Covalent Organic Framework–Covalent Organic Framework Bilayer Membranes for Highly Selective Gas Separation

Fluorescent selenium covalent organic framework sensor of ofloxacin antibiotics

As the main novel pollutants in water environment, antibiotics pose great threat to human-beings. It is of pivotal significance to develop sensitive and selective sensing materials to monitor them in natural environment. For this purpose, we design and synthesize Se-based covalent organic framework (TPE-BSD-COF). It showed excellent fluorescence quenching and linear relationship, rapid response, wide detection range, and low limit-of-detection (LOD) for ofloxacin. Most importantly, it demonstrated very good selectivity without response to other typical antibiotics. Besides, ofloxacin in real environment samples was detected and excellent recovery rates were obtained. We conducted in-depth structure-activity experiments and density-function-theory (DFT) theoretical studies to unearth the fluorescence quenching mechanism. Together, this work will provide new insights into the design of fluorescent COF sensors for the detection of various environmentally interested antibiotics pollutants.

Supramolecularly Confined Catalysis in Polyphthalocyanine-Crown-Ether Frameworks Boosts Sulfur Redox Kinetics

Metal phthalocyanines are considered as potent catalysts in lithium-sulfur (Li-S) chemistry. However, their adsorption capability is deficient to inhibit polysulfides from shuttling, which in turn retards the S-redox reaction in the cathode. Here we report flexible, two-dimensional (2D) polyphthalocyanine-crown-ether (PPc-CE) frameworks that provide a supramolecularly confined space created with the single-atom catalytic nickel phthalocyanine nodes and crown-ether linkers as Li host. Electrochemical and theoretical analyses reveal that a cooperative redox catalysis with the enhanced lithiophilicity of PPc-CE-coated carbon nanotubes (PPc-CE/CNTs) boosts Li-S redox kinetics and, meanwhile, suppresses the growth of Li dendrites for the long term. A Li||S cell employing PPc-CE/CNT catalysts delivers a high discharge capacity of 1,363 mAh g-1 at 0.1C and still retains a specific capacity of ∼700 mAh g-1 over 500 cycles at 1C. Our work provides insights into the molecular design of redox catalysts for Li-S batteries based on 2D polymers.

In-situ growth of olefin-linked covalent organic framework nanofiber membranes via surface-mediated condensation for sustainable removal of bisphenol A

Developing multifunctional adsorbents with exceptional capture performance and outstanding stability is significant for pollutant removal. Olefin-linked sp² carbon-conjugated covalent organic frameworks (sp²c-COFs) exhibit high chemical stability and robust framework properties. This work developed a facile in-situ growth strategy to synthesize sp²c-COF films on polyacrylonitrile (PAN) nanofibers via aldol condensation. The abundant nucleation sites on the surface of functionalized PAN nanofibers (APAN) facilitated the synthesis of sp²c-COF nanofibers (APAN@TMT-TFPT) through a surface-mediated condensation reaction between 2,4,6-trimethyl-1,3,5-triazine (TMT) and 1,3,5-tris(4-formylphenyl) triazine (TFPT). APAN@TMT-TFPT exhibited a high specific surface area, porosity, and exceptional chemical stability, rendering it a highly promising adsorbent for aqueous environments. APAN@TMT-TFPT demonstrated exceptional adsorption performance for bisphenol A (BPA), and the adsorption behavior conformed to the Langmuir model, with a maximum adsorption capacity of 284.98 mg g-1. The abundant benzene rings and triazine units in APAN@TMT-TFPT provide numerous active sites for BPA interaction, while the robust sp²c-COF framework and macroscopic membrane structure ensure excellent reusability and recyclability. Notably, the in-situ growth of COFs on nanofibers proposed in this work can be extended to construct other highly stable sp²c-COF-based nanofiber membranes.

Simultaneous High-Efficiency Photocatalytic Production of H2O2 and Synthesis of 3-HBA Using COFs Broadened by π Conjugate Networks and Photogenerated Charges

The coupling of H2O2 photosynthesis with photogenerated electrons and holes, when used separately, not only realizes the production of value-added products but also maximizes energy utilization. However, achieving this remains a challenge. Here, we report three variations of triazine-based covalent organic frameworks (COFs) featuring phenyl π-π-conjugated structures and furthermore introduce the phenoxy COF with a p-π-conjugated structure as a control to effectively produce hydrogen peroxide. It is found that the strong π-π conjugate network triggers a pronounced optical response and provides channels for charge transfer and electron enrichment. The results showed that the H2O2 yield of TTBA-TP-COF (TTBA = 4,4,4-(1,3,5-triazine-2,4,6-triyl) tris (([1,1-biphenyl]-4-amine)), TP = 2,4,6-triformylphloroglucinol) reached an impressive 11,982 μmol/g/h, which was 1.49 times, 8.77 times, and 7.20 times that of TAPT-TP-COF (TAPT = 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline), MA-TP-COF (MA = melamine), and TTTT-TP-COF (TTTT = 4,4',4″-((1,3,5-triazine-2,4,6-triyl)tris(oxy))trianiline), respectively. Considering the atomic economy, we pioneeringly put forward the strategy of using photogenerated electrons and holes separately. Concurrently, with the photocatalytic production of hydrogen peroxide, acetaldehyde produced by hole oxidation, sacrificial ethanol served as primary feedstock, and aldol condensation of acetaldehyde was successfully catalyzed to produce 3-hydroxybutyraldehyde (3-HBA) under alkaline conditions provided by a small amount of triethylamine. The concentration of 3-HBA produced was confirmed to be 23.84 mmol/L by means of mass spectrometry and high-performance liquid chromatography.

Machine Learning Accelerated Discovery of Covalent Organic Frameworks for Environmental and Energy Applications

Covalent organic frameworks (COFs) are porous crystalline materials obtained by linking organic ligands covalently. Their high surface area and adjustable pore sizes make them ideal for a range of applications, including CO2 capture, CH4 storage, gas separation, catalysis, etc. Traditional methods of material research, which mainly rely on manual experimentation, are not particularly efficient, while with advancements in computer science, high-throughput computational screening methods based on molecular simulation have become crucial in material discovery, yet they face limitations in terms of computational resources and time. Currently, machine learning (ML) has emerged as a transformative tool in many fields, capable of analyzing large data sets, identifying underlying patterns, and predicting material performance efficiently and accurately. This approach, termed "materials genomics", combines high-throughput computational screening with ML to predict and design high-performance materials, significantly speeding up the discovery process compared to traditional methods. This review discusses the functions of ML in the screening, design, and performance prediction of COFs and highlights their applications across various domains like CO2 capture, CH4 storage, gas separation, and catalysis, thereby providing new research directions and enhancing the understanding of COF materials and their applications.

