Covalent organic frameworks (COFs) are covalently linked organic porous networks or covalent porous crystalline polymers that enable the elaborate combination of organic building blocks into well-organized structures with atomic accuracy, which were first presented by Yaghi et al. in 2005. COFs consist of light elements, representatively including H, B, C, N, and O, that crystallize into polymeric frameworks with highly ordered internal structures held together via robust covalent bonds (see Fig.1). Different from amorphous organic polymers of linear polymers, hyperbranched polymers, cross-linked polymers, and biopolymers, COFs possess a long-range well organized structure, in which the organic building blocks are positionally controlled in two or three dimensions, increasing the people's ability to predesign primary and high-order structures. Moreover, COFs can be readily synthesized under mild conditions and may include the unbonded functional groups at the terminus of the COF matrix. The structural characteristics and unbonded functional groups make COFs achieve different properties and functions. Since COFs have an unprecedented incorporation of extraordinary features, they are currently being used in a wide range of applications for the various fields.
Fig.1 Optimized crystal structure of the pure COF (COF-IITI-0)
COFs'unique combination of crystallinity and organic functionality provide these materials with a variety of special properties.
Firstly, porosity is most important property of COFs, they have been repeatedly shown to be permanently porous by measurement of gas sorption isotherms and their pore size is also highly tunable.
Secondly, the covalently bonded structure of light elements provides the rigidity, thermal stability, low mass density, easy alteration in the framework, and most importantly porosity with structural flexibility to the COFs.
Thirdly, the COFs have high surface areas, which are comparable to those of MOFs. For example, the earliest reported COF-1 and COF-5 exhibited a surface area of SBET=711 m2/g and the subsequently reported 2D COF-6, COF-8, and COF-10 demonstrated the surface areas of SBET=1049, 968, and 976 m2/g.
Finally, COFs materials have π-stacked layered structures that cause electronic interactions between neighboring layers resulting in electronic properties, which render them highly desirable as organic semiconductors for electronic applications.
COFs desirable properties of above mentioned make these materials promising for use in the fields of gas storage and separation, catalysis, optoelectronics, sensing, small molecules adsorption, drug delivery and so on[3-4].
Fig.2 The applications of COFs
- Gas storage and separation: Their large surface areas and low densities make COFs promising for use in gas storage applications. COFs have exhibited high capacities for the storage of important gases such as methane, hydrogen, and carbon dioxide. In particular, COFs have proven to be exceptional methane storage materials, even at room temperature. COFs are also useful for the capture and storage of harmful gases. For instance, ammonia is a widely employed chemical that needs to be safely transported due to its toxicity and corrosiveness. As a Lewis base, ammonia can be effectively captured through interactions with Lewis acidic groups, such as the boron atoms present in boroxine or boronate ester based COFs. In addition, the gases permeation flux across the as-fabricated some COFs membranes were different between among gases like H2, CH4 and N2, so that these gases can be separated by COFs membranes.
- Catalysis: All COFs show an open network structure, which offers accessible channels or nanopores with uniform sizes ranging from angstroms to nanometers for guest molecules. In addition, the organic skeletons of COFs make it easy to be decorated with functional groups. So, the accessible channels and easy modification of COFs make them ideal materials used as catalysts. Compared with traditional activated carbon and zeolites, the "designable" assembly of building blocks enables the spatial separation of multiple catalytic sites in the framework, endowing COFs with cooperative catalysis character and thus enhanced catalyst reactivity. COFs enable the integration of various catalytic systems or sites into the structures and are tolerant to different types of catalytic reactions such as asymmetric catalysis, photocatalysis, electrocatalysis, metal-based catalysis, and others.
Fig.3 Schematics of sp2 carbon-conjugated COFs for photocatalytic hydrogen evolution from water
- Sensing: One of the biggest advantages of COFs than other materials is their easy chemical modification that can affect not only their intrinsic physical and chemical properties but also reactivities with other molecules, which allows the modulation of selectivity of sensitivity of COFs sensors to various target materials. And COFs have outstanding high surface area like other porous materials, these two factors are benefit for efficient sensing applications of COFs such as detecting explosives, volatile chemicals and toxic metals.
- Small molecules adsorption: COFs enable the designed synthesis of porous structures through the topology diagram and pore-surface engineering, offering a unique porous platform with different pore size, pore shape, and pore environment for exploring small molecular adsorption. Owing to their structural designability and diversity, COFs have been developed for various target molecules, from iodine, organic compounds, metal ions, and so on, offering a chemical tool to tackle environmental and energy issues.
- Optoelectronics: Most COFs contain semiconducting and luminescent properties. Because COFs contain rigid π-units in their skeletons as they employ π-units for designing and constructing skeletons to form topologically ordered columnar π-arrays, so the COFs can obtain luminescent behavior. Moreover, these columnar π-arrays trigger intracolumn electronic coupling and provide preorganized pathways for promoting charge-carrier transport, making COFs with semiconductivity and facilitating the applications of COFs for energy storage (capacitive energy storage and battery) and conversion.
Fig.4 Luminescent behavior of COFs for treating cells
- Drug delivery: Combining outstanding thermal and chemical stabilities, structural designability, and inherent porosity in one material, COFs have shown their potential in drug delivery applications. Compared to nanoparticles, biocompatible polymeric materials, liposomes, and MOFs, COFs are used as vectors for drug delivery usually have more functionality and chemical stability.
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