Covalent Organic Frameworks: Linkage Chemistry and Its Critical Role in The Evolution of π Electronic Structures and Functions.

Covalent organic frameworks (COFs) provide a molecular platform for designing a novel class of functional materials with well-defined structures. A crucial structural parameter is the linkage, which dictates how knot and linker units are connected to form two-dimensional polymers and layer frameworks, shaping ordered π-array and porous architectures. However, the roles of linkage in the development of ordered π electronic structures and functions remain fundamental yet unresolved issues. Here we report the designed synthesis of COFs featuring four representative linkages: hydrazone, imine, azine, and C=C bonds, to elucidate their impacts on the evolution of π electronic structures and functions. Our observations revealed that the hydrazone linkage provides a non-conjugated connection, while imine and azine allow partial π conjugation, and the C=C bond permits full π-conjugation. Importantly, the linkage profoundly influences the control of π electronic structures and functions, unraveling its pivotal role in determining key electronic properties such as band gap, frontier energy levels, light absorption, luminescence, carrier density and mobility, and magnetic permeability. These findings highlight the significance of linkage chemistry in COFs and offer a general and transformative guidance for designing framework materials to achieve electronic functions.

Constructing Highly Emissive Covalent Organic Frameworks for Fe3+ Ion Detection via Wall Function.

Covalent organic frameworks (COFs) represent a new type of crystalline porous polymers that possess pre-designed skeletons, uniform nanopores, and ordered π structure. These attributes make them well-suited for the design of light-emitting materials. However, the majority of COFs exhibits poor luminescence due to aggregation-caused quenching (ACQ), resulting from the strong interaction between adjacent layers. To break the limitation, the building units with three methoxy groups on the walls are used to construct TM-OMe-EBTHz-COF, which suppresses the ACQ effects to improve light-emitting activity of COF. The TM-OMe-EBTHz-COF exhibits a notable emission of yellow-green luminescence in the solid state, with a remarkably high absolute quantum yield of 21.1%. The methoxy groups and hydrazine linkage form three coordination sites, contributing to excellent performance in metal ions sensing. The TM-OMe-EBTHz-COF demonstrates high sensitivity and selectivity to Fe3+ ion. Importantly, the low detection limit is below 150 nanomolar, ranking it among the best-performing Fe3+ sensor systems.

Crystalline, Porous Helicene Covalent Organic Frameworks.

Helicenes are a class of fascinating chiral helical molecules with rich chemistry developed continuously over the past 100 years. Their helical, conjugated, and twisted structures make them attractive for constructing molecular systems. However, studies over the past century are mainly focused on synthesizing helicenes with increased numbers of aromatic rings and complex heterostructures, while research on inorganic, organic, and polymeric helicene materials is still embryonic. Herein, we report the first examples of helicene covalent organic frameworks, i.e., [7]Helicene sp2 c-COF-1, by condensing [7]Helicene dialdehyde with trimethyl triazine via the C=C bond formation reaction under solvothermal conditions. The resultant [7]Helicene sp2 c-COF-1 exhibits prominent X-ray diffraction peaks and assumes a highly ordered 2D lattice structure originated from the twisted configuration of [7]Helicene unit. The C=C linked [7]Helicene sp2 c-COF-1 materials exhibited extended π conjugation and broadly tuned their absorption, emission, redox activity, photoconductivity, and light-emitting activity, demonstrating rich multifunctionalities and great potentials in developing various applications. This work opens a way to a new family of COFs as well as helicene materials, enabling the exploration of unprecedented π architectures and properties.

Infrared Image Caption Based on Object-Oriented Attention.