Crystalline, Porous Figure-Eight-Noded Covalent Organic Frameworks

Figure-eight macrocycles represent a fascinating class of π-conjugated units characterized by unique aesthetics and non-contact molecular crossing at the center. Despite progress in synthesis over the past century, research into inorganic, organic, and polymeric figure-eight materials remains in its infancy. Here we report the first examples of figure-eight covalent organic frameworks by condensing figure-eight knots to create extended porous figure-eight π architectures. A distinct feature is that polymerization interweaves figure-eight knots into double-decker layers, which upon supramolecular polymerization organize well-defined layer frameworks. The figure-eight frameworks exhibit a band gap of 2.3 eV and emit bright orange florescence with benchmark quantum yields. Remarkably, the donor-acceptor figure-eight skeletons convert the figure-eight knots into reduction centers and the linkers into oxidation sites upon light irradiation, enable charge transport and accumulation through π columns, while the built-in hydrophilic micropores allow rapid water and oxygen delivery via capillary effect. With these distinct features, the figure-eight frameworks function as a photocatalyst to produce hydrogen peroxide at high rate and efficiency with water/saltwater, oxygen/air, and light as sole inputs. This work paves a way to a new class of molecular frameworks, underpinning the study of well-defined figure-eight materials to explore unprecedented structures and functions so far we untouched.

Engineered covalent organic frameworks (COFs) for adsorption-based metal separation technologies: A critical review

Porous covalent organic frameworks (COFs) are promising materials used for separation and purification during environmental remediation. This critical review focuses on two key aspects. First, it critically examines strategies to improve COF design and structure and evaluates their impact on separation performance. Second, engineering approaches for enhancing the interactions between COF-based adsorbents and metals for enhanced separation and capture are elucidated. The latest body of research on separating metals (e.g., Li, K, Sr, Hg, Cd, Pb, Cr, Au, Ag, Pd, and U) using COF-based adsorbents is discussed to understand the factors that influence their performance. However, it is to be noted that COF-based adsorbents are still in their infancy and remain largley unexplored, mainly hindered by synthetic complexities and suboptimal crystalline structures. This highlights the need for further research and development to fully unlock the excellent potential of COFs for metal separation applications, particularly in environmental and energy applications.

Synthesis innovations for crystallizing covalent organic framework thin films on biological and non-biological substrates

Thin film technology has emerged as a pivotal field with numerous industrial applications. Depending on their properties-such as magnetic characteristics, conductivity, architectural structure, stability, and functional backbones-thin films are widely utilized in optoelectronics, thin-film coatings, solar cells, energy storage devices, semiconductors, and separation applications. However, for all these applications, thin films must be securely attached to specific substrates, and substrate compatibility with both the thin film and the film-growth process is crucial for optimal performance. In this review, we emphasize the significance of growing thin films, particularly covalent organic framework (COF) thin films, on suitable substrates tailored for various applications. For separation technologies, polymer thin films are commonly fabricated on porous polymeric or metal-based membranes. In contrast, thin films of metals and metal oxides are typically deposited on conducting substrates, serving as current collectors for energy storage devices. Semiconductor thin films, on the other hand, are often grown on silicon or glass substrates for transistor applications. Emerging COF thin films, with their tunable properties, well-defined pore channels, and versatile functional backbones, have demonstrated exceptional potential in separation, energy storage, and electronic and optoelectronic applications. However, the interplay between COF thin films and the substrates, as well as the compatibility of growth conditions, remains underexplored. Studies investigating COF thin film growth on substrates such as metals, metal oxides, glass, silicon, polymers, ITO, and FTO have provided insights into substrate properties that promote superior film growth. The quality of the film formed on these substrates significantly influences performance in applications. Additionally, we discuss the stabilization of biological substrates, like peptide-based biomimetic catalysts and enzymes, which often suffer from instability in non-aqueous environments, limiting their industrial use. Growing COF membranes on these biological substrates can enhance their stability under harsh conditions. We also highlight techniques for growing COF membranes on biological substrates, ensuring the preservation of their structural integrity and functional properties.

Single-Molecule Detection via Pore Nanoconfinement of Covalent Organic Frameworks for Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) offers significant advantages for single-molecule detection. However, stochastic molecular motion makes it challenging to consistently capture signals from single-molecule binding events, particularly in complex environments. Herein, we propose a novel SERS system via the pore nanoconfinement effect of covalent organic frameworks (COFs) to achieve reliable single-molecule detection. The self-assembled COF thin films on SERS metal substrates (Au/Ag) create a nanogap of 3 nm, allowing electric field enhancement. By precise tuning of the COF shell thickness, a molecular-scale pore volume is formed, effectively trapping individual molecules from molecular aggregates. Furthermore, the strong intermolecular forces within the COF pores significantly enhance the residence time of individual molecules, thereby increasing the probability of detecting single-molecule binding events. This innovative approach ensures consistent and reliable SERS single-molecule detection in complex mixtures, paving the way for advanced applications in biochemical sensing and diagnostics.

Promising CO2 Capture and Effective Iodine Adsorption of Hyper-Cross-Linked Conjugated Porous Organic Polymers Prepared from a Cyclopentannulation Reaction

Three novel conjugated porous organic polymers, denoted as C-POP1-3 and which consist of alternating pyrene cores with various contorted fluorene surrogates, were successfully synthesized from a versatile one-pot palladium-catalyzed [3+2] cyclocondensation reaction. The resulting polymers were obtained in excellent yields and displayed weight-average molecular weights (Mw) ranging from 12.2 to 20.2 kg/mol with polydispersity indices (Mw/Mn) ranging between 1.8 and 2.4, suggesting that the molecular masses are narrowly distributed and thus implying homogeneous polymer chains. Thermal stability exploration of C-POP1-3 by thermogravimetric analysis (TGA) revealed an impressive robustness with a 10% weight reduction temperature attaining 485 °C. Investigation of the inherent microporosity properties of C-POP1-3 via nitrogen adsorption experiments using Brunauer-Emmett-Teller (BET) theory discloses their surface areas which reach up to 560 m2 g-1 and pore volumes averaging 0.47 cm3 g-1. The target conjugated polymers were explored as adsorbents disclosing a maximum carbon dioxide adsorption of 83.0 mg g-1 at 273 K and low pressure for C-POP1, whereas iodine sorption tests portrayed prominent outcomes, notably for C-POP3 which proved to owe a strong affinity toward the hitherto mentioned halogen by achieving a maximum adsorption of 2220 mg g-1. Additionally, recyclability experiments confirmed the possibility to regenerate the polymers' adsorption capabilities even after seven consecutive cycles of adsorption-desorption cycles, which qualify them as auspicious iodine adsorbents.

Harnessing Pore Size in COF Membranes: A Concentration Gradient-Driven Molecular Dynamics Study on Enhanced H2/CH4 Separation

This work presents a novel approach for accurately predicting the gas transport properties of covalent organic framework (COF) membranes using a nonequilibrium molecular dynamics (NEMD) methodology called concentration gradient-driven molecular dynamics (CGD-MD). We first simulated the flux of hydrogen (H2) and methane (CH4) across two distinct COF membranes, COF-300 and COF-320, for which experimental data are available in the literature. Our CGD-MD simulation results aligned closely with the experimentally measured gas permeability and selectivity of these COF membranes. Leveraging the same methodology, we discovered promising COF candidates for H2/CH4 separation, including NPN-1, NPN-2, NPN-3, TPE-COF-I, COF-303, DMTA-TPB2, 3D-Por-COF, COF-921, COF-IM AA, TfpBDH, and PCOF-2. We then compared our findings with simulations utilizing the well-known approach that merges grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) to predict gas adsorption and diffusion parameters in COFs. Our results showed that when the pore sizes of COF membranes are below 10 Å, the choice of the method plays a significant role in determining the performance of the membranes. The GCMC+EMD approach suggested that COFs tend to exhibit CH4 selectivity when their pore limiting diameters are below 10 Å, whereas the CGD-MD results reveal a preference for H2. Density functional theory calculations indicate that H2 has a lower affinity for three promising COFs, NPN-1, NPN-2, and NPN-3, compared to CH4, which results in H2 remaining unbound, while CH4 occupies all of the adsorption sites, thereby facilitating the selective recovery of H2 at the end of the separation process. We proposed a relationship between adsorption time and diffusion time, highlighting the critical role of selecting an appropriate simulation method. This relationship underscores how adsorption and diffusion processes interplay, impacting material performance. Overall, these insights not only improve the accuracy of predictive models but also guide the development of more efficient COF-based membrane applications for future research and industrial applications.

