3D Microporous Base‐Functionalized Covalent Organic Frameworks for Size‐Selective Catalysis

The design and synthesis of 3D covalent organic frameworks (COFs) have been considered a challenge, and the demonstrated applications of 3D COFs have so far been limited to gas adsorption. Herein we describe the design and synthesis of two new 3D microporous base‐functionalized COFs, termed BF‐COF‐1 and BF‐COF‐2, by the use of a tetrahedral alkyl amine, 1,3,5,7‐tetraaminoadamantane (TAA), combined with 1,3,5‐triformylbenzene (TFB) or triformylphloroglucinol (TFP). As catalysts, both BF‐COFs showed remarkable conversion (96 % for BF‐COF‐1 and 98 % for BF‐COF‐2), high size selectivity, and good recyclability in base‐catalyzed Knoevenagel condensation reactions. This study suggests that porous functionalized 3D COFs could be a promising new class of shape‐selective catalysts.     

Direct Synthesis of a Covalent Triazine‐Based Framework from Aromatic Amides

There have been extensive efforts to synthesize crystalline covalent triazine‐based frameworks (CTFs) for practical applications and to realize their potential. The phosphorus pentoxide (P₂O₅)‐catalyzed direct condensation of aromatic amide instead of aromatic nitrile to form triazine rings. P₂O₅‐catalyzed condensation was applied on terephthalamide to construct a covalent triazine‐based framework (pCTF‐1). This approach yielded highly crystalline pCTF‐1 with high specific surface area (2034.1 m² g^?1). At low pressure, the pCTF‐1 showed high CO₂ (21.9 wt % at 273 K) and H₂ (1.75 wt % at 77 K) uptake capacities. The direct formation of a triazine‐based COF was also confirmed by model reactions, with the P₂O₅‐catalyzed condensation reaction of both benzamide and benzonitrile to form 1,3,5‐triphenyl‐2,4,6‐triazine in high yield.     

A Photoresponsive Smart Covalent Organic Framework

Ordered π‐columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external‐stimuli‐responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π‐building blocks. The COF is switchable upon photoirradiation to yield a concavo‐convex polygon skeleton through the interlayer [4π+4π] cycloaddition of anthracene units stacked in the π‐columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external‐stimuli‐induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π‐electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.     

Graphene‐Like Covalent Organic Framework with a Wide Band Gap Synthesized On Surface via Stepwise Reactions

Developing graphene‐like two‐dimensional materials naturally possessing a band gap has sparked enormous interest. Thanks to the inherent wide band gap and high mobility in the 2D plane, covalent organic frameworks containing triazine rings (t‐COFs) hold great promise in this regard, whilst the synthesis of single‐layer t‐COFs remains highly challenging. Herein, we present the fabrication of a well‐defined graphene‐like t‐COF on Au(111). Instead of single/multiple‐step single‐type reactions commonly applied for on‐surface synthesis, distinct stepwise on‐surface reactions, including alkynyl cyclotrimerization, C?O bond cleavage, and C?H bond activation, are triggered on demand, leading to product evolution in a controlled step‐by‐step manner. Aside from the precise control in sophisticated on‐surface synthesis, this work proposes a single‐atomic‐layer organic semiconductor with a wide band gap of 3.41 eV.     

A Tetrathiafulvalene‐Based Electroactive Covalent Organic Framework

Two‐dimensional covalent organic frameworks (2D COFs) provide a unique platform for the molecular design of electronic and optoelectronic materials. Here, the synthesis and characterization of an electroactive COF containing the well‐known tetrathiafulvalene (TTF) unit is reported. The TTF‐COF crystallizes into 2D sheets with an eclipsed AA stacking motif, and shows high thermal stability and permanent porosity. The presence of TTF units endows the TTF‐COF with electron‐donating ability, which is characterized by cyclic voltammetry. In addition, the open frameworks of TTF‐COF are amenable to doping with electron acceptors (e.g., iodine), and the conductivity of TTF‐COF bulk samples can be improved by doping. Our results open up a reliable route for the preparation of well‐ordered conjugated TTF polymers, which hold great potential for applications in fields from molecular electronics to energy storage.     