With the ongoing development of image technology, the deployment of various intelligent applications on embedded devices has attracted increased attention in the industry. One such application is automatic image captioning for infrared images, which involves converting images into text. This practical task is widely used in night security, as well as for understanding night scenes and other scenarios. However, due to the differences in image features and the complexity of semantic information, generating captions for infrared images remains a challenging task. From the perspective of deployment and application, to improve the correlation between descriptions and objects, we introduced the YOLOv6 and LSTM as encoder-decoder structure and proposed infrared image caption based on object-oriented attention. Firstly, to improve the domain adaptability of the detector, we optimized the pseudo-label learning process. Secondly, we proposed the object-oriented attention method to address the alignment problem between complex semantic information and embedded words. This method helps select the most crucial features of the object region and guides the caption model in generating words that are more relevant to the object. Our methods have shown good performance on the infrared image and can produce words explicitly associated with the object regions located by the detector. The robustness and effectiveness of the proposed methods were demonstrated through evaluation on various datasets, along with other state-of-the-art methods. Our approach achieved BLUE-4 scores of 31.6 and 41.2 on KAIST and Infrared City and Town datasets, respectively. Our approach provides a feasible solution for the deployment of embedded devices in industrial applications.

Linkage Design in Two-Dimensional Covalent Organic Frameworks for High Iodine Uptake.

Radioactive iodine waste in the nuclear field is harmful to the environment and human health. Covalent organic frameworks (COFs) are a novel kind of porous organic material with a well-fine and unformal structure, which is an excellent candidate as a solid adsorbent for iodine adsorption. Herein, a linkage design is proposed for effective iodine adsorption. Imine-linkage COF (I-COF) and hydrazone-linked COF (H-COF) are constructed under solvothermal conditions. The Brunauer-Emmett-Teller surface area of H-COF is as high as 1747 m2 g-1 . Furthermore, the H-COF shows high porosity, stability, and rich atoms in the linkage. As a result, the work outperforms most of the previous reports in iodine capture with a high capture value at 5.72 g g-1 .

Molecular engineering of dihydroxyanthraquinone-based electrolytes for high-capacity aqueous organic redox flow batteries.

Aqueous organic redox flow batteries (AORFBs) are a promising technology for large-scale electricity energy storage to realize efficient utilization of intermittent renewable energy. In particular, organic molecules are a class of metal-free compounds that consist of earth-abundant elements with good synthetic tunability, electrochemical reversibility and reaction rates. However, the short cycle lifetime and low capacity of AORFBs act as stumbling blocks for their practical deployment. To circumvent these issues, here, we report molecular engineered dihydroxyanthraquinone (DHAQ)-based alkaline electrolytes. Via computational studies and operando measurements, we initially demonstrate the presence of a hydrogen bond-mediated degradation mechanism of DHAQ molecules during electrochemical reactions. Afterwards, we apply a molecular engineering strategy based on redox-active polymers to develop capacity-boosting composite electrolytes. Indeed, by coupling a 1,5-DHAQ/poly(anthraquinonyl sulfide)/carbon black anolyte and a [Fe(CN)6]3-/4- alkaline catholyte, we report an AORFB capable of delivering a stable cell discharge capacity of about 573 mAh at 20 mA/cm2 after 1100 h of cycling and an average cell discharge voltage of about 0.89 V at the same current density.

Porous organic polymers for light-driven organic transformations.

As a new generation of porous materials, porous organic polymers (POPs), have recently emerged as a powerful platform of heterogeneous photocatalysis. POPs are constructed using extensive organic synthesis methodologies, with various functional organic units being connected via high-energy covalent bonds. This review systematically presents the recent advances in POPs for visible-light driven organic transformations. Herein, we firstly summarize the common construction strategies for POP-based photocatalysts based on two major approaches: pre-design and post-modification; secondly, we categorize and summarize the synthesis methods and organic reaction types for constructing various types of POPs. We then classify and introduce the specific reactions of current light-driven POP-mediated organic transformations. Finally, we outline the current state of development and the problems faced in light-driven organic transformations by POPs, and we present some perspectives to motivate the reader to explore solutions to these problems and confront the present challenges in the development process.

Frequency domain maximum correntropy criterion spline adaptive filtering.

A filtering algorithm based on frequency domain spline type, frequency domain spline adaptive filters (FDSAF), effectively reducing the computational complexity of the filter. However, the FDSAF algorithm is unable to suppress non-Gaussian impulsive noises. To suppression non-Gaussian impulsive noises along with having comparable operation time, a maximum correntropy criterion (MCC) based frequency domain spline adaptive filter called frequency domain maximum correntropy criterion spline adaptive filter (FDSAF-MCC) is developed in this paper. Further, the bound on learning rate for convergence of the proposed algorithm is also studied. And through experimental simulations verify the effectiveness of the proposed algorithm in suppressing non-Gaussian impulsive noises. Compared with the existing frequency domain spline adaptive filter, the proposed algorithm has better performance.

