Twist Building Blocks from Planar to Tetrahedral for the Synthesis of Covalent Organic Frameworks

External surface hydrophilic 3D-POSS-COF@GDL for the direct adsorption of bisphenols in milk samples

In case of organic frameworks (COFs) as adsorbents in the pretreatment of complex food matrices, challenges such as poor dispersion and non-specific adsorption of interfering macromolecules like proteins are often encountered. To address this issue, this work prepared a three-dimensional covalent organic framework (3D-COF) with a novel bcu topology based on polyhedral oligomeric silsesquioxane (POSS). Subsequently, gluconolactone (GDL) was modified onto the external surface of the material via the reaction with the exposed reactive residues. The resulting POSS-COF@GDL adsorbent has an enhanced hydrophilicity in the external surface, thereby significantly improves the dispersion of materials in aqueous solution and reduces the adsorption ability toward protein. Whereas, the inner of material retains hydrophobic pores that exhibit high adsorption efficiency to small hydrophobic molecules. Compared with the traditional pretreatment methods, POSS-COF@GDL can directly extract bisphenols (BPs) in milk samples without any additional treatment. The established sample pretreatment method is coupled with high-performance liquid chromatography-ultraviolet detection (HPLC-UV), resulting in recoveries of 71.8 to 93.6%, intra- and inter-day relative standard deviations (RSDs) of <8.3%, and limits of detection (LODs) of 0.042-0.16 ng∙mL-1 for four BPs.

Rational design of metal-organic cages to increase the number of components via dihedral angle control

The general principles of discrete, large self-assemblies composed of numerous components are not unveiled and the artificial formation of such entities is a challenging topic. In metal-organic cages, design strategies for tuning the coordination directions in multitopic ligands by the bend and twist angles were previously developed to solve this problem. In this study, the importance of remote geometric communications between components is emphasized, realizing several types of metal-organic assemblies based on dihedral angle control in multitopic ligands although they have the same coordination directions. Self-assembly of a tritopic ligand with dihedral angles θ = 36.5° and a cis-protected Pd(II) ion affords M9L6 and M12L8 cages as kinetic and thermodynamic products, respectively, whereas an M12L8 sheet is formed when θ = 90°. Geometric analyses of strains in the subcomponent rings reveals that remote geometric communications among neighboring multitopic ligands through coordination bonds are key for large assemblies.

Three-Dimensional Crystalline Organic Framework Stabilized by Molecular Mortise-and-Tenon Jointing

Three-dimensional (3D) crystalline organic frameworks with complex topologies, high surface area, and low densities afford a variety of application prospects. However, the design and construction of these frameworks have been largely limited to systems containing polyhedron-shaped building blocks or those relying on component interpenetration. Here, we report the synthesis of a 3D crystalline organic framework based on molecular mortise-and-tenon jointing. This new material takes advantage of tetra(4-pyridylphenyl)ethylene and chlorinated bis(benzodioxaborole)benzene as building blocks and is driven by dative B-N bonds. A single-crystal X-ray diffraction analysis of the framework reveals the presence of two-dimensional (2D) layers with helical channels that are formed presumably during the boron-nitrogen coordination process. The protrusion of dichlorobenzene units from the upper and lower surfaces of the 2D layers facilitates the key mortise-and-tenon connections. These connections enable the interlocking of adjacent layers and the stabilization of an overall 3D framework. The resulting framework is endowed with high porosity and attractive mechanical properties, rendering it potentially suitable for the removal of impurities from acetylene.

Topological Analysis and Structural Determination of 3D Covalent Organic Frameworks

3D covalent organic frameworks (3D COFs) constitute a new type of crystalline materials that consist of a range of porous structures with numerous applications in the fields of adsorption, separation, and catalysis. However, because of the complexity of the three-periodic net structure, it is desirable to develop a thorough structural comprehension, along with a means to precisely determine the actual structure. Indeed, such advancements would considerably contribute to the rational design and application of 3D COFs. In this review, the reported topologies of 3D COFs are introduced and categorized according to the configurations of their building blocks, and a comprehensive overview of diffraction-based structural determination methods is provided. The current challenges and future prospects for these materials will also be discussed.

