Covalent organic frameworks (COFs) are a class of porous polymers that form two- or three-dimensional structures through reactions between organic precursors, resulting in strong, covalent bonds to afford porous, stable, and crystalline materials. COFs emerged as a field from the overarching domain of organic materials as researchers optimized both synthetic control and precursor selection. These improvements to coordination chemistry enabled non-porous and amorphous organic materials such as organic polymers to advance into the construction of porous, crystalline materials with rigid structures that granted exceptional material stability in a wide range of solvents and conditions. Through the development of reticular chemistry, precise synthetic control was achieved and resulted in ordered, nano-porous structures with highly preferential structural orientation and properties which could be synergistically enhanced and amplified. With judicious selection of COF secondary building units (SBUs), or precursors, the final structure could be predetermined, and modified with exceptional control enabling fine-tuning of emergent properties. This level of control facilitates the COF material to be designed, synthesized, and utilized in various applications, many times with metrics on scale or surpassing that of the current state-of-the-art approaches. COFs are classified as reticular materials. Additionally, their tunable porosity and high surface area make COFs particularly attractive for applications in gas storage, catalysis, sensing, and energy-related technologies.
History While at the University of Michigan, Omar M. Yaghi (currently at UC Berkeley) and Adrien P Cote published the first paper of COFs in 2005, reporting a series of 2D COFs. They reported the design and successful synthesis of COFs by condensation reactions of phenyl diboronic acid (C6H4[B(OH)2]2) and hexahydroxytriphenylene (C18H6(OH)6). Powder X-ray diffraction studies of the highly crystalline products having empirical formulas (C3H2BO)6·(C9H12)1 (COF-1) and C9H4BO2 (COF-5) revealed 2-dimensional expanded porous graphitic layers that have either staggered conformation (COF-1) or eclipsed conformation (COF-5). Their crystal structures are entirely held by strong bonds between B, C, and O atoms to form rigid porous architectures with pore sizes ranging from 7 to 27 angstroms. COF-1 and COF-5 exhibit high thermal stability (to temperatures up to 500 to 600 °C), permanent porosity, and high surface areas (711 and 1590 square meters per gram, respectively).
The synthesis of 3D COFs has been hindered by longstanding practical and conceptual challenges until it was first achieved in 2007 by Omar M. Yaghi and colleagues, which received the Newcomb Cleveland Prize. The research team synthesized and designed the first 3D-COF ever; COF-103 and COF-108, helping unleash this new field. Unlike 0D and 1D systems, which are soluble, the insolubility of 2D and 3D structures precludes the use of stepwise synthesis, making their isolation in crystalline form very difficult. This first challenge, however, was overcome by judiciously choosing building blocks and using reversible condensation reactions to crystallize COFs.
Structure Porous crystalline solids consist of secondary building units (SBUs), which assemble to form a periodic and porous framework. An almost infinite number of frameworks can be formed through various SBU combinations, leading to unique material properties for applications in separations, storage, and heterogeneous catalysis.
Types of porous crystalline solids include zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs). Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. MOFs are a class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands and are a new development on the interface between molecular coordination chemistry and materials science.
COFs are another class of porous polymeric materials, consisting of porous, crystalline, covalent bonds that usually have rigid structures, exceptional thermal stabilities (to temperatures up to 600 °C), are stable in water, and have low densities. They exhibit permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates.
Secondary building units
The term secondary building unit has been used for some time to describe conceptual fragments, which can be compared to bricks used to build a house of zeolites; in the context of this page, it refers to the geometry of the units defined by the points of extension.
Reticular synthesis Reticular synthesis enables facile bottom-up synthesis of the framework materials to introduce precise perturbations in chemical composition, resulting in the highly controlled tunability of framework properties. Through a bottom-up approach, a material is built from atomic or molecular components synthetically as opposed to a top-down approach, which forms a material from the bulk through approaches such as exfoliation, lithography, or other varieties of post-synthetic modification. The bottom-up approach is especially advantageous with respect to materials such as COFs because the synthetic methods are designed to directly result in an extended, highly crosslinked framework that can be tuned with exceptional control at the nanoscale level. Geometrical and dimensional principles govern the framework's resulting topology as the SBUs combine to form predetermined structures. This level of synthetic control has also been termed "molecular engineering", abiding by the concept termed by Arthur R. von Hippel in 1956.
