Isolation of the Secondary Building Unit of a 3D Metal-Organic Framework through Clip-Off Chemistry, and Its Reuse To Synthesize New Frameworks by Dynamic Covalent Chemistry

Herein, we present a novel methodology for synthesizing metal clusters or secondary building units (SBUs) that are subsequently employed to construct innovative metal-organic frameworks (MOFs) via dynamic covalent chemistry. Our approach entails extraction of SBUs from preformed MOFs through complete disassembly by clip-off chemistry. The initial MOF precursor is designed to incorporate the desired SBU, connected exclusively by cleavable linkers (in this study, with olefinic bonds). Cleavage of all the organic linkers (in this study, via ozonolysis under reductive conditions) liberates the SBUs functionalized with aldehyde groups. Once synthesized, these SBUs can be further reacted with amines in dynamic covalent chemistry to build new, rationally designed MOFs.

Covalent organic polyrotaxanes based on β-cyclodextrin for iodine capture

Herein, covalent organic polyrotaxanes (COPRs) were integrated with supermolecule self-assembly and dynamic imine bond formation to act as absorbents that captured radioactive iodine from water. The aromatic building blocks were initially complexed with β-cyclodextrin (β-CD) to form pseudorotaxanes, which were then condensed with aromatic tri-aldehyde via mechanical grinding and solvothermal synthesis in sequence. The threading of β-CD throughout the polymer skeleton effectively reduced the usage of expensive building blocks and significantly lowered the cost, while also remarkably enhancing the skeleton polarity, which is closely related to many special applications. Impressively, the threading of CD improved the water dispersibility of COPRs, which displayed an abnormally high iodine adsorption capacity. This novel synthetic strategy allows the incorporation of mechanically interlocked CDs into porous polymeric materials, which provides access to low-cost preparations of COPRs with a brand new structure for specific applications.

Continuous Synthesis and Processing of Covalent Organic Frameworks in a Flow Reactor

Covalent organic frameworks (COFs) are typically prepared in the form of insoluble microcrystalline powders using batch solvothermal reactions that are energy-intensive and require long annealing periods (>120 °C, >72 h). Thus, their wide-scale adoption in a variety of potential applications is impeded by complications related to synthesis, upscaling, and processing, which also compromise their commercialization. Here we report a strategy to address both the need for scalable synthesis and processing approaches through the continuous, accelerated synthesis, and processing of imine- and hydrazone-linked COFs using a flow microreactor. The flow microreactor is capable of unprecedented COF productivities, up to 61,111 kg m-3 day-1, and provides control over key stages of COF formation, including nanoparticle growth, self-assembly, and precipitation. Additionally, the technique successfully yields highly crystalline and porous COFs in versatile macroscopic structures such as monoliths, membranes, prints, and packed beds. We also show that a COF synthesized using the flow microreactor acts as an excellent photocatalyst for the photocatalytic degradation of perfluorooctanoic acid (PFOA) outperforming the degradation efficiency of its batch analogue and other classical photocatalysts such as titanium dioxide (TiO2).

Interpenetrated Donor-Acceptor Heterojunctions in 2D Conjugated Dibenzo[g,p]chrysene-Based Kagome Covalent Organic Frameworks

Dibenzo[g,p]chrysene can be viewed as a constrained propeller-shaped tetraphenylethylene with reduced curvature and has been utilized to construct dual-pore kagome covalent organic frameworks (COFs) with tightly packed two-dimensional (2D) layers owing to its rigid and more planar structural characteristics. Here, we introduce 2D COFs based on the node 4,4',4″,4‴-(dibenzo[g,p]chrysene-2,7,10,15-tetraphenyl)tetraamine (DBCTPTA) featuring extended conjugation compared to the dibenzo[g,p]chrysene-3,6,11,14-tetraamine (DBCTA) node. We establish two exceptionally crystalline imine-linked 2D COFs with a hexagonal dual-pore kagome structure based on the DBCTPTA core. The newly synthesized thienothiophene (TT) and benzodithiophene (BDT)-based DBCTPTA COFs show a tight stacking behavior between adjacent layers. Furthermore, we obtained an unprecedented, interpenetrated electron-donor/acceptor host-guest system with an electron-donating BDT DBCTPTA COF synthesized in situ with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) serving as molecular acceptor. The BDT DBCTPTA COF@PCBM film shows a much shorter amplitude-averaged PL lifetime of 7 ± 2 ps compared to 30 ± 4 ps of the BDT DBCTPTA COF film, indicating the light-induced charge transfer process. The successful in situ formation of interpenetrated donor-acceptor heterojunctions within 2D COFs offers a promising strategy for establishing D-A heterojunctions in diverse framework materials with open channel systems.

Covalent organic frameworks in tribology - A perspective

Two-dimensional covalent organic frameworks (2D COFs) are an emerging class of crystalline porous materials formed through covalent bonds between organic building blocks. COFs uniquely combine a large surface area, an excellent stability, numerous abundant active sites, and tunable functionalities, thus making them highly attractive for numerous applications. Especially, their abundant active sites and weak interlayer interaction make these materials promising candidates for tribological research. Recently, notable attention has been paid to COFs as lubricant additives due to their excellent tribological performance. Our review aims at critically summarizing the state-of-art developments of 2D COFs in tribology. We discuss their structural and functional design principles, as well as synthetic strategies with a special focus on tribology. The generation of COF thin films is also assessed in detail, which can alleviate their most challenging drawbacks for this application. Subsequently, we analyze the existing state-of-the-art regarding the usage of COFs as lubricant additives, self-lubrication composite coatings, and solid lubricants at the nanoscale. Finally, critical challenges and future trends of 2D COFs in tribology are outlined to initiate and boost new research activities in this exciting field.

Comparative study of metal-organic frameworks synthesized via imide condensation and coordination assembly

A series of metal-organic frameworks (1-XDI) have been synthesized by imide condensation reactions between an amine-functionalized pentanuclear zinc cluster, Zn4Cl5(bt-NH2)6, (bt-NH2 = 5-aminobenzotriazolate), and organic dianhydrides (pyromellitic dianhydride (PMDA), naphthalenetetracarboxylic dianhydride (NDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (HFIPA)). The properties of the 1-XDI MOFs have been compared with analogues (2-XDI) prepared using traditional coordination assembly. The resulting materials have been characterized by ATR-IR spectroscopy, acid-digested 1H NMR spectroscopy, elemental analysis, and gas adsorption measurements. N2 adsorption isotherm data reveal modest porosities and BET surface areas (30-552 m2 g-1). All of the new 1-XDI and 2-XDI MOFs show selective adsorption of C2H2 over CO2 while 2-PMDI and 2-BPDI exhibit high selectivity toward C3H6/C3H8 separation. This study establishes imide condensation of preformed metal-organic clusters with organic linkers as a viable route for MOF design.

