Vinylene-Bridged Two-Dimensional Covalent Organic Frameworks via Knoevenagel Condensation of Tricyanomesitylene

Linkages Chemistry of Covalent Organic Frameworks in Photocatalysis and Electrocatalysis

Covalent organic frameworks (COFs) have emerged as promising candidates for electrocatalysis and photocatalysis applications due to their structurally ordered architectures and tunable physicochemical properties. In COFs, organic building blocks are linked via covalent bonds, and the structural and electronic characteristics of COFs are critically governed by their linkage chemistry. These linkages influence essential material attributes including surface area, crystallinity, hydrophobicity, chemical stability, and the optoelectronic behavior (e.g., photoelectron separation efficiency, electron conductivity, and reductive activity), which collectively determine catalytic performance in energy conversion systems. A systematic understanding of linkage engineering in COFs not only advances synthetic methodologies but also provides innovative solutions to global energy and environmental challenges, thereby accelerating the development of sustainable technologies for clean energy production and environmental remediation.

Azadiene-Linked Carbon-Organic Frameworks for Enhanced Photocatalytic Efficiency in Organic Synthesis

2D covalent organic frameworks (2D-COFs) have garnered huge attention as a promising platform for solar to chemical energy conversion. In this work, two novel azadiene-linked COFs (COF-JLU236 and COF-JLU237) were designed and successfully synthesized under solvothermal conditions for the first time. And new materials have high crystallinity, large surface area, good durability, and broad absorption range, as well as excellent photoelectric properties. Importantly, they can effectively induce selective oxidation of sulfides and phenylboronic acids under visible light, and show wide substrate adaptability and superior reusability.

Neighboring Effect of Adjacent Nitrogen Sites on Vinylene Linkage in Covalent Organic Frameworks for Regulating Oxygen Reduction Reaction

Vinylene-linked covalent organic frameworks (COFs) are attractive electrocatalysts owing to their corresponding high chemical stability and excellent conjugated frameworks. In this study, for the first time, the methyl group of the pyrimidine ring was used to synthesize conjugated COFs (TB-TFT-COF and TB-TFC-COF) with vinylene linkages, which were employed as catalysts for the oxygen reduction reaction (ORR). In addition, local electronic structures of the vinylene linkages could be regulated by the adjacent nitrogen atomic sites of various functional moieties (triazine, pyridine, and pyrimidine), resulting in tunable electrocatalytic activity and selectivity of the COFs. Notably, the TB-TFT-COF attained a half-wave potential of 0.74 V relative to RHE alongside superior electrochemical stability, matching the performance of metal-free COF-based catalysts for ORR. Furthermore, as evidenced by density functional theory (DFT) calculations, the adjacent nitrogen sites of the pyrimidine unit around the vinylene linkage are crucial for enhancing the utilization of electrocatalytic active sites. This work establishes that the precise modulation of electronic coupling between neighboring active sites enables the development of efficient oxygen reduction reaction catalysts.

End-capping fluorination of vinylene-linked covalent organic frameworks for photocatalytic hydrogen peroxide production

Inspired by the concept of an "antenna", a simple end-capping fluorination strategy was proposed, facilitating the introduction of electron-withdrawing domains and oxygen adsorption sites. Following fluorination, the photocatalytic H2O2 production performance of Post-v-COF-F2 was 1.75 times higher than that of the pristine Homo-v-COF.

Benzophenone-Based Polymers and Covalent Organic Framework for Photocatalytic Molecular Oxygen Activation

Photosensitization by photoactive materials requires well-designed molecular engineering to enable continuous photochemical processes. However, developing a heavy-atom-free (HAF) strategy to enhance the photoactivity of photosensitizers remains a significant challenge. In this study, we introduce a novel strategy to enhance photosensitization by incorporating benzophenone-rich components into 3D polymers (BQP1 and BQP2) and the 2D covalent organic framework (BQ-TMT COF). This incorporation accelerates both charge carrier separation and intersystem crossing, thereby significantly improving photo-to-chemical energy conversion and electron transfer reactions. Notably, the crystalline BQ-TMT COF enables efficient photocatalytic molecular O2 activation, producing both 1O2 and O2·- with high efficiency and recyclability. It demonstrates selective photocatalytic oxidation in 1O2-mediated sulfide transformations. Moreover, the material performs well in O2·--mediated oxidation, including the hydroxylation of boronic acids and oxidation of amines to imines. The BQ-TMT COF-based photoelectrode generates a photocurrent of approximately 20.7 μA·cm-2 at 0.4 V vs RHE and achieves a high photocatalytic hydrogen production rate. Our study demonstrates a HAF heterogeneous photosensitizer with efficient photoactive small molecule activation through molecular engineering.

Multifunctional solar-driven interfacial evaporation system for simultaneous clean water production and high-value-added ion extraction

The utilization of solar-driven interfacial evaporation (SIE) technology for clean water production has rapidly expanded, driven by global clean water scarcity and the energy crisis. Recent developments have demonstrated that combining SIE technology with the ion extraction process enables the effective use of abundant sunlight to economically and sustainably harvest high-value minerals from the ocean while simultaneously producing clean water. This synergy not only maximizes resource recovery but also enhances the ecological and economic benefits of solar energy utilization. In this review, we provide a comprehensive overview of the materials and methodologies used in designing multifunctional SIE systems for simultaneous clean water production and high-value ion extraction. The design rationale behind these multifunctional SIE systems, along with various ion extraction strategies and mechanisms, has been thoroughly discussed, identifying both the prevailing challenges and the potential research opportunities in this evolving field. This review aims to highlight the significant potential of SIE technology not only in enhancing clean water availability but also in contributing to sustainable energy and resource management.

Rational Design Two- or Four-Electron Reaction Pathway Covalent Organic Frameworks for Efficient and Selective Electrocatalytic Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are often employed in oxygen reduction reactions (ORR) for hydrogen peroxide production due to their tunable structures and compositions. However, COF electrocatalysts require precise structural engineering, such as heteroatoms or metal site doping, to modulate the reaction pathway during the ORR process. In this work, we designed a tetraphenyl-p-phenylenediamine based COF electrocatalyst, namely TPDA-BDA, which exhibited excellent two-electron (2e) ORR performance with high H2O2 selectivity of 89.7 % and faraday efficiency (FE) of 86.7 %, higher than the reported COFs to date for H2O2 electrosynthesis. The theoretical and experimental results showed that the rate-determining step energy barrier for reduction of O2 to OOH* intermediates was significantly reduced by replacing of bipyridine with biphenyl blocks, changing from 4e to 2e ORR reaction pathway. Also, the donor-acceptor characteristic and narrower optical band gap of TPDA-BDA COF enhanced the electronic conductivity and reduction ability, thus elevating the catalytic activity. As a result, the H2O2 selectivity was maintained above 85 % even after 50 h stability test. This work reveals the structure-property relationship of COF electrocatalysts and provides a new strategy for rational design of high performance 2e ORR COF electrocatalysts for efficient and selective hydrogen peroxide production.