共价有机框架材料在吸附领域的研究进展

共价有机框架材料(Covalent Organic Frameworks,COFs)是由有机结构单元通过共价键连接的具有期性结构的多孔化合物。共价有机框架材料具有永久的孔隙、高的比表面积、可调的孔径、易于功能化和高的水热稳定性等优点,广泛应用于许多领域。本文总结了COFs目前主要的合成方法,介绍了COFs在吸附领域的应用和发展。最后,文章指出未来的研究重点是发展更多有机反应和键连方式,合成具有高度稳定性和结晶度、成本低廉的功能性材料。     

Recent Advances in Covalent Organic Frameworks for Catalysis

Covalent organic frameworks (COFs) as an emerging type of crystalline porous materials, have obtained considerable attention recently. They have exhibited diverse structure and attractive performance in various catalytic reactions. It is highly expected to have a systematic and comprehensive review summing up COFs‐derived catalysts in homogeneous and heterogeneous catalysis, which is favorable to the judicious design of an efficient catalyst for targeted reaction. Herein, we focus on summarizing recent and significant developments in COFs materials, with an emphasis on both the synthetic strategies and targeted functionalization, and categorize it in accordance with the different types of catalytic reactions. Their potential catalysis applications are reviewed thoroughly. Moreover, a personal view about the future development of COFs catalysts with respect to the large‐scale production is also discussed.     

Graphene-Like Covalent Organic Framework with a Wide Band Gap Synthesized On Surface via Stepwise Reactions

Developing graphene-like two-dimensional materials naturally possessing a band gap has sparked enormous interest. Thanks to the inherent wide band gap and high mobility in the 2D plane, covalent organic frameworks containing triazine rings (t-COFs) hold great promise in this regard, whilst the synthesis of single-layer t-COFs remains highly challenging. Herein, we present the fabrication of a well-defined graphene-like t-COF on Au(111). Instead of single/multiple-step single-type reactions commonly applied for on-surface synthesis, distinct stepwise on-surface reactions, including alkynyl cyclotrimerization, C−O bond cleavage, and C−H bond activation, are triggered on demand, leading to product evolution in a controlled step-by-step manner. Aside from the precise control in sophisticated on-surface synthesis, this work proposes a single-atomic-layer organic semiconductor with a wide band gap of 3.41 eV.     

Electronic Structure of a Semiconducting Imine‐Covalent Organic Framework

Imine COF (covalent organic framework) based on the Schiff base reaction between p‐phenylenediamine (PDA) and benzene‐1,3,5‐tricarboxaldehyde (TCA) was prepared on the HOPG‐air (air=humid N₂) interface and characterized using different probe microscopies. The role of the molar ratio of TCA and PDA has been explored, and smooth domains of imine COF up to a few μm are formed for a high TCA ratio (>2) compared to PDA. It is also observed that the microscopic roughness of imine COF is strongly influenced by the presence of water (in the reaction chamber) during the Schiff base reaction. The electronic property of imine COF obtained by tunneling spectroscopy and dispersion corrected density functional theory (DFT) calculation are comparable and show semiconducting nature with a band gap of ≈1.8 eV. Further, we show that the frontier orbitals are delocalized entirely over the framework of imine COF. The calculated cohesive energy shows that the stability of imine COF is comparable to that of graphene.     