TpBD/UiO-66-NH2 micro-mesoporous hybrid material as a stationary phase for open tubular capillary electrochromatography

The excellent stability of covalent organic frameworks (COFs) and the diversity of metal organic frameworks (MOFs) make MOF/COF hybrid materials promising candidates for chromatographic stationary phases. In this paper, a TpBD/UiO-66-NH2 hybrid material was synthesized through a Schiff-base reaction between TpBD COFs and UiO-66-NH2 MOFs; characterized using Fourier-transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy; and bonded to a capillary to prepare a TpBD/UiO-66-NH2-bonded open tubular capillary electrochromatography (OT-CEC) column. Results suggested that the hybrid material had the crystal morphology of a single COF and MOF, a micro-mesoporous structure, and good thermal stability. The inner surface of the OT-CEC column was tightly and uniformly distributed with the stationary phase (∼1.5 μm). The baseline separation of 13 amino acids and three families (4 acidic antibiotics, 4 preservatives and 6 sulfonamides) of emerging pollutant mixtures was achieved due to the synergistic effect of TpBD and UiO-66-NH2 in the stationary phase. The OT-CEC column showed good reproducibility and stability with relative standard deviations of migration time and resolutions in the range of 1.17-3.93% and 1.79-4.31%, respectively.

Self-Exfoliating Benzotristriazine Macrocyclic Network: A New 2D Material for High-Performance Electrochemical Energy Storage

Aza-fused aromatic π-conjugated networks are an important class of 2D graphitic analogs, which are generally constructed using aromatic precursors. Herein, the study describes a new synthetic approach and electrochemical properties of a self-exfoliating benzotristriazine 2D network (BTTN) constructed using aliphatic precursors, under relatively mild conditions. The obtained BTTN exhibits a nanodisc-like morphology, the self-exfoliation tendency of which is ascribed to the presence of structurally different macrocycles with high electronic repulsion between the layers. The benzotristriazine repeat units of BTTN is electroactive and holds higher carbon/nitrogen ratio when compared with the conventional graphitic aza-fused π-conjugated networks. The self-exfoliated BTTN nanodiscs show excellent electrochemical energy storage of 485 and 333 F g-1 at 1 A g-1 in three-electrode and two-electrode measurements, respectively. BTTN in a symmetric coin-cell architecture exhibits a high specific energy value of 46 Wh kg-1 at a power density of 1 kW kg-1 and shows excellent cyclic stability of 96% for 10 000 and 90% for 30 000 charge-discharge cycles at a higher current density of 5 A g-1, surpassing the device performance of most of the reported all-organic pseudocapacitive 2D networks.

Linkage-Mixed Covalent Organic Frameworks Synthesized by a Liquid-Solid Two-Phase Strategy for Photoenhanced Uranium Extraction

Synthesizing COFs with hybrid linkage coupling with both reversible and irreversible natures remains a challenging issue. Herein, we report the synthesis of two rare COFs constructed by both reversible and irreversible linkages through a liquid-solid two-phase strategy. A systematic study reveals a one-pot, two-step reaction mechanism for the two COFs, the first step being a reversible Schiff base reaction and the second step being an irreversible Knoevenagel reaction. Interestingly, this hybrid linkage COF is found to show an outstanding photoenhanced uranium extraction performance. The results not only provide a general and green approach to develop the linkage chemistry of COFs but also enrich the synthesis toolboxes and application of COFs.

Enhancing structural control in covalent organic frameworks through steric interaction-driven linker design

Covalent Organic Frameworks (COFs) exhibiting kagome (kgm) structures are promising crystalline porous materials with two distinct pores. However, there are no reliable synthetic methods to exclusively target the kgm over the polymorphic square-lattice (sql) structure. To address this, we introduce a linker design strategy featuring bulky functional groups, which through steric interactions can hinder the sql net formation, thereby leading to a kgm structure. By rigid attachment of the methyl benzoate groups to a tetradentate COF linker, steric interactions with neighbouring linkers depending on the pore size become possible. The steric interaction was tuned by varying the complementary bidentate linear linker lengths, where the shorter phenylenediamine linker leads to steric hindrance and the formation of the kgm lattice, while with the longer benzidine linker, steric interaction is reduced leading to the sql lattice. Thus, control over the net can be exerted through steric interaction strengths. Additionally, structural analysis revealed the formation of the kgm COF with an unusual ABC stacking, leading to pearl string type pores instead of two distinct pore sizes. This COF system shows that steric interaction-driven design enhances control over COF structures, expanding the design toolbox, but also provides valuable insights into network formation and polymorphism.

Ion transport mechanisms in covalent organic frameworks: implications for technology

Covalent organic frameworks (COFs) have emerged as promising materials for ion conduction due to their highly tunable structures and excellent electrochemical stability. This review paper explores the mechanisms of ion conduction in COFs, focusing on how these materials facilitate ion transport across their ordered structures, which is crucial for applications such as solid electrolytes in batteries and fuel cells. We discuss the design strategies employed to enhance ion conductivity, including pore size optimization, functionalization with ionic groups, and the incorporation of solvent molecules and salts. Additionally, we examine the various applications of ion-conductive COFs, particularly in energy storage and conversion technologies, highlighting recent advancements and future directions in this field. This review paper aims to provide a comprehensive overview of the current state of research on ion-conductive COFs, offering insights into their potential to design highly ion-conductive COFs considering not only fundamental studies but also practical perspectives for advanced electrochemical devices.

Catalyst-Free Dynamic Covalent C=C/C=N Metathesis Reaction for Associative Covalent Adaptable Networks

Covalent adaptable networks (CANs), leveraging the dynamic exchange of covalent bonds, emerge as a promising material to address the challenge of irreversible cross-linking in thermosetting polymers. In this work, we explore the introduction of a catalyst-free and associative C=C/C=N metathesis reaction into thermosetting polyurethanes, creating CANs with superior stability, solvent resistance, and thermal/mechanical properties. By incorporating this dynamic exchange reaction, stress-relaxation is significantly accelerated compared to imine-bond-only networks, with the rate adjustable by modifying substituents in the ortho position of the dynamic double bonds. The obtained plasticity enables recycle without altering the chemical structure or mechanical properties, and is also found to be vital for achieving shape memory functions with complex spatial structures. This metathesis reaction as a new dynamic crosslinker of polymer networks has the potential to accelerate the ongoing exploration of malleable and functional thermoset polymers.