Room-Temperature Single-Phase Synthesis of Semiconducting Metal-Covalent Organic Frameworks With Microenvironment-Tuned Photocatalytic Efficiency

In order to improve the solubility of metallated monomers and product crystallinity, metal-covalent organic frameworks (MCOFs) are commonly prepared via high-temperature sol-vothermal synthesis. However, it hampers the direct extraction of crystallization evolution information. Exploring facile room-temperature strategies for both synthesizing MCOFs and exploiting the crystallinity mechanism is extremely desired. Herein, by a novel single-phase synthetic strategy, three MCOFs with different microstructure is rapidly prepared based on the Schiff base reaction between planarity-tunable C3v monomers and metallated monomers at room temperature. Based on detailed time-dependent experiments and theoretical calculations, it is found that there is a planarity-tuned and competitive growth relationship between disordered structures and crystal nucleus for the first time. The high planarity of monomers boosts the formation of crystal nucleus and rapid growth, suppressing the forming of amorphous structures. In addition, the microenvironment effect on selective photocatalytic coupling of benzylamine (BA) is investigated. The strong donor-acceptor (D-A) MCOF exhibits efficient photocatalytic activity with a high conversion rate of 99% and high selectivity of 99% in 5 h under the 520 nm light irradiation. This work opens a new pathway to scalable and efficient synthesis of highly crystalline MCOFs.

sp2 Carbon-Conjugated Covalent Organic Frameworks (sp2c-COFs): Synthesis and Application in Photocatalytic Water Splitting

Preparation of irreversible sp2 carbon-conjugated covalent organic frameworks (sp2c-COFs) with specific porosity, easy structural functionalization, high chemical stability, and unique π-electron conjugation structure (especially the combination of π-π stacking interactions and conjugation system), can remove the barrier of electron transfer and provide a unique advantage for photocatalytic water splitting. Herein, based on three kinds of reactions (Aldol condensation reaction, Knoevenagel condensation reaction, and Horner-Wadsworth-Emmons reaction) and guided by the precise modulation of ligand structure and topology, this review summarizes the synthesis of sp2c-COFs and their applications in photoelectrocatalytic water splitting (hydrogen evolution and oxygen evolution reactions). Furthermore, challenges and possible research directions for sp2c-COFs in photocatalytic water splitting are also provided.

p-π Conjugated Covalent Organic Frameworks Expedite Molecular Triplet Excitons for H2O2 Production Coupled with Biomass Upgrading

High-efficiency production of triplet states in covalent organic framework photocatalysts is crucial for high-selectivity oxygen (O2) reduction to hydrogen peroxide (H2O2). Herein, fluorine and partial fluorine atoms are incorporated into an olefin-linked triazine covalent organic framework (F-ol-COF and HF-ol-COF), in which the adjacent fluorine (F) atoms-olefinic bond forms p-π conjugation that induces spin-polarization under irradiation, thus expediting triplet excitons for activating O2 to singlet oxygen (1O2) and contributing to a high H2O2 selectivity (91%). Additionally, the feasibility of coupling H2O2 production with the valorization of 5-hydroxymethylfurfural (HMF) is exhibited. The F-ol-COF demonstrates a highly stable H2O2 yield rate of 12558 µmol g-1 h-1 with the HMF-to-functionalized furan conversion yield of 95%, much higher than the partially fluorinated COF (HF-ol-COF) and the non-fluorinated COF (H-ol-COF). Mechanistic studies reveal that F-incorporation promotes charge separation, intensifies the Lewis acidity of the carbon atoms on the olefinic bond as active sites for O2 adsorption, and provides highly concentrated holes at the triazine unit for HMF oxidation upgrading. This study suggests the attractive potential of rational design of porous-crystalline photocatalysts for high-efficiency photocatalytic O2 reduction to H2O2 and biomass upgrading.

Olefin-Linked Covalent Organic Frameworks as Prospective Artificial Platforms for Efficient Photocatalysis

The development of semiconducting materials for photoredox catalysis holds great promise for sustainable utilization of solar energy. Olefin-linked covalent organic frameworks (COFs), which are built by linking organic structs into crystalline frameworks through C=C bonds, have attracted tremendous attention in photocatalysis due to their saliant advantages such as extended π-conjugation, permanent porosity, exceptional chemical stability, light-harvesting and charge separation abilities. This review offers a comprehensive overview of recent new advances toward the development of olefin-linked COFs and their uses as artificial platforms for photocatalytic applications, like hydrogen evolution, carbon dioxide reduction and organic transformations. Structural design strategies, preparation methods and structure-function relationships in various photoredox reactions are summarized, which is accompanied by various approaches to boost their catalytic performance. The challenges and future prospectives are further discussed.

Photocatalytic Hydroxylation and Oxidative Coupling Reactions Mediated by Multinuclear Au(I) Supramolecular Clusters

Polynuclear Au(I) cluster photocatalysts, known for their high activity and stability, hold substantial potential in organic synthetic chemistry. This study synthesized two Au(I) supramolecular cluster catalysts with different nuclearities: a tetranuclear cluster, C1 ([(dppmAu2)2L1] • PF6 -), and a hexadecanuclear cluster, C2 [(dppmAu2)6(Au4)(L1)4] • 4PF6 -, through a multicomponent stepwise self-assembly approach. Both cluster structures feature aurophilicity interaction motifs that endow them with exceptional photocatalytic performance, exhibiting optical band gaps of 2.27 eV and 2.41 eV, respectively. Upon photoexcitation, these clusters efficiently generate reactive oxygen species, significantly enhancing their photocatalytic efficacy for the oxidative hydroxylation of phenylboronic acids and oxidative coupling of benzylamines under mild conditions. Catalytic efficiencies exceeding 90 % were achieved. Turnover frequencies for C2 and C1 were measured at 52.045 h-1 and 6.030 h-1, respectively, representing the highest efficiencies reported for photocatalysts to date. Compared to C1, C2 exhibited superior photocatalytic activity, attributed to its higher photoelectric sensitivity and greater exposure of active metal sites. Using a combination of experimental data and density functional theory calculations, the plausible mechanisms were proposed for two photocatalytic reactions. This study demonstrates that the use of multicomponent cooperative self-assembly strategy to synthesize high-nuclearity Au(I) clusters offers innovative pathways for the development of efficient, green, light-driven organic synthesis.

Vinylene-Linking of Polycyclic Aromatic Hydrocarbons to π-Extended Two-Dimensional Covalent Organic Framework Photocatalyst for H2O2 Synthesis

Polycyclic aromatic hydrocarbons (PAHs) hold the predominant role either as individual molecules or building blocks in the field of organic semiconductors or nanocarbons. Connecting PAHs via sp2-carbon bridges to form high-crystalline π-extended structures is highly desired not only for enlarging the regimes of two-dimensional materials but also for achieving exceptional properties/functions. In this work, we developed 5,10-dimethyl-4,9-diazapyrene as a key monomer, whose two methyl groups at the positions adjacent to nitrogen atoms, can helpfully increase the solubility, and serve as the active connection sites. In the presence of organic acids, this monomer enables smoothly conducting Knoevenagel condensation to form two vinylene-linked PAH-cored COFs, which show high-crystalline honeycomb structures with large surface areas up to 1238 m2 g-1. Owing to the direct connection mode of PAH building blocks with vinylene, the as-prepared COFs possess spatially extended π-conjugation and substantial semiconducting properties. Consequently, their visible-light photocatalysis with exceptional activity and durability was manifested to generate H2O2 up to 3820 μmol g-1 h-1 in pure water, and even 17080 μmol g-1 h-1 using benzyl alcohol as a hole sacrificial agent.

