Implications of Solvent Vapor Annealing on Crystallinity and Orientation of Covalent Organic Framework Thin Films

Covalent organic frameworks (COFs) are a promising class of crystalline organic polymers that can be processed into many different forms, with a majority existing as insoluble powders and thin films. Still, the optimization of crystallinity and orientation of COF thin films, a precondition for many applications, remains particularly challenging. To address this gap, this study employs grazing-incidence wide-angle X-ray scattering (GIWAXS) to examine solvent vapor annealing (SVA) as a postsynthetic method to enhance the structural ordering of imine-linked COF thin films. The SVA process was first systematically optimized for 1,3,5-tris-(4-aminophenyl)-benzene-p-phthalaldehyde (TAPB-PDA) COF thin films through kinetic and thermal studies, with an annealing temperature of 90 °C for 60 min being identified as the optimal treatment condition, which yielded a maximum increase in crystallinity. These optimized conditions were subsequently applied to TAPB-PDA thin films of varying thicknesses and those grown on a diverse range of substrates to evaluate the versatility of the SVA process. These conditions were then utilized for a series of six different COFs with varying chemical compositions and pore sizes. GIWAXS analysis was used to verify the crystalline structure of each COF as well as to obtain a comparative measurement of crystalline volume and orientational order before and after SVA. The SVA process significantly enhances crystallinity in four of the six COFs examined, particularly for those with larger pore sizes (>2.2 nm) and already exhibiting preferential alignment. In particular, 1,3,5-tris-(4-aminophenyl)-benzene-2,5-dihydroxyterephthalaldehyde (TAPB-DHPDA) and TAPB-PDA, both with a pore size of 3.6 nm, demonstrate significant increases in crystalline ordering, with up to a 9-fold increase in intensity and a 61% decrease in full width at half-maximum (FWHM) of the crystalline (100) diffraction peak. The solvent mixture (1,4-dioxane, mesitylene, and 11 M acetic acid) is hypothesized to facilitate this enhancement by enabling imine bond reversibility and creating an environment that promotes crystallite rearrangement. This work demonstrates SVA as an effective method to improve COF thin film quality.