Unraveling Early-Stage Dynamics of Cage-to-Covalent Organic Framework Transformation at Liquid-Liquid Interface

Postsynthetic linker exchange (PLE) has emerged as an emerging synthetic strategy for constructing high-quality covalent organic frameworks (COFs) from preassembled entities such as linear polymers, amorphous networks, COFs, and porous organic cages by using the principles of dynamic covalent chemistry. The PLE strategy has recently been extended at the liquid-liquid interface to fabricate highly crystalline two-dimensional (2D)-COF membranes at a faster time scale (24 h). Examining the early stages of the interfacial PLE dynamics becomes essential to understanding the expedited COF growth process. In this regard, pendant drop tensiometry has been employed to probe the initial reaction dynamics of the imine cage-to-COF transformation through dynamic interfacial tension (IFT) measurements. The contrasting trends in IFT profiles between PLE-mediated (from cage) and direct COF synthesis (from parent monomers) are in qualitative agreement with the kinetics of bulk-scale interfacial polymerizations. Further, the distinct early-stage interfacial behaviors between the diverse synthetic routes have been experimentally demonstrated using tensiometry, optical microscopy, electron microscopy, and powder X-ray diffraction (PXRD) analysis. The pivotal role of in situ generated imine intermediates (ImIs) and the phenomenon of spontaneous emulsification toward accelerated interfacial COF growth process was delineated. The current study on deploying the pendant drop tensiometric technique to examine early-stage interfacial polymerization dynamics opens up a gripping avenue for mechanistic exploration in PLE-based COF synthesis. The generality of the developed methodology to study the initial COF growth kinetics was extended to a new interfacial PLE-mediated cage-to-COF transformation.

Hierarchical single-crystal-to-single-crystal transformations of a monomer to a 1D-polymer and then to a 2D-polymer

Designing and synthesizing flawless two-dimensional polymers (2D-Ps) via meticulous molecular preorganization presents an intriguing yet challenging frontier in research. We report here the single-crystal-to-single-crystal (SCSC) synthesis of a 2D-P via thermally induced topochemical azide-alkyne cycloaddition (TAAC) reaction. A designed monomer incorporating two azide and two alkyne units is synthesized. The azide and alkyne groups are preorganized in the monomer crystal in reactive geometries for polymerizations in two orthogonal directions. On heating, the polymerizations proceed in a hierarchical manner; at first, the monomer reacts regiospecifically in a SCSC fashion to form a 1,5-triazolyl-linked 1D polymer (1D-P), which upon further heating undergoes another SCSC polymerization to a 2D-P through a second regiospecific TAAC reaction forming 1,4-triazolyl-linkages. Two different linkages in orthogonal directions make this an architecturally attractive 2D-P, as determined, at atomic resolution, by single-crystal X-ray diffraction. The 2D-P reported here is thermally stable in view of the robust triazole-linkages and can be exfoliated as 2D-sheets.

Engineering the physical properties and photocatalytic activities of a β-ketoenamine COF using continuous flow synthesis

Covalent Organic Frameworks (COF) having conjugated backbone are an interesting class of metal-free, visible light active, heterogeneous photocatalysts. Interestingly, synthesis of COF using continuous flow process has emerged as an efficient, alternative method when compared to the traditional batch process. Here, we demonstrate the possibility to engineer the physical properties and hence the adsorption and catalytic activities of a β-ketoenamine COF by varying monomer flow rate and microreactor design during the continuous flow synthesis. Crystallinity of the COF increases on varying the monomer flow rate from 100 (S-100) to 500 (S-500) and up to 1000 μLmin-1 (S-1000), in an S-shaped microreactor, resulting in an enhanced surface area: 525, 722 and 1119 m2g-1 respectively. The photophysical properties of the COF are also found to vary significantly with the change in flow synthesis conditions. S-1000 is characterized by the highest adsorption of MB, due to its high surface area and accessible pores. On the other hand, S-500 shows the highest photocurrent, a low recombination of photogenerated charges and the lowest charge transfer resistance. Thus, S-500 is found to be the best photocatalyst for the removal of a model pollutant (methylene blue, MB). Further, enhanced photocatalytic removal of MB using S-500 could be achieved by performing the photocatalysis in continuous flow.

An Alkyne-Bridged Covalent Organic Framework Featuring Interactive Pockets for Bromine Capture

The high degree of corrosivity and reactivity of bromine, which is released from various sources, poses a serious threat to the environment. Moreover, its coexistence with iodine forming an equilibrium compound, iodine monobromide (IBr) necessitates the selective capture of bromine from halogen mixtures. The electrophilicity of halogens to π-electron rich structures enabled us to strategically design a covalent organic framework for halogen capture, featuring a defined pore environment with localized sorption sites. The higher capture capacity of bromine (4.6 g g-1) over iodine by ~41 % shows its potential in selective capture. Spectroscopic results uncovering the preferential interaction sites are supported by theoretical investigations. The alkyne bridge is a core functionality promoting the selectivity in capture by synergistic physisorption, rationalized by the higher orbital overlap of bromine due to its smaller atomic size as well as reversible chemical interactions. The slip stacking in the structure has further promoted this phenomenon by creating clusters of molecular interaction sites with bromine intercalated between the layers. The inclusion of unsaturated moieties, i.e. triple bonds and the complementary pore geometry offer a promising design strategy for the construction of porous materials for halogen capture.

Enzyme Immobilization using Covalent Organic Frameworks: From Synthetic Strategy to COFs Functional Role

Enzymes, a class of biocatalysts, exhibit remarkable catalytic efficiency, specificity, and selectivity, governing many reactions that are essential for various cascades within living cells. The immobilization of structurally flexible enzymes on appropriate supports holds significant importance in facilitating biomimetic transformations in extracellular environments. Covalent organic frameworks (COFs) have emerged as ideal candidates for enzyme immobilization due to high surface tunability, diverse chemical/structural designs, exceptional stability, and metal-free nature. Various immobilization techniques have been proposed to fabricate COF-enzyme biocomposites, offering significant enhancements in activity and reusability for COF-immobilized enzymes as well as new insights into developing advanced enzyme-based applications. In this review, we provide a comprehensive overview of state-of-the-art strategies for immobilizing enzymes within COFs by focusing on their applicability and versatility. These strategies are systematically summarized and compared by categorizing them into postsynthesis immobilization and in situ immobilization, where their respective strengths and limitations are thoroughly discussed. Combined with an overview of critical emerging applications, we further elucidate the multifaceted roles of COFs in enzyme immobilization and subsequent applications, highlighting the advanced biofunctionality achievable through COFs.

