A General Synthetic Strategy for the Synthesis of Imine-Linked Covalent Organic Frameworks in Choline Chloride–Hexafluoroisopropanol-Based Deep Eutectic Solvents

The imine-linked covalent organic frameworks (COFs) have garnered significant attention in various fields due to exceptional stability and polyamide structures, etc. However, the conventional synthetic protocols for the high-crystallinity imine-linked COFs often necessitate the use of harmful organic solvent and extra catalyst, rendering their synthesis environmentally detrimental. Therefore, it is imperative to develop facile methods for COF preparation that employ green solvents without the need of extra catalyst. Herein, an environmentally friendly, efficient, general synthetic strategy was developed to synthesize five imine-linked COFs with high yield and high crystallinity by using choline chloride (ChCl)–hexafluoroisopropanol (HFIP) linked deep eutectic solvents (DESs) as general solvents for the first time. Unlike other reactions that required repeated exploration of the types of solvents to synthesize COFs with high crystallinity, the as-developed method only required changing the molar ratio of ChCl and HFIP in DES; various imine-linked COFs with high crystallinity can be synthesized. To further elucidate the formation of imine-linked COFs in ChCl-HFIP-based DESs, COF-CH-1, which was prepared using 1,3,5-tris(4-aminophenyl)benzene (TPB) and terephthalaldehyde (TPDD) as precursors, was regarded as an example. The formation process of COF-CH-1 was preliminarily deduced linked on the changes of morphology, functional groups, element compositions, etc., with the change of reaction temperature and reaction time in optimal DES. Finally, according to the results, we also preliminarily speculated on the possible role of DESs in COFs' formation.