Construction of Covalent Organic Frameworks via a Visible-Light-Activated Photocatalytic Multicomponent Reaction

Harnessing Electro-Descriptors for Mechanistic and Machine Learning Analysis of Photocatalytic Organic Reactions

Photocatalysis has emerged as an effective tool for addressing the contemporary challenges in organic synthesis. However, the trial-and-error-based screening of feasible substrates and optimal reaction conditions remains time-consuming and potentially expensive in industrial practice. Here, we demonstrate an electrochemical-based data-acquisition approach that derives a simple set of redox-relevant electro-descriptors for effective mechanistic analysis and performance evaluation through machine learning (ML) in photocatalytic synthesis. These electro-descriptors correlate to the quantification of shifted charge transfer processes in response to the photoirradiation and enabled construction of reactivity diagram where high-yield reactive "hot zones" can reflect subtle changes of the reaction system. For the model reaction of photocatalytic deoxygenation reaction, the influence of varying carboxylic acids (substrate A, oxidation-intended) and alkenes (substrate B, reduction-intended) and varying reaction conditions on the reaction yield can be visualized, while mathematical analysis of the electro-descriptor patterns further revealed distinct mechanistic/kinetic impacts from different substrates and conditions. Additionally, in the application of ML algorithms, the experimentally derived electro-descriptors reflect an overall redox kinetic outcome contributed from vast reaction parameters, serving as a capable means to reduce the dimensionality in the case of complex multiparameter chemical space. As a result, utilization of electro-descriptors enabled efficient and robust quantitative evaluation of chemical reactivity, demonstrating promising potential of introducing operando-relevant experimental insights in the data-driven chemistry.

Unprecedented POSS-Linked 3D Covalent Organic Frameworks with 2-Fold Interpenetrated scu or sqc Topology Regulated by Porphyrin Center for Photocatalytic CO2 Reduction

The synthesis and characterization of porphyrin center regulated three-dimensional covalent organic frameworks (COFs) with 2-fold interpenetrated scu or sqc topology have been investigated. These COFs exhibit unique structural features and properties, making them promising candidates for photocatalytic applications in CO2 reduction and artemisinin synthesis. The porphyrin center serves as an anchor for metal ions, allowing precise control over structures and functions of the frameworks. Furthermore, the metal coordination within the framework imparts desirable catalytic properties, enabling their potential use in photocatalytic reactions. Overall, these porphyrin center regulated metal-controlled COFs offer exciting opportunities for the development of advanced materials with tailored functionalities.

A sensing platform for ascorbic acid based on the spatial confinement of covalent organic frameworks

Essential to many activities in our bodies, ascorbic acid is a small molecule essential to human health and physiological processes. In this study, a covalent organic framework called TpNda-COF was synthesized, which is composed of Tp (triformylephloroglucinol) and Nda (1, 5-napthalenediamine). This framework acts as a mimic enzyme and displays excellent oxidase-like activity when stimulated with purple light (at = 405 nm). It catalyzes the oxidation of 3,3',5,5'-tetramethylbenzydine (TMB) by generating O2- free radicals in the presence of oxygen. The resulting oxTMB shows a characteristic absorption peak at 652 nm. The biomimetic catalysis efficiency is significantly improved due to spatial restriction. By introducing ascorbic acid (AA) in the system, the blue oxTMB is reduced to colorless TMB. The decrease in absorption peak intensity can be quantitatively measured using a UV-Vis spectrophotometer, enabling the detection of AA. The sensing platform demonstrates excellent selectivity and sensitivity. It has a wide linear detection range from 5 μM to 50 μM, with a low detection limit of 1.44 μM. Advantages such as the easy control of light, high stability and efficient oxidation are provided by the TpNda-COF mimic oxidase. This innovative method presents a promising and cost-effective approach for rapid detection of ascorbic acid, with potential applications across various fields.