Intrareticular Exciton Effects Regulate Photocatalytic Activity in Donor-Acceptor Olefin-Linked Covalent Organic Frameworks

Olefin-linked covalent organic frameworks (OL-COFs) show great promise for visible-light-driven photocatalysis. Manipulating atomic-level donor-acceptor interactions in OL-COFs is key to understanding their exciton effects in this system. Here, three OL-COFs are presented with orthorhombic lattice structures, synthesized via Knoevenagel polycondensation reaction of terephthalaldehyde and tetratopic monomers featuring phenyl, benzo[c][1,2,5]oxadiazole, and benzo[c][1,2,5]thiadiazole moieties. These OL-COFs feature tunable donor-acceptor interactions, making them ideal for studying exciton effects in olefin-linked systems. Comprehensive analyses, including temperature-dependent photoluminescence spectra, ultrafast spectroscopy, and theoretical calculations, reveal that stronger donor-acceptor interactions lead to reduced exciton binding energy (Eb), accelerated exciton dissociation, and longer-lived photogenerated charges, thereby enhancing photocatalytic performance. Notably, The TMO-BDA COF, with the lowest Eb, demonstrates superior photocatalytic activity in one-pot sequential organic transformation and excellent catalytic performance in gram-scale reactions, highlighting its potential for practical applications. This work provides valuable insights into regulating the exciton effect at the molecular level in OL-COFs, offering pathways to enhance photocatalytic efficiency.

Near-IR absorbing tetraene-linked π-conjugated porous polymers for energy storage and electrical conductivity

Tetraene-linked diketopyrrolopyrrole (DPP)-based CMPs were developed via Knoevenagel condensation of ditopic active hydrogen containing DPP with tritopic aryl aldehydes. The "tetra-ene" π-arrangement in the molecular framework promotes uninterrupted π-delocalization, resulting in near-infrared (NIR) absorption (∼red edge of 1200 nm), high electrical conductivity in the pristine (10-3 S m-1) and doped states (0.2 S m-1), and moderate energy storage (70 F g-1).

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Vinyl-bearing sp2 carbon-conjugated covalent organic framework composites for enhanced electrochemical performance in hydrogen evolution reaction and lithium-sulfur batteries

Vinyl-bearing triazine-functionalized covalent organic frameworks (COFs) have emerged as promising materials for electrocatalysis and energy storage. Guided by density functional theory calculations, a vinyl-enriched COF (VCOF-1) featuring a donor-acceptor structure was synthesized based on the Knoevenagel reaction. Moreover, the VCOF-1@Ru without pyrolysis was obtained through chemical coordination interactions between VCOF-1 and RuCl3, exhibiting enhanced electrocatalytic performance in the hydrogen evolution reaction when exposed to 0.5 M H2SO4. The results demonstrated that the protonation of VCOF-1@Ru enhanced the electrical conductivity and accelerated the generation of H2 on the catalytically active site Ru. Additionally, VCOF-1@CNT with a tubular structure was prepared by uniformly wrapping VCOF-1 onto carbon nanotubes (CNTs) and using it as a cathode for lithium-sulfur batteries by chemically and physically encapsulating S. The enhanced performance of VCOF-1@CNT was attributed to the effective suppression of lithium polysulfide migration. This suppression was achieved through several mechanisms, including the inverse vulcanization of vinyl on VCOF-1@CNT, the enhancement of material conductivity, and the interaction between N in the materials and Li ions. This study demonstrated a strategy for enhancing material performance by precisely modulating the COF structure at the molecular level.

Tris(triazolo)triazine-Based Covalent Organic Frameworks for Efficiently Photocatalytic Hydrogen Peroxide Production

Two-dimensional covalent organic frameworks (2D-COFs) have recently emerged as fascinating scaffolds for solar-to-chemical energy conversion because of their customizable structures and functionalities. Herein, two tris(triazolo)triazine-based COF materials (namely COF-JLU51 and COF-JLU52) featuring large surface area, high crystallinity, excellent stability and photoelectric properties were designed and constructed for the first time. Remarkably, COF-JLU51 gave an outstanding H2O2 production rate of over 4200 μmol g-1 h-1 with excellent reusability in pure water and O2 under one standard sun light, that higher than its isomorphic COF-JLU52 and most of the reported metal-free materials, owing to its superior generation, separation and transport of photogenerated carriers. Experimental and theoretical researches prove that the photocatalytic process undergoes a combination of indirect 2e- O2 reduction reaction (ORR) and 4e- H2O oxidation reaction (WOR). Specifically, an ultrahigh yield of 7624.7 μmol g-1 h-1 with apparent quantum yield of 18.2 % for COF-JLU52 was achieved in a 1 : 1 ratio of benzyl alcohol and water system. This finding contributes novel, nitrogen-rich and high-quality tris(triazolo)triazine-based COF materials, and also designate their bright future in photocatalytic solar transformations.

Constructing Donor-Acceptor-Linked COFs Electrolytes to Regulate Electron Density and Accelerate the Li+ Migration in Quasi-Solid-State Battery

Regulation the electronic density of solid-state electrolyte by donor-acceptor (D-A) system can achieve highly-selective Li+ transportation and conduction in solid-state Li metal batteries. This study reports a high-performance solid-state electrolyte thorough D-A-linked covalent organic frameworks (COFs) based on intramolecular charge transfer interactions. Unlike other reported COF-based solid-state electrolyte, the developed concept with D-A-linked COFs not only achieves electronic modulation to promote highly-selective Li+ migration and inhibit Li dendrite, but also offers a crucial opportunity to understand the role of electronic density in solid-state Li metal batteries. The introduced strong electronegativity F-based ligand in COF electrolyte results in highly-selective Li+ (transference number 0.83), high ionic conductivity (6.7 × 10-4 S cm-1), excellent cyclic ability (1000 h) in Li metal symmetric cell and high-capacity retention in Li/LiFePO4 cell (90.8% for 300 cycles at 5C) than substituted C- and N-based ligands. This is ascribed to outstanding D-A interaction between donor porphyrin and acceptor F atoms, which effectively expedites electron transferring from porphyrin to F-based ligand and enhances Li+ kinetics. Consequently, we anticipate that this work creates insight into the strategy for accelerating Li+ conduction in high-performance solid-state Li metal batteries through D-A system.

