Designed Synthesis of a 2D Porphyrin‐Based sp 2 Carbon‐Conjugated Covalent Organic Framework for Heterogeneous Photocatalysis

Benzotrithiophene-based Covalent Organic Framework Photocatalysts with Controlled Conjugation of Building Blocks for Charge Stabilization

Covalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure-property-activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)-based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT-COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts.

Pyrene-Based D-A Molecules as Efficient Heterogeneous Catalysts for Visible-Light-Induced Aerobic Organic Transformations

In this work, an efficient visible light promoted aerobic dehydro-coupling of amines, oxidation of thioethers and hydroxylation of arylboronic acids under benign conditions by using pyrene-based donor-acceptor (D-A) conjugated organic molecules was described. Donor-acceptor structure influences their π-conjugation and band gap a lot, and thereby enhances their visible light absorption ability, single electron transfer and oxidative behaviors. Alkynyl units in PS-IV play a crucial role in the catalyst which could serve as electron transferring bridge to strengthen electron delocalization, thus facilitating the single electron transfer from photosensitizer to substrates, and making it an efficient ⋅O₂ ^- generator. While PS-III without alkynyl units tends to produce ¹ O₂ . Therefore, these molecules can serve as efficient catalysts for different kinds of visible-light-induced aerobic organic reactions. More importantly, the simply structured molecule is insoluble and stable in various solvents, and thus could be recycled as heterogeneous catalyst for many rounds with slight catalytic activity degradation. Besides, large scale (1 mol) reaction of benzylamine coupling proceeded smoothly under the standard conditions.

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 (H₂ O₂ ) via molecular design. However, precise construction of COFs toward overall photosynthetic H₂ O₂ 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 H₂ O₂ production from O₂ and pure water. The spatially and orderly separated active sites in HEP-COFs can efficiently promote charge separation and enhance photocatalytic H₂ O₂ production. Compared with HEP-TAPB-COF, HEP-TAPT-COF exhibits higher H₂ O₂ 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.

Enhancement of Visible-Light-Driven Hydrogen Evolution Activity of 2D π-Conjugated Bipyridine-Based Covalent Organic Frameworks via Post-Protonation

Photocatalytic hydrogen (H₂ ) evolution represents a promising and sustainable technology. Covalent organic frameworks (COFs)-based photocatalysts have received growing attention. A 2D fully conjugated ethylene-linked COF (BTT-BPy-COF) was fabricated with a dedicated designed active site. The introduced bipyridine sites enable a facile post-protonation strategy to fine-tune the actives sites, which results in a largely improved charge-separation efficiency and increased hydrophilicity in the pore channels synergically. After modulating the degree of protonation, the optimal BTT-BPy-PCOF exhibits a remarkable H₂ evolution rate of 15.8 mmol g^-1  h^-1 under visible light, which surpasses the biphenyl-based COF 6 times. By using different types of acids, the post-protonation is proved to be a potential universal strategy for promoting photocatalytic H₂ evolution. This strategy would provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

Redox-Active Azulene-based 2D Conjugated Covalent Organic Framework for Organic Memristors

As a conjugated and unsymmetric building block composed of an electron-poor seven-membered sp² carbon ring and an electron-rich five-membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5-tris(4-aminophenyl)-benzene and azulene-1,3-dicarbaldehyde as the starting materials, an azulene(Azu)-based 2D conjugated covalent organic framework, COF-Azu, is prepared through liquid-liquid interface polymerization strategy for the first time. The as-fabricated Al/COF-Azu/indium tin oxide (ITO) memristor shows typical non-volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80 % for a neutral network trained on a larger data set is achieved.

Boosting Electroreduction of CO2 over Cationic Covalent Organic Frameworks: Hydrogen Bonding Effects of Halogen Ions

We present the first example of charged imidazolium functionalized porphyrin-based covalent organic framework (Co-iBFBim-COF-X) for electrocatalytic CO₂ reduction reaction, where the free anions (e.g., F^- , Cl^- , Br^- , and I^- ) of imidazolium ions nearby the active Co sites can stabilize the key intermediate *COOH and inhibit hydrogen evolution reaction. Thus, Co-iBFBim-COF-X exhibits higher activity than the neutral Co-BFBim-COF, following the trend of F^- <Cl^- <Br^- <I^- . Particularly, the Co-iBFBim-COF-I^- showed nearly 100 % CO₂ selectivity at a low full-cell voltage of 2.3 V, and achieved a high CO₂ partial current density of 52 mA cm^-2 with a turnover frequency of 3018 h^-1 at 2.4 V in the anion membrane electrode assembly, which is 3.57 times larger than that of neutral Co-BFBim-COF. This work provides new insight into the importance of free anions in the stabilization of intermediates and decreasing the local binding energy of H₂ O with active moiety to enhance CO₂ reduction reaction.

Advanced Covalent Organic Frameworks for Multi-Valent Metal Ion Batteries

Covalent organic frameworks (COFs) have received increased interest in recent years as an advanced class of materials. By virtue of the available monomers, multiple conformations and various linkages, COFs offer a wide range of opportunities for complex structural design and specific functional development of materials, which has facilitated the widespread application in many fields, including multi-valent metal ion batteries (MVMIBs), described as the attractive candidate replacing lithium-ion batteries (LIBs). With their robust skeletons, diverse pores, flexible structures and abundant functional groups, COFs are expected to help realize a high performance MVMIBs. In this review, we present an overview of COFs, describe advances in topology design and synthetic reactions, and study the application of COFs in MVMIBs, as well as discuss challenges and solutions in the preparation of COFs electrodes, in the hope of providing constructive insights into the future direction of COFs.

Designing Thiophene-Enriched Fully Conjugated 3D Covalent Organic Framework as Metal-Free Oxygen Reduction Catalyst for Hydrogen Fuel Cells

The application of three-dimensional (3D) covalent organic frameworks (COFs) in renewable energy fields is greatly limited due to their non-conjugated skeletons. Here, we design and successfully synthesize a thiophene-enriched fully conjugated 3D COF (BUCT-COF-11) through an all-thiophene-linked saddle-shaped building block (COThTh-CHO). The BUCT-COF-11 exhibits excellent semiconducting property with intrinsic metal-free oxygen reduction reaction (ORR) activity. