Pyrene-Based Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production

Four highly porous covalent organic frameworks (COFs) containing pyrene units were prepared and explored for photocatalytic H₂O₂ production. The experimental studies are complemented by density functional theory calculations, proving that the pyrene unit is more active for H₂O₂ production than the bipyridine and (diarylamino)benzene units reported previously. H₂O₂ decomposition experiments verified that the distribution of pyrene units over a large surface area of COFs plays an important role in catalytic performance. The Py-Py-COF though contains more pyrene units than other COFs which induces a high H₂O₂ decomposition due to a dense concentration of pyrene in close proximity over a limited surface area. Therefore, a two-phase reaction system (water-benzyl alcohol) was employed to inhibit H₂O₂ decomposition. This is the first report on applying pyrene-based COFs in a two-phase system for photocatalytic H₂O₂ generation.     

咔唑基共价有机框架用于光催化析氢

光催化析氢反应是获得高纯氢气的一种具有广阔应用前景的技术. 目前, 开发经济高效、 经久耐用的催化剂仍然是一个巨大的挑战. 我们以3,3'',6,6''-四醛-9,9''-双咔唑和3,7-二氨基二苯并[b,d]噻吩-5,5-二氧化物为基本构建单元, 通过经典的席夫碱反应, 合成了供体-受体型咔唑基共价有机框架(CZ-COF), 对其结构进行了表征, 并探究了其光催化析氢性能. CZ-COF展现了优异的光催化活性, 平均产氢速率为31 μmol·h^-1.     

共价有机框架材料在光催化领域中的应用

共价有机框架(COFs)材料是有机构筑基元通过共价键连接而形成的晶态有机多孔材料. COFs具有孔道结构规整、及比表面积高等特点,被广泛地应用于气体储存与分离、催化、传感、储能及光电转化等领域.将具有可调吸光能力的有机构筑基元引入到COFs中,可使其展现出强大的光催化潜力.近年来, COFs在光催化领域中发展迅猛.本文总结了COFs在光催化产氢、光催化二氧化碳还原、光催化有机反应以及光催化污染物降解等方面的研究进展,并展望了其在光催化领域的应用前景.     

Covalent Organic Frameworks for Photocatalytic Organic Transformation

Photocatalytic organic transformation is an efficient, energysaving and environmentally friendly strategy for organic synthesis. The key to developing a green and economical route for photocatalytic organic synthesis lies in the construction of optimal photocatalysts. Covalent organic frameworks(COFs), a kind of porous crystalline materials with characteristics of high surface area, excellent porosity, and superior thermo-chemical stability, have driven people to explore their potential as photocatalysts in photocatalytic organic transformations by virtue of their structural versatility and designability. Furthermore, the insolubility of COFs makes it possible to recycle the catalysts by simple technical means. In recent years, researchers have made great efforts to develop both the design strategies of COFs as heterogeneous photocatalysts and the reaction types of photocatalytic organic transformations. In this review, we focus on the design of COF-based photocatalytic materials and analyze the influence factors of photocatalytic performance. Moreover, we summarize the application of COFbased photocatalysts in photocatalytic organic conversion. Finally, the perspectives on new opportunities and challenges in the field are discussed.     

Substituent Effects on the Photocatalytic Properties of a Symmetric Covalent Organic Framework

Symmetric covalent organic framework (COF) photocatalysts generally suffer from inefficient charge separation and short-lived photoexcited states. By performing density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we find that partial substitution with one or two substituents (N or NH\begin{document}$_2$\end{document}) in the linkage of the representative symmetric COF (N\begin{document}$_0$\end{document}-COF) gives rise to the separation of charge carriers in the resulting COFs (\emph{i.e}., N\begin{document}$_1$\end{document}-COF, N\begin{document}$_2$\end{document}-COF, (NH\begin{document}$_2$\end{document})\begin{document}$_1$\end{document}-N\begin{document}$_0$\end{document}-COF, and (NH\begin{document}$_2$\end{document})\begin{document}$_2$\end{document}-N\begin{document}$_0$\end{document}-COF). Moreover, we also find that the energy levels of the highest occupied crystal orbital (HOCO) and the lowest unoccupied crystal orbital (LUCO) of the N\begin{document}$_0$\end{document}-COF can shift away from or toward the vacuum level, depending on the electron-withdrawing or electron-donating characters of the substituent. Therefore, we propose that partial substitution with carefully chosen electron-withdrawing or electron-donating substituents in the linkages of symmetric COFs can lead to efficient charge separation as well as appropriate HOCO and LUCO positions of the generated COFs for specific photocatalytic reactions. The proposed rule can be utilized to further boost the photocatalytic performance of many symmetric COFs.     