Benzothiadiazole-Based Ordered Mesoporous Polymer as a Versatile, Metal-Free Heterogeneous Photocatalyst

Visible-light active heterogeneous organophotocatalysts have recently gained considerable interest in organic synthetic community. Ordered mesoporous polymers (OMPs) are highly promising as heterogeneous alternative to traditional precious metal/organic dyes-based photocatalysts. Herein, we report the preparation of a benzothiadiazole functionalized OMPs (BT-MPs) through a "bottom-up" strategy. High ordered periodic porosity, large surface area, excellent stability and rational energy-band structures guarantee the high catalytic activity of BT-MPs. As a result, at least six conversions, e. g., the [3+2] cycloaddition of phenols with olefins, the selective oxidation of sulfides, the C-3 thiocyanation of indole and the aminothiocyanation of β-keto ester, could be promoted smoothly by BT-MPs. In addition, BT-MPs was readily recovered with well maintaining its photocatalytic activity and could be reused for at least eight times. This study highlights the potential of exploiting photoactive OMPs as recyclable, robust and metal-free heterogeneous photocatalysts.

Restricted Growth of Vinylene-Linked Covalent Organic Frameworks along Two-Dimensional Plane Using Heterogeneous Catalysis

The vinylene-linked covalent organic frameworks (viCOFs) have been generally synthesized in the presence of homogeneous catalysts such as KOH or trifluoroacetic acid. However, highly ordered viCOFs cannot always be obtained due to the uncommitted growth of viCOF layers in the homogeneous system with ubiquitous catalysts. Here, we propose a scalable protocol to restrict the growth of viCOFs along the two-dimensional (2D) plane by introducing a heterogeneous catalyst, polyoxometalates (POMs). With the unique Brønsted alkalinity and catalytic surface, POMs induce the growth of 2D viCOF layers along the surface of the catalytic substrate and restrain the generation of out-of-plane branches. Based on this protocol, six typical 2D viCOFs with high crystallinity and porosity were synthesized within a shorter reaction time as compared with the reported works using the common homogeneous catalysts for viCOF synthesis. On the basis of the density functional theory calculations and experimental results, a bottom intercalation growth pattern of viCOFs was revealed during the heterogeneous reaction. The unique growth pattern greatly promotes the orderly assembly of monomers, thus shortening the reaction time and improving the crystallinity of viCOFs. Furthermore, this heterogeneous catalysis strategy is suitable for the gram-scale preparation of 2D viCOFs. These results provide a novel avenue for the synthesis of high-quality viCOFs and may bring new insights into the synthetic methodology of COFs.

Ambipolar conjugated ladder polymers by room-temperature Knoevenagel polymerization

Two soluble conjugated ladder polymers (cLPs), decorated with multiple electron-poor species (i.e., cyano groups, fused pentagons, and N-heterocyclic rings), have been synthesized from the newly developed tetraketo-functionalized double aza[5]helicene building blocks using a single-step Knoevenagel polycondensation strategy. This facile approach features mild conditions (e.g., room temperature) and high efficiency, allowing us to quickly access a nonalternant ladder-like conjugated system with the in situ formation of multicyano substituents in the backbone. Analysis by 1H NMR, FT-Raman, and FT-IR spectra confirms the successful synthesis of the resulting cLPs. The combination of theoretical calculations and experimental characterizations reveals that the slightly contorted geometry coupled with a random assignment of trans- and cis-isomeric repeating units in each main chain contributes to improving the solubility of such rigid, multicyano nanoribbon systems. Apart from outstanding thermal stability, the resulting cLPs exhibit attractive red fluorescence, excellent redox properties, and strong π-π interactions coupled with orderly face-on packing in their thin-film states. They are proven to be the first example of ambipolar cLPs that show satisfactory hole and electron mobilities of up to 0.01 and 0.01 cm2 V-1 s-1, respectively. As we demonstrate, the Knoevenagel polycondensation chemistries open a new window to create complex and unique ladder-like nanoribbon systems under mild reaction conditions that are otherwise challenging to achieve.

The development of catalysts and auxiliaries for the synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) have recently seen significant advancements. Large quantities of structurally & functionally oriented COFs with a wide range of applications, such as gas adsorption, catalysis, separation, and drug delivery, have been explored. Recent achievements in this field are primarily focused on advancing synthetic methodologies, with catalysts playing a crucial role in achieving highly crystalline COF materials, particularly those featuring novel linkages and chemistry. A series of reviews have already been published over the last decade, covering the fundamentals, synthesis, and applications of COFs. However, despite the pivotal role that catalysts and auxiliaries play in forming COF materials and adjusting their properties (e.g., crystallinity, porosity, stability, and morphology), limited attention has been devoted to these essential components. In this Critical Review, we mainly focus on the state-of-the-art progress of catalysts and auxiliaries applied to the synthesis of COFs. The catalysts include four categories: acid catalysts, base catalysts, transition-metal catalysts, and other catalysts. The auxiliaries, such as modulators, oxygen, and surfactants, are discussed as well. This is then followed by the description of several specific applications derived from the utilization of catalysts and auxiliaries. Lastly, a perspective on the major challenges and opportunities associated with catalysts and auxiliaries is provided.

Tuning the Pyridine Units in Vinylene-Linked Covalent Organic Frameworks Boosting 2e- Oxygen Reduction Reaction

The N-doped carbon materials are supposed to be the efficient oxygen reduction reaction (ORR) catalysts with the undefined N-doped carbon ring groups. It is essential to well define the role of the nitrogen atoms of these carbon structures in active behavior. Even though, the covalent organic frameworks (COFs) with precise structures are well developed, but unable to exclude the polar linkages influence. This study presents a series of pyridine-containing COFs linked via nonpolar carbon-carbon double bonds (C = C). Their catalytic activity and selectivity for 2e- ORR are successfully modulated by locating the embedded pyridine nitrogen in the backbones through the linking modes of pyridine moieties within the frameworks. Such phenomena can be attributed to their different binding abilities toward O2, leading to the different binding strength of the intermediate OH* to the catalytic sites, also verified by the theoretical calculation. This work provides us a new insight to design high-efficiency ORR catalysts through the exact location of pyridine nitrogen.

Novel Olefin-Linked Covalent Organic Framework with Multifunctional Group Modification for the Fluorescence/Smartphone Detection of Uranyl Ion

Monitoring and purification of uranium contamination are of great importance for the rational utilization of uranium resources and maintaining the environment. In this work, an olefin-linked covalent organic framework (GC-TFPB) and its amidoxime-modified product (GC-TFPB-AO) are synthesized with 3-cyano-4,6-dimethyl-2-hydroxypyridine (GC) and 1,3,5-tris(4-formylphenyl) benzene (TFPB) by Knoevenagel condensation. GC-TFPB-AO results in specificity for rapid fluorescent/smartphone uranyl ion (UO22+) detection based on the synergistic effect of multifunctional groups (amidoxime, pyridine, and hydroxyl groups). GC-TFPB-AO features a rapid and highly sensitive detection and adsorption of UO22+ with a detection limit of 21.25 nM. In addition, it has a good recovery (100-111%) for fluorescence detection in real samples, demonstrating an excellent potential of predesigned olefin-linked fluorescent COFs in nuclear contaminated wastewater detection and removal.

