Covalent organic frameworks (COFs) are a class of crystalline porous polymers that allow the atomically precise integration of organic units to create predesigned skeletons and nanopores. They have recently emerged as a new molecular platform for designing promising organic materials for gas storage, catalysis, and optoelectronic applications. The reversibility of dynamic covalent reactions, diversity of building blocks, and geometry retention are three key factors involved in the reticular design and synthesis of COFs. This tutorial review describes the basic design concepts, the recent synthetic advancements and structural studies, and the frontiers of functional exploration.

Covalent organic frameworks

Covalent organic frameworks (COFs) enable precise reticulation of organic building units into extended 2D and 3D open networks using strong covalent bonds to constitute predesignable topologies and tunable pore structures, presenting an emerging class of crystalline porous polymers. Although rapid progress and substantial achievements in COF chemistry over the past 15 years have been realised, highly efficient strategies and reproducible procedures still play a central role in achieving high-quality COFs and serve as a major driving force for the further advancement of this promising field. In this review, we focused on the key progress in synthesising high-quality COF crystallites and films by highlighting their uniqueness from the viewpoints of synthetic strategies and procedures. We discussed representative synthetic methods including mechanochemical synthesis, microwave synthesis, multicomponent reaction, multistep synthesis and linker exchange strategies to compare their features in producing COFs. We scrutinised the recently developed “two-in-one” molecular design strategy to showcase advantages in optimising synthetic conditions such as catalyst, monomer feeding rate and tolerance to functional groups. We analysed interfacial polymerisation for fabricating various COF films by emphasising their scope and applicability. Moreover, we proposed key underlying challenges to be solved and predicted future frontiers from the perspectives of synthesising high quality crystallites and films that are key to practical applications.

New synthetic strategies toward covalent organic frameworks.

The excessive depletion of fossil fuels and consequent energy crisis combined with environmental issues call for inexhaustible, clean and renewable energy sources and environmentally friendly energy technologies, such as solar energy and novel electrochemical energy conversion and storage devices. Developing supporting platforms for energy conversion and storage ameliorating mass transfer and electron transfer has stepped into the center of the energy research arena. Covalent organic frameworks (COFs) are emerging crystalline porous materials linked via covalent bonding possessing flexible molecular design and synthetic strategies, high conjugated and modifiable structures, large surface area and porosity. Due to these merits, COFs have shown promising perspectives in energy applications including photocatalysis, electrocatalysis, supercapacitors, metal-ion/sulfur batteries, etc. This critical review imparts a comprehensive summary of the fast-developing COF field in terms of molecular design and subsequent synthetic strategies to prepare COFs with highly conjugated and modifiable structures and their applications in energy conversion and storage. Furthermore, challenges and perspectives according to previous contributions are also discussed for developing more efficient energy conversion and storage COF materials. It is anticipated that this review could boost further research enthusiasm for COF-based materials in energy applications.

Covalent organic framework-based materials for energy applications

Metal-organic framework (MOF) and covalent organic framework (COF) nanosheets are a new type of two-dimensional (2D) materials with unique design principles and various synthesis methods. They are considered ideal electrochemical devices due to the ultrathin thickness, easily tunable molecular structure, large porosity and other unique properties. There are two common methods to synthesize 2D MOF/COF nanosheets: bottom-up and top-down. The top-down strategy mainly includes ultrasonic assisted exfoliation, electrochemical exfoliation and mechanical exfoliation. Another strategy mainly includes interface synthesis, modulation synthesis, surfactant-assisted synthesis. In this Review, the development of ultrathin 2D nanosheets in the field of electrochemistry (supercapacitors, batteries, oxygen reduction, and hydrogen evolution) is introduced, and their unique dimensional advantages are highlighted.

Two-Dimensional MOF and COF Nanosheets: Synthesis and Applications in Electrochemistry.

