Humidity-Guided In Situ Polymerization for Controlled Formation of Smooth, Oriented Covalent Organic Framework Films

Efficient, reliable preparation of high-quality films of covalent organic frameworks (COFs) is essential for fully exploring the potential of this class of porous crystalline compounds in, for example, sensing and catalysis. To this end, in situ polymerization on substrates offers a promising route; however, it remains a challenge to achieve highly crystalline, smooth, and oriented COF films through this method. Here, we present a humidity-controlled, drop-casting-based approach for the formation of imine-COF films, using the monomer pair 1,3,6,8-tetrakis(4-aminophenyl)pyrene and terephthalaldehyde as a representative example, along with additional pairs to highlight the method's versatility. This method affords homogeneous, face-on-oriented films with excellent reproducibility and efficiency. The key to this effect is regulating the environmental humidity during solvent evaporation, which presumably enables spatiotemporal modulation of the polymerization and crystallization processes. Such control over the film structure allows statistical evaluation of morphology-dependent variations in chemiresistive response, demonstrating that this operationally simple approach provides a robust platform for both fundamental studies and technological development of COF films.