Constrution of Quinazoline-Linked Covalent Organic Frameworks via a Multicomponent Reaction for Photocatalysis

Quinazoline (Qz)-linked covalent organic frameworks (COFs) have been constructed via a three-component reaction of ortho-acylanilines, benzaldehydes and NH4OAc. The structure of Qz-COFs has been confirmed by solid-state nuclear magnetic resonance spectroscopy, Fourier transform infrared and powder X-ray diffraction patterns. The Qz-COFs possess high chemical stability, showing good endurance to strong acid, strong base, oxidant, reductant and other conditions. Particularly, Qz-COF-3 can catalyze the aerobic photooxidation of toluene and other compounds containing C(sp3)-H bonds.

Chemistries and materials for atmospheric water harvesting

Atmospheric water harvesting (AWH) is recognized as a crucial strategy to address the global challenge of water scarcity by tapping into the vast reserves of atmospheric moisture for potable water supply. Within this domain, sorbents lie in the core of AWH technologies as they possess broad adaptability across a wide spectrum of humidity levels, underpinned by the cyclic sorption and desorption processes of sorbents, necessitating a multi-scale viewpoint regarding the rational material and chemical selection and design. This Invited Review delves into the essential sorption mechanisms observed across various classes of sorbent systems, emphasizing the water-sorbent interactions and the progression of water networks. A special focus is placed on the insights derived from isotherm profiles, which elucidate sorbent structures and sorption dynamics. From these foundational principles, we derive material and chemical design guidelines and identify key tuning factors from a structural-functional perspective across multiple material systems, addressing their fundamental chemistries and unique attributes. The review further navigates through system-level design considerations to optimize water production efficiency. This review aims to equip researchers in the field of AWH with a thorough understanding of the water-sorbent interactions, material design principles, and system-level considerations essential for advancing this technology.

Porous organic crystals crosslinked by free-radical reactions

Two hydrogen-bonded crosslinked organic frameworks (HCOFs) were synthesized via free radical reactions utilizing butadiene and isoprene as crosslinkers. These HCOFs exhibit high crystallinity, enabling detailed structural characterization via single-crystal X-ray diffraction analysis. Subsequently, one of the olefin-rich HCOFs was converted to a hydroxylated framework through hydroboration-oxidation while maintaining the high crystallinity.

Linkage-Mediated Electronic Structure Modulation in Multicomponent Covalent Organic Frameworks for Dramatically Promoted Photocatalytic Hydrogen Evolution

Linkage chemistry is an essential aspect to covalent organic framework (COF) applications; it is highly desirable to precisely modulate electronic structure mediated directly by linkage for efficient COF-based photocatalytic hydrogen evolution, which however, remains substantially challenging. Herein, as a proof of concept, a collection of robust multicomponent pyrene-based COFs with abundant donor-acceptor (D-A) interactions has been judiciously designed and synthesized through molecularly engineering linkage for photogeneration of hydrogen. Controlled locking and conversion of linkage critically contribute to continuously regulating COFs' electronic structures further to optimize photocatalytic activities. Remarkably, the well-modulated optoelectronic properties turn on the average hydrogen evolution rate from zero to 15.67 mmol g-1 h-1 by the protonated quinoline-linked COF decorated with the trifluoromethyl group (TT-PQCOF-CF3). Using diversified spectroscopy and theoretical calculations, we show that multiple modifications toward linkage synergistically lead to the redistribution of charge on COFs with extended π-conjugation and reinforced D-A effect, making TT-PQCOF-CF3 a promising material with significantly boosted carrier separation and migration. This study provides important guidance for the design of high-performance COF photocatalysts based on the strategy of linkage-mediated electronic structure modulation in COFs.

Enzyme-Immobilized Porous Crystals for Environmental Applications

Developing efficient technologies to eliminate or degrade contaminants is paramount for environmental protection. Biocatalytic decontamination offers distinct advantages in terms of selectivity and efficiency; however, it still remains challenging when applied in complex environmental matrices. The main challenge originates from the instability and difficult-to-separate attributes of fragile enzymes, which also results in issues of compromised activity, poor reusability, low cost-effectiveness, etc. One viable solution to harness biocatalysis in complex environments is known as enzyme immobilization, where a flexible enzyme is tightly fixed in a solid carrier. In the case where a reticular crystal is utilized as the support, it is feasible to engineer next-generation biohybrid catalysts functional in complicated environmental media. This can be interpreted by three aspects: (1) the highly crystalline skeleton can shield the immobilized enzyme against external stressors. (2) The porous network ensures the high accessibility of the interior enzyme for catalytic decontamination. And (3) the adjustable and unambiguous structure of the reticular framework favors in-depth understanding of the interfacial interaction between the framework and enzyme, which can in turn guide us in designing highly active biocomposites. This Review aims to introduce this emerging biocatalysis technology for environmental decontamination involving pollutant degradation and greenhouse gas (carbon dioxide) conversion, with emphasis on the enzyme immobilization protocols and diverse catalysis principles including single enzyme catalysis, catalysis involving enzyme cascades, and photoenzyme-coupled catalysis. Additionally, the remaining challenges and forward-looking directions in this field are discussed. We believe that this Review may offer a useful biocatalytic technology to contribute to environmental decontamination in a green and sustainable manner and will inspire more researchers at the intersection of the environment science, biochemistry, and materials science communities to co-solve environmental problems.

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.

Microwave Assisted Fast Synthesis of a Donor-Acceptor COF Towards Photooxidative Amidation Catalysis

The synthesis of covalent organic frameworks (COFs) at bulk scale require robust, straightforward, and cost-effective techniques. However, the traditional solvothermal synthetic methods of COFs suffer low scalability as well as requirement of sensitive reaction environment and multiday reaction time (2-10 days) which greatly restricts their practical application. Here, we report microwave assisted rapid and optimized synthesis of a donor-acceptor (D-A) based highly crystalline COF, TzPm-COF in second (10 sec) to minute (10 min) time scale. With increasing the reaction time from seconds to minutes crystallinity, porosity and morphological changes are observed for TzPm-COF. Owing to visible range light absorption, suitable band alignment, and low exciton binding energy (Eb=64.6 meV), TzPm-COF can efficaciously produce superoxide radical anion (O2 .-) after activating molecular oxygen (O2) which eventually drives aerobic photooxidative amidation reaction with high recyclability. This photocatalytic approach works well with a variety of substituted aromatic aldehydes having electron-withdrawing or donating groups and cyclic, acyclic, primary or secondary amines with moderate to high yield. Furthermore, catalytic mechanism was established by monitoring the real-time reaction progress through in situ diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) study.