Asymmetric Potential Model of Two-Dimensional Imine Covalent Organic Frameworks with Enhanced Quantum Efficiency for Photocatalytic Water Splitting

Two-dimensional (2D) covalent organic frameworks (COFs) assembled using building blocks have been widely employed in photocatalysis due to their customizable optoelectronic characteristics and porous structure, which facilitate charge carrier and mass movement. Nevertheless, the development of COF photocatalysts encounters a continuous obstacle in enhancing the efficiency of photocatalysis, impeded by a limited comprehension of the orbital interaction between molecular fragments and linkers. In this study, we present a model that examines the interaction between molecular fragments in an imine-based COF at the frontier molecular orbital level, enabling us to comprehend the impact of manipulating linkers on light adsorption, exciton efficiency, and catalytic activity. Our findings demonstrate that altering the connecting orientation of 14 R-C=N-R imine linkers in 2D COFs can enhance solar-to-hydrogen (STH) efficiency under visible light from 2.76% to 4.24%. This research has the potential to provide a valuable model for refining photocatalysts with enhanced photocatalytic performance.

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.

Synthetic Leaves Based on Crystalline Olefin-Linked Covalent Organic Frameworks for Efficient CO2 Photoreduction with Water

We report, for the first time, a new synthetic strategy for the preparation of crystalline two-dimensional olefin-linked covalent organic frameworks (COFs) based on aldol condensation between benzodifurandione and aromatic aldehydes. Olefin-linked COFs can be facilely crystallized through either a pyridine-promoted solvothermal process or a benzoic anhydride-mediated organic flux synthesis. The resultant COF leaf with high in-plane π-conjugation exhibits efficient visible-light-driven photoreduction of carbon dioxide (CO2) with water (H2O) in the absence of any photosensitizer, sacrificial agents, or cocatalysts. The production rate of carbon monoxide (CO) reaches as high as 158.1 μmol g-1 h-1 with near 100% CO selectivity, which is accompanied by the oxidation of H2O to oxygen. Both theoretical and experimental results confirm that the key lies in achieving exceptional photoinduced charge separation and low exciton binding. We anticipate that our findings will facilitate new possibilities for the development of semiconducting COFs with structural diversity and functional variability.

Strong Second-Harmonic Generation Induced by a Triphenylamine-Based Bismuth-Organic Framework for Photocatalytic Activity Enhancement

Taking advantage of the well-defined geometry of metal centers and highly directional metal-ligand coordination bonds, metal-organic frameworks (MOFs) have emerged as promising candidates for nonlinear optical (NLO) materials. In this work, taking a photoresponsive carboxylate triphenylamine derivative as an organic ligand, a bismuth-based MOF, Bi-NBC, NBC = 4',4‴,4‴″-nitrilotris(([1,1'-biphenyl]-4-carboxylic acid)) is obtained. Structure determination reveals that it is a potential NLO material derived from its noncentrosymmetric structure, which is finally confirmed by its rarely strong second harmonic generation (SHG) effect. Theoretical calculations reveal that the potential difference around Bi atoms is large; therefore, it leads to a strong local built-in electric field, which greatly facilitates the charge separation and transfer and finally improves the photocatalytic performance. Our results provide a reference for the exploration of MOFs with NLO properties.

Tetrazine-Linked Covalent Organic Frameworks With Acid Sensing and Photocatalytic Activity

The first synthesis and comprehensive characterization of two vinyl tetrazine-linked covalent organic frameworks (COF), TA-COF-1 and TA-COF-2, are reported. These materials exhibit high crystallinity and high specific surface areas of 1323 and 1114 m2 g-1. The COFs demonstrate favorable band positions and narrow band gaps suitable for light-driven applications. These advantages enable TA-COFs to act as reusable metal-free photocatalysts in the arylboronic acids oxidation and light-induced coupling of benzylamines. In addition, these TA-COFs show acid sensing capabilities, exhibiting visible and reversible color changes upon exposure to HCl solution, HCl vapor, and NH3 vapor. Further, the TA-COFs outperform a wide range of previously reported COF photocathodes. The tetrazine linker in the COF skeleton represents a significant advancement in the field of COF synthesis, enhancing the separation efficiency of charge carriers during the photoreaction and contributing to their photocathodic properties. TA-COFs can also degrade 5-nitro-1,2,4-triazol-3-one (NTO), an insensitive explosive present in industrial wastewater, in 20 min in a sunlight-driven photocatalytic process; thus, revealing dual functionality of the protonated TA-COFs as both photodegradation and Brønsted acid catalysts. This pioneering work opens new avenues for harnessing the potential of the tetrazine linker in COF-based materials, facilitating advances in catalysis, sensing, and other related fields.

Photoenzymatic CO2 Reduction Dominated by Collaborative Matching of Linkage and Linker in Covalent Organic Frameworks

Artificial photoenzymatic systems based on covalent organic frameworks (COFs) provide an interesting platform for converting CO2 to value-added fuels. However, the dual roles of COFs as photocatalysts and enzyme hosts showcase contradictory preferences for structures, which poses a great challenge for their rational design. Herein, we report the collaborative matching of linkages and linkers in COFs on their ability to exert both photocatalytic activity and enzyme loading, which has been neglected until now. The linkage-dependent linker regulation pattern was elucidated, and the optimal match showed a record-breaking apparent quantum efficiency at 420 nm for photocatalytic cofactor regeneration of 13.95% with a high turnover frequency of 5.3 mmol g-1 h-1, outperforming other reported crystalline framework photocatalysts. Moreover, theoretical calculations and experiments revealed the mechanism underlying the effects of matching the linkage and linker on exciton dissociation and charge migration in photocatalysis. This newfound understanding enabled the construction of COFs with both high photoactivity and large pores closer in size to the formate dehydrogenase, achieving high loading capacity and a suitable confinement effect. Remarkably, the artificial photoenzymatic system constructed according to optimal linkage-linker matching exhibited highly efficient CO2 reduction, yielding formic acid with a specific activity as high as 1.46 mmol g-1 catalyst h-1 and good reusability, paving the way for sustainable CO2 conversion driven by visible light.

Reticular framework materials for photocatalytic organic reactions

Photocatalytic organic reactions, harvesting solar energy to produce high value-added organic chemicals, have attracted increasing attention as a sustainable approach to address the global energy crisis and environmental issues. Reticular framework materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are widely considered as promising candidates for photocatalysis owing to their high crystallinity, tailorable pore environment and extensive structural diversity. Although the design and synthesis of MOFs and COFs have been intensively developed in the last 20 years, their applications in photocatalytic organic transformations are still in the preliminary stage, making their systematic summary necessary. Thus, this review aims to provide a comprehensive understanding and useful guidelines for the exploration of suitable MOF and COF photocatalysts towards appropriate photocatalytic organic reactions. The commonly used reactions are categorized to facilitate the identification of suitable reaction types. From a practical viewpoint, the fundamentals of experimental design, including active species, performance evaluation and external reaction conditions, are discussed in detail for easy experimentation. Furthermore, the latest advances in photocatalytic organic reactions of MOFs and COFs, including their composites, are comprehensively summarized according to the actual active sites, together with the discussion of their structure-property relationship. We believe that this study will be helpful for researchers to design novel reticular framework photocatalysts for various organic synthetic applications.

