Water cluster in hydrophobic crystalline porous covalent organic frameworks

Spontaneous water oozing of a soft drain bed via energy-free atmospheric water harvesting

Atmospheric water harvesting has emerged as an efficient strategy for addressing the global challenge of freshwater scarcity. However, the in being energy-consuming water-collecting process has obstructed its practicality. In this work, a soft drain bed, which was composed of hydrophilic cloth and hygroscopic gel, has been demonstrated to capture atmospheric water effectively, followed by converting it into liquid water spontaneously and sustainably, under all-weather humidity conditions. Under the optimal working condition of 30°C with a relative humidity level of 75%, the bed can provide a spontaneous water oozing ability of 1.25 g (liquid water)/hour within the 8 h of working time. More importantly, after 5 working cycles, 80% of the oozing ability can be reserved, suggesting the high potential for practical freshwater supply application. The proposed design strategy is expected to provide new hints for the development of future energy-saving decentralized freshwater supply systems.

Accelerating Anhydrous Proton Transport in Covalent Organic Frameworks: Pore Chemistry and its Impacts

Proton conduction is important in both fundamental research and technological development. Here we report designed synthesis of crystalline porous covalent organic frameworks as a new platform for high-rate anhydrous proton conduction. By developing nanochannels with different topologies as proton pathways and loading neat phosphoric acid to construct robust proton carrier networks in the pores, we found that pore topology is crucial for proton conduction. Its effect on increasing proton conductivity is in an exponential mode other than linear fashion, endowing the materials with exceptional proton conductivities exceeding 10-2 S cm-1 over a broad range of temperature and a low activation energy barrier down to 0.24 eV. Remarkably, the pore size controls conduction mechanism, where mesopores promote proton conduction via a fast-hopping mechanism, while micropores follow a sluggish vehicle process. Notably, decreasing phosphoric acid loading content drastically reduces proton conductivity and greatly increases activation energy barrier, emphasizing the pivotal role of well-developed proton carrier network in proton transport. These findings and insights unveil a new general and transformative guidance for designing porous framework materials and systems for high-rate ion conduction, energy storage, and energy conversion.

Covalent Organic Frameworks for Water Harvesting from Air

Despite approximately 70 % of the earth being covered by water, water shortage has emerged as an urgent social challenge. Sorbent-based atmospheric water harvesting stands out as a potent approach to alleviate the situation, particularly in arid regions. This method requires adsorbents with ample working capacity, rapid kinetics, low energy costs, and long-term stability under operating conditions. Covalent organic frameworks (COFs) are a novel class of crystalline porous materials and offer distinct advantages due to their high specific surface area, structural diversity, and robustness. These properties enable the rational design and customization of their water-harvesting capabilities. Herein, the basic concepts about the water sorption process within COFs, including the parameters that qualitatively or quantitatively describe their water isotherms and the mechanism are summarized. Then, the recent methods used to prepare COFs-based water harvesters are reviewed, emphasizing the structural diversity of COFs and presenting the common empirical understandings of these endeavors. Finally, challenges and research concepts are proposed to help develop next-generation COFs-based water harvesters.

High-Performance Trimethylamine Sensor Based on an Imine Covalent Organic Framework

As trimethylamine (TMA) is widely used in agriculture and industry, inhalation of TMA can cause very serious negative effects on human health. However, most of the current gas sensors for detecting TMA are commonly performed at high temperatures and cannot meet market needs. Inspired by this, we prepared imine covalent organic frameworks (TB-COF) synthesized from two monomers, 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 1,3,5-benzotricarboxaldehyde (BTCA), using acetic acid as a catalyst at room temperature. Based on this, three sensors were prepared for gas sensitivity testing, namely, TA, BT, and TB-COF sensors. The three sensors were tested for 15 different gases at room temperature. From the whole gas sensitivity data, the TB-COF sensor made by compositing TA and BT has a higher sensitivity (6845.9%) to TMA at 500 ppm, which is 6.1 and 5.4 times higher than the response of TA and BT sensors, respectively. The TB-COF sensor adsorbs and desorbs TMA in a controlled 23 s cycle with a low detection limit of 28.6 ppb. This result indicates that TB-COF prepared at room temperature can be used as a gas-sensitive sensing material for real-time monitoring of TMA. The gas sensing results demonstrate the great potential of COFs for sensor development and application and provide ideas for further development of COFs-based gas sensors.