Enaminone-Linked Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Production

Covalent organic frameworks (COFs), possessing pre-designable structures and tailorable functionalities, are promising candidates for photocatalysis. Nevertheless, the most studied imine-linked COFs (Im-COFs) usually suffer from unsatisfactory stability and photocatalytic performance. To meet this challenge, a series of highly stable enaminone-linked COFs (En-COFs) have been synthesized and afford much improved visible-light-driven hydrogen production activities, ranging from 44 to 1078 times that of isoreticular Im-COFs, with the only difference being the linkages (enaminone vs. imine) in their structures. The enhanced light-harvesting ability, facilitated exciton dissociation and improved chemical stability account for the superior activity. Furthermore, quinoline-linked COFs (Qu-COFs) have been further obtained via the post-modification of Im-COFs. Compared with Im-COFs, the photocatalytic activities of Qu-COFs are significantly improved after modification, but still below those of the corresponding En-COFs (3–107 times). The facile synthesis, excellent activity, and high chemical stability demonstrate that En-COFs are a promising platform for photocatalysis.     

Vinyl-Group-Anchored Covalent Organic Framework for Promoting the Photocatalytic Generation of Hydrogen Peroxide

The artificial photosynthesis of H₂O₂ from water and oxygen using semiconductor photocatalysts is attracting increasing levels of attention owing to its green, environmentally friendly, and energy-saving characteristics. Although covalent organic frameworks (COFs) are promising materials for promoting photocatalytic H₂O₂ production owing to their structural and functional diversity, they typically suffer from low charge-generation and -transfer efficiencies as well as rapid charge recombination, which restricts their use as catalysts for photocatalytic H₂O₂ production. Herein, we report a strategy for anchoring vinyl moieties to a COF skeleton to facilitate charge separation and migration, thereby promoting photocatalytic H₂O₂ generation. This vinyl-group-bearing COF photocatalyst exhibits a H₂O₂-production rate of 84.5 μmol h⁻¹ (per 10 mg), which is ten-times higher than that of the analog devoid of vinyl functionality and superior to most reported COF photocatalysts. Both experimental and theoretical studies provide deep insight into the origin of the improved photocatalytic performance. These findings are expected to facilitate the rational design and modification of organic semiconductors for use in photocatalytic applications.     

Two-dimensional Covalent Organic Frameworks: Tessellation by Synthetic Art

Covalent organic frameworks(COFs) featuring designable nanoporous structures exhibit many fascinating properties and have attracted great attention in recent years for their intriguing application potential in sensing, catalysis, gas storage and separation, optoelectronics, etc. Rational design of twodimensional(2D) COFs through judiciously selecting chemical building blocks is critical to acquiring predetermined skeleton and pore structures. In this perspective, we review the reticular synthesis of 2D COFs with different topologies, highlighting the important role of various characterization techniques in crystal structure determination. 2D COFs with simple tessellations have been widely investigated, while the synthesis of complex tessellated COFs is still a great challenge. Some recent examples of 2D COFs with novel topological structures are also surveyed.     

Cobaloxime-Integrated Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution Coupled with Alcohol Oxidation

We report an azide-functionalized cobaloxime proton-reduction catalyst covalently tethered into the Wurster-type covalent organic frameworks (COFs). The cobaloxime-modified COF photocatalysts exhibit enhanced photocatalytic activity for hydrogen evolution reaction (HER) in alcohol-containing solution with no presence of a typical sacrificial agent. The best performing cobaloxime-modified COF hybrid catalyzes hydrogen production with an average HER rate up to 38 μmol h⁻¹ in ethanol/phosphate buffer solution under 4 h illumination. Ultrafast transient optical spectroscopy characterizations and charge carrier analysis reveal that the alcohol contents functioning as hole scavengers could be oxidized by the photogenerated holes of COFs to form aldehydes and protons. The consumption of the photogenerated holes thus suppresses exciton recombination of COFs and improves the ratio of free electrons that were effectively utilized to drive catalytic reaction for HER. This work demonstrates a great potential of COF-catalyzed HER using alcohol solvents as hole scavengers and provides an example toward realizing the accessibility to the scope of reaction conditions and a greener route for energy conversion.     

Covalent Organic Frameworks Containing Dual O2 Reduction Centers for Overall Photosynthetic Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are highly desirable for achieving high-efficiency overall photosynthesis of hydrogen peroxide (H₂O₂) via molecular design. However, precise construction of COFs toward overall photosynthetic H₂O₂ remains a great challenge. Herein, we report the crystalline s-heptazine-based COFs (HEP-TAPT-COF and HEP-TAPB-COF) with separated redox centers for efficient H₂O₂ production from O₂ and pure water. The spatially and orderly separated active sites in HEP-COFs can efficiently promote charge separation and enhance photocatalytic H₂O₂ production. Compared with HEP-TAPB-COF, HEP-TAPT-COF exhibits higher H₂O₂ production efficiency for integrating dual O₂ reduction active centers of s-heptazine and triazine moieties. Accordingly, HEP-TAPT-COF bearing dual O₂ reduction centers exhibits a remarkable solar-to-chemical energy efficiency of 0.65 % with a high apparent quantum efficiency of 15.35 % at 420 nm, surpassing previously reported COF-based photocatalysts.     

基于醌基的共价有机框架用于电催化析氧反应

将蒽醌作为构筑单元设计合成了醌基功能化的新型2,6-二氨基蒽醌共价有机框架(DAAQ-COF). 粉末X射线衍射、 氮气吸附-脱附、 红外和热重等分析结果表明, DAAQ-COF具有高的结晶度和比表面积(577 m²/g). 此外, 醌基功能化的无金属DAAQ-COF显示出高的析氧反应(OER)活性(10 mA/cm²下, 过电位389 mV, Tafel斜率135 mV/dec). 这源于引入的醌基基团有效改变了COF框架的电子结构和化学特性, 加上COF材料本身的高结晶度和比表面积, 使得反应物能更有效地与活性位点接触, 从而促进OER进程. 这些结果表明合理地设计功能化的COF材料能够进一步推动此类材料在电催化领域的应用.     

Heterogenization of Salen Metal Molecular Catalysts in Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

Integrating a molecular catalyst with a light harvester into a photocatalyst is an effective strategy for solar light conversion. However, it is challenging to establish a crystallized framework with well-organized connections that favour charge separation and transfer. Herein, we report the heterogenization of a Salen metal complex molecular catalyst into a rigid covalent organic framework (COF) through covalent linkage with the light-harvesting unit of pyrene for photocatalytic hydrogen evolution. The chemically conjugated bonds between the two units contribute to fast photogenerated electron transfer and thereby promote the proton reduction reaction. The Salen cobalt-based COF showed the best hydrogen evolution activity (1378 μmol g⁻¹ h⁻¹), which is superior to the previously reported nonnoble metal based COF photocatalysts. This work provides a strategy to construct atom-efficient photocatalysts by the heterogenization of molecular catalysts into covalent organic frameworks.     

Cyanide-based Covalent Organic Frameworks for Enhanced Overall Photocatalytic Hydrogen Peroxide Production

Photocatalytic oxygen reduction to produce hydrogen peroxide (H₂O₂) is a promising route to providing oxidants for various industrial applications. However, the lack of well-designed photocatalysts for efficient overall H₂O₂ production in pure water has impeded ongoing research and practical thrusts. Here we present a cyanide-based covalent organic framework (TBTN-COFs) combining 2,4,6-trimethylbenzene-1,3,5-tricarbonitrile (TBTN) and benzotrithiophene-2,5,8-tricarbaldehyde (BTT) building blocks with water-affinity and charge-separation. The ultrafast intramolecular electron transfer (<500 fs) and prolonged excited state lifetime (748 ps) can be realized by TBTN-COF, resulting in a hole accumulated BTT and electron-rich TBTN building block. Under one sun, the 11013 μmol h⁻¹ g⁻¹ yield rate of H₂O₂ can be achieved without any sacrificial agent, outperforming most previous reports. Furthermore, the DFT calculation and in situ DRIFTS spectrums suggesting a Yeager-type absorption of *O₂⋅⁻ intermediate in the cyanide active site, which prohibits the formation of superoxide radical and revealing a favored H₂O₂ production pathway.     

Strategies for Improving the Catalytic Performance of 2D Covalent Organic Frameworks for Hydrogen Evolution and Oxygen Evolution Reactions

Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been deemed as clean and sustainable strategies to solve the energy crisis and environmental problems. Various catalysts have been developed to promote the process of HER and OER. Among them, two-dimensional covalent organic frameworks (2D COFs) have received great attention due to their diverse and designable structure. In this minireview, we mainly summarize the diverse linkages of 2D COFs and strategies for enhancing the catalytic performance of 2D COFs for HER and OER, such as introducing active building blocks, metal ions and tailored linkages. Furthermore, a brief outlook for the development directions of COFs in the field of HER and OER is provided, expecting to stimulate new opportunities in future research.     

Tuning Local Charge Distribution in Multicomponent Covalent Organic Frameworks for Dramatically Enhanced Photocatalytic Uranium Extraction

Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron-donating or withdrawing groups in the pores. Intramolecular donor-acceptor (D-A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro-functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high-activity COF photocatalysts with intramolecular D-A structures not easily accessible using traditional synthetic approaches.     