3D Covalent Organic Frameworks from Design, Synthesis to Applications in Optoelectronics

Covalent organic frameworks (COFs), a new class of crystalline materials connected by covalent bonds, have been developed rapidly in the past decades. However, the research on COFs is mainly focused on two-dimensional (2D) COFs, and the research on three-dimensional (3D) COFs is still in the initial stage. In 2D COFs, the covalent bonds exist only in the 2D flakes and can form 1D channels, which hinder the charge transport to some extent. In contrast, 3D COFs have a more complex pore structure and thus exhibit higher specific surface area and richer active sites, which greatly enhance the 3D charge carrier transport. Therefore, compared to 2D COFs, 3D COFs have stronger applicability in energy storage and conversion, sensing, and optoelectronics. In this review, it is first introduced the design principles for 3D COFs, and in particular summarize the development of conjugated building blocks in 3D COFs, with a special focus on their application in optoelectronics. Subsequently, the preparation of 3D COF powders and thin films and methods to improve the stability and functionalization of 3D COFs are summarized. Moreover, the applications of 3D COFs in electronics are outlined. Finally, conclusions and future research directions for 3D COFs are presented.

Purification of Waste-Generated Biogas Mixtures Using Covalent Organic Framework's High CO2 Selectivity

Development of crystalline porous materials for selective CO2 adsorption and storage is in high demand to boost the carbon capture and storage (CCS) technology. In this regard, we have developed a β-keto enamine-based covalent organic framework (VM-COF) via the Schiff base polycondensation technique. The as-synthesized VM-COF exhibited excellent thermal and chemical stability along with a very high surface area (1258 m2 g-1) and a high CO2 adsorption capacity (3.58 mmol g-1) at room temperature (298 K). The CO2/CH4 and CO2/H2 selectivities by the IAST method were calculated to be 10.9 and 881.7, respectively, which were further experimentally supported by breakthrough analysis. Moreover, theoretical investigations revealed that the carbonyl-rich sites in a polymeric backbone have higher CO2 binding affinity along with very high binding energy (-39.44 KJ mol-1) compared to other aromatic carbon-rich sites. Intrigued by the best CO2 adsorption capacity and high CO2 selectivity, we have utilized the VM-COF for biogas purification produced by the biofermentation of municipal waste. Compared with the commercially available activated carbon, VM-COF exhibited much better purification ability. This opens up a new opportunity for the creation of functionalized nanoporous materials for the large-scale purification of waste-generated biogases to address the challenges associated with energy and the environment.

Morphology Tuning via Linker Modulation: Metal-Free Covalent Organic Nanostructures with Exceptional Chemical Stability for Electrocatalytic Water Splitting

The development of synthetic routes for the formation of robust porous organic polymers (POPs) with well-defined nanoscale morphology is fundamentally significant for their practical applications. The thermodynamic characteristics that arise from reversible covalent bonding impart intrinsic chemical instability in the polymers, thereby impeding their overall potential. Herein, a unique strategy is reported to overcome the stability issue by designing robust imidazole-linked POPs via tandem reversible/irreversible bond formation. Incorporating inherent rigidity into the secondary building units leads to robust microporous polymeric nanostructures with hollow-spherical morphologies. An in-depth analysis by extensive solid-state NMR (1D and 2D) study on 1H, 13C, and 14N nuclei elucidates the bonding and reveals the high purity of the newly designed imidazole-based POPs. The nitrogen-rich polymeric nanostructures are further used as metal-free electrocatalysts for water splitting. In particular, the rigid POPs show excellent catalytic activity toward the oxygen evolution reaction (OER) with long-term durability. Among them, the most efficient OER electrocatalyst (TAT-TFBE) requires 314 mV of overpotential to drive 10 mA cm-2 current density, demonstrating its superiority over state-of-the-art catalysts (RuO2 and IrO2).

Expanding Olefin-Linked Covalent Organic Frameworks toward Selective Photocatalytic Oxidation of Organic Sulfides

