Modifying the structure to meet the demands
Covalent organic frameworks (COFs) are robust crystalline porous polymers with predictable structural composition and functions. The bottom-up synthesis of COFs allows for incorporation of reactive pendant groups and other active sites within their skeleton. On the other hand, the strong covalent bonding stitching together the organic building blocks in COF frameworks secure high chemical and thermal stabilities of the compound to a large extent. These facts lead to the judicious postsynthetic functionalization in COFs to introduce new functions in a controlled manner.
Recent development of the postsynthetic functionalization of COFs have successfully introduced wide ranges of organic to inorganic functional constituents into COFs to obtain the desired functionalized COFs. This functionalization involves the formation of specific bonds (covalent and coordination/ionic bonds), chemical reaction (oxidation/reduction reaction) and host-guest interaction between the COFs and functional constituents. As a result, several organic functionalized COFs, metal/COF hybrids and other material based-COFs have been reported so far.
In a new review published in the Beijing-based National Science Review, a research group at the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, chemistry department of Jilin University, China summarizes the recent progress of postsynthetic functionalization of COFs. Co-authors Yusran Yusran, Hui Li, Xinyu Guan, Qianrong Fang and Shilun Qiu trace the course of postsynthetic functional development in COFs and comparatively elaborate the contribution those functionalizations over the structural qualities and performance of the resultant functionalized COFs towards specific applications.
These authors emphasize the paramount importance of the functional development of COFs for creation of COF-based smart materials and outline the challenges in their future development
"The most prominent property of COFs is their high chemical stability that surpasses former porous materials such as inorganic zeolites and MOFs, which signals the robust postsynthetic functionalization in COFs to be executed," the authors emphasize in an article titled "Postsynthetic Functionalization of Covalent Organic Frameworks." "Although the desired functionalized COFs can be achieved via a bottom-up approach, postsynthetic functionalization provides a more reliable approach that is better controlled," they add.
The first example of postsynthetic functionalization of COFs is traced back to 2011, when the prepared COF-LZU1 was modified with a Pd complex to obtain a Pd/COF-LZU-1 hybrid. Furthermore, the authors include their works in the article for functionalization of 3-D-COF-OH into 3-D-COF-COOH and incorporation of metal species into 3-D-Salphen-COF (JUC-509) to fabricate JUC-509-Y. "Generally, 2-D COF variants are more widely synthesized and functionalized compared to 3-D COFs. However, in our lab, we demonstrated the functionalization of 3-D COFs with reactive pendant groups," adds the leading author.
The resultant functionalized COFs clearly demonstrated stability enhancement as well as better performance and activity towards gas storage/separation, molecules uptake-release, catalysis, electrical storage sensing, and other important applications. "We all witness how the resultant functionalized COFs with specific functions could do more than the bare COFs in the field of catalysis, adsorbent and other applications," the authors state.
"In the future development of COFs, there are still many things needed to develop, such as functionalization of the available linkages which is still less explored," the authors forecast. "Functionalization of COFs via host-guest chemistry so far, are mainly explored for electronic and energy storage applications, thus investigations in wider sophisticated applications are still highly engage. Last but not the least, continuous efforts in development of functionalized 3-D COFs may still need special concern," they conclude.
More information:
Postsynthetic Functionalization of Covalent Organic Frameworks, National Science Review (2019). DOI: 10.1093/nsr/nwz122
Provided by Science China Press
