Regioselective One-Step Cyclization and Aromatization towards Directly Amino-Functionalized Covalent Organic Framework with Stable Benzodiimidazole Linkage

The compatibility of crystallinity, stability, and functionality in covalent organic frameworks (COFs) is challenging but significant in reticular chemistry and materials science. Herein, it is presented for the first time a strategy to synthesize directly amino-functionalized COF with stable benzodiimidazole linkage by regioselective one-step cyclization and aromatization. Bandrowski's base with two types of amino groups is used as a unique monomer, providing not only construction sites for the material framework through specific region-selective reaction, but also amino active sites for functionality, which is usually difficult to achieve directly in COF synthesis because amino groups are the participants in COF bonding. In addition, the aromatic benzodiimidazole rings and the large conjugated system of the product effectively improve the crystallinity and stability, so that the as-prepared BBCOF remains unchanged in both acid and base solutions, which is obviously better than the conventional imine-linked COF. Impressively, the significantly enhanced conjugation degree by the benzodiimidazole structure also endows BBCOF with an efficient photocatalytic reduction of uranyl ion, with removal rate as high as 96.6% in single-ion system and 95% in multi-ion system. This study is of great importance to the design and synthesis of functional COFs with a commendable trade-off among crystallinity, stability, and functionality.

Automated summary

This study presents a novel strategy for synthesizing stable amino-functionalized COFs with benzodiimidazole linkage. The use of Bandrowski's base as a unique monomer provides construction sites for the material framework and amino active sites for functionality in COF synthesis, which is usually challenging to achieve. The resulting BBCOF exhibits improved crystallinity and stability due to the aromatic benzodiimidazole rings and large conjugated system. Furthermore, the enhanced conjugation degree enables efficient photocatalytic reduction of uranyl ions. This research is of great importance for the design and synthesis of functional COFs with a commendable trade-off among crystallinity, stability, and functionality.