Olefin-linked covalent organic frameworks: synthesis and applications

Covalent organic frameworks (COFs) with high specific porosity, easy functionalization, and tailored structure are an emerging class of crystalline porous polymers that have been extensively exploited as ideal materials in various fields. Among them, sp2-carbon linked COFs with high chemical stability, porous backbone, and unique π-electron conjugated architectures structure have raised widespread attention. Specifically, the porous channels of olefin-linked COFs could be packed with active sites for catalysis and guest molecules, while π-π stacking interactions and conjugation systems pave the way for electron transfer. In recent years, many efforts have been devoted to the development of sp2-carbon linked COFs for applications in catalysis, energy storage, gas adsorption, and separation. In this review, we highlight the design principles, synthesis strategies, and impactful applications of olefin-linked COFs. We are looking forward to this review to deepen the understanding of the synthesis of olefin-linked COFs and motivate the further development of these novel conjugated organic materials with distinctive physicochemical properties, as well as their applications in a variety of fields.

Relative Local Electron Density Tuning in Metal-Covalent Organic Frameworks for Boosting CO2 Photoreduction

The high local electron density and efficient charge carrier separation are two important factors to affect photocatalytic activity, especially for the CO2 photoreduction reaction. However, the systematic studies on the structure-functional relationship regarding the above two factors based on precisely structure model are rarely reported. Herein, as a proof-of-concept, we developed a new strategy on the evaluation of local electron density by controlling the relative electron-deficient (ED) and electron-rich (ER) intensity of monomer at a molecular level based on three rational-designed vinylene-linked sp2 carbon-covalent organic frameworks (COFs). As expected, the as-prepared vinylene-linked sp2 carbon-conjugated metal-covalent organic framework (MCOFs) (VL-MCOF-1) with molecular junction exhibited excellent activities for CO2 -to-HCOOH conversion (283.41 μmol g-1  h-1 ) and high selectivity of 97.1 %, much higher than the VL-MCOF-2 and g-C34 N6 -COF, which is due to the synergistic effect of the multi-electronic metal clusters (Cu3 (PyCA)3 ) (PyCA=pyrazolate-4-carboxaldehyde) as strong ER roles and cyanopyridine units as ED roles and active sites, as well as the boosted photo-induced charge separation efficiency of vinyl connection and increased light utilization ability. These results not only provide a strategy for regulating the electron-density distribution of photocatalysts at the molecular level but also offers profound insights for metal clusters-based COFs to effective CO2 conversion.

Narrow-Pore Engineering of Vinylene-Linked Covalent Organic Frameworks with Weak Interaction-Triggered Multiple Responses

Vinylene-linked covalent organic frameworks (COFs) are emerging as promising crystalline materials, but their narrow pore engineering is severely impeded by the weak reversibility of the carbon-carbon double bond formation reaction, which has led to less exploration of their ultramicroporous structures and properties. Herein, we developed a single aromatic ring-based tetratopic monomer, tetramethylpyrazine, which undergoes a smooth Knoevenegal condensation at its four arylmethly carbon atoms with linear aromatic dialdehyde monomers upon the self-catalyzed activation of pyridine nitrogen-containing monomers in the presence of an organic anhydride. This has resulted in the formation of two vinylene-linked COFs, which both crystallized in orthorhombic lattices, and layered in AA stacking fashions along the vertical directions. They exhibit high surface areas and well-tailored ultramicropore sizes up to 0.5 nm. The unique cross-linking mode at two pairs of para-positions of each pyrazine unit through carbon-carbon double bonds afford them with π-extended conjugation over the in-plane backbones and substantial semiconducting characters. The resultant COFs can be well-dispersed in water to form stable sub-microparticles with negative charges (zeta potentials: ca. -30 mV), and exhibiting tunable aggregation behaviors through protonation/deprotonation. As a consequence, they exhibit pore-size-dependent colorimetric responses to various anions with different pKa values in high selectivity.

General Approach to Construct C-C Single Bond-Linked Covalent Organic Frameworks

C-C single bond-linked covalent organic frameworks (CSBL-COFs) are extremely needed because of their excellent stabilities and potential applications in harsh conditions. However, strategies to generate CSBL-COFs are limited to the acetylenic self-homocoupling Glaser-Hay reaction or post-synthetic reduction of vinylene-based COFs. Exploring new strategies to expand the realm of CSBL-COFs is urgently needed but extremely challenging. To address the synthetic challenges, we for the first time developed a general approach via the reaction between aromatic aldehydes and active methyl group-involving monomers with enhanced acidity, which realized the successful construction of a series of CSBL-COFs. As expected, the obtained CSBL-COFs exhibited outstanding chemical stability, which can stabilize in 6 M NaOH, 3 M HCl, boiling water, and 100 mg/mL NaBH4 for at least 3 days. It is important to mention that CSBL-COFs possess a large amount of ionic sites distributed throughout the networks; gentle shaking allowed our COFs to easily self-disperse as nanoparticles and suspend in water for at least 12 h without reprecipitating. As far as we know, such self-dispersed COFs with high water dispersity are rare to date, and few examples are mainly limited to the guanidinium- and pseudorotaxane-based COFs. Our work thus developed a family of self-dispersed COFs for potential applications in different sorts of fields. Our contribution would thus pave a new avenue for constructing a broader class of CSBL-COFs for their wide applications in various fields.

Fabricating s-collidine-derived vinylene-linked covalent organic frameworks for photocatalysis

Research into vinyl-linked covalent organic frameworks (COFs) has grown significantly in recent years due to various attractive properties. Herein, we design and synthesize two highly crystalline and stable 2,4,6-collidine-derived vinylene-linked 2D COFs. Both COFs can act as efficient photocatalysts to facilitate visible-light-driven aerobic oxidation. The TM-TBT-COF was observed to exhibit superior activity and recyclability owing to its excellent semiconducting properties.

Olefin-Linked Covalent Organic Frameworks with Electronegative Channels as Cationic Highways for Sustainable Lithium Metal Battery Anodes

Despite the enormous interest in Li metal as an ideal anode material, the uncontrollable Li dendrite growth and unstable solid electrolyte interphase have plagued its practical application. These limitations can be attributed to the sluggish and uneven Li+ migration towards Li metal surface. Here, we report olefin-linked covalent organic frameworks (COFs) with electronegative channels for facilitating selective Li+ transport. The triazine rings and fluorinated groups of the COFs are introduced as electron-rich sites capable of enhancing salt dissociation and guiding uniform Li+ flux within the channels, resulting in a high Li+ transference number (0.85) and high ionic conductivity (1.78 mS cm-1 ). The COFs are mixed with a polymeric binder to form mixed matrix membranes. These membranes enable reliable Li plating/stripping cyclability over 700 h in Li/Li symmetric cells and stable capacity retention in Li/LiFePO4 cells, demonstrating its potential as a viable cationic highway for accelerating Li+ conduction.