Olefin-linked covalent organic frameworks (COFs) have exhibited great potential in visible-light photocatalysis. In principle, expanding fully conjugated COFs can facilitate light absorption and charge transfer, leading to improved photocatalysis. Herein, three olefin-linked COFs with the same topology are synthesized by combining 2,4,6-trimethyl-1,3,5-triazine (TMT) with 1,3,5-triformylbenzene (TFB), 1,3,5-tris(4-formylphenyl)benzene (TFPB), and 1,3,5-tris(4-formylphenylethynyl)benzene (TFPEB), namely, TMT-TFB-COF, TMT-TFPB-COF, and TMT-TFPEB-COF, respectively. From TMT-TFB-COF to TMT-TFPB-COF, expanding phenyl rings provides only limited expansion for π-conjugation due to the steric effect of structural twisting. However, from TMT-TFPB-COF to TMT-TFPEB-COF, the insertion of acetylenes eliminates the steric effect and provides more delocalized π-electrons. As such, TMT-TFPEB-COF exhibits the best optoelectronic properties among these three olefin-linked COFs. Consequently, the photocatalytic performance of TMT-TFPEB-COF is much better than those of TMT-TFB-COF and TMT-TFPB-COF on the oxidation of organic sulfides into sulfoxides with oxygen. The desirable reusability and substrate compatibility of the TMT-TFPEB-COF photocatalyst are further confirmed. The selective formation of organic sulfoxides over TMT-TFPEB-COF under blue light irradiation proceeds via both electron- and energy-transfer pathways. This work highlights a rational design of expanding the π-conjugation of fully conjugated COFs toward selective visible-light photocatalysis.

Three-dimensional covalent organic frameworks with nia nets for efficient separation of benzene/cyclohexane mixtures

The synthesis of three-dimensional covalent organic frameworks with highly connected building blocks presents a significant challenge. In this study, we report two 3D COFs with the nia topology, named JUC-641 and JUC-642, by introducing planar hexagonal and triangular prism nodes. Notably, our adsorption studies and breakthrough experiments reveal that both COFs exhibit exceptional separation capabilities, surpassing previously reported 3D COFs and most porous organic polymers, with a separation factor of up to 2.02 for benzene and cyclohexane. Additionally, dispersion-corrected density functional theory analysis suggests that the good performance of these 3D COFs can be attributed to the incorporation of highly aromatic building blocks and the presence of extensive pore structures. Consequently, this research not only expands the diversity of COFs but also highlights the potential of functional COF materials as promising candidates for environmentally-friendly separation applications.

Revolutionizing the structural design and determination of covalent-organic frameworks: principles, methods, and techniques

Covalent organic frameworks (COFs) represent an important class of crystalline porous materials with designable structures and functions. The interconnected organic monomers, featuring pre-designed symmetries and connectivities, dictate the structures of COFs, endowing them with high thermal and chemical stability, large surface area, and tunable micropores. Furthermore, by utilizing pre-functionalization or post-synthetic functionalization strategies, COFs can acquire multifunctionalities, leading to their versatile applications in gas separation/storage, catalysis, and optoelectronic devices. Our review provides a comprehensive account of the latest advancements in the principles, methods, and techniques for structural design and determination of COFs. These cutting-edge approaches enable the rational design and precise elucidation of COF structures, addressing fundamental physicochemical challenges associated with host-guest interactions, topological transformations, network interpenetration, and defect-mediated catalysis.

Single-crystal polymers (SCPs): from 1D to 3D architectures

Single-crystal polymers (SCPs) with unambiguous chemical structures at atomic-level resolutions have attracted great attention. Obtaining precise structural information of these materials is critical as it enables a deeper understanding of the potential driving forces for specific packing and long-range order, secondary interactions, and kinetic and thermodynamic factors. Such information can ultimately lead to success in controlling the synthesis or engineering of their crystal structures for targeted applications, which could have far-reaching impact. Successful synthesis of SCPs with atomic level control of the structures, especially for those with 2D and 3D architectures, is rare. In this review, we summarize the recent progress in the synthesis of SCPs, including 1D, 2D, and 3D architectures. Solution synthesis, topochemical synthesis, and extreme condition synthesis are summarized and compared. Around 70 examples of SCPs with unambiguous structure information are presented, and their synthesis methods and structural analysis are discussed. This review offers critical insights into the structure-property relationships, providing guidance for the future rational design and bottom-up synthesis of a variety of highly ordered polymers with unprecedented functions and properties.

Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures

Rational control and understanding of isomerism are of significance but still remain a great challenge in reticular frameworks, in particular, for covalent organic frameworks (COFs) due to the complicated synthesis and energy factors. Herein, reaction of 3,3',5,5'-tetra(4-formylphenyl)-2,2',6,6'-tetramethoxy-1,1'-biphenyl (TFTB) with 3,3',5,5'-tetrakis(4-aminophenyl)bimesityl (TAPB) under different reaction conditions affords single crystals of two 3D COF isomers, namely, USTB-20-dia and USTB-20-qtz. Their structures with resolutions up to 0.9-1.1 Å have been directly solved by three-dimensional electron diffraction (3D ED) and synchrotron single crystal X-ray diffraction, respectively. USTB-20-dia and USTB-20-qtz show rare 2 × 2-fold interpenetrated dia-b nets and 3-fold interpenetrated qtz-b frameworks. Comparative studies of the crystal structures of these COFs and theoretical simulation results indicate the crucial role of the flexible molecular configurations of building blocks in the present interpenetrated topology isomerism. This work not only presents the rare COF isomers but also gains an understanding of the formation of framework isomerism from both single crystal structures and theoretical simulation perspectives.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