Advanced Porous Aromatic Frameworks: A Comprehensive Overview of Emerging Functional Strategies and Potential Applications

Porous aromatic frameworks (PAFs) are a fundamental group of porous materials characterized by their distinct structural features and large surface areas. These materials are synthesized from aromatic building units linked by strong carbon-carbon bonds, which confer exceptional rigidity and long-term stability. PAFs functionalities may arise directly from the intrinsic chemistry of their building units or through the postmodification of aromatic motifs using well-defined chemical processes. Compared to other traditional porous materials such as zeolites and metallic-organic frameworks, PAFs demonstrate superior stability under severe chemical treatments due to their robust carbon-carbon bonding. Even in challenging environments, the chemical stability and ease of functionalization of PAFs demonstrate their flexibility and specificity. Research on PAFs has significantly expanded and accelerated over the past decade, necessitating a comprehensive overview of key advancements in this field. This review provides an in-depth analysis of the recent advances in the synthesis, functionalization, and dimensionality of PAFs, along with their distinctive properties and wide-ranging applications. This review explores the innovative methodologies in PAFs synthesis, the strategies for functionalizing their structures, and the manipulation of their dimensionality to tailor their properties for specific potential applications. Similarly, the key application areas, including batteries, absorption, sensors, CO2 capture, photo-/electrocatalytic usages, supercapacitors, separation, and biomedical are discussed in detail, highlighting the versatility and potential of PAFs in addressing modern scientific and industrial challenges.

Covalent Organic Frameworks for Green Energy: Synthesis, Properties, and Applications

Covalent organic frameworks (COFs) are a new type of porous organic crystalline material, which have become an emerging platform for promoting the development of green energy technology due to their high surface area, adjustable pores, low skeleton density, and easy functionalization. In recent years, with the continuous advancement of synthesis technology, the synthesis efficiency and sustainability of COFs have been significantly improved, from traditional solvothermal methods to the emergence of various green synthesis strategies such as ion thermal, mechanochemical, and ultrasound assisted methods. This article reviews the main synthesis methods of COFs and explores their applications in the field of green energy, such as photocatalysis, gas adsorption and separation, electrocatalysis, battery, supercapacitor and Proton exchange membrane fuel cell. By analyzing the performance and mechanism of COFs in these applications in detail, this article further looks forward to the challenges and future development trends faced by COFs in green energy technology, aiming to provide valuable reference and inspiration for researchers in related fields.

A Dynamic Covalent Organic Framework with Entangled 2D Layers

Dynamic covalent organic frameworks (COFs) represent an emerging class of porous materials with an inherent structural flexibility. However, due to the challenges in their synthesis and structural characterization, research on dynamic COFs remains at an early stage and requires further exploration. Herein, we report the designed synthesis of a novel COF with entangled 2D layers that exhibits interesting dynamic behavior in response to organic vapor exposure. By employing the continuous rotation electron diffraction technique, we precisely resolved the crystal structures of the COF before and after vapor adsorption. Structural analysis revealed that the vapor-induced conformational changes, such as anthracene unit rotation, triggered layer adjustments and reduced entanglement angles, leading to significant pore structure alterations. This study not only introduces a new class of dynamic COFs but also provides a foundation for the rational design of entangled frameworks with structural flexibility for diverse applications.

Vacuum-Assisted Confined Growth of MOF@COF Composite Membranes with Enhanced Hydrogen Permselectivity

With ordered and periodic network structures, adjustable pore sizes and high chemical stability, covalent organic frameworks (COFs) have drawn much attention for the fabrication of superior separation membranes. However, it is challenging to prepare COF membranes with a molecular sieving property for gas separation due to their relatively large pore size. In this work, we develop the MOF-in-COF concept for vacuum-assisted synthesis of metal-organic framework (MOF) ZIF-8 inside the pores of TB-COF formed from trihydroxy-benzene-tricarbaldehyde (T) and diamino-biphenyl-disulfonic acid (B), thus constructing a novel ZIF-8@TB-COF membrane. Attributing to the formation of a well-defined one-dimensional (1D) nanoscale transport channel, the ZIF-8@TB-COF membrane displays a high hydrogen permselectivity. At 100 °C and 200 kPa, the mixture separation factors of H2/CO2, H2/CH4 and H2/C3H8 are 21.9, 63.1 and 134.4, respectively, which are much higher than those of the pristine TB-COF membrane due to the precise size sieving channels brought about by the incorporated MOF. The synthesis of ZIF-67@TB-COF membrane demonstrates the versatility of the synthesis strategy.

Emerging MOFs, COFs, and their derivatives for energy and environmental applications

Traditional fossil fuels significantly contribute to energy supply, economic development, and advancements in science and technology. However, prolonged and extensive use of fossil fuels has resulted in increasingly severe environmental pollution. Consequently, it is imperative to develop new, clean, and pollution-free energy sources with high energy density and versatility as substitutes for conventional fossil fuels, although this remains a considerable challenge. Simultaneously, addressing water pollution is a critical concern. The development, design, and optimization of functional nanomaterials are pivotal to advancing new energy solutions and pollutant remediation. Emerging porous framework materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), recognized as exemplary crystalline porous materials, exhibit potential in energy and environmental applications due to their high specific surface area, adjustable pore sizes and structures, permanent porosity, and customizable functionalities. This work provides a comprehensive and systematic review of the applications of MOFs, COFs, and their derivatives in emerging energy technologies, including the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, lithium-ion batteries, and environmental pollution remediation such as the carbon dioxide reduction reaction and environmental pollution management. In addition, strategies for performance adjustment and the structure-effect relationships of MOFs, COFs, and their derivatives for these applications are explored. Interaction mechanisms are summarized based on experimental discussions, theoretical calculations, and advanced spectroscopy analyses. The challenges, future prospects, and opportunities for tailoring these materials for energy and environmental applications are presented.

Predesign of Covalent-Organic Frameworks for Efficient Photocatalytic Dehydrogenative Cross-Coupling Reaction

The dehydrogenative cross-coupling reaction is the premier route for synthesizing important 4-quinazolinone drugs. However, it usually requires high reaction temperature and long reaction time, and the yield of the final product is low. Here two stable and photosensitive covalent-organic frameworks (COFs), TAPP-An and TAPP-Cu-An are purposefully designed and constructed to serve as unprecedented heterogeneous tandem catalysts to complete dehydrogenative cross-coupling reactions in a short time and under mild reaction conditions (room temperature and light), leading to the high-efficient photosynthesis of 4-quinazolinones. Particularly, TAPP-Cu-An is the best heterogeneous catalyst currently available for the synthesis of 4-quinazolinones, even surpassing all the catalysts reported so far. It also enables one-step photosynthesis of 4-quinazolinones with higher conversion (>99%) and selectivity (>99%) in a shorter time, and the product can be easily prepared on a gram scale. Extensive experiments combined with theoretical calculations show that the excellent photogenerated charge separation and transport capability, as well as the synergistic An-Cu catalysis in TAPP-Cu-An are the main driving forces for this efficient reaction.