Analysis of COF-300 synthesis: probing degradation processes and 3D electron diffraction structure

Although COF-300 is often used as an example to study the synthesis and structure of (3D) covalent organic frameworks (COFs), knowledge of the underlying synthetic processes is still fragmented. Here, an optimized synthetic procedure based on a combination of linker protection and modulation was applied. Using this approach, the influence of time and temperature on the synthesis of COF-300 was studied. Synthesis times that were too short produced materials with limited crystallinity and porosity, lacking the typical pore flexibility associated with COF-300. On the other hand, synthesis times that were too long could be characterized by loss of crystallinity and pore order by degradation of the tetrakis(4-aminophenyl)methane (TAM) linker used. The presence of the degradation product was confirmed by visual inspection, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As TAM is by far the most popular linker for the synthesis of 3D COFs, this degradation process might be one of the reasons why the development of 3D COFs is still lagging compared with 2D COFs. However, COF crystals obtained via an optimized procedure could be structurally probed using 3D electron diffraction (3DED). The 3DED analysis resulted in a full structure determination of COF-300 at atomic resolution with satisfying data parameters. Comparison of our 3DED-derived structural model with previously reported single-crystal X-ray diffraction data for this material, as well as parameters derived from the Cambridge Structural Database, demonstrates the high accuracy of the 3DED method for structure determination. This validation might accelerate the exploitation of 3DED as a structure determination technique for COFs and other porous materials.

Rationally Designed Benzobisthiadiazole-Based Covalent Organic Framework for High-Performance NIR-II Fluorescence Imaging-Guided Photodynamic Therapy

Although being applied as photosensitizers for photodynamic therapy, covalent organic frameworks (COFs) fail the precise fluorescence imaging in vivo and phototherapy in deep-tissue, due to short excitation/emission wavelengths. Herein, this work proposes the first example of NIR-II emissive and benzobisthiadiazole-based COF-980. Comparing to its ligands, the structure of COF-980 can more efficiently reducing the energy gap (ΔES1-T1) between the excited state and the triplet state to enhance photodynamic therapy efficiency. Importantly, COF-980 demonstrates high photostability, good anti-diffusion property, superior reactive oxygen species (ROS) generation efficiency, promising imaging ability, and ROS production in deep tissue (≈8 mm). Surprisingly, COF-980 combined with laser irradiation could trigger larger amount of intracellular ROS to high efficiently induce cancer cell death. Notably, COF-980 NPs precisely enable PDT guided by NIR-II fluorescence imaging that effectively inhibit the 4T1 tumor growth with negligible adverse effects. This study provides a universal approach to developing long-wavelength emissive COFs and exploits its applications for biomedicine.

Covalent organic frameworks (COFs) as carrier for improved drug delivery and biosensing applications

Porous organic frameworks (POFs) represent a significant subclass of nanoporous materials in the field of materials science, offering exceptional characteristics for advanced applications. Covalent organic frameworks (COFs), as a novel and intriguing type of porous material, have garnered considerable attention due to their unique design capabilities, diverse nature, and wide-ranging applications. The unique structural features of COFs, such as high surface area, tuneable pore size, and chemical stability, render them highly attractive for various applications, including targeted and controlled drug release, as well as improving the sensitivity and selectivity of electrochemical biosensors. Therefore, it is crucial to comprehend the methods employed in creating COFs with specific properties that can be effectively utilized in biomedical applications. To address this indispensable fact, this review paper commences with a concise summary of the different methods and classifications utilized in synthesizing COFs. Second, it highlights the recent advancements in COFs for drug delivery, including drug carriers as well as the classification of drug delivery systems and biosensing, encompassing drugs, biomacromolecules, small biomolecules and the detection of biomarkers. While exploring the potential of COFs in the biomedical field, it is important to acknowledge the limitations that researchers may encounter, which could impact the practicality of their applications. Third, this paper concludes with a thought-provoking discussion that thoroughly addresses the challenges and opportunities associated with leveraging COFs for biomedical applications. This review paper aims to contribute to the scientific community's understanding of the immense potential of COFs in improving drug delivery systems and enhancing the performance of biosensors in biomedical applications.

On-Water Surface Synthesis of Vinylene-Linked Cationic Two-Dimensional Polymer Films as the Anion-Selective Electrode Coating

Vinylene-linked two-dimensional polymers (V-2DPs) and their layer-stacked covalent organic frameworks (V-2D COFs) featuring high in-plane π-conjugation and robust frameworks have emerged as promising candidates for energy-related applications. However, current synthetic approaches are restricted to producing V-2D COF powders that lack processability, impeding their integration into devices, particularly within membrane technologies reliant upon thin films. Herein, we report the novel on-water surface synthesis of vinylene-linked cationic 2DPs films (V-C2DP-1 and V-C2DP-2) via Knoevenagel polycondensation, which serve as the anion-selective electrode coating for highly-reversible and durable zinc-based dual-ion batteries (ZDIBs). Model reactions and theoretical modeling revealed the enhanced reactivity and reversibility of the Knoevenagel reaction on the water surface. On this basis, we demonstrated the on-water surface 2D polycondensation towards V-C2DPs films that show large lateral size, tunable thickness, and high chemical stability. Representatively, V-C2DP-1 presents as a fully crystalline and face-on oriented film with in-plane lattice parameters of a=b≈43.3 Å. Profiting from its well-defined cationic sites, oriented 1D channels, and stable frameworks, V-C2DP-1 film possesses superior bis(trifluoromethanesulfonyl)imide anion (TFSI-)-transport selectivity (transference, t_=0.85) for graphite cathode in high-voltage ZDIBs, thus triggering additional TFSI--intercalation stage and promoting its specific capacity (from ~83 to 124 mAh g-1) and cycling life (>1000 cycles, 95 % capacity retention).

Fully Conjugated Covalent Organic Framework Nanosheets for Visible-Light-Driven Organic Synthesis in Water

Covalent organic framework (COF) nanosheets have recently garnered great attention as a new class of functional materials. As one of the sustainable processes, however, the photocatalytic organic synthesis in water has not been investigated using COF nanosheets as a photocatalyst to date. Herein, we reported the synthesis of a fully conjugated COF nanosheets with carboxyl functional group through a cooperative strategy of chemical exfoliation and group transformation. The new COF nanosheets was found to be an efficient heterogeneous photocatalyst for a wide range of organic synthesis including selective oxidation of sulfides and oxidative coupling of benzylamines in water under visible-light illumination. This work contributes a new roadmap for the design and synthesis of functional COF-based nanosheets, but also further extends the application boundary of the ultrathin COF nanosheets.

