectronic devices, and more. COFs are typically synthesized as microcrystalline powders, a morphology that limits their performance in these applications, and their limited solubility precludes large-scale processing into more useful morphologies and devices. Here, we report a general, scalable method to exfoliate two-dimensional imine-linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous, crystalline COF films up to 10 cm in diameter. This strategy was successfully applied to three different COF structures, and excellent film thickness control (50 nm to 20 µm) was achieved by modifying the suspension composition, concentration, and casting protocol. Acid-mediated exfoliation is a promising strategy for solution processing readily accessible imine-linked COF powders into functional devices.