Intermediate Formation of Macrocycles for Efficient Crystallization of 2D Covalent Organic Frameworks with Enhanced Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) have gained significant attention as key photocatalysts for efficient solar light conversion into hydrogen production. Unfortunately, the harsh synthetic conditions and intricate growth process required to obtain highly crystalline COFs greatly hinder their practical application. Herein, we report a simple strategy for the efficient crystallization of 2D COFs based on the intermediate formation of hexagonal macrocycles. Mechanistic investigation suggests that the use of 2,4,6-triformyl resorcinol (TFR) as the asymmetrical aldehyde build block allows the equilibration between irreversible enol-to-keto tautomerization and dynamic imine bonds to produce the hexagonal β-ketoenamine-linked macrocycles, the formation of which could provide COFs with high crystallinity in half hour. We show that COF-935 with 3 wt % Pt as cocatalyst exhibit a high hydrogen evolution rate of 67.55 mmol g⁻¹ h⁻¹ for water splitting when exposed to visible light. More importantly, COF-935 exhibits an average hydrogen evolution rate of 19.80 mmol g⁻¹ h⁻¹ even at a low loading of only 0.1 wt % Pt, which is a significant breakthrough in this field. This strategy would provide valuable insights into the design of highly crystalline COFs as efficient organic semiconductor photocatalysts.     

Two-dimensional Covalent Organic Frameworks for Electrochromic Switching

The electrochromic materials have received immense attention for the fabrication of smart optoelectronic devices. The alteration of the redox states of the electroactive functionalities results in the color change in response to electrochemical potential. Even though transition metal oxides, redox-active small organic molecules, conducting polymers, and metallopolymers are known for electrochromism, advanced materials demonstrating multicolor switching with fast response time and high durability are of increasing demand. Recently, two-dimensional covalent organic frameworks (2D COFs) have been demonstrated as electrochromic materials due to their tunable redox functionalities with highly ordered structure and large specific surface area facilitating fast ion transport. Herein, we have discussed the mechanistic insights of electrochromism in 2D COFs and their structure-property relationship in electrochromic performance. Furthermore, the state-of-the-art knowledge for developing the electrochromic 2D COFs and their potential application in next-generation display devices are highlighted.     

Solar-to-H2O2 Catalyzed by Covalent Organic Frameworks

Benefiting from the excellent structural tunability, robust framework, ultrahigh porosity, and rich active sites, covalent organic frameworks (COFs) are widely recognized as promising photocatalysts in chemical conversions, and emerged in the hydrogen peroxide (H₂O₂) photosynthesis in 2020. H₂O₂, serving as an environmental-friendly oxidant and a promising liquid fuel, has attracted increasing researchers to explore its potential. Over the past few years, numerous COFs-based photocatalysts are developed with encouraging achievements in H₂O₂ production, whereas no comprehensive review articles exist to summarize this specific and significant area. Herein we provide a systematic overview of the advances and challenges of COFs in photocatalytic H₂O₂ production. We first introduce the priorities of COFs in H₂O₂ photosynthesis. Then, various strategies to improve COFs photocatalytic efficiency are discussed. The perspective and outlook for future advances of COFs in this emerging field are finally offered. This timely review will pave the way for the development of highly efficient COFs photocatalysts for practical production of value-added chemicals not limited to H₂O₂.     

Recent Advances of Covalent Organic Frameworks for Chiral Separation

Covalent organic frameworks(COFs), orderly assembled from the building blocks via covalent bonds, are a novel type of porous materials with rich functional sites and permanent porosity. At present, most of COFs are achiral networks, nevertheless, chiral COFs(CCOFs) have become a research hotspot in recent years, due to their unique chiral sites and microenvironment. As one of the most important applications of CCOFs, chiral separation has attracted huge attention for the convenient, rapid and efficient feature. In this review, recent progresses of covalent organic frameworks for chiral separation are covered. And we also present the challenges and outlooks of CCOFs in the future for this field.     

二维共价有机框架材料的可控合成及其光催化应用研究进展

通过模拟自然界光合作用, 将太阳能转化为方便存储的化学能是缓解未来能源短缺和环境污染问题的理想途径之一. 二维共价有机框架材料(2D COFs)是近年来发展起来的一类新型有机半导体材料, 具有结晶度高、结构精确以及化学组分灵活可调等优势, 在光催化领域展现出巨大应用潜力, 受到了研究者们的广泛关注. 对2D COFs的可控制备以及电子结构调控方法进行了系统总结, 并重点介绍了它们在光催化水分解、CO₂还原以及H₂O₂合成领域的最近研究进展, 讨论了材料结构和催化性能之间的关系, 最后对2D COFs在光催化应用领域存在的机遇和挑战进行了展望.