A 3D Covalent Organic Framework with In-situ Formed Pd Nanoparticles for Efficient Electrochemical Oxygen Reduction

Non-platinum noble metals are highly desirable for the development of highly active, stable oxygen reduction reaction (ORR) electrocatalysts for fuel cells and metal-air batteries. However, how to improve the utilization of non-platinum noble metals is an urgent issue. Herein, a highly efficient catalyst for ORR was prepared through homogeneous loading of Pd precursors by a domain-limited method in a three-dimensional covalent organic framework (COF) followed by pyrolysis. The morphology of the Pd nanoparticles (Pd NPs) was well maintained after carbonization, which was attributed to the rigid structure of the 3D COF. Thanks to the uniform distribution of Pd NPs in the carbon, the catalyst exhibited a remarkable half-wave potential of 0.906 V and a Tafel slope of 70 mV dec-1 in 0.1 M KOH, surpassing the commercial Pt/C catalyst (0.863 V and 75 mV dec-1 ). Furthermore, a maximum power density of 144.0 mW cm-2 was achieved at 252 mA cm-2 , which was significantly higher than the control battery (105.1 mW cm-2 ). This work not only provides a simple strategy for in-situ preparation of highly dispersible metal catalysts in COFs, but also offers new insights into the ORR electrocatalysis.

Combination of covalent organic frameworks (COFs) and polyoxometalates (POMs): the preparation strategy and potential application of COF-POM hybrids

Both covalent organic frameworks (COFs) and polyoxometalates (POMs) show excellent properties and application potential in many fields, thus receiving widespread attention. In recent years, COF-POM hybrid materials were prepared by combining COFs and POMs through physical or chemical methods. COF-POM hybrids have shown high performance in many fields, such as catalysis, sensing, energy storage, and biomedicine. In this review, we introduced the preparation strategy and application of COF-POM hybrids in detail. We believe that the combination of COFs and POMs will provide more abundant functions and broad application prospects.

Single-Crystal Three-Dimensional Covalent Organic Framework Constructed from 6-Connected Triangular Prism Node

The limited structural diversity of three-dimensional covalent organic frameworks (3D COFs) greatly restricts their application exploration. Therefore, there is an urgent need to expand their library of molecular building blocks, such as the development of highly connected (>4 reaction sites) polyhedral nodes. Herein, by precisely controlling the precursor conformation, we rationally designed a new 6-connected triangular prism node derived from the triphenylbenzene molecule and further used it to construct a novel 3D COF (3D-TMTAPB-COF) via imine condensation reaction. Surprisingly, without the addition of competing reagents, 3D-TMTAPB-COF crystallized directly into single crystals of ∼15 μm in size and was determined to adopt a rare 6-fold interpenetrated (Class IIIa interpenetration) acs topology. In addition, 3D-TMTAPB-COF showed a high SF6 adsorption capacity (60.9 cm3 g-1) and good SF6/N2 selectivity (335) at 298 K and 1 bar, superior to those of most crystalline porous materials. This work not only confirms the possibility of growing large-size single-crystal 3D COFs formed with strong covalent bonds by a solvothermal method in the absence of modulators, but also reports a novel triangular prism node for future construction of 3D COFs with interesting applications.

Constructing a 3D Covalent Organic Framework from 2D hcb Nets through Inclined Interpenetration

Three-dimensional covalent organic frameworks (3D COFs) have been of great interest due to their inherent numerous open sites and pore confinement effect. However, it has remained challenging to build 3D frameworks via interdigitation (also known as inclined interpenetration) by generating an entangled network formed by multiple 2D layers inclined with respect to each other. Herein, we report the first case of constructing a 3D COF, termed COF-904, through interdigitating 2D hcb nets, which was formed via [3+2] imine condensation reactions by the use of 1,3,5-triformylbenzene and 2,3,5,6-tetramethyl-1,4-phenylenediamine. The single-crystal structure of COF-904 is solved, and the locations of all non-hydrogen atoms are determined by 3D electron diffraction with a resolution up to 0.8 Å. These results not only broaden the strategy for achieving 3D COFs via interdigitation but also demonstrate that structurally complex extended frameworks can arise from simple molecules.