Dynamic Covalent Chemistry of Enamine-Ones: Exploring Tunable Reactivity in Vitrimeric Polymers and Covalent Organic Frameworks

Dynamic covalent chemistry (DCC) has revolutionized the field of polymer science by offering new opportunities for the synthesis, processability, and recyclability of polymers as well as in the development of new materials with interesting properties such as vitrimers and covalent organic frameworks (COFs). Many DCC linkages have been explored for this purpose, but recently, enamine-ones have proven to be promising dynamic linkages because of their facile reversible transamination reactions under thermodynamic control. Their high stability, stimuli-responsive properties, and tunable kinetics make them promising dynamic cross-linkers in network polymers. Given the rapid developments in the field in recent years, this review provides a critical and up-to-date overview of recent developments in enamine-one chemistry, including factors that control their dynamics. The focus of the review will be on the utility of enamine-ones in designing a variety of processable and self-healable polymers with important applications in vitrimers and recyclable closed-loop polymers. The use of enamine-one linkages in crystalline polymers, known as COFs and their applications are also summarized. Finally, we provide an outlook for future developments in this field.

Early stages of covalent organic framework formation imaged in operando

Covalent organic frameworks (COFs) are a functional material class able to harness, convert and store energy. However, after almost 20 years of research, there are no coherent prediction rules for their synthesis conditions. This is partly because of an incomplete picture of nucleation and growth at the early stages of formation. Here we use the optical technique interferometric scattering microscopy (iSCAT)1-3 for in operando studies of COF polymerization and framework formation. We observe liquid-liquid phase separation, pointing to the existence of structured solvents in the form of surfactant-free (micro)emulsions in conventional COF synthesis. Our findings show that the role of solvents extends beyond solubility to being kinetic modulators by compartmentation of reactants and catalyst. Taking advantage of these observations, we develop a synthesis protocol for COFs using room temperature instead of elevated temperatures. This work connects framework synthesis with liquid phase diagrams and thereby enables an active design of the reaction environment, emphasizing that visualization of chemical reactions by means of light-scattering-based techniques can be a powerful approach for advancing rational materials synthesis.

Metal-organic framework (MOF)/C-dots and covalent organic framework (COF)/C-dots hybrid nanocomposites: Fabrications and applications in sensing, medical, environmental, and energy sectors

Developing new hybrid materials is critical for addressing the current needs of the world in various fields, such as energy, sensing, health, hygiene, and others. C-dots are a member of the carbon nanomaterial family with numerous applications. Aggregation is one of the barriers to the performance of C-dots, which causes luminescence quenching, surface area decreases, etc. To improve the performance of C-dots, numerous matrices including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and polymers have been composited with C-dots. The porous crystalline structures, which are constituents of metal nodes and organic linkers (MOFs) or covalently attached organic units (COFs) provide privileged features such as high specific surface area, tunable structures, and pore diameters, modifiable surface, high thermal, mechanical, and chemical stabilities. Also, the MOFs and COFs protect the C-dots from the environment. Therefore, MOF/C-dots and COF/C-dots composites combine their features while retaining topological properties and improving performances. In this review, we first compare MOFs with COFs as matrices for C-dots. Then, the recent progress in developing hybrid MOFs/C-dots and COFs/C-dots composites has been discussed and their applications in various fields have been explained briefly.

Decoding Excimer Formation in Covalent-Organic Frameworks Induced by Morphology and Ring Torsion

A thorough and quantitative understanding of the fate of excitons in covalent-organic frameworks (COFs) after photoexcitation is essential for their augmented optoelectronic and photocatalytic applications via precise structure tuning. The synthesis of a library of COFs having identical chemical backbone with impeded conjugation, but varied morphology and surface topography to study the effect of these physical properties on the photophysics of the materials is herein reported. The variation of crystallite size and surface topography substantified different aggregation pattern in the COFs, which leads to disparities in their photoexcitation and relaxation properties. Depending on aggregation, an inverse correlation between bulk luminescence decay time and exciton binding energy of the materials is perceived. Further transient absorption spectroscopic analysis confirms the presence of highly localized, immobile, Frenkel excitons (of diameter 0.3-0.5 nm) via an absence of annihilation at high density, most likely induced by structural torsion of the COF skeletons, which in turn preferentially relaxes via long-lived (nanosecond to microsecond) excimer formation (in femtosecond scale) over direct emission. These insights underpin the importance of structural and topological design of COFs for their targeted use in photocatalysis.

Porous isoreticular non-metal organic frameworks

Metal-organic frameworks (MOFs) are useful synthetic materials that are built by the programmed assembly of metal nodes and organic linkers1. The success of MOFs results from the isoreticular principle2, which allows families of structurally analogous frameworks to be built in a predictable way. This relies on directional coordinate covalent bonding to define the framework geometry. However, isoreticular strategies do not translate to other common crystalline solids, such as organic salts3-5, in which the intermolecular ionic bonding is less directional. Here we show that chemical knowledge can be combined with computational crystal-structure prediction6 (CSP) to design porous organic ammonium halide salts that contain no metals. The nodes in these salt frameworks are tightly packed ionic clusters that direct the materials to crystallize in specific ways, as demonstrated by the presence of well-defined spikes of low-energy, low-density isoreticular structures on the predicted lattice energy landscapes7,8. These energy landscapes allow us to select combinations of cations and anions that will form thermodynamically stable, porous salt frameworks with channel sizes, functionalities and geometries that can be predicted a priori. Some of these porous salts adsorb molecular guests such as iodine in quantities that exceed those of most MOFs, and this could be useful for applications such as radio-iodine capture9-12. More generally, the synthesis of these salts is scalable, involving simple acid-base neutralization, and the strategy makes it possible to create a family of non-metal organic frameworks that combine high ionic charge density with permanent porosity.

Atomic-level imaging of beam-sensitive COFs and MOFs by low-dose electron microscopy

Electron microscopy, an important technique that allows for the precise determination of structural information with high spatiotemporal resolution, has become indispensable in unravelling the complex relationships between material structure and properties ranging from mesoscale morphology to atomic arrangement. However, beam-sensitive materials, particularly those comprising organic components such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), would suffer catastrophic damage from the high energy electrons, hindering the determination of atomic structures. A low-dose approach has arisen as a possible solution to this problem based on the integration of advancements in several aspects: electron optical system, detector, image processing, and specimen preservation. This article summarizes the transmission electron microscopy characterization of MOFs and COFs, including local structures, host-guest interactions, and interfaces at the atomic level. Revolutions in advanced direct electron detectors, algorithms in image acquisition and processing, and emerging methodology for high quality low-dose imaging are also reviewed. Finally, perspectives on the future development of electron microscopy methodology with the support of computer science are presented.

The Application of Porous Organic Polymers as Metal Free Photocatalysts in Organic Synthesis

Concerns about increasing greenhouse gas emissions and their effect on our environment highlight the urgent need for new sustainable technologies. Visible light photocatalysis allows the clean and selective generation of reactive intermediates under mild conditions. The more widespread adoption of the current generation of photocatalysts, particularly those using precious metals, is hampered by drawbacks such as their cost, toxicity, difficult separation, and limited recyclability. This is driving the search for alternatives, such as porous organic polymers (POPs). This new class of materials is made entirely from organic building blocks, can possess high surface area and stability, and has a controllable composition and functionality. This review focuses on the application of POPs as photocatalysts in organic synthesis. For each reaction type, a representative material is discussed, with special attention to the mechanism of the reaction. Additionally, an overview is given, comparing POPs with other classes of photocatalysts, and critical conclusions and future perspectives are provided on this important field.

Synthetic Strategies to Extended Aromatic Covalent Organic Frameworks

π-Conjugated materials are highly attractive owing to their unique optical and electronic properties. Covalent organic frameworks (COFs) offer a great opportunity for precise arrangement of building units in a π-conjugated crystalline matrix and tuning of the properties through choice of functionalities or post-synthetic modification. With this review, we aim at summarizing both the most representative as well as emerging strategies for the synthesis of π-conjugated COFs. We give examples of direct synthesis using large, π-extended building blocks. COFs featuring fully conjugated linkages such as vinylene, pyrazine, and azole are discussed. Then, post-synthetic modification methods that result in the extension of the COF π-system are reviewed. Throughout, mechanistic insights are presented when available. In the context of their utilization as film devices, we conduct a concise survey of the prominent COF layer deposition techniques reported and their aptness for the deposition of fused aromatic systems.

Simultaneous Inside and Outside Functionalization of Single-Walled Carbon Nanotubes

Functionalizing single-walled carbon nanotubes (SWCNTs) in a robust way that does not affect the sp2 carbon framework is a considerable research challenge. Here we describe how triiodide salts of positively charged macrocycles can be used not only to functionalize SWCNTs from the outside, but simultaneously from the inside. We employed disulfide exchange in aqueous solvent to maximize the solvophobic effect and therefore achieve a high degree of macrocycle immobilization. Characterization by Raman spectroscopy, EDX-STEM and HR-TEM clearly showed that serendipitously this wet-chemical functionalization procedure also led to the encapsulation of polyiodide chains inside the nanotubes. The resulting three-shell composite materials are redox-active and experience an intriguing interplay of electrostatic, solvophobic and mechanical effects that could be of interest for applications in energy storage.

Orientation and morphology control in acid-catalyzed covalent organic framework thin films

As thin films of semiconducting covalent organic frameworks (COFs) are demonstrating utility for ambipolar electronics, channel materials in organic electrochemical transistors (OECTs), and broadband photodetectors, control and modulation of their thin film properties is paramount. In this work, an interfacial growth technique is utilized to synthesize imine TAPB-PDA COF films at both the liquid-liquid interface as well as at the liquid-solid interface on a Si/SiO2 substrate. The concentration of acetic acid catalyst in the aqueous phase is shown to significantly influence the thin film morphology of the liquid-solid growth, with concentrations below 1 M resulting in no film nucleation, concentrations of 1-4 M enabling smooth film formation, and concentrations greater than 4 M resulting in films with a higher density of particulates on the surface. Importantly, while the films grown at the liquid-liquid interface are mixed-orientation, those grown directly at the liquid-solid interface on the Si/SiO2 surface have highly oriented COF layers aligned parallel to the substrate surface. Moreover, this liquid-solid growth process affords TAPB-PDA COF thin films with p-type charge transport having a transconductance of 10 μS at a gate voltage of -0.9 V in an OECT device structure.

Pore-Engineered Hydrogen-Bonded Supramolecular Fluorosensor for Ultrasensitive Determination of Copper Ions

The selective quantification of copper ions (Cu2+) in biosamples holds great importance for disease diagnosis, treatment, and prognosis since the Cu2+ level is closely associated with the physiological state of the human body. While it remains a long-term challenge due to the extremely low level of free Cu2+ and the potential interference by the complex matrices. Here, a pore-engineered hydrogen-bonded organic framework (HOF) fluorosensor is constructed enabling the ultrasensitive and highly selective detection of free Cu2+. Attributing to atomically precise functionalization of active amino "arm" within the HOF pores and the periodic π-conjugated skeleton, this porous HOF fluorosensor affords high affinity toward Cu2+ through double copper-nitrogen (Cu─N) coordination interactions, resulting in specific fluorescence quenching of the HOF as compared with a series of substances ranging from other metal ions, metabolites, amino acids to proteins. Such superior fluorescence quenching effect endows the Cu2+ quantification by this new HOF sensor with a wide linearity of 50-20 000 nm, a low detection limit of 10 nm, and good recoveries (89.5%-115%) in human serum matrices, outperforming most of the reported approaches. This work highlights the practicability of hydrogen-bonded supramolecular engineering for designing facile and ultrasensitive biosensors for clinical free Cu2+ determination.

An Azulene-Based Crystalline Porous Covalent Organic Framework for Efficient Photothermal Conversion

The designed synthesis of a crystalline azulene-based covalent organic framework (COF-Azu-TP) is presented and its photothermal property is investigated. Azulene, a distinctive 5-7 fused ring non-benzenoid aromatic compound with a large intramolecular dipole moment and unique photophysical characteristics, is introduced as the key feature in COF-Azu-TP. The incorporation of azulene moiety imparts COF-Azu-TP with broad-spectrum light absorption capability and interlayer dipole interactions, which makes COF-Azu-TP a highly efficient photothermal conversion material. Its polyurethane (PU) composite exhibits a solar-to-vapor conversion efficiency (97.2%) and displays a water evaporation rate (1.43 kg m-2 h-1) under one sun irradiation, even at a very low dosage of COF-Azu-TP (2.2 wt%). Furthermore, COF-Azu-TP is utilized as a filler in a polylactic acid (PLA)/polycaprolactone (PCL) composited shape memory material, enabling rapid shape recovery under laser stimulation. A comparison study with a naphthalene-based COF isomer further emphasizes the crucial role of azulene in enhancing photothermal conversion efficiency. This study demonstrates the significance of incorporating specific building blocks into COFs for the development of functional porous materials with enhanced properties, paving the way for future applications in diverse fields.