Construction of functional covalent organic framework films by modulator and solvent induced polymerization

Covalent organic frameworks are attractive candidates for the next generation films in technical applications. However, due to their crystallization nature, insolubility in common solvents as well as infusible at high temperatures make it challenging to grow them spontaneously or process them into films. Herein, we report an efficient strategy to fabricate covalent organic framework films based on a modulator-solvent induced polymerization process. The addition of modulator slows down the nucleation rate during the initial stages of covalent organic framework growth, resulting in the formation of fluidic precursors that are easy to process. Subsequently, a suitable drying process is introduced to balance the evaporation rate of solvent and the crystallization rate of modulator induced, resulting in the formation of covalent organic framework films with a mixture of amorphous and crystalline structures. This strategy is universal for the fabrication of several types of covalent organic framework films with large-scale and freestanding state. Moreover, covalent organic framework films with asymmetric structure can function as organic vapor-triggered actuators, offering excellent repeatability and reversibility. By introducing functional molecules such as fluorescence, chirality and catalyst during the nucleation process, versatile functional covalent organic framework films can be easily fabricated, which endow them with broader application prospects.

Advancements in framework materials for enhanced energy harvesting

Energy harvesting, the process of capturing ambient energy from various sources and converting it into usable electrical power, has attracted a lot of attention due to its potential to provide long-term and self-sufficient energy solutions. This comprehensive review thoroughly explores the use of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) for energy harvesting by piezoelectric and triboelectric nanogenerators (PENGs and TENGs). It begins by classifying and outlining the structural diversity of MOFs and COFs, which is key to understanding their importance in energy applications. Key characterization techniques are focused on emphasizing their importance in optimizing material properties for efficient energy conversion. The working mechanisms of PENGs and TENGs are discussed, focusing on their ability to transform mechanical energy into electrical energy and their advantages in operation. The use of MOFs and COFs in energy harvesting applications is then discussed, including synthesis procedures, unique characteristics relevant to electricity conversion, and various practical applications such as self-powered sensors and wearable electronics. Current challenges such as stability, scalability, and performance improvements are explored, as well as proposed future improvements to help advance current research. Finally, the study highlights the importance of framework materials for the development of energy harvesting systems, providing an invaluable resource for academics and engineers seeking to exploit the potential of these materials for renewable energy sources. The goal of this article is to stimulate further invention and implementation of efficient materials-based energy harvesting framework devices by integrating recent advances and mapping future possibilities.

Two-Dimensional Organic-Inorganic van der Waals Hybrids

Two-dimensional organic-inorganic (2DOI) van der Waals hybrids (vdWhs) have emerged as a groundbreaking subclass of layer-stacked (opto-)electronic materials. The development of 2DOI-vdWhs via systematically integrating inorganic 2D layers with organic 2D crystals at the molecular/atomic scale extends the capabilities of traditional 2D inorganic vdWhs, thanks to their high synthetic flexibility and structural tunability. Constructing an organic-inorganic hybrid interface with atomic precision will unlock new opportunities for generating unique interfacial (opto-)electronic transport properties by combining the strengths of organic and inorganic layers, thus allowing us to satisfy the growing demand for multifunctional applications. Here, this review provides a comprehensive overview of the latest advancements in the chemical synthesis, structural characterization, and numerous applications of 2DOI-vdWhs. Firstly, we introduce the chemistry and the physical properties of the recently rising organic 2D crystals (O2DCs), which feature crystalline 2D nanostructures comprising carbon-rich repeated units linked by covalent/noncovalent bonds and exhibit strong in-plane extended π-conjugation and weak interlayer vdWs interaction. Simultaneously, representative inorganic 2D crystals (I2DCs) are briefly summarized. After that, the synthetic strategies will be systematically summarized, including synthesizing single-component O2DCs with dimensional control and their vdWhs with I2DCs. With these synthetic approaches, the control in the dimension, the stacking modes, and the composition of the 2DOI-vdWhs will be highlighted. Subsequently, a special focus will be given on the discussion of the optical and electronic properties of the single-component 2D materials and their vdWhs, which will be closely relevant to their structures, so that we can establish a general structure-property relationship of 2DOI-vdWhs. In addition to these physical properties, the (opto-)electronic devices such as transistors, photodetectors, sensors, spintronics, and neuromorphic devices as well as energy devices will be discussed. Finally, we provide an outlook to discuss the key challenges for the 2DOI-vdWhs and their future development. This review aims to provide a foundational understanding and inspire further innovation in the development of next-generation 2DOI-vdWhs with transformative technological potential.

Staggered-Stacking Two-Dimensional Covalent Organic Framework Membranes for Molecular and Ionic Sieving

Two-dimensional covalent organic frameworks (2D COFs), a family of crystalline materials with abundant porous structures offering nanochannels for molecular transport, have enormous potential in the applications of separation, energy storage, and catalysis. However, 2D COFs remain limited by relatively large pore sizes (>1 nm) and weak interlayer interactions between 2D nanosheets, making it difficult to achieve efficient membranes to meet the selective sieving requirements for water molecules (0.3 nm) and hydrated salt ions (>0.7 nm). Here, we report a high-performance 2D COF membrane with narrowed channels (0.7 × 0.4 nm2) and excellent mechanical performance constructed by the staggered stacking of cationic and anionic 2D COF nanosheets for selectively sieving of water molecules and hydrated salt ions. The mechanical performance has been improved by two times than that of single-phase 2D COF membranes due to the enhanced interlayer interactions between nanosheets. The stacked 2D COF membranes exhibit significantly improved monovalent salt ions rejection ratio (up to 77.9%) compared with single-phase COF membranes (∼49.2%), while maintaining comparable water permeability. The design of stacked 2D COF membranes provides a potential strategy for constructing high-performance nanoporous membranes to achieve precise molecular and ionic sieving.

Photocatalytically self-cleaning graphene oxide nanofiltration membranes reinforced with bismuth oxybromide for high-performance water purification

Graphene oxide (GO) membranes have emerged as promising candidates for water purification applications, owing to their unique physicochemical attributes. Nevertheless, the trade-off between permeability and selectivity, coupled with their vulnerability to membrane fouling, poses significant challenges to their widespread industrial deployment. In this study, we introduce an innovative in-situ growth and layer-by-layer assembly technique for fabricating multilayer GO membranes reinforced with bismuth oxybromide (BiOBr) on commonly employed Nylon substrates. This method allows for the creation of two-dimensional lamellar membranes capable of photocatalytic self-cleaning and tunable nanochannel dimensions. The synthesized GO/BiOBr composite membranes exhibit remarkable water permeance rates (approximately 493.9 LMH/bar) and high molecular rejection efficiency (>99 % for Victoria Blue B and Congo Red dyes). Notably, these membranes showcase an enhanced photocatalytic self-cleaning performance upon exposure to visible light. Our work provides a viable route for the fabrication of functionalized GO-based nanofiltration membranes with BiOBr inclusions, offering a synergistic combination of high water permeability, modifiable nanochannels, and effective self-cleaning capabilities through photocatalysis.