Covalent organic frameworks (COFs) core@shell nanohybrids: Novel nanomaterial support towards environmental sustainability applications

Covalent organic frameworks (COFs) based on core@shell nanohybrids have recently received significant attention and have become one of the most promising strategies for improving the stability and catalytic activity of COFs. Compared with traditional core@shell, COF-based core@shell hybrids own remarkable advantages, including size-selective reactions, bifunctional catalysis, and integration of multiple functions. These properties could enhance the stability and recyclability, resistance to sintering, and maximize the electronic interaction between the core and the shell. The activity and selectivity of COF-based core@shell could be simultaneously improved by taking benefit of the existing synergy between the functional encapsulating shell and the covered core material. Considering that, we have highlighted various topological diagrams and the role of COFs in COF-based core@shell hybrid for activity and selectivity enhancement. This concept article provides all-inclusive advances in the design and catalytic applications of COF-based core@shell hybrids. Various synthetic techniques have been developed for the facile tailoring of functional core@shell hybrids, including novel seed growth, in-situ, layer-by-layer, and one-pot method. Importantly, charge dynamics and structure-performance relationships are investigated through different characterization techniques. Different COF-based core@shell hybrids with established synergistic interactions have been detailed, and their influence on stability and catalytic efficiency for various applications is explained and discussed in this contribution. A comprehensive discussion on the remaining challenges associated with COF-based core@shell nanoparticles and research directions has also been provided to deliver insightful ideas for additional future developments.

Crystallization of Dimensional Isomers in Covalent Organic Frameworks

Dimensional isomers, defined in reticular chemistry as frameworks consisting of identical molecular building blocks but extended in two or three dimensions (2D or 3D), are an important type of framework isomers that have never been isolated. Herein, we report the crystallization of dimensional isomers in covalent organic frameworks (COFs) for the first time. By polymerization of the same molecular building blocks at different temperatures, both 2D and 3D COFs were successfully constructed due to the temperature-induced conformational changes of precursors from planar to tetrahedral. In addition, the non-fluorescent 2D COF can be gradually converted into the fluorescent 3D COF by increasing the temperature under solvothermal conditions. Therefore, it is reasonable to crystallize the dimensional isomers of reticular materials by controlling the conformation of molecular building blocks, and more examples can be expected. Since the obtained dimensional isomers show different properties and functions, this work will definitely motivate us to design reticular materials for target applications in the future.

Covalent Organic Frameworks: From Structures to Applications

Three-dimensional covalent organic frameworks possess hierarchical nanopores, enormous surface areas with high porosity, and open positions. The synthesis of large crystals of three-dimensional covalent organic frameworks is a challenge, since different structures are generated during the synthesis. Presently, their synthesis with new topologies for promising applications has been developed by the use of building units with varied geometries. Covalent organic frameworks have multiple applications: chemical sensing, fabrication of electronic devices, heterogeneous catalysts, etc. We have presented the techniques for the synthesis of three-dimensional covalent organic frameworks, their properties, and their potential applications in this review.

Interfacial Synthesis of an Ultrathin Two-Dimensional Polymer Film via [2 + 2] Photocycloaddition

A carbon-carbon-linked, ultrathin, two-dimensional (2D) polymer film was prepared at the air/water interface through photochemically triggered [2 + 2] cycloaddition. The preorganization of the monomers on the water surface and the subsequent photo-polymerization led to the successful preparation of the ultrathin 2D polymer film. The obtained film is continuous, free standing, and has a large area (over 50 μm²). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) give clear evidence of the ultrathin film morphology. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) indicate successful photo-induced [2 + 2] polymerization.

Designed Synthesis of Three-Dimensional Covalent Organic Frameworks: A Mini Review

Covalent organic frameworks are porous crystals of polymers with two categories based on their covalent linkages: layered structures with two dimensions and networks with three-dimensional structures. Three-dimensional covalent organic frameworks are porous, have large surface areas, and have highly ordered structures. Since covalent bonds are responsible for the formation of three-dimensional covalent organic frameworks, their synthesis has been a challenge and different structures are generated during the synthesis. Moreover, initially, their topologies have been limited to dia, ctn, and bor which are formed by the condensation of triangular or linear units with tetrahedral units. There are very few building units available for their synthesis. Finally, the future perspective of 3D COFs has been designated for the future development of three-dimensional covalent organic frameworks.