Design and Synthesis of a Triazine-Based sp2 Carbon-Conjugated Covalent Organic Framework for Blue Light Photocatalysis

Fully conjugated covalent organic frameworks (COFs) can exhibit great potential in semiconductor photocatalysis. But their syntheses remain elusive due to the low reversibility of vinylene linkage. Herein, by tuning the amount of base and temperature, a novel triazine-based sp2  carbon-conjugated COF (TA-sp2 c-COF) is successfully constructed over Cs2 CO3 . Besides, the influence of modulating factors on the chemical and optoelectronic properties of TA-sp2 c-COF is thoroughly investigated. TA-sp2 c-COF adopts an eclipsed AA stacking structure with uniform micropores (1.4 nm). The blue light photocatalysis of the highly crystalline TA-sp2 c-COF is established for the selective oxidative coupling of amines with oxygen, and the predominant role of superoxide is identified in forming imines. This work foretells that meticulous modulation of reaction conditions is the key to constructing sp2  carbon-conjugated COFs toward solar photocatalysis.

From Supramolecular Organic Cages to Porous Covalent Organic Frameworks for Enhancing Iodine Adsorption Capability by Fully Exposed Nitrogen-Rich Sites

In order to overcome the limitations of supramolecular organic cages for their incomplete accessibility of active sites in the solid state and uneasy recyclability in liquid solution, herein a nitrogen-rich organic cage is rationally linked into framework systems and four isoreticular covalent organic frameworks (COFs), that is, Cage-TFB-COF, Cage-NTBA-COF, Cage-TFPB-COF, and Cage-TFPT-COF, are successfully synthesized. Structure determination reveals that they are all high-quality crystalline materials derived from the eclipsed packing of related isoreticular two-dimensional frameworks. Since the nitrogen-rich sites usually have a high affinity toward iodine species, iodine adsorption investigations are carried out and the results show that all of them display an enhancement in iodine adsorption capacities. Especially, Cage-NTBA-COF exhibits an iodine adsorption capacity of 304 wt%, 14-fold higher than the solid sample packed from the cage itself. The strong interactions between the nitrogen-rich sites and the adsorbed iodine species are revealed by spectral analyses. This work demonstrates that, utilizing the reticular chemistry strategy to extend the close-packed supramolecular organic cages into crystalline porous framework solids, their inherent properties can be greatly exploited for targeted applications.

Recent Advances in the Use of Covalent Organic Frameworks as Heterogenous Photocatalysts in Organic Synthesis

Organic photochemistry is intensely developed in the 1980s, in which the nature of excited electronic states and the energy and electron transfer processes are thoroughly studied and finally well-understood. This knowledge from molecular organic photochemistry can be transferred to the design of covalent organic frameworks (COFs) as active visible-light photocatalysts. COFs constitute a new class of crystalline porous materials with substantial application potentials. Featured with outstanding structural tunability, large porosity, high surface area, excellent stability, and unique photoelectronic properties, COFs are studied as potential candidates in various research areas (e.g., photocatalysis). This review aims to provide the state-of-the-art insights into the design of COF photocatalysts (pristine, functionalized, and hybrid COFs) for organic transformations. The catalytic reaction mechanism of COF-based photocatalysts and the influence of dimensionality and crystallinity on heterogenous photocatalysis performance are also discussed, followed by perspectives and prospects on the main challenges and opportunities in future research of COFs and COF-based photocatalysts.

Sulfonic-Pendent Vinylene-Linked Covalent Organic Frameworks Enabling Benchmark Potential in Advanced Energy

Both proton exchange membrane fuel cells and uranium-based nuclear techniques represent two green and advanced energies. However, both of them still face some intractable scientific and industrial problems. For the former, established proton-conduction materials always suffer one or another defect such as low proton conductivity, high activation energy, bad durability, or just small-scale product; while for the later, there still lacks available adsorbent to selectively recover of UO₂ ²⁺ from concentrated nitric acid (>1 M) during the spent fuel reprocessing due to the deactivation of the adsorption site or the decomposition of adsorbent under such rigorous conditions. It is found that the above two issues can be well solved by the construction of sulfonic-pendent vinylene-linked covalent organic frameworks (COFs), since these COFs contain abundant sulfonic units for both intrinsic proton conduction and UO₂ ²⁺ capture through strong coordination fixation and vinylene linkage that enhances the stability up to 12 M nitric acid (one of the best materials surviving in 12 M HNO₃ ).

Mixed-Component Metal-Organic Framework for Boosting Synergistic Photoactivation of C(sp3)-H and Oxygen

Synergistic catalysis is an efficient and powerful strategy for simultaneously activating reactants by multiple active sites to promote the efficiency of difficult and challenging catalytic reactions. Meanwhile, enzymes with multi-active-site synergistic catalytic properties possessing high efficiency and high selectivity have become the goal pursued in the field of catalytic chemistry in recent years. Metal-organic frameworks (MOFs), as an effective heterogeneous catalytic platform, that can integrate multiple active sites for synergistic catalysis like enzymatic systems have recently attracted interest. Herein, we report a doubly interpenetrated metal-organic framework with dual active sites, Mn^III-porphyrin sites to directly activate molecular oxygen and fluoren-9-one sites to produce a hydrogen atom transfer (HAT) agent by the proton-coupled electron transfer (PCET) process to simultaneously activate inert C(sp³)-H bonds for efficient inert C(sp³)-H bond oxidation under mild conditions. The bifunctional mixed-component MOF structure forced the two catalytic sites closer together to a more suitable distance, exhibiting high photocatalytic activity for inert C(sp³)-H bond oxidation with almost unique selectivity under mild conditions. The density functional theory (DFT) calculation of free energy during the whole catalytic process demonstrated that it is likely that the synergistic catalytic process occurred in the interframework to accelerate the catalytic reaction. The assembling mixed-component MOF for synergistic catalysis would be a prospective approach for the inert C(sp³)-H photoactivation and functionalization.

Construction of Monophosphine-Metal Complexes in Privileged Diphosphine-Based Covalent Organic Frameworks for Catalytic Asymmetric Hydrogenation

Privileged diphosphine ligands that chelate many transition metals to form stable chelation complexes are essential in a variety of catalytic processes. However, the exact identity of the catalytically active moieties remains ambiguous because the chelated metal catalysts may undergo rearrangement during catalysis to produce monophosphine-metal complexes, which are hard to isolate and evaluate the activities. By taking advantage of the isolation of two phosphorus atoms, we demonstrate here the successful construction of chiral monophosphine-Ir/Ru complexes of diphosphine ligands in covalent organic frameworks (COFs) for enantioselective hydrogenation. By condensation of the tetraaldehyde of enantiopure MeO-BIPHEP and linear aromatic diamines, we prepare two homochiral two-dimensional COFs with ABC stacking, in which the two P atoms of each diphosphine are separated and fixed far apart. Post-synthetic metalations of the COFs thus afford the single-site Ir/Ru-monophosphine catalysts, in contrast to the homogeneous chelated analogues, that demonstrated excellent catalytic and recyclable performance in the asymmetric hydrogenation of quinolines and β-ketoesters, affording up to 99.9% enantiomeric excess. Owing to the fact that the porous catalyst is capable of adsorbing and concentrating hydrogen, the catalytic reactions are promoted under ambient/medium pressure, which are typically performed under high pressure for homogeneous catalysis. This work not only shows that monophosphine-metal complexes of diphosphines can be catalytically active centers for asymmetric hydrogenation reactions but also provides a new strategy to prepare new types of privileged phosphine-based heterogeneous catalysts.

