Elucidating descriptors for crystallinity control in halogenated covalent organic frameworks

Covalent organic frameworks (COFs) represent an extensively studied material featuring crystalline scaffolds drawing direct inspiration from dynamic covalent chemistry. Fluorinated COFs (F-COFs) exhibit unique properties, promising applications, and enhanced crystallinity and porosity compared to their non-fluorinated counterparts. However, specific descriptors for precisely controlling crystallinity through bottom-up synthesis remain lacking. In this work, systematic studies have been performed to dig out the critical factors governing the crystallinity and porosity of diverse halogenated COF scaffolds functionalized by F, Cl, and Br-involved moieties. By combining experimental COF construction and characterization with first principles simulations, it is demonstrated that the imine bond formation energy plays a pivotal role in determining the quality of halogenated COFs derived from aromatic aldehyde and amine monomers employed herein, with the optimal reaction enthalpies lying in the range of −1.27 to −1.54 kcal mol⁻¹, using mesitylene/dioxane as the solvent and acetic acid as the catalyst. The precise correlation between crystallinity and bond formation energy established through these systematic synthesis-simulation studies offers a novel guiding principle for constructing highly crystalline COF scaffolds.