Construction of Covalent Organic Frameworks via Multicomponent Reactions

Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.

Conjugated Three-Dimensional High-Connected Covalent Organic Frameworks for Lithium-Sulfur Batteries

Developing conjugated three-dimensional (3D) covalent organic frameworks (COFs) still remains an extremely difficult task due to the lack of enough conjugated 3D building blocks. Herein, condensation between an 8-connected pentiptycene-based D_2h building block (DMOPTP) and 4-connected square-planar linkers affords two 3D COFs (named 3D-scu-COF-1 and 3D-scu-COF-2). A combination of the 3D homoaromatic conjugated structure of the former building block with the 2D conjugated structure of the latter linking units enables the π-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2-3.5 × 10^-5 S cm^-1. In particular, the 3D rigid quadrangular prism shape of DMOPTP guides the formation of a twofold interpenetrated scu 3D topology and high-connected permanent porosity with a large Brunauer-Emmett-Teller (BET) surface area of 2340 and 1602 m² g^-1 for 3D-scu-COF-1 and 3D-scu-COF-2, respectively, ensuring effective small molecule storage and mass transport characteristics. This, in combination with their good charge transport properties, renders them promising sulfur host materials for lithium-sulfur batteries (LSBs) with high capacities (1035-1155 mA h g^-1 at 0.2 C, 1 C = 1675 mA g^-1), excellent rate capabilities (713-757 mA h g^-1 at 5.0 C), and superior cycling stability (71-83% capacity retention at 2.0 C after 500 cycles), surpassing the most of organic LSB cathodes reported thus far.

Efficient Adsorption of Acetylene over CO2 in Bioinspired Covalent Organic Frameworks

Rational design of covalent organic frameworks (COFs) to broaden their diversity is highly desirable but challenging due to the limited, expensive, and complex building blocks, especially compared with other easily available porous materials. In this work, we fabricated two novel bioinspired COFs, namely, NUS-71 and NUS-72, using reticular chemistry with ellagic acid and triboronic acid-based building blocks. Both COFs with AB stacking mode exhibit high acetylene (C₂H₂) adsorption capacity and excellent separation performance for C₂H₂/CO₂ mixtures, which is significant but rarely explored using COFs. The impressive affinities for C₂H₂ appear to be related to the sandwich structure formed by C₂H₂ and the host framework via multiple host-guest interactions. This work not only represents a new avenue for the construction of low-cost COFs but also expands the variety of the COF family using natural biochemicals as building blocks for broad application.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

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.

2D Microporous Covalent Organic Frameworks as Cobalt Nanoparticle Supports for Electrocatalytic Hydrogen Evolution Reaction

Covalent organic frameworks (COFs) are a new class of porous crystalline polymers, which are considered to be excellent supports for metal nanoparticles (MNPs) due to their highly ordered structure, chemical tunability, and porosity. In this work, two novel ultra-microporous COFs, JUC−624 and JUC−625, with narrow pore size distribution have been synthesized and used for the confined growth of ultrafine Co nanoparticles (CoNPs) with high loading. In an alkaline environment, the produced materials were investigated as electrocatalysts for the hydrogen evolution reaction (HER). Electrochemical test results show that CoNPs@COFs have a Tafel slope of 84 mV·dec−1, an onset overpotential of 105 mV, and ideal stability. Remarkably, CoNPs@JUC−625 required only 146 mV of overpotential to afford a current density of 10 mA cm−2. This research will open up new avenues for making COF-supported ultrafine MNPs with good dispersity and stability for extensive applications.

Highly conjugated two-dimensional covalent organic frameworks for efficient iodine uptake

Radioactive iodine in nuclear waste would be harmful to nature and human health. The design of adsorbents for iodine capture with high efficiency still remains a challenge. Herein, two highly conjugated two-dimensional covalent organic frameworks (TFPB-BPTA-COF and TFPB-PyTTA-COF) have been successfully constructed. Both COFs possess high porosity, stability, and high π-conjugated framework. Impressively, TFPB-PyTTA-COF exhibits an excellent iodine uptake value up to 5.6 g g -1 , which is superior to most of reported COF-based adsorbents for iodine capture.

