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

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.

Recently, novel core-shell MOF@COF hybrids display excellent performance in various fields because of their inherited advantages from their parent MOFs and/or COFs. However, it is still a grand challenge to adjust the morphology of MOFs and/or COFs for consequent performance improvement. Herein, a Ti-MOF@TpTt hybrid coated with ultra-thin COF nanobelt, which is different from the fibrillar-like parent COF, is successfully synthesized through a sequential growth strategy. The as-obtained Pd decorated Ti-MOF@TpTt catalyst exhibits much higher photocatalytic performance than those of Ti-MOF, TpTt-COF, and Ti-MOF@TpTt hybrids with fibrillar-like COF shell for the photocatalytic cascade reactions of ammonia borane (AB) hydrolysis and nitroarenes hydrogenation. These can be attributed to its high BET surface area, core-shell structure, and type II heterojunction, which offers more accessible active sites and improves the separation efficiency of photo-generated carriers. Finally, the possible mechanisms of the cascade reaction are also proposed to well explain the improved performance of this photocatalytic system. This work presents a constructive route for designing core-shell MOF@COF hybrids with controllable morphology adjustment of COF shell, leading to the improved photocatalytic ability to broaden the applications of MOF/COF hybrid materials.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202101884

Construction of Core-Shell MOF@COF Hybrids with Controllable Morphology Adjustment of COF Shell as a Novel Platform for Photocatalytic Cascade Reactions.

Photoresponsive materials have drawn much attention and are widely applied in daily life for their reversible changes in luminous color or appearance color under light irradiation. In this work, a new photoresponsive system based on triarylamine derivatives is developed. With the changed aryl substituents, adjustable photoresponsive properties, including photoactivated phosphorescence and photochromism after being dispersed into the poly(methyl methacrylate) (PMMA) matrix, are demonstrated. According to the theoretical calculations and experimental data, the competition between the formations of triplet excitons and cationic radicals under photoirradiation should be the main reason for their different photoresponsive properties. Excitingly, the applications of rewritable photopatterning, anticounterfeiting, information encryption, and decryption are realized conveniently, in addition to the successful model of sunglasses to protect eyes away from ultraviolet radiation and strong light in the sunlight. These studies present a simple and efficient design strategy for the development of photoresponsive materials on modulating the phosphorescence and photochromic property.

Tunable Photoresponsive Behaviors Based on Triphenylamine Derivatives: The Pivotal Role of π-Conjugated Structure and Corresponding Application.

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

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