Light-Emitting Conjugated Organic Polymer as an Efficient Fluorescent Probe for Cu2+ Ions Detection and Cell Imaging.

Conjugated organic polymers (COPs) have been excellent candidates because the conjugated structure occupied π structure that is useful to develop light-emitting materials. However, most COPs emitt weak luminescence owing to the H-aggregation effect. Light-emitting conjugated organic polymers (LCOP-1) possess rich butyl groups anchored in the skeleton to enhance light-emitting activity via reducing the H-aggregation effect. Owing to abundant hydroxyl and nitrogen atoms, LCOP-1 exhibits high sensitivity, selectivity, and fast response to Cu2+ ions within 1 min in comparison with the cations of Na+ , Mg2+ , Al3+ , Zn2+ , Cd2+ , Ni2+ , Cr3+ , Hg2+ , Fe3+ , Fe2+ , Pb2+ , Co2+ , etc. The detection limit can be down to nanomolar. Moreover, the sensor exhibits detection toward Cu2+ ions via a naked eye colorful change from pale-yellow to yellowish-brown. Furthermore, the light-emitting probe also successfully achieves the detection of Cu2+ ions in cells without cytotoxicity, indicating its great potentials in biological function.

Diyne-linked and fully π-conjugated polymetalloporphyrin nanosheets for outstanding heterogeneous catalysis.

Two-dimensional, ultrathin, robust, and fully π-conjugated organic nanomaterials are highly desirable for application in various fields due to their unique photoelectric characteristics and great number of exposed active sites. However, such matters combining excellent stability, full π-conjugation and adjustability are rare, which has become a bottleneck for their practical application. Herein, we present a novel kind of diyne-linked polymetalloporphyrin nanosheet featuring permanent porosity and full π-conjugation, which exhibits a high-aspect-ratio, outstanding stability and convenient tailoring for electronic structures. Importantly, the novel nanosheets with monodisperse nickel atoms were found to be outstanding heterogeneous catalyst with unprecedented catalytic activity and selectivity for 4-nitrophenol reduction to 4-aminophenol under mild conditions. The findings recommend that diyne-linked polymetalloporphyrin nanosheets may offer new platforms for the conversion of photoelectricity and energy in the future.

Recent Progress in Metal-Free Covalent Organic Frameworks as Heterogeneous Catalysts.

Covalent organic frameworks (COFs), connecting different organic units into one system through covalent bonds, are crystalline organic porous materials with 2D or 3D networks. Compared with conventional porous materials such as inorganic zeolite, active carbon, and metal-organic frameworks, COFs are a new type of porous materials with well-designed pore structure, high surface area, outstanding stability, and easy functionalization at the molecular level, which have attracted extensive attention in various fields, such as energy storage, gas separation, sensing, photoluminescence, proton conduction, magnetic properties, drug delivery, and heterogeneous catalysis. Herein, the recent advances in metal-free COFs as a versatile platform for heterogeneous catalysis in a wide range of chemical reactions are presented and the synthetic strategy and promising catalytic applications of COF-based catalysts (including photocatalysis) are summarized. According to the types of catalytic reactions, this review is divided into the following five parts for discussion: achiral organic catalysis, chiral organic conversion, photocatalytic organic reactions, photocatalytic energy conversion (including water splitting and the reduction of carbon dioxide), and photocatalytic pollutant degradation. Furthermore, the remaining challenges and prospects of COFs as heterogeneous catalysts are also presented.

Conjugated Microporous Polymers as Heterogeneous Photocatalysts for Efficient Degradation of a Mustard-Gas Simulant.

