Conversion of Linkages in Covalent Organic Frameworks
Chapter 1: Linking small organic molecules into an extended solid typically generates amorphous, disordered polymer. The introduction of order into such a material is of interest because it allows for fine control of the overall structure. Covalent Organic Frameworks (COFs) represent a solution to this problem. They are a new class of porous highly crystalline solids formed by combining organic precursors into an extended structure. Notably, the judicious choice of linkers can give rise to highly regular and predictable materials. Much research is directed toward the development of new methods of linking building blocks together to generate crystalline COFs. These efforts promise not only new materials, but also the chance to extend the principles of molecular covalent chemistry to the formation of organic crystals.
Chapter 2: The formation of a COF linked through carbon-carbon bonds is targeted via the Diels-Alder reaction between furan and maleimide. This reaction was chosen since it is thermally reversible, allowing for error correction and therefore, the synthesis of crystalline material. This material would constitute the first example of a COF linked by carbon-carbon bonds. However, these efforts were not rewarded with success; no crystalline material was obtained. This is likely due to the large number of diastereomers that could result from the combination of any given pair of furan- and maleimide-functionalized linkers.
Chapter 3: The imine linkages of TPB-TP-COF, formed from the union of 1,3,5-tris(4-aminophenyl)benzene and terephthalaldehyde, were oxidized to amide linkages. The completeness of this reaction was verified by both infrared and solid-state NMR spectroscopy. This reaction can be performed under mild conditions, allowing for the retention of crystallinity and permanent porosity. The resulting amide-linked COF shows greater stability under both aqueous acid and base than the imine-functionalized one. In a greater sense, this report serves as a proof of concept for the discovery of new COF linkages via postsynthetic conversion. This means that frameworks with less reversible linkages can be obtained from easily synthesized COFs with highly reversible ones. This is a new approach to the synthesis of COFs, and has been fruitfully used in the formation of a variety of new linkages.
Chapter 4: Imine-linked ILCOF-1 based on 1,4-phenylenediamine and 1,3,6,8-tetrakis(4-formylphenyl)pyrene was converted through consecutive linker substitution and oxidative cyclization to two isostructural COFs, having thiazole and oxazole linkages. The completeness of the conversion was assessed by infrared and solid-state NMR spectroscopy, and the crystallinity of the COFs was confirmed by powder X-ray diffraction. The materials derived in this way demonstrate increased chemical stability, relative to the imine-linked starting material. This represents the introduction of additional control to our previous approach since new functional groups can be installed after COF synthesis, allowing for a broader array of postsynthetic reactions to be performed.
Chapter 5: This section provides an outlook on the state of COF postsynthetic linkage modification and summarizes the work performed in this area by other groups.