Covalent organic frameworks (COFs) with precise porous framework structures are formed by polymerization of organic monomers. Several condensation reactions such as the Schiff base reaction, spiro-borane condensation, Knoevenagel condensation and imide condensation were occurred on organic monomers to form a linkage for the construction of COFs, therefore, the COFs monomers are figuratively called COFs linkers. The COFs linkers can be classified into following major categories including aldehyde COFs linkers, amido COFs linkers, nitro COFs linkers, boric COFs linkers, halogen COFs linkers, hydroxy COFs linkers, cyan COFs linkers and others. Some of the most commonly used COF linkers are portrayed in Fig.1.
Fig.1 Some of the most widely used COFs linkers
Linkage forms of COFs linkers
Different COFs linkers have been applied for COFs, and those formed boronate-ester linkage, boroxine linkage, imine linkage, hydrazone linkage, azine linkage, imide linkage, C=C linkage, 1,4-dioxin linkage and others (as shown in Fig.2) by condensation reactions in the construction of COFs.
Fig.2 Various linkages formed by COFs linkers for the synthesis of COFs
- Boronate-Ester Linkage: The cyclic five-membered boronate ester is a planar linkage and can be formed by the condensation of boronic acids and catechol derivatives, and can also be formed through the reaction of boronic acids and acetonide monomers in the presence of a Lewis acid (BF3∙OEt2) catalyst. Boronate ester linkage, owing to its planarity and high reversibility yields highly crystalline COFs. Remarkably, boronate ester linkage has been explored for the integration of electron donor and acceptor units into donor-acceptor COFs in which the donor and acceptor moieties are segregated into bicontinuous donor-on-donor and acceptor-on-acceptor arrays.
- Boroxine Linkage: The self-condensation of boronic acids yields cyclic six-membered boroxine linkage with a planar structure and water byproduct. For example, The boroxine-linked 2D COF-1 and PPy-COF (as shown in Fig.3 ) have been synthesized from the self-condensation of 1,4-benzenediboronic acid (BDBA) and pyrene-2,7-diboronic acid (PDBA), respectively, in a sealed Pyrex tube at 120℃.
Fig.3 Schematic for the synthesis of PPy-COF
- Imine Linkage: An imine linkage can be formed by the reaction of aromatic amine and aldehyde in the presence of organic acid or Lewis acid catalyst. The imine-linked COFs usually can be classified into five types, including hexagonal, tetragonal, rhombic, kagome, and trigonal architectures according to the topology diagrams.
- Hydrazone Linkage: A hydrazone linkage is formed via the reaction of aldehyde with hydrazide in the presence of AcOH catalyst. Developing a hydrazone linkage to synthesize COFs requires the integration of an ethoxy group to the ortho position of edge units which helps to secure the conformation and promotes the formation of crystalline products.
- Azine Linkage: The azine linkage takes advantage of the shortest hydrazine monomer to connect two aldehydes into polygon skeletons that form the smallest pores among various linkages. A variety of knots for forming azine linkage have been explored, including substituted benzene, triphenyl benzene, triphenyl triazine, and pyrene monomers, which lead to the synthesis of hexagonal, rhombic, and trigonal COFs.
- Imide Linkage: The imide linkage can be synthesized via the reaction between amine derivatives and acetic anhydride, this reaction is less reversible and requires a high reaction temperature as high as 250℃. For example, the PI-COF-1 and PI-COF-2 have been prepared upon reaction at 200℃ for 5 days, whereas PI-COF-3 is obtained under 250℃ for 7 days.
- C=C Linkage: The C=C-linked fully π-conjugated COFs have been achieved via the Knoevenagel condensation of aldehydes and benzyl cyanides in the presence of a base catalyst. The example of C=C-linked COF is prepared by the condensation of tetrakis(4-formylphenyl)pyrene (TFPPy) and 1,4-phenylenediacetonitrile in the mixture to yield COF in 89% yield.
- 1,4-Dioxin Linkage: The 1,4-dioxin linkage have been synthesized by the condensation of ortho-difluoro benzene or pyridine and catechol in the present of a base catalyst, the resulting COFs exhibit high chemical stability owing to the irreversible dioxin linkage.
Fig.4 Structural formula of 1,4-dioxin linked COFs-316
As the raw materials of COFs, the COFs linkers are critical to the construction of COFs. Firstly, using various COF linkers can form different linkage units by and covalent bond forming reactions, which have an important impact on physical properties of the COFs. Because the pore size of the COFs can be controlled by the linkage units with different molecular lengths, and the topology of the COFs can be controlled by the linkage units with different shape. Secondly, the COFs linkers with different functional groups can effect the chemical properties of COFs such as electrical property, adsorption property and others. The above mentioned two points directly determine the applications of COFs.
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- Ahmed I.; Jhung S.H. Covalent organic framework-based materials: Synthesis, modification, and application in environmental remediation[J]. Coordination Chemistry Reviews, 2021, 441(30): 213989.