Celebrating 15 Years of Covalent Organic Frameworks

Professor Xiaodong Zou is a full professor and chair of Inorganic and Structural Chemistry and deputy head of the Department of Materials and Environmental Chemistry at Stockholm University. After completing her PhD in structural chemistry and electron crystallography, she started at the beginning of her independent career to apply these techniques in solving structures of porous materials. She first worked on development of inorganic porous materials, such as zeolites and related open-frameworks. A joint project with Prof. Michael O’Keeffe drew her attention to metal organic frameworks (MOFs), covalent organic frameworks (COFs) and reticular synthesis. Since then, she is working on developing structure characterization techniques for porous materials.

Organic materials are highly sensitive to electron beam irradiation and thus easily damaged upon imaging by electron microscopy. Now, low-dose aberration-corrected high resolution transmission electron microscopy allows for less invasive near-atomic-scale imaging of a two-dimensional polymer.

The manner in which adjacent sheets stack in layered covalent organic frameworks largely influences their material properties, including chemical stability, crystallinity and porosity. The layer stacking of a COF has now been probed locally, showing disorder that is not detected through long-range characterization.

Using a small organic cage structure as a node has enabled the preparation of a new 3D covalent organic framework.

Three-dimensional covalent organic frameworks are attractive functional materials, although there are fewer examples than their two-dimensional counterparts. Here, the authors review the synthetic approaches yielding these compounds, and highlight key challenges facing researchers in the field.

Covalent organic frameworks are crystalline porous polymers with precisely ordered polygon architectures. In this Review we summarize recent advances in the design principles and synthetic reactions, highlight the current status in structural construction and functionality design, and predict challenging issues and future directions.

Covalent organic frameworks (COFs) have potential applications in, for example, gas storage and separation. The pore sizes in these materials are tunable by selection of the building blocks, and materials with multiple pore sizes are desirable. This Perspective considers synthetic approaches to 2D COFs that rely on tessellation to prepare such multiporous materials.

The development of porous, crystalline materials with high chemical stability is crucial for their practical uses. Now, polyarylether-based covalent organic frameworks (PAE-COFs) have been synthesized that show high crystallinity and porosity, as well as good stability against harsh chemical environments including boiling water and strong acids and bases.

Covalent organic frameworks (COFs) are attractive multifunctional porous materials that can be generated with atomic precision. However, endowing them with long-range order—desirable for applications—has remained challenging. Now, propeller-shaped building units have been used that allow consecutive layers to lock in position, resulting in highly crystalline COFs.

Two-dimensional covalent organic frameworks (2D COFs) are commonly synthesised through dynamic covalent chemistry, as it allows for thermodynamic ‘error correction' which enhances crystallinity. Here a crystalline 2D COF with amine and hydroxyl functional groups within the pores is synthesised through kinetically-controlled reactions.

The incorporation of mechanical bonds into porous materials offers opportunities for new functionality. Here a covalent organic framework is synthesized by polymerization of a cucurbituril-viologen complex, imparting improved thickness, stability, and luminescence compared to the unrotaxanated viologen-based COF.

Using complementary linkers as building blocks in the design of 2D covalent organic frameworks (COFs) limits the formation of compositionally and structurally complex networks. Here, the authors demonstrate a COF with a bex topology by combining non-complementary triangular and rectangular linkers.

Hollow, spherical nano/microstructures are potentially useful for energy and drug delivery applications. Here, the authors show that these structures can be fabricated from covalent organic frameworks, and exploit their chemical stability and mesoporous structures for enzyme encapsulation.

The discovery of new covalent organic framework (COF) topologies is often led by trial-and-error experiments. Here, the authors present a methodology for high throughput construction of COFs based on a materials genomics strategy and demonstrate the synthesis of the generated 2D and 3D-COFs.

Layered COFs are attractive precursors for two-dimensional materials but they are difficult to cleave into mono- or few-layer sheets. Pseudorotaxane moieties have now been embedded into layered COFs to facilitate their cleavage into sheets of uniform thickness. Crown-ether macrocycles within the COF backbone bind to ionic viologen guests, leading to electrostatic repulsion between layers.

