The dream to prepare well-defined materials drives the methodological evolution for molecular synthesis, structural control and materials manufacturing. Among various methods, chemical approaches to design, synthesize, control and engineer small molecules, polymers and networks offer the fundamental strategies. Merging covalent bonds and non-covalent interactions into one method to establish a complex structural composition for specific functions, mimicking biological systems such as DNA, RNA and proteins, is at the centre of chemistry and materials science. Covalent organic frameworks (COFs) are a class of crystalline porous polymers that enable the integration of organic units into highly ordered structures via polymerization. This polymerization system is unique as it deploys covalent bonds to construct the primary order structures of polymeric backbones via polycondensation and leverages on non-covalent interactions to create the high order structures of polymeric networks via supramolecular polymerization in a one-pot reaction system. This Primer covers all aspects of the field of COFs from chemistry to physics, materials and applications, and outlines the design principle, experimental methods, characterization and applications, with an aim to show a concise yet full picture of the field. The key fundamental issues to be addressed are analysed with an outlook on the future major directions from different perspectives. Covalent organic frameworks (COFs) are a class of crystalline porous polymers consisting of highly ordered organic structures formed by polymerization. In this Primer, Tan et al. discuss the design principle, experimental methods, characterization and applications of COFs. 

https://mts.intechopen.com/storage/books/11903/authors_book/authors_book.pdf

Covalent organic frameworks

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications.

Porphyrin- and porphyrinoid-based covalent organic frameworks (COFs): From design, synthesis to applications

A new kind of piperazine-linked covalent organic framework (COF) was synthesized through the nucleophilic substitution reaction between octaminophthalocyanines and hexadecafluorophthalocyanines. The two-dimensional (2D) frameworks are in tetragonally shaped polygon sheets, which stack in an AA stacking mode to constitute periodically ordered metallophthalocyanine columns and one-dimensional (1D) microporous channels. The piperazine-linked COFs exhibit excellent chemical stability and permanent porosity. By virtue of the neatly arrayed phthalocyanine columns and inbuilt cationic radicals, the piperazine-linked frameworks are highly conductive. The conductivity values of NiPc-NH-CoPcF8 COF reached up to 2.72 and 12.7 S m-1 for pellet and film samples, respectively. Moreover, this p-type conductive COF exhibited a high carrier mobility of 35.4 cm2 V-1 s-1. Both the electric conductivity and carrier mobility set new records for conductive COFs.

Piperazine-Linked Covalent Organic Frameworks with High Electrical Conductivity.

H2 O2 is a significant chemical widely utilized in the environmental and industrial fields, with growing global demand. Without sacrificial agents, simultaneous photocatalyzed H2 O2 synthesis through the oxygen reduction reaction (ORR) and water oxidation reaction (WOR) dual channels from seawater is green and sustainable but still challenging. Herein, two novel thiophene-containing covalent organic frameworks (TD-COF and TT-COF) were first constructed and served as catalysts for H2 O2 synthesis via indirect 2e- ORR and direct 2e- WOR channels. The photocatalytic H2 O2 production performance can be regulated by adjusting the N-heterocycle modules (pyridine and triazine) in COFs. Notably, with no sacrificial agents, just using air and water as raw materials, TD-COF exhibited high H2 O2 production yields of 4060 μmol h-1 g-1 and 3364 μmol h-1 g-1 in deionized water and natural seawater, respectively. Further computational mechanism studies revealed that the thiophene was the primary photoreduction unit for ORR, while the benzene ring (linked to the thiophene by the imine bond) was the central photooxidation unit for WOR. The current work exploits thiophene-containing COFs for overall photocatalytic H2 O2 synthesis via ORR and WOR dual channels and provides fresh insight into creating innovative catalysts for photocatalyzing H2 O2 synthesis. 

Thiophene-Containing Covalent Organic Frameworks for Overall Photocatalytic H2O2 Synthesis in Water and Seawater.

Semiconductive Covalent Organic Frameworks: Structural Design, Synthesis, and Application

Covalent organic frameworks (COFs) have aroused extensive attention from various fields owing to their numerous advantages, including permanent porosity, high crystallinity, strong robustness, and well-ordered channels. However, the poor processability of the crystallite powder has greatly impeded their further utilization in many advanced devices and frontier areas. In this work, we fabricate a series of COF films using an interfacial polymerization strategy at a liquid-liquid interface under ambient conditions. The as-synthesized freestanding films are continuous, flexible, and defect-free and have large areas of up to 4 × 6 cm2. In addition, the pore sizes of these COF films can be well controlled based on the principle of reticular chemistry. These films exhibit high chemical stability even in acidic and basic aqueous solutions. More significantly, the highly robust COF films can serve as a nanofiltration membrane for efficient separation of pollutant molecules with different dimensions. These films show high selectivity for the separation of mixed molecule feed and excellent recyclability without a significant loss in the rejection rate.

De Novo Fabrication of Large-Area and Self-Standing Covalent Organic Framework Films for Efficient Separation.

Driven by green and sustainable methodologies for chemical manufacturing, enzymes are forecast to have much to prospect in the coming years due to their high activity and selectivity. However, the fragile active spatial‐conformation of an enzyme is predominantly maintained by weak intermolecular interactions, showing quite sensitive to thermal, pH, and chemical chemoreception. Thus, the low activity and recyclability result from structural instability can hardly meet the prototype of green chemistry in commercial process. Covalent organic frameworks (COFs) are stable extended porous network materials assembled by organic linkers in a moderate way. Due to their pre‐designable structures, an enzyme possesses the access to be immobilized to the interior or surface of COF skeletons via covalent or noncovalent processes, which can afford protection to enzymes in harsh conditions. Moreover, with the improved stability and cyclicity, the formed COF/enzyme biocomposites can facilitate broader horizon. Herein, this review mainly canvasses the advances in the emerging field of COF/enzyme biocomposites, sketching the factors that are critical in building COF/enzyme systems and the applications of registered enzyme host platforms reported in recent years.

Engineering of Covalent Organic Framework‐Based Advanced Platforms for Enzyme Immobilization: Strategies, Research Progress, and Prospects

A porphyrin-based two-dimensional (2D) covalent organic framework (COF) was developed by a C4 + C4 topological diagram. It was constructed by the condensation of zinc 5,10,15,20-tetra(4-aminophenyl...

Semiconductive Porphyrin-Based Covalent Organic Frameworks for Sensitive Near-Infrared Detection.

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Semiconductive Covalent Organic Frameworks: Structural Design, Synthesis, and Application

De Novo Fabrication of Large-Area and Self-Standing Covalent Organic Framework Films for Efficient Separation.

Engineering of Covalent Organic Framework‐Based Advanced Platforms for Enzyme Immobilization: Strategies, Research Progress, and Prospects

Semiconductive Porphyrin-Based Covalent Organic Frameworks for Sensitive Near-Infrared Detection.