An Ultrastable Crystalline Acylhydrazone-Linked Covalent Organic Framework for Efficient Removal of Organic Micropollutants from Water

As an important member of crystalline porous polymers, acylhydrazone-linked covalent organic frameworks (COFs) have gained much attention in recent years. However, the low structural stability imparts a limit on their practical applications. To tackle this problem, we report a simple strategy to increase the chemical stability of acylhydrazone-linked COFs by incorporating azobenzene groups in the conjugated framework. Through reinforcing the π-π stacking interactions between the adjacent layers with increased π-surface, it is surprising to find that the resulting materials exhibit extreme stability in harsh environments, such as in strong acid, strong base, aqueous educing agent and boiling water, even exposed to air for one year. As a proof-of-concept, such frameworks have been used to remove various organic micropollutants such as antibiotics, plastic components, endocrine disruptors, and carcinogens from water with high capacity, fast speed and excellent reusability over a wide pH range at environmentally relevant concentrations. The results provide a new avenue to significantly enhance the stability of COFs for practical applications.     

Direct Solar‐to‐Electrochemical Energy Storage in a Functionalized Covalent Organic Framework

A covalent organic framework integrating naphthalenediimide and triphenylamine units (NT‐COF) is presented. Two‐dimensional porous nanosheets are packed with a high specific surface area of 1276 m² g^?1. Photo/electrochemical measurements reveal the ultrahigh efficient intramolecular charge transfer from the TPA to the NDI and the highly reversible electrochemical reaction in NT‐COF. There is a synergetic effect in NT‐COF between the reversible electrochemical reaction and intramolecular charge transfer with enhanced solar energy efficiency and an accelerated electrochemical reaction. This synergetic mechanism provides the key basis for direct solar‐to‐electrochemical energy conversion/storage. With the NT‐COF as the cathode materials, a solar Li‐ion battery is realized with decreased charge voltage (by 0.5 V), increased discharge voltage (by 0.5 V), and extra 38.7 % battery efficiency.     

Embedding Ultrasmall Au Clusters into the Pores of a Covalent Organic Framework for Enhanced Photostability and Photocatalytic Performance

Gold clusters loaded on various supports have been widely used in the fields of energy and biology. However, the poor photostability of Au clusters on support interfaces under prolonged illumination usually results in loss of catalytic performance. Covalent organic frameworks (COFs) with periodic and ultrasmall pore structures are ideal supports for dispersing and stabilizing Au clusters, although it is difficult to encapsulate Au clusters in the ultrasmall pores. In this study, a two‐dimensional (2D) COF modified with thiol chains in its pores was prepared. With ?SH groups as nucleation sites, Au nanoclusters (NCs) could grow in situ within the COF. The ultrasmall pores of the COF and the strong S?Au binding energy combine to improve the dispersibility of Au NCs under prolonged light illumination. Interestingly, Au–S–COF bridging as observed in this artificial Z‐scheme photocatalytic system is deemed to be an ideal means to increase charge‐separation efficiency.     

Postsynthetic Functionalization of Three‐Dimensional Covalent Organic Frameworks for Selective Extraction of Lanthanide Ions

Chemical functionalization of covalent organic frameworks (COFs) is critical for tuning their properties and broadening their potential applications. However, the introduction of functional groups, especially to three‐dimensional (3D) COFs, still remains largely unexplored. Reported here is a general strategy for generating a 3D carboxy‐functionalized COF through postsynthetic modification of a hydroxy‐functionalized COF, and for the first time exploration of the 3D carboxy‐functionalized COF in the selective extraction of lanthanide ions. The obtained COF shows high crystallinity, good chemical stability, and large specific surface area. Furthermore, the carboxy‐functionalized COF displays high metal loading capacities together with excellent adsorption selectivity for Nd³⁺ over Sr²⁺ and Fe³⁺ as confirmed by the Langmuir adsorption isotherms and ideal adsorbed solution theory (IAST) calculations. This study not only provides a strategy for versatile functionalization of 3D COFs, but also opens a way to their use in environmentally related applications.     

Imine‐Linked Covalent Organic Framework on Surface for Biosensor

Imine‐linked covalent organic framework on amino functionalized silicon substrate was constructed via a step‐wise reaction between 1,3,5‐benzenetricarboxaldehyde and 1,4‐diaminobenzene. The obtained material was used as biosensor for bovine serum albumin (BSA) adsorption and probe DNA immobilization, which extended the application of covalent organic frameworks (COFs) to a new field.     