Dr. Yi Liu is the facility director at the Molecular Foundry, Lawrence Berkeley National Laboratory. Trained as a supramolecular chemist and after a postdoctoral stay and working on click chemistry in Sharpless’s group he started his independent research career working on organic electronics, porous materials, and covalent organic frameworks (COFs). His aim is to develop nanostructured electronic materials through the design, synthesis and manipulation of tailor-made molecular constituents.

The electrical conductivity of covalent organic frameworks may be limited by a low degree of π-conjugation or structural disorder. Here the authors produce uniform and highly conjugated COFs via a reversible dynamic imine condensation and demonstrate enhanced electrical conductivity in their product.

Semiconducting covalent organic frameworks (COF) with conjugated structures are desired for optoelectric conversion but poor reversibility of bond forming reactions hampers the formation of crystalline structures. Here, the authors synthesise COFs with disubstituted C = C linkages which enable water splitting under visible light irradiation.

Topological connection of organic chromophores is an attractive way to design light-emitting covalent organic frameworks but the synthesis of stable light-emitting frameworks remains challenging. Here the authors report the designed synthesis of sp2 carbon conjugated frameworks that combine stability with light-emitting activity

Covalent organic frameworks (COFs) find increasing application as sensor material, but fast switching solvatochromism was not realized. Here the authors demonstrate that combination of electron-rich and -deficient building blocks leads to COFs which fast and reversibly change of their electronic structure depending on the surrounding atmosphere.

The donor–acceptor (D-A) conjugation has been adopted for two-dimensional (2D) covalent organic frameworks (COFs) for efficient generation of free charge carriers. Here, the authors investigate the dynamics of photogenerated charge carriers in 2D D-A COFs by combining femtosecond optical spectroscopy and non-adiabatic molecular dynamics simulation.

3D covalent organic frameworks (COF) show interesting hierarchical arrangements of nanopores and open sites but their synthesis remains challenging. Here the authors report a fluorescent AIEgen-based 3D COF and demonstrate application as a coating material for white LEDs and for sensing of picric acid

Encoding functionalities in covalent organic frameworks (COFs) is important for widening their application field but the development of fluorescent COFs is hampered by a lack of guiding design principles. Here the authors demonstrate tuning and switching of the photoluminescence in 2D COFs made of non-emissive building blocks.

Controlling chirality and function in metal organic frameworks has been an achievement, but very difficult to carry out in covalent organic frameworks. Here the authors show chiral covalent organic frameworks that are crystallized from achiral precursors by chiral catalytic induction.

Covalent organic frameworks are crystalline porous polymers integrating molecular building blocks into periodic structures. Here, the authors report a general multiple-component condensation strategy that enables the use of one knot and two or three linkers to synthesize complex, anisotropic frameworks.

Tuning of molecular switches in solid state toward stimuli-responsive materials attracted attention in recent years but has not yet been realized in three-dimensional (3D) covalent organic frameworks (COFs). Herein, the authors demonstrate a stable and switchable 3D COF which undergoes reversible transformation through a hydroquinone/quinone redox reaction.

Electrochromic materials are important for different optical applications but often these materials show low stability. Here, the authors demonstrate a stable donor-acceptor covalent organic framework which shows a stable dark-to-transmissive switching behaviour.

Prof. Donglin Jiang is a full professor at the National University of Singapore and is recognized as a pioneer in the field of 2D polymers and covalent organic frameworks (COFs). With a background in chemistry and studies on dendrimers for ten years, the beauty of dendrimers inspired him to consider the possibility of constructing other types of polymers with well-defined shapes and structures. After taking up an associate professorship in 2005 to set up an independent laboratory at the Institute for Molecular Science at the National Institute for Natural Science, he started to dedicate his work on the design, synthesis and functional exploration of 2D polymers, COFs and conjugated microporous polymers.

Catalysts for CO₂ photo- or electroreduction must balance activity, selectivity and efficiency. Here, the authors discuss the use of metal–organic frameworks for these processes and the role reticular chemistry may play in designing new catalysts.

The inherent synthetic tuneability of organic materials makes them attractive in photocatalysis, but they tend to have low quantum efficiencies for water splitting. A crystalline covalent organic framework featuring a benzo-bis(benzothiophene sulfone) moiety has now been shown to exhibit high activity for photochemical hydrogen evolution from water.