Covalent Organic Frameworks: Opportunities for Rational Materials Design in Cancer Therapy

Nanomedicines are extensively used in cancer therapy. Covalent organic frameworks (COFs) are crystalline organic porous materials with several benefits for cancer therapy, including porosity, design flexibility, functionalizability, and biocompatibility. This review examines the use of COFs in cancer therapy from the perspective of reticular chemistry and function-oriented materials design. First, the modification sites and functionalization methods of COFs are discussed, followed by their potential as multifunctional nanoplatforms for tumor targeting, imaging, and therapy by integrating functional components. Finally, some challenges in the clinical translation of COFs are presented with the hope of promoting the development of COF-based anticancer nanomedicines and bringing COFs closer to clinical trials.

Ultra-fast supercritically solvothermal polymerization for large single-crystalline covalent organic frameworks

Crystalline polymer materials, e.g., hyper-crosslinked polystyrene, conjugate microporous polymers and covalent organic frameworks, are used as catalyst carriers, organic electronic devices and molecular sieves. Their properties and applications are highly dependent on their crystallinity. An efficient polymerization strategy for the rapid preparation of highly or single-crystalline materials is beneficial not only to structure-property studies but also to practical applications. However, polymerization usually leads to the formation of amorphous or poorly crystalline products with small grain sizes. It has been a challenging task to efficiently and precisely assemble organic molecules into a single crystal through polymerization. To address this issue, we developed a supercritically solvothermal method that uses supercritical carbon dioxide (sc-CO2) as the reaction medium for polymerization. Sc-CO2 accelerates crystal growth due to its high diffusivity and low viscosity compared with traditional organic solvents. Six covalent organic frameworks with different topologies, linkages and crystal structures are synthesized by this method. The as-synthesized products feature polarized photoluminescence and second-harmonic generation, indicating their high-quality single-crystal nature. This method holds advantages such as rapid growth rate, high productivity, easy accessibility, industrial compatibility and environmental friendliness. In this protocol, we provide a step-by-step procedure including preparation of monomer dispersion, polymerization in sc-CO2, purification and characterization of the single crystals. By following this protocol, it takes 1-5 min to grow sub-mm-sized single crystals by polymerization. The procedure takes ~4 h from preparation of monomer dispersion and polymerization in sc-CO2 to purification and drying of the product.

A Review on Covalent Organic Frameworks as Artificial Interface Layers for Li and Zn Metal Anodes in Rechargeable Batteries

Li and Zn metals are considered promising negative electrode materials for the next generation of rechargeable metal batteries because of their non-toxicity and high theoretical capacity. However, the uneven deposition of metal ions (Li+ , Zn2+ ) and the uncontrolled growth of dendrites result in poor electrochemical stability, unsatisfactory cycle life, and rapid capacity decay of batteries assembled with Li and Zn electrodes. Owing to the unique internal directional channels and abundant redox active sites of covalent organic frameworks (COFs), they can be used to promote uniform deposition of metal ions during stripping/electroplating through interface modification strategies, thereby inhibiting dendrite growth. COFs provide a new perspective in addressing the challenges faced by the anodes of Li metal batteries and Zn ion batteries. This article discusses the stability and types of COFs, and summarizes some novel COF synthesis methods. Additionally, it reviews the latest progress and optimization methods of using COFs for metal anodes to improve battery performance. Finally, the main challenges faced in these areas are discussed. This review will inspire future research on metal anodes in rechargeable batteries.

Rational design of bimetallic sites in covalent organic frameworks for efficient photocatalytic oxidative coupling of amines

The conversion of organic compounds by photocatalysis under mild conditions is an environment-friendly alternative for organic transformations. In this work, the bimetallic covalent organic framework coordinated by Sr2+ and Fe2+ in the porphyrin centers with molar ratio of 2:1 (COF-Sr2Fe1) was synthesized through a two-step reaction. Under the synergistic regulation of Sr2+ and Fe2+, the separation of photogenerated charges and visible light absorption for COF-Sr2Fe1 were significantly promoted, and thus COF-Sr2Fe1 exhibited efficient photocatalytic performance towards benzylamine oxidative coupling reaction with a yield of 97 %, much higher than that of the nonmetallic covalent organic framework COF-366. Moreover, it was found that the Fe site displayed higher dehydrogenation ability and the Sr site displayed higher CN coupling ability through the density functional theory (DFT) calculations, thereby making the dehydrogenation and CN coupling steps more controllable for benzylamine oxidative coupling reaction by COF-Sr2Fe1. This work provides a strategy for designing efficient covalent organic frameworks photocatalysts, and helps to understand the oxidative coupling of amines more deeply.