2D Covalent Organic Framework for Water Harvesting with Fast Kinetics and Low Regeneration Temperature

Atmospheric water harvesting represents a promising technique to address water stress. Advanced adsorbents have been rationally designed to achieve high water uptake, yet their water sorption kinetics and regeneration temperature greatly limit water production efficiency. Herein, we demonstrated that 2D covalent organic frameworks (COFs), featuring hydrophobic skeleton, proper hydrophilic site density, and 1D open channels significantly lowered the water diffusion and desorption energy barrier. DHTA-Pa COF showed a high water uptake of 0.48 g/g at 30 % R.H. with a remarkable adsorption rate of 0.72 L/Kg/h (298 K) and a desorption rate of 2.58 L/Kg/h (333 K). Moreover, more than 90 % adsorbed water could be released within 20 min at 313 K. This kinetic performance surpassed the reported porous materials and boosted the efficiency for multiple water extraction cycles. It may shed light on the material design strategy to achieve high daily water production with low-energy input.     

Covalent Organic Frameworks and Cage Compounds: Design and Applications of Polymeric and Discrete Organic Scaffolds

Porous organic materials are an emerging class of functional nanostructures with unprecedented properties. Dynamic covalent assembly of small organic building blocks under thermodynamic control is utilized for the intriguingly simple formation of complex molecular architectures in one‐pot procedures. In this Review, we aim to analyze the basic design principles that govern the formation of either covalent organic frameworks as crystalline porous polymers or covalent organic cage compounds as shape‐persistent molecular objects. Common synthetic procedures and characterization techniques will be discussed as well as more advanced strategies such as postsynthetic modification or self‐sorting. When appropriate, comparisons are drawn between polymeric frameworks and discrete organic cages in terms of their underlying properties. Furthermore, we highlight the potential of these materials for applications ranging from gas storage to catalysis and organic electronics.     

A Carboxyl-Functionalized Covalent Organic Framework Synthesized in a Deep Eutectic Solvent for Dye Adsorption

Instead of using organic solvents, a deep eutectic solvent (DES) composed of tetrabutylammonium bromide and imidazole (Bu₄NBr/Im) was employed as a solvent for the first time to synthesize covalent organic frameworks (COFs). Due to the low vapor pressure of the Bu₄NBr/Im-based DES, a new carboxyl-functionalized COF (TpPa-COOH) was synthesized under environmental pressure. The as-synthesized TpPa-COOH has open channels, and the DES can be removed completely from the pores. The dye adsorption performance of TpPa-COOH was examined for three organic dyes with similar molecular sizes: one anionic dye (eosin B, EB) and two cationic dyes (methylene blue, MB and safranine T, ST). TpPa-COOH showed an excellent selective adsorption effect on MB and ST. The electronegative keto form in TpPa-COOH might help to form electrostatic and π–π interactions between the π-stacking frameworks of TpPa-COOH and the positive plane MB and ST molecules. The adsorption isotherms of MB and ST on TpPa-COOH were further investigated in detail, and the equilibrium adsorption was well modeled by using a Langmuir isotherm model. Together with hydrogen bonding, TpPa-COOH showed higher adsorption capacity for ST than for MB (1135 vs. 410 mg g⁻¹). These results could provide a guidance for the green synthesis of adsorbents in removing organic dyes from wastewater.     

Synthesis of Two-Dimensional Covalent Organic Frameworks in Ionic Liquids

Two-dimensional covalent organic frameworks were synthesized in high yields by polycondensation in nonvolatile ionic liquids. The resulting crystallites are highly porous and exhibit exceptional capability of removing bisphenol A from water. The one reported is a general method to synthesize microporous and mesoporous frameworks, it allows to achieve regular macroscopic shapes, and it is effective in a wide range of reaction temperatures.