Postsynthetic Construction of Single-Crystal Formamide-Linked Covalent Organic Frameworks.

Postsynthetic modification (PSM) of covalent organic frameworks (COFs) provides a powerful strategy to promote the development of this emerging class of crystalline reticular materials. However, obtaining precise structural information regarding the postsynthetic installation of functional groups into the skeletons of COFs remains a significant challenge, primarily because of the loss of crystallinity of the parent crystals during the PSM process. Herein, by carefully selecting the starting COF crystal and optimizing the reaction conditions, we report the first example of a multistep single-crystal-to-single-crystal chemical transformation for COFs, in which imine-linked COFs are successfully transformed into novel formamide-linked COFs. The structural details of these COFs are precisely characterized via single-crystal X-ray diffraction analysis, revealing a mechanical contraction of up to 13.4% in volume for the formamide-linked COF compared with the parent amine-linked COF. Moreover, the chemical stability of the resulting COFs toward strong acids was significantly enhanced. This protocol opens a door for the design of novel COFs and presents a potential pathway for exploring their properties and applications.