Summary Covalent organic frameworks (COFs) are a new type of crystalline porous polymers known for chemical stability, excellent structural regularity, robust framework, and inherent porosity, making them promising materials for capturing various types of pollutants from aqueous solutions. This review thoroughly presents the recent progress and advances of COFs and COF-based materials as superior adsorbents for the efficient removal of toxic heavy metal ions, radionuclides, and organic pollutants. Information about the interaction mechanisms between various pollutants and COF-based materials are summarized from the macroscopic and microscopic standpoints, including batch experiments, theoretical calculations, and advanced spectroscopy analysis. The adsorption properties of various COF-based materials are assessed and compared with other widely used adsorbents. Several commonly used strategies to enhance COF-based materials’ adsorption performance and the relationship between structural property and sorption ability are also discussed. Finally, a summary and perspective on the opportunities and challenges of COFs and COF-based materials are proposed to provide some inspiring information on designing and fabricating COFs and COF-based materials for environmental pollution management.

Orderly Porous Covalent Organic Frameworks-based Materials: Superior Adsorbents for Pollutants Removal from Aqueous Solutions

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended two- and three-dimensional polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores. This polymer architecture is unique as it enables predesigning both primary- and high-order structures, greatly enhancing our capabilities of designing organic materials to produce predictable structures and to achieve unique properties and functions. Progress over the past 15 years in the design, synthesis and functional exploration of COFs has successively established the basis of the COF field and COFs have shown the great potential of chemistry in developing a class of amazing organic materials. In this review, we focus on analysing the historic developments of COFs to uncover a full materials and application picture by providing comprehensive yet clear guidance for molecular design, synthetic control and functional exploration. We scrutinise the structural components of COFs including building blocks, reactive sites and functional groups with the aim of finding the origins of structural designability and diversity, as well as multiple functionalities. We disclose strategies for designing and synthesising frameworks to construct various tailor-made interfaces, and for exploring skeletons and pores to design properties and functions. With well-defined skeletons, pores and interfaces that offer a chemical basis to trigger and control interactions with photons, excitons, phonons, polarons, electrons, holes, spins, ions and molecules, we illustrate the current status of our understandings of structure-property correlations, and unveil the principles for establishing a regime to design unique functions that originate from and are inherent to structures. We predict the key central issues in design and synthesis, the challenges in functional design and the future directions from the perspectives of chemistry, physics and materials science.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

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.

Excitonic effects caused by Coulomb interactions between electrons and holes play subtle and significant roles on photocatalysis, yet have been long ignored. Herein, porphyrinic covalent organic frameworks (COFs, specifically DhaTph-M), in the absence or presence of different metals in porphyrin centers, have been shown as ideal models to regulate excitonic effects. Remarkably, the incorporation of Zn2+ in the COF facilitates the conversion of singlet to triplet excitons, whereas the Ni2+ introduction promotes the dissociation of excitons to hot carriers under photoexcitation. Accordingly, the discriminative excitonic behavior of DhaTph-Zn and DhaTph-Ni enables the activation of O2 to 1O2 and O2•-, respectively, under visible light irradiation, resulting in distinctly different activity and selectivity in photocatalytic terpinene oxidation. Benefiting from these results, DhaTph-Ni exhibits excellent photocatalytic activity in O2•--engaged hydroxylation of boronic acid, while DhaTph-Zn possesses superior performance in 1O2-mediated selective oxidation of organic sulfides. This work provides in-depth insights into molecular oxygen activation and opens an avenue to the regulation of excitonic effects based on COFs.

Photocatalytic Molecular Oxygen Activation by Regulating Excitonic Effects in Covalent Organic Frameworks.

Porous organic polymers (POPs) are a class of multi-dimensional porous network materials, which are built via strong covalent linkages between various organic building blocks with different geometries and topologies, and have recently become a rising research field in porous materials. POPs can be generally divided into two categories based on their degree of long-range order, including amorphous (e.g., CMPs, HCPs, PIMs, and PAFs) and crystalline (e.g., COFs). Owning to their advantages of light-weight, superior inherent porosity, excellent stability, pre-designable and tunable structures and functions, POPs have received increasing attention and research interest for their tremendous potential applications in gas storage/separation, heterogeneous catalysis, photoelectric conversion, chemical- and bio-sensing, energy storage and conversion, etc. Their porous structure, pore size, specific surface areas and functions can be directly designed and facilely tuned by introducing specific functional building blocks. In this review article, we summarize the latest representative advances in the field of POPs, focusing on their design and synthetic strategies, with emphasis on their specific applications in photocatalysis, including photocatalytic chemical transformation, photodetoxification of contaminants from water, and water splitting.

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Porous organic polymers: a promising platform for efficient photocatalysis

A new porphyrin covalent organic framework (A2B2-Por-COF) was readily constructed via self-polycondensation of an A2B2 porphyrin monomer. In contrast to the previously reported porphyrin COFs prepa...

Facile Synthesis of Porphyrin Based Covalent Organic Frameworks via an A2B2 Monomer for Highly Efficient Heterogeneous Catalysis

Two imine-based two-dimensional covalent organic frameworks (2D COFs: TPT-Azine-COF and TPT-TAPB-COF) which exhibit large surface areas and good crystallinity were synthesized from flexible building blocks. Both of them exhibit a prominent adsorption capacity for Rhodamine B (970 mg g–1) and volatile iodine (225 wt %) with good recyclability.

Covalent Organic Frameworks Constructed from Flexible Building Blocks with High Adsorption Capacity for Pollutants

Open 1D channels found in covalent organic frameworks are unique and promising to serve as pathways for proton conduction; how to develop high-rate yet stable transporting systems remains a substantial challenge. Herein, this work reports a strategy for exploring proton-conducting frameworks by engineering pore walls and installing proton-containing polymers into the pores. Amide-linked and sulfonated frameworks were synthesized from imine-linked precursors via sequentially engineering to oxidize into amide linkages and to further anchor sulfonic acid groups onto the pore walls, enabling the creation of sulfonated frameworks with high crystallinity and channel ordering. Integrating sulfonated polyether ether ketone chains into the open channels enables proton hopping to across the channels, greatly increases proton conductivity and enables a stable continuous run. These results suggest a way to explore proton-conducting COFs via systematic engineering of the wall and space of the open nanochannels.

Wenjing Hao

Papers

Facile Synthesis of Porphyrin Based Covalent Organic Frameworks via an A2B2 Monomer for Highly Efficient Heterogeneous Catalysis

Covalent Organic Frameworks Constructed from Flexible Building Blocks with High Adsorption Capacity for Pollutants

Sulfonated 2D Covalent Organic Frameworks for Efficient Proton Conduction.

Efficient degradation of chlortetracycline in water by ultrasound activation of flexible ZnCoO doped PVDF membranes with high β phase

Effect of Enterococcus faecalis on osteoclastogenesis under cobalt-mimicked hypoxia in vitro.