Compared with traditional metal-based photosensitizers, heterogeneous and organic photocatalysts with visible-light activity are more environmentally friendly and sustainable. The simultaneous introduction of electron-rich and electron-deficient units in donor-acceptor typed conjugated microporous polymer (CMP) photocatalysts can significantly enhance their visible-light harvesting and separation efficiency of photogenerated carriers. Here, two carbazole-based CMPs (CzBSe-CMP and CzBQn-CMP) were successfully constructed through a cost-effective process. They show inherent porosity with large Brunauer-Emmett-Teller surface area and excellent thermal and chemical stability. Their photoelectric properties, energy levels, optical band gaps, transient photocurrent response, and photocatalytic activity could be conveniently tailored through tuning the electron-deficient moiety in polymer networks. More importantly, CzBSe-CMP was found to be a superior solid photocatalyst for selective photo-oxidation of mustard gas simulant 2-chloroethyl ethyl sulfide into a nontoxic product by using molecular oxygen as a sustainable oxygen source under visible-light illumination. In addition, the obtained CMP-based photocatalysts also showed excellent recyclability and could be reutilized through adding more simulants or a simple separation procedure. The current contribution provides great application prospects for CMPs as metal-free, solid photocatalysts in organic transformation and environmental protection.

Metalloporphyrin-Based Hypercrosslinked Polymers Catalyze Hetero-Diels-Alder Reactions of Unactivated Aldehydes with Simple Dienes: A Fascinating Strategy for the Construction of Heterogeneous Catalysts.

We describe a novel and intriguing strategy for the construction of efficient heterogeneous catalysts by hypercrosslinking catalyst molecules in a one-pot Friedel-Crafts alkylation reaction. The new hypercrosslinked polymers (HCPs) as porous solid catalysts exhibit the combined advantages of homogeneous and heterogeneous catalysis, owing to their high surface area, good stability, and tailoring of catalytic centers on the frameworks. Indeed, a new class of metalloporphyrin-based HCPs were successfully synthesized using modified iron(III) porphyrin complexes as building blocks, and the resulting networks were found to be excellent recyclable heterogeneous catalysts for the hetero-Diels-Alder reaction of unactivated aldehydes with 1,3-dienes. Moreover, this new strategy showed wide adaptability, and many kinds of homogeneous-like solid-based catalysts with high catalytic performance and excellent recyclability were also constructed.

The robustness of multiplex networks under layer node-based attack.

From transportation networks to complex infrastructures, and to social and economic networks, a large variety of systems can be described in terms of multiplex networks formed by a set of nodes interacting through different network layers. Network robustness, as one of the most successful application areas of complex networks, has attracted great interest in a myriad of research realms. In this regard, how multiplex networks respond to potential attack is still an open issue. Here we study the robustness of multiplex networks under layer node-based random or targeted attack, which means that nodes just suffer attacks in a given layer yet no additional influence to their connections beyond this layer. A theoretical analysis framework is proposed to calculate the critical threshold and the size of giant component of multiplex networks when nodes are removed randomly or intentionally. Via numerous simulations, it is unveiled that the theoretical method can accurately predict the threshold and the size of giant component, irrespective of attack strategies. Moreover, we also compare the robustness of multiplex networks under multiplex node-based attack and layer node-based attack, and find that layer node-based attack makes multiplex networks more vulnerable, regardless of average degree and underlying topology.

An Azine-Linked Covalent Organic Framework: Synthesis, Characterization and Efficient Gas Storage.

A azine-linked covalent organic framework, COF-JLU2, was designed and synthesized by condensation of hydrazine hydrate and 1,3,5-triformylphloroglucinol under solvothermal conditions for the first time. The new covalent organic framework material combines permanent micropores, high crystallinity, good thermal and chemical stability, and abundant heteroatom activated sites in the skeleton. COF-JLU2 possesses a moderate BET surface area of over 410 m(2) g(-1) with a pore volume of 0.56 cm(3) g(-1) . Specifically, COF-JLU2 displays remarkable carbon dioxide uptake (up to 217 mg g(-1) ) and methane uptake (38 mg g(-1) ) at 273 K and 1 bar, as well as high CO2 /N2 (77) selectivity. Furthermore, we further highlight that it exhibits a higher hydrogen storage capacity (16 mg g(-1) ) than those of reported COFs at 77 K and 1 bar.