Probing Self-Diffusion of Guest Molecules in a Covalent Organic Framework: Simulation and Experiment

Covalent organic frameworks (COFs) are a class of porous materials whose sorption properties have so far been studied primarily by physisorption. Quantifying the self-diffusion of guest molecules inside their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus essential for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance measurements and molecular dynamics simulations, we have studied the self-diffusion of acetonitrile and chloroform in the 1D pore channels of two imine-linked COFs (PI-3-COF) with different levels of crystallinity and porosity. The higher crystallinity and porosity sample exhibited anisotropic diffusion for MeCN parallel to the pore direction, with a diffusion coefficient of Dpar = 6.1(3) × 10-10 m2 s-1 at 300 K, indicating 1D transport and a 7.4-fold reduction in self-diffusion compared to the bulk liquid. This finding aligns with molecular dynamics simulations predicting 5.4-fold reduction, assuming an offset-stacked COF layer arrangement. In the low-porosity sample, more frequent diffusion barriers result in isotropic, yet significantly reduced diffusivities (DB = 1.4(1) × 10-11 m2 s-1). Diffusion coefficients for chloroform at 300 K in the pores of the high- (Dpar = 1.1(2) × 10-10 m2 s-1) and low-porosity (DB = 4.5(1) × 10-12 m2 s-1) samples reproduce these trends. Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species while suggesting efficient intracrystalline transport at short diffusion times.

A Sulfonated Covalent Organic Framework for Atmospheric Water Harvesting

We report a sulfonated covalent organic framework (COF) capable of atmospheric water harvesting in arid conditions. The isothermal water uptake profile of the framework was studied, and the network displayed steep water sorption at low relative humidity (RH) in temperatures of up to 45 °C, reaching a water uptake of 0.12 g g-1 at 10 % RH and even 0.08 g g-1 at just 5 % RH, representing some of the most extreme conditions on the planet. We found that the inclusion of sulfonate moieties shifted uptake in the water isotherm profiles to lower RH compared to non-sulfonated equivalents, demonstrating well the benefits of including these hydrophilic sites for water uptake in hot, arid locations. Repeated uptake and desorption cycles were performed on the material without significant detriment to its adsorption performance, demonstrating the potential of the sulfonated COF for real-world implementation.

Solvent Effects on Metal-free Covalent Organic Frameworks in Oxygen Reduction Reaction

Binding water molecules to polar sites in covalent organic frameworks (COFs) is inevitable, but the corresponding solvent effects in electrocatalytic process have been largely overlooked. Herein, we investigate the solvent effects on COFs for catalyzing the oxygen reduction reaction (ORR). Our designed COFs incorporated different kinds of nitrogen atoms (imine N, pyridine N, and phenazine N), enabling tunable interactions with water molecules. These interactions play a crucial role in modulating electronic states and altering the catalytic centers within the COFs. Among the synthesized COFs, the one with pyridine N atoms exhibits the highest activity, with characterized by a half-wave potential of 0.78 V and a mass activity of 0.32 A mg-1, which surpass those from other metal-free COFs. Theoretical calculations further reveal that the enhanced activity can be attributed to the stronger binding ability of *OOH intermediates to the carbon atoms adjacent to the pyridine N sites. This work sheds light on the significance of considering solvent effects on COFs in electrocatalytic systems, providing valuable insights into their design and optimization for improved performance.

Boosting CO2 Photoreduction to Formate or CO with High Selectivity over a Covalent Organic Framework Covalently Anchored on Graphene Oxide