Given their modular synthesis, unique structural features and rich functionality, structurally ordered covalent organic frameworks (COFs) and covalent monolayers have shown great potential in a broad range of applications, such as catalysis, molecular separation, energy storage, light harvesting, etc The synthesis of COF thin films and covalent monolayers mainly utilizes dynamic covalent chemistry (DCvC), which relies on the reversible formation and breaking of rather strong covalent bonds within molecules under certain external stimuli Such reversible reaction conditions enable a self-correction mechanism, which can selectively resolve defect sites leading to the formation of highly ordered COF films under thermodynamic control Novel techniques to obtain single-layer covalent nanosheets have spread throughout recent literature Emerging interfacial polymerization techniques (eg, air–water, liquid–liquid, liquid–solid, etc) have been employed to successfully synthesize crystalline COF thin films from a variety of starting building blocks Although the growth of ordered frameworks at the interface represents a rapidly developing field, the reversible reactions suitable for the synthesis of thin films or monolayers are still very limited The identification and development of new dynamic reactions and interfacial polymerization conditions would be critical for the further development of COF thin films and covalent monolayer materials This review covers the recent design and synthesis of COF thin films and covalent monolayers as well as their property study The fundamental working mechanisms of different surface and interfacial polymerization and the current challenges and opportunities in this rapidly growing field are presented

Confined growth of ordered organic frameworks at an interface

By using an oriented electric field in a scanning tunneling microscope, one can locally control the condensation of boronic acids at the liquid/solid interface. The phase transition between self-as...

Electric-Field-Mediated Reversible Transformation between Supramolecular Networks and Covalent Organic Frameworks

The quality of crystalline two-dimensional (2D) polymers1-6 is intimately related to the elusive polymerization and crystallization processes. Understanding the mechanism of such processes at the (sub)molecular level is crucial to improve predictive synthesis and to tailor material properties for applications in catalysis7-10 and (opto)electronics11,12, among others13-18. We characterize a model boroxine 2D dynamic covalent polymer, by using in situ scanning tunnelling microscopy, to unveil both qualitative and quantitative details of the nucleation-elongation processes in real time and under ambient conditions. Sequential data analysis enables observation of the amorphous-to-crystalline transition, the time-dependent evolution of nuclei, the existence of 'non-classical' crystallization pathways and, importantly, the experimental determination of essential crystallization parameters with excellent accuracy, including critical nucleus size, nucleation rate and growth rate. The experimental data have been further rationalized by atomistic computer models, which, taken together, provide a detailed picture of the dynamic on-surface polymerization process. Furthermore, we show how 2D crystal growth can be affected by abnormal grain growth. This finding provides support for the use of abnormal grain growth (a typical phenomenon in metallic and ceramic systems) to convert a polycrystalline structure into a single crystal in organic and 2D material systems. 

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Observing polymerization in 2D dynamic covalent polymers

The in situ on-surface conversion process from boroxine-linked covalent organic frameworks (COFs) to boronate ester-linked COFs is triggered and catalyzed at room temperature by an electric field and monitored with scanning tunneling microscopy (STM). The adaptive behavior within the generated dynamic covalent libraries (DCLs) was revealed, providing in-depth understanding of the dynamic network switching process.

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Real-Time Molecular-Scale Imaging of Dynamic Network Switching between Covalent Organic Frameworks.

Reducing particle size in supported metal catalysts to single‐atom level isolates the active metal sites and maximizes the atomic utilization efficiency. However, the large inter‐atom distance, particularly in low‐loading single‐atom catalyst (SAC), is not favorable for a complex reaction where two (or more) reactants have to be activated. A key question is how to control the inter‐atom distances to promote dinuclear‐type coactivation at the adjacent metal sites. Here, it is reported that reducing the average inter‐atom distance of copper SACs supported on carbon nitride (C3N4) to 0.74 ± 0.13 nm allows these catalysts to exhibit a dinuclear‐type catalytic mechanism in the nitrile–azide cycloaddition. Operando X‐ray absorption fine structure study reveals a dynamic ligand exchange process between nitrile and azide, followed by their coactivation on dinuclear Cu SAC sites to form the tetrazole product. This work highlights that reducing the nearest‐neighbor distance of SAC allows the mechanistic pathway to diversify from single‐site to multisite catalysis.

Gaolei Zhan

Papers

Electric-Field-Mediated Reversible Transformation between Supramolecular Networks and Covalent Organic Frameworks

Observing polymerization in 2D dynamic covalent polymers

Real-Time Molecular-Scale Imaging of Dynamic Network Switching between Covalent Organic Frameworks.

Promoting Dinuclear‐Type Catalysis in Cu1–C3N4 Single‐Atom Catalysts

Constructing ambivalent imidazopyridinium-linked covalent organic frameworks