Three-Dimensional Crystalline Covalent Triazine Frameworks via a Polycondensation Approach

The growth of crystalline covalent triazine frameworks (CTFs) is still considered as a great challenge due to the less reversible covalent bonds of triazine linkages. The research studies of crystalline CTFs to date have been limited to two-dimensional (2D) structures, and the three-dimensional (3D) crystalline CTFs have never been reported before. Herein we report the design and synthesis of two 3D crystalline CTFs, termed 3D CTF-TPM and 3D CTF-TPA through a reversible/irreversible polycondensation approach. The targeted 3D CTFs adopt ctn topology, and show moderate crystallinity, relatively large surface area (ca. 2000 m²  g^-1 ), and high CO₂ uptake capacity (23.61 wt.%). Moreover, these 3D CTFs exhibit ultrastability in the presence of boiling water, strong acid (1 M HCl) and strong base (1 M NaOH). This contribution represents the first report of 3D crystalline CTFs, which not only extends their structural diversity but also offers a synthetic strategy and structural basis for expanding practical applications of CTF materials.

Bottom-Up Synthesis of 8-Connected Three-Dimensional Covalent Organic Frameworks for Highly Efficient Ethylene/Ethane Separation

Developing cost-/energy-efficient separation techniques for purifying ethylene from an ethylene/ethane mixture is highly important but very challenging in the industrial process. Herein, using a bottom-up [8 + 2] construction approach, we rationally designed and synthesized three three-dimensional covalent organic frameworks (COFs) with 8-connected bcu networks, which can selectively remove ethane from an ethylene/ethane mixture with high efficiency. These COF materials, which are fabricated by the condensation reaction of a customer-designed octatopic aldehyde monomer with linear diamino linkers, possess high crystallinity, good structural robustness, and high porosity. Attributed to the well-organized micro-sized pores with a nonpolar/inert pore environment, these COFs display high ethane adsorption capacity and good selectivity over ethylene, making them among the best ethane-selective adsorbents for ethylene purification. Their excellent ethylene/ethane separation performance is validated by dynamic breakthrough experiments with high-purity ethylene (>99.99%) produced through a single adsorption process. The separation performance surpasses all reported C₂H₆-selective COFs and even some benchmark metal-organic frameworks. This work provides important guidance for the design of new adsorbents for value-added gas purification.

Three-Dimensional Triptycene-Functionalized Covalent Organic Frameworks with hea Net for Hydrogen Adsorption

Owing to the finite building blocks and difficulty in structural identification, it remains a tremendous challenge to elaborately design and synthesize three-dimensional covalent organic frameworks (3D COFs) with predetermined topologies. Herein, we report the first two cases of 3D COFs with the non-interpenetrated hea net, termed JUC-596 and JUC-597, by using the combination of tetrahedral and triangular prism building units. Due to the presence of triptycene functional groups and fluorine atoms, JUC-596 exhibits an exceptional performance in the H₂ adsorption up to 305 cm³  g^-1 (or 2.72 wt%) at 77 K and 1 bar, which is higher than previous benchmarks from porous organic materials reported so far. Furthermore, the strong interaction between H₂ and COF materials is verified through the DFT theoretical calculations. This work represents a captivating example of rational design of functional COFs based on a reticular chemistry guide and demonstrates its promising application in clean energy storage.

A physicochemical introspection of porous organic polymer photocatalysts for wastewater treatment

A detailed physicochemical explanation for experimental observations is provided for POPs as powerful photocatalysts for organic transformations and wastewater decontamination.

Tailoring the Pore Surface of 3D Covalent Organic Frameworks via Post-Synthetic Click Chemistry

Three-dimensional covalent organic frameworks (3D COFs) have gained increasing attention for their attractive features. However, the development of 3D COFs is strongly restricted, mainly due to their synthetic difficulty and complicated structure determination. Post-synthetic modification, which can avoid these problems by incorporating functional moieties into a predetermined framework, provides an alternative way to construct 3D COFs with specific functions. Herein, we report the designed synthesis and characterization of a series of highly crystalline 3D COFs with different loadings of ethynyl groups. Notably, these alkyne-tagged 3D COFs provide a platform for targeted anchoring various specific groups onto the pore walls via click reactions. Moreover, the pore surface engineering can accordingly change their properties, for example, the obtained click products exhibited higher CO₂ /N₂ selectivity. We describe a simple but powerful strategy to build functional 3D COFs, which will certainly advance them for a ranging of interesting applications in the future.

Three-dimensional covalent organic frameworks based on a π-conjugated tetrahedral node

The construction of three-dimensional (3D) covalent organic frameworks (COFs), especially fully conjugated 3D COFs, is a long-standing challenge. Herein, we report a saddle-like, π-conjugated cyclooctatetrathiophene (COTh) as a tetrahedral node to construct fully conjugated 3D COFs. The present work enriches the structural diversities and potential applications of 3D COFs.