Covalent organic frameworks (COFs) have been widely studied in photocatalytic CO2 reduction reaction (CO2 RR). However, pristine COFs usually exhibit low catalytic efficiency owing to the fast recombination of photogenerated electrons and holes. In this study, we fabricated a stable COF-based composite (GO-COF-366-Co) by covalently anchoring COF-366-Co on the surface of graphene oxide (GO) for the photocatalytic CO2 reduction. Interestingly, in absolute acetonitrile (CH3 CN), GO-COF-366-Co shows a high selectivity of 94.4 % for the photoreduction of CO2 to formate, with a formate yield of 15.8 mmol/g, which is approximately four times higher than that using the pristine COF-366-Co. By contrast, in CH3 CN/H2 O (v : v=4 : 1), the main product for the photocatalytic CO2 reduction over GO-COF-366-Co is CO (96.1 %), with a CO yield as high as 52.2 mmol/g, which is also approximately four times higher than that using the pristine COF-366-Co. Photoelectrochemical experiments demonstrate the covalent bonding of COF-366-Co and GO to form the GO-COF-366-Co composite facilitates charge separation and transfer significantly, thereby accounting for the enhanced catalytic activity. In addition, theoretical calculations and in situ Fourier transform infrared spectroscopy reveal H2 O can stabilize the *COOH intermediate to further form a *CO intermediate via O-H(aq)⋅⋅⋅O(*COOH) hydrogen bonding, thus explaining the regulated photocatalytic performance.

Vascular Bundle for Exceptional Water Confinement, Transport, and Evaporation

Nature, through billions of years of evolution, has constructed extremely efficient biosystems for transporting, confining, and vaporizing water. Mankind's ability to master water, however, is far from impeccable, and a sustainable supply of clean fresh water remains a global challenge. Here, we learn from Nature and prepare papyrus carbon (PC) from Egyptian papyrus paper as a sustainable solar desalination material. By taking advantage of the capillary pores from vascular bundles that are inherently built for transporting water in plants, PC achieves an evaporation rate of 4.1 kg m-2 h-1 in a passive single-stage device. Raman spectroscopy and thermal calorimetry show that the capillary pores pose a confinement effect to generate loosely hydrogen-bonded intermediate water, which substantially reduces the enthalpy of vaporization, allowing for exceptionally high energy efficiencies. The understanding is applicable to all nature-designed vascular plants and man-made separation and purification systems.

Smart Home Sleep Respiratory Monitoring System Based on a Breath-Responsive Covalent Organic Framework

A smart home sleep respiratory monitoring system based on a breath-responsive covalent organic framework (COF) was developed and utilized to monitor the sleep respiratory behavior of real sleep apnea patients in this work. The capacitance of the interdigital electrode chip coated with COFTPDA-TFPy exhibits thousands-level reversible responses to breath humidity gases, with subsecond response time and robustness against environmental humidity. A miniaturized printed circuit board, an open-face-mask-based respiratory sensor, and a smartphone app were constructed for the wearable wireless smart home sleep respiratory monitoring system. Leveraging the sensitive and rapid reversible response of COFs, the COF-based respiratory monitoring system can effectively record normal breath, rapid breath, and breath apnea, enabling over a thousand cycles of hour-level continuous monitoring during daily wear. Next, we took the groundbreaking step of advancing the humidity sensor to the clinical trial stage. In clinical experiments on real sleep apnea patients, the COF-based respiratory monitoring system successfully recorded hour-level sleep respiratory data and differentiated the breathing behavior characteristics and severity of sleep apnea patients and subjects with normal sleep function and primary snoring patients. This work successfully advanced humidity sensors into clinical research for real patients and demonstrated the enormous application potential of COF materials in clinical diagnosis.

Covalent Organic Frameworks: Reversible 3D Coalesce via Interlocked Skeleton-Pore Actions and Impacts on π Electronic Structures

Covalent organic frameworks (COFs) create extended two-dimensional (2D) skeletons and aligned one-dimensional (1D) channels, constituting a class of novel π architectures with predesignable structural ordering. A distinct feature is that stacks of π building units in skeletons shape the pore walls, onto which a diversity of different units can be assembled to form various pore interfaces, opening a great potential to trigger a strong structural correlation between the skeleton and the pore. However, such a possibility has not yet been explored. Herein, we report reversible three-dimensional (3D) coalescence and interlocked actions between the skeleton and pore in COFs by controlling hydrogen-bonding networks in the pores. Introducing carboxylic acid units to the pore walls develops COFs that can confine water molecular networks, which are locked by the surface carboxylic acid units on the pore walls via multipoint, multichain, and multidirectional hydrogen-bonding interactions. As a result, the skeleton undergoes an interlocked action with pores to shrink over the x-y plane and to stack closer along the z direction upon water uptake. Remarkably, this interlocked action between the skeleton and pore is reversibly driven by water adsorption and desorption and triggers profound effects on π electronic structures and functions, including band gap, light absorption, and emission.

