Crystalline Dioxin-Linked Covalent Organic Frameworks from Irreversible Reactions

Triangular 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and linear tetrafluorophthalonitrile (TFPN) or 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile (TFPC) were linked by 1,4-dioxin linkages to form crystalline 2D covalent organic frameworks, termed COF-316 and -318. Unlike the condensation reactions commonly used to crystallize the great majority of COFs, the reactions used in this report are based on nucleophilic aromatic substitution reactions (S_NAr) that are considered irreversible. Our studies show that the reactivity of TFPN and TFPC with HHTP is enhanced by the nitrile substituents leading to facile reactions of planar building units to yield the present 1,4-dioxin linked COFs. Because these reactions are irreversible, the resultant frameworks have high chemical stability in both acid and base. This has led to postsynthetic modifications of COF-316 by reactions necessitating extreme conditions to covalently install functionalities not otherwise accessible. We also report the permanent porosity of these COFs.