Hypoxia-Activated Biodegradable Porphyrin-Based Covalent Organic Frameworks for Photodynamic and Photothermal Therapy of Wound Infection

Wound infections have gradually become a major threat to human health. Recently, covalent organic frameworks (COFs) have shown great potential in antibacterial and wound healing; however, difficult biodegradability and long-time in vivo retention limit their further application. Herein, biodegradable COFs containing porphyrin backbones and hypoxia-sensitive azobenzene group, namely, HRCOFs, are fabricated for photodynamic therapy (PDT) and photothermal therapy (PTT) of wound infection. Due to the introduction of a porphyrin molecule, HRCOFs can produce singlet oxygen (1O2) under 660 nm laser irradiation. The prepared HRCOFs can also generate thermal energy under 808 nm NIR laser irradiation. HRCOFs show excellent synergetic antibacterial ability against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) in vitro. The in vivo experiments also demonstrate synergistic PDT and PTT effects of HRCOFs against wound infection. Importantly, HRCOFs are response to wound microenvironment, can be degraded for clearance, and avoid some adverse effects caused by long-time retention in vivo, exhibiting good biocompatibility. In general, the obtained biodegradable HRCOFs with both photodynamic and photothermal effects can be used for antibacterial infections and provide great value for promoting wound healing.

Robust Computation and Analysis of Vibrational Spectra of Layered Framework Materials Including Host-Guest Interactions

Layered framework materials, a rapidly advancing class of porous materials, are composed of molecular components stitched together via covalent bonds and are usually synthesized through wet-chemical methods. Computational infrared (IR) and Raman spectra are among the most important characterization tools for this material class. Besides the a priori known spectra of the molecular building blocks and the solvent, they allow for in situ monitoring of the framework formation during synthesis. Therefore, they need to capture the additional peaks from host-guest interactions and the bands from emerging bonds between the molecular building blocks, verifying the successful synthesis of the desired material. In this work, we propose a robust computational framework based on ab initio molecular dynamics (AIMD), where we compute IR and Raman spectra from the time-correlation functions of dipole moments and polarizability tensors, respectively. As a case study, we apply our methodology to a covalent organic framework (COF) material, COF-1, and present its AIMD-computed IR and Raman spectra with and without 1,4-dioxane solvent molecules in its pores. To determine robust settings, we meticulously validate our model and explore how stacking disorder and different methods for computing dipole moments and polarizabilities affect IR and Raman intensities. Using our robust computational protocol, we achieve excellent agreement with experimental data. Furthermore, we illustrate how the computed spectra can be dissected into individual contributions from the solvent molecules, the molecular building blocks of COF-1, and the bonds connecting them.

Assembly-Dissociation-Reconstruction Synthesis of Covalent Organic Framework Membranes with High Continuity for Efficient CO2 Separation

Covalent organic frameworks (COFs), at the forefront of porous materials, hold tremendous potential in membrane separation; however, achieving high continuity in COF membranes remains crucial for efficient gas separation. Here, we present a unique approach termed assembly-dissociation-reconstruction for fabricating COF membranes tailored for CO2/N2 separation. A parent COF is designed from two-node aldehyde and three-node amine monomers and dissociated to high-aspect-ratio nanosheets. Subsequently, COF nanosheets are orderly reconstructed into a crack-free membrane by surface reaction under water evaporation. The membrane exhibits high crystallinity, open pores and a strong affinity for CO2 adsorption over N2, resulting in CO2 permeance exceeding 1060 GPU and CO2/N2 selectivity surpassing 30.6. The efficacy of this strategy offers valuable guidance for the precise fabrication of gas-separation membranes.

Probing Defects in Covalent Organic Frameworks

Defects in covalent organic frameworks (COFs) play a pivotal role in determining their properties and performance, significantly influencing interactions with adsorbates, guest molecules, and substrates as well as affecting charge carrier dynamics and light absorption characteristics. The present review focuses on the diverse array of techniques employed for characterizing and quantifying defects in COFs, addressing a critical need in the field of materials science. As will be discussed in this review, there are basically two types of defects referring either to missing organic moieties leaving free binding groups in the material or structural imperfections resulting in lower crystallinity, grain boundary defects, and incomplete stacking. The review summarizes an in-depth analysis of state-of-the-art characterization techniques, elucidating their specific strengths and limitations for each defect type. Key techniques examined in this review include powder X-ray diffraction (PXRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), scanning transmission electron microscopy (STEM), scanning tunneling microscope (STM), high resolution transmission electron microcoe (HRTEM), gas adsorption, acid-base titration, advanced electron microscopy methods, and computational calculations. We critically assess the capability of each technique to provide qualitative and quantitative information about COF defects, offering insights into their complementary nature and potential for synergistic use. The last section summarizes the main concepts of the review and provides perspectives for future development to overcome the existing challenges.

Triazine-Carbazole-Based Covalent Organic Frameworks as Efficient Heterogeneous Photocatalysts for the Oxidation of N-aryltetrahydroisoquinolines

Covalent organic frameworks (COFs) have attracted growing interests as new material platform for a range of applications. In this study, a triazine-carbazole-based covalent organic framework (COF-TCZ) was designed as highly porous material with conjugated donor-acceptor networks, and feasibly synthesized by the Schiff condensation of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tr ianiline (TAPB) and 9-(4-formylphenyl)-9H-carbazole-3,6-dicarbaldehyde (CZTA) under the solvothermal condition. Considering the effect of linkage, the imine-linked COF-TCZ was further oxidized to obtain an amide-linked covalent organic framework (COF-TCZ-O). The as-synthesized COFs show high crystallinity, good thermal and chemical stability, and excellent photoactive properties. Two π-conjugated triazine-carbazole-based COFs with tunable linkages are beneficial for light-harvesting capacity and charge separation efficiency, which are empolyed as photocatalysts for the oxidation reaction of N-aryltetrahydroisoquinoline. The COFs catalyst systems exhibit the outstanding photocatalytic performance with high conversion, photostability and recyclability. Photoelectrochemical tests were employed to examine the behavior of photogenerated charge carriers in photo-illumination system. The control experiments provide further insights into the nature of photocatalysis. In addition, the current research also provided a valuable approach for developing photofunctional COFs to meet challenge in achieving the great potential of COFs materials in organic conversion.

Sulfonated Chitosan Gel Membrane with Confined Amine Carriers for Stable and Efficient Carbon Dioxide Capture

Capturing carbon dioxide (CO2) from flue gases is a crucial step towards reducing CO2 emissions. Among the various carbon capture methods, facilitated transport membranes (FTMs) have emerged as a promising technology for CO2 capture owing to their high efficiency and low energy consumption in separating CO2. However, FTMs still face the challenge of losing mobile carriers due to weak interaction between the carriers and membrane matrix. Herein, we report a sulfonated chitosan (SCS) gel membrane with confined amine carriers for effective CO2 capture. In this structure, diethylenetriamine (DETA) as a CO2-mobile carrier is confined within the SCS gel membrane via electrostatic forces, which can react reversibly with CO2 and thus greatly facilitate its transport. The SCS ion gel membrane allows for the fast diffusion of amine carriers within it while blocking the diffusion of nonreactive gases, like N2. Thus, the prepared membrane exhibits exceptional CO2 separation capabilities when tested under simulated flue gas conditions with CO2 permeance of 1155 GPU and an ultra-high CO2/N2 selectivity of above 550. Moreover, the membrane retains a stable separation performance during the 170 h continuous test. The excellent CO2 separation performance demonstrates the high potential of gel membranes for CO2 capture from flue gas.