Single Cobalt Ion-Immobilized Covalent Organic Framework for Lithium-Sulfur Batteries with Enhanced Rate Capabilities

Covalent organic frameworks (COFs) are notable for their remarkable structure, function designability, and tailorability, as well as stability, and the introduction of "open metal sites" ensures the efficient binding of small molecules and activation of substrates for heterogeneous catalysis and energy storage. Herein, we use the postsynthetic metal sites to catalyze polysulfide conversion and to boost the binding affinity to active matter for lithium-sulfur batteries (LSBs). A dual-pore COF, USTB-27, with hxl topology has been successfully assembled from the imine chemical reaction between 2,3,8,9,14,15-hexa(4-formylphenyl)diquinoxalino [2,3-a:2',3'-c]phenazine and [2,2'-bipyridine]-5,5'-diamine. The chelating nitrogen sites of both modules are able to postsynthetically functionalize with single cobalt sites to generate USTB-27-Co. The discharge capacity of the sulfur-loaded S@USTB-27-Co composite in a LSB is 1063, 945, 836, 765, 696, and 644 mA h g-1 at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 C, respectively, much superior to that of non-cobalt-functionalized species S@USTB-27. Following the increased current densities, the rate performance of S@USTB-27-Co is much better than that of S@USTB-27. In particular, the capacity retention at 5.0 C has a magnificent increase from 19% for the latter species to 61% for the former one. Moreover, S@USTB-27-Co exhibits a higher specific capacity of 543 mA h g-1 than that of S@USTB-27 (402 mA h g-1) at a current density of 1.0 C after electrochemical cycling for 500 runs. This work illustrates the "open metal sites" strategy to engineer the active chemical component conversion in COF channels as well as their binding strength for specific applications.

A Spirobifluorene-Based Covalent Organic Framework for Dual Photoredox and Nickel Catalysis

Covalent organic frameworks (COFs) have emerged as tunable, crystalline, and porous functional organic materials, but their application in photocatalysis has been limited by rapid excited-state quenching. Herein, we report the first example of dual photoredox/nickel catalysis by an sp2 carbon-conjugated spirobifluorene-based COF. Constructed from spirobifluorene and nickel-bipyridine linkers, the NiSCN COF adopted a two-dimensional structure with staggered stacking. Under light irradiation, NiSCN catalyzed amination and etherification/esterification reactions of aryl halides through the photoredox mechanism, with a catalytic efficiency more than 23-fold higher than that of its homogeneous control. NiSCN was used in five consecutive reactions without a significant loss of catalytic activity.

Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures

Rational control and understanding of isomerism are of significance but still remain a great challenge in reticular frameworks, in particular, for covalent organic frameworks (COFs) due to the complicated synthesis and energy factors. Herein, reaction of 3,3',5,5'-tetra(4-formylphenyl)-2,2',6,6'-tetramethoxy-1,1'-biphenyl (TFTB) with 3,3',5,5'-tetrakis(4-aminophenyl)bimesityl (TAPB) under different reaction conditions affords single crystals of two 3D COF isomers, namely, USTB-20-dia and USTB-20-qtz. Their structures with resolutions up to 0.9-1.1 Å have been directly solved by three-dimensional electron diffraction (3D ED) and synchrotron single crystal X-ray diffraction, respectively. USTB-20-dia and USTB-20-qtz show rare 2 × 2-fold interpenetrated dia-b nets and 3-fold interpenetrated qtz-b frameworks. Comparative studies of the crystal structures of these COFs and theoretical simulation results indicate the crucial role of the flexible molecular configurations of building blocks in the present interpenetrated topology isomerism. This work not only presents the rare COF isomers but also gains an understanding of the formation of framework isomerism from both single crystal structures and theoretical simulation perspectives.

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.

