Covalent organic frameworks form processible colloids

Covalent organic frameworks (COFs) are crystalline porous polymers with features that include having a pore size that can be tailored by choosing suitable building blocks. Researchers are examining COFs and other types of framework compounds for use as selective membranes, catalyst supports, and other applications. But COFs tend to form tough-to-process insoluble microcrystalline powders, thwarting those efforts. Help may now be on the way thanks to a study reporting a method to prepare COFs as stable colloidal suspensions by inhibiting the usual irreversible crystallite aggregation and precipitation (ACS Cent. Sci. 2017, DOI: 10.1021/acscentsci.6b00331). A team led by William R. Dichtel of Northwestern University and Nathan C. Gianneschi of the University of California, San Diego, studied a boronate ester-linked hydroxytriphenylene material known as COF-5. On the basis of X-ray scattering, liquid-cell microscopy, and other analytical methods, the team determined that nitrile solvents impede crystallite aggregation yet do not inhibit framework polymerization. They found that adding even a small amount of acetonitrile to the dioxane-mesitylene solvent used to synthesize COF-5 prevents precipitation, yielding instead a colloidal suspension that remains stable for weeks. The team used those suspensions to cast high-quality free-standing transparent films, a key step toward applications.