A 3D Anionic Metal Covalent Organic Framework with soc Topology Built from an Octahedral TiIV Complex for Photocatalytic Reactions

The construction of three-dimensional (3D) covalent organic frameworks (COFs) remains challenging due to the limited types of organic building blocks. With octahedral Ti^IV complex as the building unit, this study reports on the first 3D anionic titanium-based COF (Ti-COF-1) with an edge-transitive (6, 4)-connected soc topology. Ti-COF-1 exhibits high crystallinity, superior stability, and large specific surface area (1000.4 m²  g^-1 ). Moreover, Ti-COF-1 has a broad absorption band in the UV spectrum with an optical energy gap of 1.86 eV, and exhibits high photocatalytic activity toward Meerwein addition reactions. This research demonstrates an attractive strategy for the design of 3D functional COFs.

Substoichiometric 3D Covalent Organic Frameworks Based on Hexagonal Linkers

Covalent organic frameworks (COFs), a fast-growing field in crystalline porous materials, have achieved tremendous success in structure development and application exploration over the past decade. The vast majority of COFs reported to date are designed according to the basic concept of reticular chemistry, which is rooted in the idea that building blocks are fully connected within the frameworks. We demonstrate here that sub-stoichiometric construction of 2D/3D COFs can be accomplished by the condensation of a hexagonal linker with 4-connected building units. It is worth noting that the partially connected frameworks were successfully reticulated for 3D COFs for the first time, representing the highest BET surface area among imine-linked 3D COFs to data. The unreacted benzaldehydes in COF frameworks can enhance C₂H₂ and CO₂ adsorption capacity and selectivities between C₂H₂/CH₄ and C₂H₂/CO₂ for sub-stoichiometric 2D COFs, while the reserved benzaldehydes control the interpenetrated architectures for the 3D case, achieving a rare non-interpenetrated pts topology for 3D COFs. This work not only paves a new avenue to build new COFs and endows residual function groups with further applications but also prompts redetermination of reticular frameworks in highly connected and symmetrical COFs.

Saddle-Shaped Building Blocks: A New Concept for Designing Fully Conjugated 3D Organic Semiconducting Materials

Currently, most organic semiconducting materials (OSMs) are π-conjugated structures in one or two dimension (2D), where the lack of layer-layer π-conjugation connection greatly blocks their electron delocalization and transport. The 3D fully conjugated materials could solve this issue because they can provide efficient charge-transport pathways throughout the whole 3D skeleton, in which the suitable 3D building block is the key to the development of fully conjugated 3D OSMs. Cyclooctatetraene (COT) and its derivatives are good candidates due to their π-conjugation with 3D saddle-shaped architecture. In this Concept, we discuss the key features of saddle-shaped COT-based derivatives and their synthetic strategy, then we present the current development of using the COT derivatives as building blocks to construct the 3D fully conjugated organic small compound- and polymer-based OSMs. The properties and perspectives of these OSMs in photovoltaics, electro-catalysis and electrical conductivities are also discussed. These recent advances in the developing 3D fully conjugated materials could potentially open up a new frontier in the design of OSMs.

Tuning the Topology of Three-Dimensional Covalent Organic Frameworks via Steric Control: From to Unprecedented

Whether or not the topology of three-dimensional covalent organic frameworks (3D COFs) can be tuned via steric control remains a big question and has never been reported. Herein, we describe the designed synthesis of two highly crystalline 3D COFs (3D-TPB-COF-OMe and 3D-TPB-COF-Ph), through the polycondensation of tetra(p-aminophenyl)methane and methoxy- or phenyl- substituted 1,2,4,5-tetrakis(4-formylphenyl)benzene on the 3- and 6-positions. Amazingly, by using the continuous rotation electron diffraction technique, 3D-TPB-COF-OMe is determined to have a 5-fold interpenetrated structure with a reported pts net, while 3D-TPB-COF-Ph adopts an unprecedented self-penetrated ljh topology (ljh = Luojia Hill) that does not exist in the database of ToposPro. Therefore, by altering the substituents from methoxy to phenyl groups, the topology of designed 3D COFs changes accordingly, and a rare net is now available. This result clearly demonstrates that such COF structures need to be carefully determined due to its complexity, and moreover, it is promising to design 3D COFs with new topology for interesting application by increasing the steric hindrance of molecular building blocks.