Ultrathin 2D Cerium-Based Metal-Organic Framework Nanosheet That Boosts Selective Oxidation of Inert C(sp3 )H Bond through Multiphoton Excitation

The development of methodologies for inducing and tailoring activities of catalysts is an important issue in various catalysis. The ultrathin 2D monolayer metal-organic framework (MOF) nanosheets with more accessible active sites and faster diffusion obtained by exfoliating 3D layered MOFs are of great potential as heterogeneous catalysts, but the rational design and preparation of 3D layered MOFs remains a grand challenge. Herein, a novel weak electrostatic interaction strategy to construct a 3D layered cerium-bearing MOF by coordinating chlorine-capped cerium nodes and linear photoactive methyl viologen (MV⁺ ) organic linkers is used. Under multiphoton excitation, the MV⁺ ligands and CeCl chromophores are triggered consecutively to form the high activity chlorine radical (Cl^• ) for activation of inert C(sp³ )H bond through a hydrogen atom transfer. Benefiting from framework confinement effects, synergistic effects of two active sites and/or flexibility of the ultrathin framework nanosheets with high surface utilization, the observed activities increase in the order CeCl₃ /MV⁺  < bulk 3D MOF crystals < 2D MOF nanosheets in photocatalysis. This work not only contributes a new strategy to construct 3D layered MOFs and their ultrathin nanosheets but also paves the way to use nanostructured MOFs to handle synergy of multiple molecular catalysts.

Three-Component Donor-π-Acceptor Covalent-Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

Two-dimensional covalent-organic frameworks (2D COFs) have recently emerged as great prospects for their applications as new photocatalytic platforms in solar-to-hydrogen conversion; nevertheless, their inefficient solar energy capture and fast charge recombination hinder the improvement of photocatalytic hydrogen production performance. Herein, two photoactive three-component donor-π-acceptor (TCDA) materials were constructed using a multicomponent synthesis strategy by introducing electron-deficient triazine and electron-rich benzotrithiophene moieties into frameworks through sp² carbon and imine linkages, respectively. Compared with two-component COFs, the novel TCDA-COFs are more convenient in regulating the inherent photophysical properties, thereby realizing outstanding photocatalytic activity for hydrogen evolution from water. Remarkably, the first sp² carbon-linked TCDA-COF displays an impressive hydrogen evolution rate of 70.8 ± 1.9 mmol g^-1 h^-1 with excellent reusability in the presence of 1 wt % Pt under visible-light illumination (420-780 nm). Utilizing the combination of diversified spectroscopy and theoretical prediction, we show that the full π-conjugated linkage not only effectively broadens the visible-light harvesting of COFs but also enhances charge transfer and separation efficiency.

Monolayer-Assisted Surface-Initiated Schiff-Base-Mediated Aldol Polycondensation for the Synthesis of Crystalline sp2 Carbon-Conjugated Covalent Organic Framework Thin Films

sp² carbon-conjugated covalent organic frameworks (sp²c-COFs) with superb in-plane π-conjugations, high chemical stability, and robust framework structure are expected to be ideal films/membranes for a wide range of applications including energy-related devices and optoelectronics. However, so far, sp²c-COFs have been mainly limited to microcrystalline powders, and this consequently hampered their performances in devices. Herein, we report a simple and robust methodology to fabricate large-area, free-standing, and crystalline sp²c-COF films (TFPT-TMT and TB-TMT) on various solid substrates (e.g., fluorine-doped tin oxide, aluminum sheet, polyacrylonitrile membrane) by self-assembly monolayer-assisted surface-initiated Schiff-base-mediated aldol polycondensation (namely, SI-SBMAP). The resultant sp²c-COF films show lateral sizes up to 120 cm² and tunable thickness from tens of nanometers to a few micrometers. Owing to the robust framework and highly ordered quasi-1D channels, the sp²c-COF membrane-based osmotic power generator presents an output power density of 14.1 W m^-2 under harsh conditions, outperforming most reported COF membranes as well as commercialized benchmark devices (5 W m^-2). This work demonstrates a simple and robust interfacial methodology for the fabrication of sp²c-COF films/membranes for green energy applications and potential optoelectronics.

Interface-confined synthesis of a nonplanar redox-active covalent organic framework film for synaptic memristors

The development of novel synthetic methodologies and unprecedented structures of covalent organic framework (COF) films is of great importance for exploring their potential applications in optoelectronic devices, sensors, and membrane separation. From the point of view of monomer selection, rigid building blocks are always the first choice for synthesizing crystalline COF films. However, the preparation of COF films with flexible building units remains challenging. Herein, by introducing flexible triphenylamine-based building units, a nonplanar COF film (TFPA-TAPA film) is fabricated via liquid-liquid interface-confined synthesis at room temperature and atmospheric pressure. The growth mechanism of the flexible building units at the liquid-liquid interface is related to the transformation of strip-type slices into free-standing COF films by dynamic covalent chemistry. As a proof-of-concept, the as-fabricated Al/TFPA-TAPA/ITO device shows excellent multilevel storage and history-dependent memristive switching behavior. The synaptic potentiation/depression, human learning and memorization functions, as well as the transition from short-term synaptic plasticity to long-term plasticity, are successfully emulated by using this synaptic memristor.

Integrating Bifunctionality and Chemical Stability in Covalent Organic Frameworks via One-Pot Multicomponent Reactions for Solar-Driven H2O2 Production

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1-3 and DMCR-1NH) equipped with an acid-base bifunctionality. These DMCR-COFs show superior photocatalytic H₂O₂ evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

Application of fully conjugated covalent organic frameworks in photocatalytic carbon dioxide reduction performance

Photocatalytic reduction of carbon dioxide into useful feedstocks has attracted increasing attention. In this study, a fully conjugated COF material COF-TMT-A with the main structure containing an alkyne group and triazine part was synthesized using sp²-carbon-carbon double bond (CC) linked COF as a research target. The prepared COF materials were characterized in detail by FT-IR, PXRD, and ¹³C solid-state NMR. The introduction of an alkyne group not only enhanced the conjugated π-electron leaving domain but also optimized the electronic band structure and significantly improved the photocatalytic activity. The selectivity for the product HCOO was as high as 99%. A 10 h photocatalytic CO₂ reduction experiment was carried out, and COF-TMT-A showed a significantly higher HCOO^- yield of about 43 μmol compared with COF-701 and the ligand.