Nanofibrous Porous Organic Polymers and Their Derivatives: From Synthesis to Applications

Engineering porous organic polymers (POPs) into 1D morphology holds significant promise for diverse applications due to their exceptional processability and increased surface contact for enhanced interactions with guest molecules. This article reviews the latest developments in nanofibrous POPs and their derivatives, encompassing porous organic polymer nanofibers, their composites, and POPs-derived carbon nanofibers. The review delves into the design and fabrication strategies, elucidates the formation mechanisms, explores their functional attributes, and highlights promising applications. The first section systematically outlines two primary fabrication approaches of nanofibrous POPs, i.e., direct bulk synthesis and electrospinning technology. Both routes are discussed and compared in terms of template utilization and post-treatments. Next, performance of nanofibrous POPs and their derivatives are reviewed for applications including water treatment, water/oil separation, gas adsorption, energy storage, heterogeneous catalysis, microwave absorption, and biomedical systems. Finally, highlighting existent challenges and offering future prospects of nanofibrous POPs and their derivatives are concluded.

Powders to Thin Films: Advances in Conjugated Microporous Polymer Chemical Sensors

Chemical sensing of harmful species released either from natural or anthropogenic activities is critical to ensuring human safety and health. Over the last decade, conjugated microporous polymers (CMPs) have been proven to be potential sensor materials with the possibility of realizing sensing devices for practical applications. CMPs found to be unique among other porous materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) due to their high chemical/thermal stability, high surface area, microporosity, efficient host-guest interactions with the analyte, efficient exciton migration along the π-conjugated chains, and tailorable structure to target specific analytes. Several CMP-based optical, electrochemical, colorimetric, and ratiometric sensors with excellent selectivity and sensing performance were reported. This review comprehensively discusses the advances in CMP chemical sensors (powders and thin films) in the detection of nitroaromatic explosives, chemical warfare agents, anions, metal ions, biomolecules, iodine, and volatile organic compounds (VOCs), with simultaneous delineation of design strategy principles guiding the selectivity and sensitivity of CMP. Preceding this, various photophysical mechanisms responsible for chemical sensing are discussed in detail for convenience. Finally, future challenges to be addressed in the field of CMP chemical sensors are discussed.

3D Covalent Organic Framework with "the" Topology

Discovery of new topology covalent organic frameworks (COFs) is a mainstay in reticular chemistry and materials research because it not only serves as a stepwise guide to the designed construction of covalent-organic architectures but also helps to comprehend function from structural design point-of-view. Proceeding on this track, the first 3D COF, TUS-38, with the topology is constructed by reticulating a planar triangular 3-c node of D3h symmetry with a tetragonal prism 8-c node of D2h symmetry via [3 + 8] reversible imine condensation reaction. TUS-38 represents a twofold interpenetrated multidirectional pore network with a high degree of crystallinity and structural integrity. Interestingly, stemming from the nitrogen-rich s-triazine rings with electron-deficient character and ─C ═ N─ linkages composing the TUS-38 framework that benefit to the charge-transfer and hence dipole-dipole electrostatic interactions between the framework and iodine in addition to exclusive topological characteristics of the exotic the net, TUS-38 achieves an exemplary capacity for iodine vapor uptake reaching 6.3 g g-1. Recyclability studies evidence that TUS-38 can be reused at least five times retaining 95% of its initial adsorption capacity; while density functional theory (DFT) calculations have heightened the understanding of the interactions between iodine molecules and the framework.

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (β) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (β = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.

Photoelectron Migration Boosted by Hollow Double-Shell Dyads Based on Covalent Organic Frameworks for Highly Efficient Photocatalytic Hydrogen Generation

Photocatalytic hydrogen production based on noble metal-free systems is a promising technology for the conversion of solar energy into green hydrogen, it is pivotal and challenging to tailor-make photocatalysts for achieving high photocatalytic efficiency. Herein, we reported a hollow double-shell dyad through uniformly coating covalent organic frameworks (COFs) on the surface of hollow Co9S8. The double shell architecture enhances the scattering and refraction efficiency of incident light, shortens the transmission distance of the photogenerated charge carriers, and exposes more active sites for photocatalytic conversion. The hydrogen evolution rate is as high as 23.15 mmol g-1 h-1, which is significantly enhanced when compared with that of their physical mixture (0.30 mmol g-1 h-1) and Pt-based counterpart (11.84 mmol g-1 h-1). This work provides a rational approach to the construction of noble-metal-free photocatalytic systems based on COFs to enhance hydrogen evolution performance.

COF-Topological Quantum Material Nano-heterostructure for CO2 to Syngas Production under Visible Light

Efficient solar-driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF-topological quantum material nano-heterostructure, COF@TI with a newly synthesized donor-acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron-hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile-water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano-heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

Covalent Organic Frameworks as Single-Site Photocatalysts for Solar-to-Fuel Conversion

Single-site photocatalysts (SSPCs) are well-established as potent platforms for designing innovative materials to accomplish direct solar-to-fuel conversion. Compared to classical inorganic porous materials, such as zeolites and silica, covalent organic frameworks (COFs)─an emerging class of porous polymers that combine high surface areas, structural diversity, and chemical stability─are attractive candidates for SSPCs due to their molecular-level precision and intrinsic light harvesting ability, both amenable to structural engineering. In this Perspective, we summarize the design concepts and state-of-the-art strategies for the construction of COF SSPCs, and we review the development of COF SSPCs and their applications in solar-to-fuel conversion from their inception. Underlying pitfalls concerning photocatalytic characterization are discussed, and perspectives for the future development of this burgeoning field are given.

Sustainable remediation of pesticide pollutants using covalent organic framework - A review on material properties, synthesis methods and application

Covalent organic frameworks (COF) have emerged as a potential class of materials for a variety of applications in a wide number of sectors including power storage, environmental services, and biological applications due to their ordered and controllable porosity, large surface area, customizable structure, remarkable stability, and diverse electrical characteristics. COF have received a lot of attention in recent years in the field of environmental remediation, It also find its way to eliminate the emerging pollutant from the environment notably pesticide from polluted water. This review more concentrated on the application of COF in pesticide removal by modifying COF structure, COF synthesis and material properties. To increase the adsorption ability and selectivity of the material towards certain pesticides removal, the synthesis of COF involves organic linkers with various functional groups such as amine, carboxylic acid groups etc. The COF have a high degree of stability and endurance make them suitable for intermittent usage in water treatment applications. This review manifests the novel progress where modified COFs employed in a prominent manner to remove pesticides from polluted water. Some examples of COF application in the eradication of pesticides are triformyl phenylene framework functionalized with amine groups has capacity to remove up to 50 mg/l of Organophosphorus - chlorpyrifos. COF modified to improve their photocatalytic capacity to breakdown the pesticide under visible light irradiation. COF tetraphenyl ethylene linked with carboxylic acid group shows efficient photocatalytic degradation of 90% of organochlorine insecticide endosulfan when subjected to visible light. Atrazine and imidacloprid are reduced from 100 ppm to 1 ppm in aqueous solutions by COF based on high adsorption capacity. In addition, the strategies, technique, synthesis and functional group modification design of COF are discussed.