ChatGPT Research Group for Optimizing the Crystallinity of MOFs and COFs

We leveraged the power of ChatGPT and Bayesian optimization in the development of a multi-AI-driven system, backed by seven large language model-based assistants and equipped with machine learning algorithms, that seamlessly orchestrates a multitude of research aspects in a chemistry laboratory (termed the ChatGPT Research Group). Our approach accelerated the discovery of optimal microwave synthesis conditions, enhancing the crystallinity of MOF-321, MOF-322, and COF-323 and achieving the desired porosity and water capacity. In this system, human researchers gained assistance from these diverse AI collaborators, each with a unique role within the laboratory environment, spanning strategy planning, literature search, coding, robotic operation, labware design, safety inspection, and data analysis. Such a comprehensive approach enables a single researcher working in concert with AI to achieve productivity levels analogous to those of an entire traditional scientific team. Furthermore, by reducing human biases in screening experimental conditions and deftly balancing the exploration and exploitation of synthesis parameters, our Bayesian search approach precisely zeroed in on optimal synthesis conditions from a pool of 6 million within a significantly shortened time scale. This work serves as a compelling proof of concept for an AI-driven revolution in the chemistry laboratory, painting a future where AI becomes an efficient collaborator, liberating us from routine tasks to focus on pushing the boundaries of innovation.

Carborane-Based Three-Dimensional Covalent Organic Frameworks

The predesignable porous structure and high structural flexibility of covalent organic frameworks (COFs) render this material desirable as a platform for addressing various cutting-edge issues. Precise control over their composition, topological structure, porosity, and stability to realize tailor-made functionality still remains a great challenge. In this work, we developed a new kind of three-dimensional (3D) carborane-based COF with a 7-fold interpenetrating dia topological diagram. The resulting COFs exhibited high crystallinity, exceptional porosity, and strong robustness. The slightly lower electronegativity of boron (2.04) than that of hydrogen (2.20) can lead to the polarization of the B-H bond into a Bδ+-Hδ- mode, which renders these COFs as high-performance materials for the adsorption and separation of hexane isomers through the B-Hδ-···Hδ+-C interaction. Significantly, the carborane content of obtained COFs reached up to 54.2 wt %, which gets the highest rank among all the reported porous materials. Combining high surface area, strong robustness, and high content of carborane, the obtained COFs can work as efficient adsorbents for the separation of the five hexane isomers with high separation factors. This work not only enhances the diversity of 3D functional COFs but also constitutes a further step toward the efficient separation of alkane isomers.

Regioselective One-Step Cyclization and Aromatization towards Directly Amino-Functionalized Covalent Organic Framework with Stable Benzodiimidazole Linkage

The compatibility of crystallinity, stability, and functionality in covalent organic frameworks (COFs) is challenging but significant in reticular chemistry and materials science. Herein, it is presented for the first time a strategy to synthesize directly amino-functionalized COF with stable benzodiimidazole linkage by regioselective one-step cyclization and aromatization. Bandrowski's base with two types of amino groups is used as a unique monomer, providing not only construction sites for the material framework through specific region-selective reaction, but also amino active sites for functionality, which is usually difficult to achieve directly in COF synthesis because amino groups are the participants in COF bonding. In addition, the aromatic benzodiimidazole rings and the large conjugated system of the product effectively improve the crystallinity and stability, so that the as-prepared BBCOF remains unchanged in both acid and base solutions, which is obviously better than the conventional imine-linked COF. Impressively, the significantly enhanced conjugation degree by the benzodiimidazole structure also endows BBCOF with an efficient photocatalytic reduction of uranyl ion, with removal rate as high as 96.6% in single-ion system and 95% in multi-ion system. This study is of great importance to the design and synthesis of functional COFs with a commendable trade-off among crystallinity, stability, and functionality.

Nanoconfined Water Clusters in Zinc White Oil Paint

Pigments in oil paint are bound by a complex oil polymer network that is prone to water-related chemical degradation. We use cryo-Fourier-transform infrared spectroscopy and differential scanning calorimetry to study how water distributes inside zinc white oil paint. By measuring water freezing and melting transitions, we show that water-saturated zinc white oil paint contains both liquid-like clustered water and nonclustered water. A comparison of titanium white paint and nonpigmented model systems indicates that water clustering happens near the pigment-polymer interface. The cluster size was estimated in the nanometer range based on the ice melting and freezing temperatures and on the position of the O-D vibration band. As liquid-like water can play a crucial role in the dissolution and transport of ions and molecules, understanding the factors that favor this phenomenon is essential for establishing safe conditions for the conservation of painted works of art.