Covalent organic frameworks (COFs) feature periodic layers and ordered pores that make them promising for applications in catalysis, but they typically suffer from poor stability. Now, adding methoxy groups to its pore walls has been shown to strengthen a COF's interlayer interactions, resulting in a stable, crystalline, porous material that can be further converted into chiral organocatalysts.

The exfoliation of covalent organic frameworks is a potential route to interesting two-dimensional materials. Here, the authors report a strategy incorporating metal ions and axial ligands to facilitate exfoliation of porphyrin COFs, and examine the photocatalytic activity of the resulting nanomaterials.

The design of large-pore proton conductors with well-defined high-order structures is challenging. Proton conduction in a crystalline covalent organic framework 2–4 orders of magnitude higher than microporous polymers is now demonstrated.

Development of porous proton-transporting materials combining stability and high performance has remained a challenge. Here, the authors report a stable covalent organic framework with excellent proton conductivity in which nitrogen sites on pore walls confine and stabilize a H3PO4 network in the channels via hydrogen-bonding interactions.

Covalent organic frameworks are receiving increasing attention as promising cathode materials for rechargeable batteries. Here the authors report a honeycomb-like nitrogen-rich COF design in which the pyrazines and carbonyls enable favorable redox chemistry and remarkable Na-ion storage performance.

Conjugated polymeric molecules are promising electrode materials for batteries. Here the authors show a two-dimensional few-layered covalent organic framework that delivers a large reversible capacity of more than 1500 mAh g⁻¹ with the storage mechanism governed by 14-electron redox chemistry.

The study of covalent organic frameworks (COFs) in electrocatalytic CO₂ reduction reaction (CO₂RR) has drawn much attention. Here the authors show a series of tetrathiafulvalene based COFs designed and exfoliated into nanosheets which exhibit high electrocatalytic CO₂RR performance.

Production of hydrogen via the photocatalytic reduction of water is an attractive source of energy, but the catalysts are often expensive and possess little room for modification. Here, the authors show that covalent organic frameworks can be tuned for optimal photocatalytic performance.

Prof. Natalia Shustova is the Peter and Bonnie McCausland Associate Professor at the University of South Carolina. After receiving her Ph.D. in Physical Chemistry from the Moscow State University and a second Ph.D. in Inorganic Chemistry from the Colorado State University, she became interested in porous frameworks during her time as a Postdoctoral Associate in the group of Prof. Mircea Dincă at MIT. After starting her independent research career first as an Assistant Professor at the University of South Carolina in 2013 and since 2017 as Associate Professor, her research interest lies in the development of materials for sustainable energy conversion, sensing and artificial biomimetic systems.

Fabrication of large scale and defect free covalent organic framework (COF) membranes with pores small enough for gas sieving remains challenging. Here, the authors report a scalable fabrication method to grow large area defect free COF membranes and to tune the pore size in the sub-nm region by adjusting the stacking modes of the COF layers.

Porous materials for uranium capture have been developed in the past, but materials for simultaneous uranium capture and detection are scarce. Here the authors develop a stable covalent organic framework capable of adsorbing and detecting uranyl ions.

Porous materials can be used in columns for gas separation applications. Here, Yan and co-workers from a bottom-up method have produced chiral covalent organic frameworks and from an in situ strategy formed capillary columns for chiral gas chromatography.

The fabrication of defect-free covalent organic framework (COF) membranes for the separation of small molecules is challenging. Here, the authors report robust COF membranes with precise molecular sieving through a mixed-dimensional assembly, exhibiting high performance for alcohol dehydration and salt rejection.

Despite their potential application as drug-delivery carriers, covalent organic frameworks (COF) have been only evaluated in vitro. Here the authors show by real time tracking in vivo the cell uptake of anticancer-drug loaded and water dispersible COFs.

Macroscopic architectures of covalent organic frameworks (COF) allow to fully exploit their chemical functionality and porosity but achieving three-dimensional hierarchical porous COF architectures remains challenging. Here, the authors present a COF/reduced graphene oxide aerogel which is synthesized by growing COF during a hydrothermal process along the surface of graphene sheets.