Ionic Liquid-Accelerated Growth of Covalent Organic Frameworks with Tunable Layer-Stacking

Layer-stacking behaviors are crucial for two-dimensional covalent organic frameworks (2D COFs) to define their pore structure, physicochemical properties, and functional output. So far, fine control over the stacking mode without complex procedures remains a grand challenge. Herein, we proposed a "key-cylinder lock mimic" strategy to synthesize 2D COFs with a tunable layer-stacking mode by taking advantage of ionic liquids (ILs). The staggered (AB) stacking (unlocked) COFs were exclusively obtained by incorporating ILs of symmetric polarity and matching molecular size; otherwise, commonly reported eclipsed (AA) stacking (locked) COFs were observed instead. Mechanistic study revealed that AB stacking was induced by a confined interlocking effect (CIE) brought by anions and bulky cations of the ILs inside pores ("key" and "cylinder", respectively). Excitingly, this strategy can speed up production rate of crystalline powders (e.g., COF-TAPT-Tf@BmimTf2N in merely 30 minutes) under mild reaction conditions. This work highlights the enabling role of ILs to tailor the layer stacking of 2D COFs and promotes further exploration of their stacking mode-dependant applications.

Light-Enhanced Tandem-Responsive Nano Delivery Platform for Amplified Anti-tumor Efficiency

Designing nanomedicines with low toxicity, high targeting, excellent therapeutic effects, and precise release is always the major challenges in clinical cancer treatment. Here, we report a light-enhanced tandem-responsive nano delivery platform COF-B@X-03 for amplified anti-tumor efficiency. Biotin-loaded COF-B@X-03 could precisely target tumor cells, and the azo and hydrazone bonds in it would be depolymerized by the overexpressed azoreductase and acidic microenvironment in hypoxic tumors. In vitro experimental results indicate mitochondrial and endoplasmic reticulum stress caused by COF-B@X-03 under light is the direct cause of tumor cell death. In vivo experimental data prove COF-B@X-03 achieves low oxygen dependent phototherapy, and the maintenance of intratumoral hypoxia provides the possibility for the continuous degradation of COF-B@X-03 to generate more reactive oxygen species for tumor photodynamic therapy by released X-03. In the end, COF-B@X-03 phototherapy group achieves higher tumor inhibition rate than X-03 phototherapy group, which is 81.37 %. Meanwhile, COF-B@X-03 significantly eliminates the risk of tumor metastasis. In summary, the construction of this tandem-responsive nano delivery platform provides a new direction for achieving efficient removal of solid tumors in clinical practice.

Cationic covalent organic framework nanosheets as the coating layer of commercial separator for high-efficiency lithium-sulfur batteries

Ion-selective separators, are crucial and in high demand for maximizing the performance of lithium-sulfur (Li-S) batteries, which can conduct lithium ions while efficiently blocking polysulfides. However, commercial separators cannot effectively block the shuttle of polysulfides after multiple cycles due to their large porosity and easy dissolution, resulting in a reduced battery life. Herein, a covalent organic framework nanosheets (CON) ion-coated separator is prepared on the commercial separator. Due to the smaller pore size of CON-TFSI compared to polysulfides, the CON-TFSI modified separator can effectively block the polysulfide from migrating across the separator. By incorporating this innovative functional layer, Li-S batteries demonstrate outstanding performance. In a Li-S battery featuring a sulfur loading of 0.6 mg/cm2, it attains an initial discharge specific capacity of up to 891.9 mA h g-1, and exhibits the capacity retention of 54.6 % after 500 cycles at a current density of 0.2 C. This work offers a fresh perspective on the advancement of high-performance membranes for Li-S batteries.

Ultrafine Spatial Modulation of Diazapyrene-Based Two-Dimensional Conjugated Covalent Organic Frameworks

Tailormade bottom-up synthesis of covalent organic frameworks (COFs) from various functional building blocks offer not only tunable topology and pore size but also multidimensional properties. High crystallinity is one of the prerequisites for their structures and associated physicochemical properties. Among different π-conjugated motifs for constructing COFs, pyrene-based tetragonal structures are effective in achieving highly ordered and crystalline states. In the present research, we demonstrated that the substitution of pyrene with 2,7-diazapyrene produces nearly "flat" structures of two-dimensional (2D) COF layers by controlling the torsional angle of linker molecules. Featuring finite pore diameters and excellent thermodynamic stability of ∼500 °C, ordered face-to-face (slipped AA) stacking arrangements were produced. Extended electrical conjugation spanning 2D frames with modest optical bandgaps (Eg) of ∼2.1 eV shows the planar character of diazapyrene-based COFs. The stacking of the conjugated 2D frames with small Eg values is also beneficial for the formation of highly stable conducting pathways in the crystalline state, which was confirmed by the results of the microwave conductivity measurements. Nitrogen centers in diazapyrene units also play a key role as the active sites for proton transfer, and the maximum proton conductivity of σ = 10-2 S cm-1 was achieved along the cocontinuous nanopore structures surrounded by the active sites. Results show that tetragonal COFs based on diazapyrene can be used as a highly crystalline two-dimensional material with special electrical and proton-conducting capabilities.

Ion-Ion Selectivity of Synthetic Membranes with Confined Nanostructures

Synthetic membranes featuring confined nanostructures have emerged as a prominent category of leading materials that can selectively separate target ions from complex water matrices. Further advancements in these membranes will pressingly rely on the ability to elucidate the inherent connection between transmembrane ion permeation behaviors and the ion-selective nanostructures. In this review, we first abstract state-of-the-art nanostructures with a diversity of spatial confinements in current synthetic membranes. Next, the underlying mechanisms that govern ion permeation under the spatial nanoconfinement are analyzed. We then proceed to assess ion-selective membrane materials with a focus on their structural merits that allow ultrahigh selectivity for a wide range of monovalent and divalent ions. We also highlight recent advancements in experimental methodologies for measuring ionic permeability, hydration numbers, and energy barriers to transport. We conclude by putting forth the future research prospects and challenges in the realm of high-performance ion-selective membranes.

Construction of Benzoxazine-linked One-Dimensional Covalent Organic Frameworks Using the Mannich Reaction

Covalent polymerization of organic molecules into crystalline one-dimensional (1D) polymers is effective for achieving desired thermal, optical, and electrical properties, yet it remains a persistent synthetic challenge for their inherent tendency to adopt amorphous or semicrystalline phases. Here we report a strategy to synthesize crystalline 1D covalent organic frameworks (COFs) composing quasi-conjugated chains with benzoxazine linkages via the one-pot Mannich reaction. Through [4+2] and [2+2] type Mannich condensation reactions, we fabricated stoichiometric and sub-stoichiometric 1D covalent polymeric chains, respectively, using doubly and singly linked benzoxazine rings. The validity of their crystal structures has been directly visualized through state-of-the-art cryogenic low-dose electron microscopy techniques. Post-synthetic functionalizations of them with a chiral MacMillan catalyst produce crystalline organic photocatalysts that demonstrated excellent catalytic and recyclable performance in light-driven asymmetric alkylation of aldehydes, affording up to 94 % enantiomeric excess.