Monomer Symmetry-Regulated Defect Engineering: In Situ Preparation of Functionalized Covalent Organic Frameworks for Highly Efficient Capture and Separation of Carbon Dioxide

Developing crystalline porous materials with highly efficient CO₂ selective adsorption capacity is one of the key challenges to carbon capture and storage (CCS). In current studies, much more attention has been paid to the crystalline and porous properties of crystalline porous materials for CCS, while the defects, which are unavoidable and ubiquitous, are relatively neglected. Herein, for the first time, we propose a monomer-symmetry regulation strategy for directional defect release to achieve in situ functionalization of COFs while exposing uniformly distributed defect-aldehyde groups as functionalization sites for selective CO₂ capture. The regulated defective COFs possess high crystallinity, good structural stability, and a large number of organized and functionalized aldehyde sites, which exhibit one of the highest selective separation values of all COF sorbing materials in CO₂/N₂ selective adsorption (128.9 cm³/g at 273 K and 1 bar, selectivity: 45.8 from IAST). This work not only provides a new strategy for defect regulation and in situ functionalization of COFs but also provides a valuable approach in the design and preparation of new adsorbents for CO₂ adsorption and CO₂/N₂ selective separation.

Covalent Organic Frameworks for Atmospheric Water Harvesting

Atmospheric water harvesting using reticular materials is an innovation that has the potential to change the world. Using covalent organic frameworks (COFs) for capturing water holds great promise because COFs are metal-free, stable under working conditions, and their structure can be intentionally designed to meet the requirements for this application. To promote the chemistry and use of COFs for atmospheric water harvesting, important features for synthesizing suitable water-harvesting COFs are discussed. The achievements of using COFs as water harvesters are then highlighted, showing how the water harvesting properties are related to the structural design. Finally, perspectives and research directions for further studies in COF chemistry are provided.

Au Nanoparticles Functionalized Covalent-Organic-Framework-Based Electrochemical Sensor for Sensitive Detection of Ractopamine

Ractopamine, as a feed additive, has attracted much attention due to its excessive use, leading to the damage of the human nervous system and physiological function. Therefore, it is of great practical significance to establish a rapid and effective method for the detection of ractopamine in food. Electrochemical sensors served as a promising technique for efficiently sensing food contaminants due to their low cost, sensitive response and simple operation. In this study, an electrochemical sensor for ractopamine detection based on Au nanoparticles functionalized covalent organic frameworks (AuNPs@COFs) was constructed. The AuNPs@COF nanocomposite was synthesized by in situ reduction and was characterized by FTIR spectroscopy, transmission electron microscope and electrochemical methods. The electrochemical sensing performance of AuNPs@COF-modified glassy carbon electrode for ractopamine was investigated using the electrochemical method. The proposed sensor exhibited excellent sensing abilities towards ractopamine and was used for the detection of ractopamine in meat samples. The results showed that this method has high sensitivity and good reliability for the detection of ractopamine. The linear range was 1.2-1600 μmol/L, and the limit of detection (LOD) was 0.12 μmol/L. It is expected that the proposed AuNPs@COF nanocomposites hold great promise for food safety sensing and should be extended for application in other related fields.

A Highly Stable Ortho-Ketoenamine Covalent Organic Framework with Balanced Hydrophilic and Hydrophobic Sites for Atmospheric Water Harvesting

Atmospheric moisture is a sustainable clean water source that can solve the shortage of fresh water in arid areas. Herein a 2D covalent organic framework (COF-ok) was reported as a promising porous sorbent for solar-driven atmospheric water harvesting. COF-ok with ortho-ketoenamine linkage was extremely stable in harsh environment, including in boiling water, strong acids and bases. Because of the balanced hydrophilic and hydrophobic sites in channels, COF-ok showed a high water uptake of 0.33 g g^-1 at a low relative humidity of 34 % featuring a characteristic S-shaped water sorption isotherm with low regeneration temperature (∼45 °C) and excellent cyclic stability. A laboratory-scale water harvesting system could collect water of 161 g kg^-1 under sunlight.