According to the literature, integration of monomeric compounds into an iso-reticular framework, such as a COF, can lead to a synergistic enhancement and amplification of their properties. COF materials can be synthesized bottom-up using reticular synthesis to attain robust, tunable crystalline frameworks that synergistically enhance the properties of the precursor compounds used for their synthesis. As a result of reticular synthesis, the COF material is highly modular and tuned efficiently by varying the SBUs' identity, length, and functionality depending on the desired property change on the framework scale. The resulting COF frameworks offer many advantages in terms of improved performance in different applications. The ability to introduce diverse functionality directly into the framework scaffold to allow for a variety of functions would be cumbersome, if not impossible, to achieve through top-down methods, such as lithographic approaches or chemical-based nanofabrication. Through reticular synthesis, it is possible to molecularly engineer modular, framework materials with highly porous scaffolds that exhibit unique electronic, optical, and magnetic properties while simultaneously integrating desired functionality into the COF skeleton.
Reticular synthesis is different from the retrosynthesis of organic compounds because the structural integrity and rigidity of the building blocks in reticular synthesis remain unaltered throughout the construction process—an important aspect that could help to fully realize the benefits of design in crystalline solid-state frameworks. Similarly, reticular synthesis should be distinguished from supramolecular assembly because, in the former, building blocks are linked by strong bonds throughout the crystal.
Synthetic chemistry
Reticular synthesis was used by Yaghi and coworkers in 2005 to construct the first two COFs reported in the literature: COF-1, using a dehydration reaction of benzenediboronic acid (BDBA), and COF-5, via a condensation reaction between hexahydroxytriphenylene (HHTP) and BDBA. These framework scaffolds were interconnected through the formation of boroxine and boronate linkages, respectively, using solvothermal synthetic methods.
COF linkages Since Yaghi and coworkers' seminal work in 2005, COF synthesis has expanded to include a wide range of organic connectivity, such as boron-, nitrogen-, and other atom-containing linkages. The linkages in the figures shown are not comprehensive, as other COF linkages exist in the literature, especially for the formation of 3D COFs.
Boron condensation The most popular COF synthesis route is a boron condensation reaction which is a molecular dehydration reaction between boronic acids. In the case of COF-1, three boronic acid molecules converge to form a planar six-membered B3O3 (boroxine) ring with the elimination of three water molecules.
Triazine-based trimerization
Another class of high-performance polymer frameworks with regular porosity and high surface area is based on triazine materials, which can be achieved by dynamic trimerization reactions of simple, cheap, and abundant aromatic nitriles in ionothermal conditions (molten zinc chloride at high temperature (400 °C)). CTF-1 is a good example of this chemistry.
Imine condensation
The imine condensation reaction which eliminates water (exemplified by reacting aniline with benzaldehyde using an acid catalyst), can be used as a synthetic route to reach a new class of COFs. The 3D COF called COF-300 and the 2D COF named TpOMe-DAQ are good examples of this chemistry. When 1,3,5-triformylphloroglucinol (TFP) is used as one of the SBUs, two complementary tautomerizations occur (an enol to keto and an imine to enamine) which result in a β-ketoenamine moiety as depicted in the DAAQ-TFP framework. Both DAAQ-TFP and TpOMe-DAQ COFs are stable in acidic aqueous conditions and contain the redox active linker 2,6-diaminoanthroquinone, which enables these materials to reversibly store and release electrons within a characteristic potential window. Consequently, both of these COFs have been investigated as electrode materials for potential use in supercapacitors.
Solvothermal synthesis The solvothermal approach is the most common one used in the literature but typically requires long reaction times due to the insolubility of the organic SBUs in nonorganic media and the time necessary to reach thermodynamic COF products.
Templated synthesis Morphological control on the nanoscale is still limited as COFs lack synthetic control in higher dimensions due to the lack of dynamic chemistry during synthesis. To date, researchers have attempted to establish better control through different synthetic methods such as solvothermal synthesis, interface-assisted synthesis, and solid templation as well as seeded growth. First one of the precursors is deposited onto the solid support, followed by the introduction of the second precursor in vapor form. This results in the deposition of the COF as a thin film on the solid support.
Properties
Porosity A defining advantage of COFs is the exceptional porosity that results from the substitution of analogous SBUs of varying sizes. Pore sizes range from 7-23 Å and feature a diverse range of shapes and dimensionalities that remain stable during the evacuation of solvent. The rigid scaffold of the COF structure enables the material to be evacuated of solvent and retain its structure, resulting in high surface areas as seen by the Brunauer–Emmett–Teller analysis. This high surface area to volume ratio and incredible stability enables the COF structure to serve as exceptional materials for gas storage and separation.
Crystallinity