Wrinkled metal-organic framework thin films with tunable Turing patterns for pliable integration

Flexible integration spurs diverse applications in metal-organic frameworks (MOFs). However, current configurations suffer from the trade-off between MOF loadings and mechanical compliance. We report a wrinkled configuration of MOF thin films. We established an interfacial synthesis confined and controlled by a polymer topcoat and achieved multiple Turing motifs in the wrinkled thin films. These films have complete MOF surface coverage and exhibit strain tolerance up to 53.2%. The enhanced mechanical properties allow film transfer onto various substrates. We obtained membranes with large H2/CO2 selectivity (41.2) and high H2 permeance (8.46 × 103 gas permeation units), showcasing negligible defects after transfer. We also achieved soft humidity sensors on delicate electrodes by avoiding exposure to harsh MOF synthesis conditions. These results highlight the potential of wrinkled MOF thin films for plug-and-play integration.

Separation of cyclohexanol from cyclohexanol/cyclohexene mixtures by crystals of pillar[6]arene containing three benzoquinone units

Here, we develop a new absorbent for efficient separation of cyclohexanol (CHA-ol) and cyclohexene (CHA-ene) by using crystals of pillar[6]arene with three benzoquinone units (P3QA). P3QA crystals are found to show remarkable selectivity for CHA-ol in 50 : 50 (v/v) CHA-ol : CHA-ene mixtures with a purity of 95.2%, along with vapochromic behavior.

Chemical fuel-driven transient 2D supramolecular organic frameworks (SOFs): catalysis for green synthesis

Mimicking the dissipative assemblies found in living systems fueled by bioenergy, we present a novel chemical fuel-driven transient 2D SOF, formed via the redox reaction-driven transient self-assembly of tetraphenylene-based structural units and cucurbit[8]uril (CB[8]). The system was initiated by adding sodium dithionite (SDT) as the fuel, leading to the formation of 2D SOFs through 2 : 1 host-guest complexation between the viologen cation radical and CB[8]. These 2D SOFs then spontaneously disassemble over time as the radicals are oxidized by air. The temporal assembly and lifetimes of these transient SOFs can be controlled by adjusting the concentrations of the fuel. Moreover, the resulting transient 2D SOFs exhibited remarkable potential as catalysts for the green synthesis of benzyl sulfones in water.

Peri-Xanthenoxanthene-Based Covalent Organic Frameworks for High-Performance Aqueous Zn-Ion Hybrid Supercapacitors

Aqueous zinc-ion hybrid supercapacitors (Zn-HSCs) are promising devices for sustainable and efficient energy storage. However, they suffer from a limited energy density compared to lithium-ion batteries. This limitation can be overcome by developing novel electrode materials, with covalent organic frameworks (COFs) standing out as a particularly intriguing option. Herein, peri-xanthenoxanthene (PXX) has been integrated for the first time into a COF scaffold to take advantage of its straightforward synthesis, chemical stability, π-conjugated backbone, and heteroatom content endowing reversible redox reactions at low potentials. Two novel hexagonal COFs have been designed and synthesized by tethering of a PXX-diamine unit having a C2 symmetry with two distinct tris-aldehydes acting as C3-symmetric cornerstones, i.e., triformyl benzene (TFB) and triformylphloroglucinol (Tp), ultimately yielding COF PXX(PhNH2)2-TFB and COF PXX(PhNH2)2-Tp, respectively. As cathodes in Zn-HSCs, COF PXX(PhNH2)2-Tp exhibits a remarkable specific capacitance, energy, and power densities (237 F g-1, 106.6 Wh kg-1, and 3.0 kW kg-1, respectively), surpassing those of COF PXX(PhNH2)2-TFB (109 F g-1, 49.1 Wh kg-1, and 0.67 kW kg-1). Importantly, both COFs display outstanding long-term stability, over 5000 charge/discharge cycles, with capacitance retention >92%. These findings underscore the potential of PXX-based COFs as high-performance cathode materials for HSCs, thereby offering a promising new avenue for energy storage technologies.

In situ construction of covalent-organic framework on hydrogen-bond organic framework: Fluorescence detection and removal of 4-nitrophenol and metamitron in aqueous media

A multifunctional COF@HOF (ETTA-DFP@TCBP-HOF) composite is prepared by adding red-fluorescent ETTA-DFP COF to the blue-fluorescent TCBP-HOF preparation system through molecular hydrogen bonding or π - π stacking interactions in situ one-pot synthesis. ETTA-DFP@TCBP-HOF is a multifunctional material for the quantitative detection and simultaneous adsorption of 4-nitrophenol (4-NP) and metamitron (MET) in aqueous solution. As a dual-emission fluorescent sensor, the ETTA-DFP@TCBP-HOF has both fluorescence of TCBP-HOF at 474 nm and ETTA-DFP COF at 592 nm, which shows a ratiometric response to 4-NP and MET with high selectivity, good sensitivity, good anti-interference performance and fast response. As a adsorbent, ETTA-DFP@TCBP-HOF displays rapid adsorption kinetics, and acceptable adsorption capacity for 4-NP and MET. In conclusion, this work constructs a novel multifunctional hybrid material with dual-emission center of HOF and COF, which can not only be used as a ratiometric fluorescent probe for detection, but also for removal of hazardous pollutants, suggesting a new strategy for environmental remediation and human health.

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Regioisomerism in Thienothiophene-Based Covalent Organic Frameworks─A Tool for Band-Gap Engineering

The craft of tuning optical properties is well-established for crystalline inorganic and hybrid solids. However, a far greater challenge is to tune the optical properties of organic materials systematically by design. We now introduce a synthesis concept that enables us to alter the optical properties of crystalline covalent organic frameworks (COFs) systematically using isomeric structures of thienothiophene-based building blocks (T23/32T) combined with a variety of tetratopic aromatic amines, e.g., the Wurster moiety (W-NH2). This concept is demonstrated for the synthesis of COFs in bulk and film forms and provides highly crystalline and porous isomeric COFs featuring predesigned photophysical properties. The band gap of the framework can be tuned continuously and precisely by chemically doping the pristine W23TT COF with its related constitutional isomer building block. Density-functional theory investigations of COF model compounds indicate that the extent of π-conjugation is among the key characteristics enabling the band-gap engineering.

Structural Design and Energy and Environmental Applications of Hydrogen-Bonded Organic Frameworks: A Systematic Review

Hydrogen-bonded organic frameworks (HOFs) are emerging porous materials that show high structural flexibility, mild synthetic conditions, good solution processability, easy healing and regeneration, and good recyclability. Although these properties give them many potential multifunctional applications, their frameworks are unstable due to the presence of only weak and reversible hydrogen bonds. In this work, the development history and synthesis methods of HOFs are reviewed, and categorize their structural design concepts and strategies to improve their stability. More importantly, due to the significant potential of the latest HOF-related research for addressing energy and environmental issues, this work discusses the latest advances in the methods of energy storage and conversion, energy substance generation and isolation, environmental detection and isolation, degradation and transformation, and biological applications. Furthermore, a discussion of the coupling orientation of HOF in the cross-cutting fields of energy and environment is presented for the first time. Finally, current challenges, opportunities, and strategies for the development of HOFs to advance their energy and environmental applications are discussed.