Single-Crystal Three-Dimensional Covalent Organic Framework Constructed from 6-Connected Triangular Prism Node

The limited structural diversity of three-dimensional covalent organic frameworks (3D COFs) greatly restricts their application exploration. Therefore, there is an urgent need to expand their library of molecular building blocks, such as the development of highly connected (>4 reaction sites) polyhedral nodes. Herein, by precisely controlling the precursor conformation, we rationally designed a new 6-connected triangular prism node derived from the triphenylbenzene molecule and further used it to construct a novel 3D COF (3D-TMTAPB-COF) via imine condensation reaction. Surprisingly, without the addition of competing reagents, 3D-TMTAPB-COF crystallized directly into single crystals of ∼15 μm in size and was determined to adopt a rare 6-fold interpenetrated (Class IIIa interpenetration) acs topology. In addition, 3D-TMTAPB-COF showed a high SF6 adsorption capacity (60.9 cm3 g-1) and good SF6/N2 selectivity (335) at 298 K and 1 bar, superior to those of most crystalline porous materials. This work not only confirms the possibility of growing large-size single-crystal 3D COFs formed with strong covalent bonds by a solvothermal method in the absence of modulators, but also reports a novel triangular prism node for future construction of 3D COFs with interesting applications.

Beyond Solvothermal: Alternative Synthetic Methods for Covalent Organic Frameworks

Covalent organic frameworks (COFs) are crystalline porous organic materials that hold a wealth of potential applications across various fields. The development of COFs, however, is significantly impeded by the dearth of efficient synthetic methods. The traditional solvothermal approach, while prevalent, is fraught with challenges such as complicated processes, excessive energy consumption, long reaction times, and limited scalability, rendering it unsuitable for practical applications. The quest for simpler, quicker, more energy-efficient, and environmentally benign synthetic strategies is thus paramount for bridging the gap between academic COF chemistry and industrial application. This Review provides an overview of the recent advances in alternative COF synthetic methods, with a particular emphasis on energy input. We discuss representative examples of COF synthesis facilitated by microwave, ultrasound, mechanic force, light, plasma, electric field, and electron beam. Perspectives on the advantages and limitations of these methods against the traditional solvothermal approach are highlighted.

A Photosensitizing Metal-Organic Framework as a Tandem Reaction Catalyst for Primary Alcohols from Terminal Alkenes and Alkynes

Owing to the wide and growing demand for primary alcohols, the development of efficient catalysts with high regioselectivity remains a worthwhile pursuit. However, according to Markovnikov's rule, it is a challenge to obtain primary alcohols with high yields and regioselectivity from terminal alkenes or alkynes. Herein, we report the synthesis of a photosensitizing two-dimensional (2D) metal-organic framework (MOF) from cyclic trinuclear copper(I) units (Cu-CTUs) and a boron dipyrro-methene (Bodipy) ligand. The MOF features broadband light absorption, excellent photoinduced charge separation efficiency, and photochemical properties. By integrating the copper-catalyzed hydroboration and photocatalyzed aerobic oxidation, it can catalyze terminal alkenes and alkynes to produce primary alcohols via a one-pot tandem reaction with excellent regioselectivity, good overall yields in two-step reactions (up to 85 %), broad substrate compatibility (32 examples) and good reusability under mild conditions.

Postsynthetic Annulation of Three-Dimensional Covalent Organic Frameworks for Boosting CO2 Photoreduction

Three-dimensional covalent organic frameworks (3D COFs), with interconnected pores and exposed functional groups, provide new opportunities for the design of advanced functional materials through postsynthetic modification. Herein, we demonstrate the successful postsynthetic annulation of 3D COFs to construct efficient CO₂ reduction photocatalysts. Two 3D COFs, NJU-318 and NJU-319Fe, were initially constructed by connecting hexaphenyl-triphenylene units with pyrene- or Fe-porphyrin-based linkers. Subsequently, the hexaphenyl-triphenylene moieties within the COFs were postsynthetically transformed into π-conjugated hexabenzo-trinaphthylene (pNJU-318 and pNJU-319Fe) to enhance visible light absorption and CO₂ photoreduction activity. The optimized photocatalyst, pNJU-319Fe, shows a CO yield of 688 μmol g^-1, representing a 2.5-fold increase compared to that of unmodified NJU-319Fe. Notably, the direct synthesis of hexabenzo-trinaphthylene-based COF catalysts was unsuccessful due to the low solubility of conjugated linkers. This study not only provides an effective method to construct photocatalysts but also highlights the unlimited tunability of 3D COFs through structural design and postsynthetic modification.