Computational Protocol for the Spectral Assignment of NMR Resonances in Covalent Organic Frameworks

Solid-state nuclear magnetic resonance spectroscopy is routinely used in the field of covalent organic frameworks to elucidate or confirm the structure of the synthesized samples and to understand dynamic phenomena. Typically this involves the interpretation and simulation of the spectra through the assumption of symmetry elements of the building units, hinging on the correct assignment of each line shape. To avoid misinterpretation resulting from library-based assignment without a theoretical basis incorporating the impact of the framework, this work proposes a first-principles computational protocol for the assignment of experimental spectra, which exploits the symmetry of the underlying building blocks for computational feasibility. In this way, this protocol accommodates the validation of previous experimental assignments and can serve to complement new NMR measurements.

One-Dimensional Covalent Organic Framework-Based Multilevel Memristors for Neuromorphic Computing

Memristors are essential components of neuromorphic systems that mimic the synaptic plasticity observed in biological neurons. In this study, a novel approach employing one-dimensional covalent organic framework (1D COF) films was explored to enhance the performance of memristors. The unique structural and electronic properties of two 1D COF films (COF-4,4'-methylenedianiline (MDA) and COF-4,4'-oxydianiline (ODA)) offer advantages for multilevel resistive switching, which is a key feature in neuromorphic computing applications. By further introducing a TiO2 layer on the COF-ODA film, a built-in electric field between the COF-TiO2 interfaces could be generated, demonstrating the feasibility of utilizing COFs as a platform for constructing memristors with tunable resistive states. The 1D nanochannels of these COF structures contributed to the efficient modulation of electrical conductance, enabling precise control over synaptic weights in neuromorphic circuits. This study also investigated the potential of these COF-based memristors to achieve energy-efficient and high-density memory devices.

Synthesis and Direct Observation of Molecules of 2D Polymers: With High Molecular Weights, Large Areas, Small Micropores, Solubility, Membrane Forming Ability, and High Oxygen Permselectivity

If ideal 2D polymer (2DP) macromolecules with small pores that are similar in size to gas molecules, large areas, small thickness, and excellent membrane-forming ability are synthesized, ultimate gas separation membranes would be obtained. However, as far it is known, such ideal well-characterized 2DP macromolecules are not isolated. In this study, an ideal 2DP macromolecule is synthesized by using the successive three reactions (Glaser coupling, SCAT reaction, and the introduction of octyl groups), in which the conjugated framework structure is maintained, from a fully conjugated 1D polymer. Because this exfoliated 2DP is soluble, the macromolecular structure can be fully characterized by 1H-NMR, GPC, SEM, AFM, and its dense membrane with no defects can be fabricated by the solvent cast method. This soluble 2DP macromolecule has very small micropores (6.0 Å) inside the macromolecule, a large area (30 × 68 nm by SEM and AFM), high molecular weight (Mn = 2.80 × 105 by GPC), and a small thickness (4.4 Å by AFM). This membrane shows the highest oxygen permselectivity exceeding Robeson's upper line because of the high molecular sieving effect of the controlled small micropores.

Construction of Core-Shelled Covalent/Metal-Organic Frameworks for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is the half-reaction in zinc-air batteries and water splitting. Developing highly efficient catalysts toward OER is a challenge due to the difficulty of removing four electrons from two water molecules. Covalent organic frameworks (COFs) provide the new chance to construct the highly active catalysts for OER, because they have controlled skeletons, porosities, and well-defined catalytic sites. In this work, core-shell hybrids of COF and metal-organic frameworks (MOFs) have first demonstrated to catalyze the OER. The synergetic effects between the COF-shell and MOF-core render the catalyst with higher activity than those from the COF and MOF. And the catalyst achieved an overpotential of 328 mV, with a Tafel slope of 43.23 mV dec-1 in 1 m KOH. The theoretical calculation revealed that the high activity is from the Fe sites in the catalyst, which has suitable binding ability of reactant intermediate (OOH*), and thus contributed high activity. This work gives a new insight to designing COFs in electrochemical energy storage and conversion systems.

Gas-Triggered Gate-Opening in a Flexible Three-Dimensional Covalent Organic Framework

The development of novel soft porous crystals (SPCs) that can be transformed from nonporous to porous crystals is significant because of their promising applications in gas storage and separation. Herein, we systematically investigated for the first time the gas-triggered gate-opening behavior of three-dimensional covalent organic frameworks (3D COFs) with flexible building blocks. FCOF-5, a 3D COF containing C-O single bonds in the backbone, exhibits a unique "S-shaped" isotherm for various gases, such as CO2, C2, and C3 hydrocarbons. According to in situ characterization, FCOF-5 undergoes a pressure-induced closed-to-open structural transition due to the rotation of flexible C-O single bonds in the framework. Furthermore, the gated hysteretic sorption property of FCOF-5 can enable its use as an absorbent for the efficient removal of C3H4 from C3H4/C3H6 mixtures. Therefore, 3D COFs synthesized from flexible building blocks represent a new type of SPC with gate-opening characteristics. This study will strongly inspire us to design other 3D COF-based SPCs for interesting applications in the future.

Template-Free Self-Assembly of Hollow Microtubular Covalent Organic Frameworks for Oral Delivery of Insulin

Covalent organic frameworks (COFs) have demonstrated versatile application potential since their discovery. Although the structure of COFs is orderly arranged, the synthesis of controllable macrostructures still faces challenges. Herein, we report, to our knowledge, the first template-free self-assembled COF-18 Å hollow microtubule (MT-COF-18 Å) structure and its use for insulin delivery that exhibits high loading capacity, gastroresistance, and glucose-responsive properties. The hollow MT-COF-18 Å was achieved by a template-free method benefiting from the mixed solvents of mesitylene and dioxane. The formation mechanism and morphology changes with insulin loading and release were observed. In Caco-2 cells, the transferrin-coated system demonstrated enhanced insulin cellular uptake and transcellular transport, which indicated great potential for oral applications. Additionally, the composites presented sustained glycemic control and effective insulin blood concentrations without noticeable toxicity in diabetic rats. This work shows that hollow microtubular COFs hold great promise in loading and delivery of biomolecules.

Hypoxia-triggered degradable porphyrinic covalent organic framework for synergetic photodynamic and photothermal therapy of cancer

Nanomedicines receive great attention in cancer treatment. Nevertheless, nonbiodegradable and long-term retention still limit their clinical translation. Herein, we successfully synthesize a hypoxia-triggered degradable porphyrinic covalent organic framework (HPCOF) for antitumor therapy in vivo. HPCOF possesses wide absorption in near infrared region (NIR) which endows HPCOF excellent photothermal conversion efficiency and photoacoustic (PA) imaging ability. Moreover, HPCOF exhibits excellent photodynamic and photothermal effect under special-wavelength laser irradiation. For the first time, the in vitro and in vivo tests demonstrate that HPCOF shows effective therapeutic effect for the combination of PDT and PTT under the monitoring of PA imaging. Importantly, in tumor region, HPCOF could be triggered by hypoxia microenvironment and collapsed gradually, then cleared from the body after treatment. This work fabricates a novel COF for cancer treatment and testifies great potential of HPCOF in clinical application with reducing long-term toxicity.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.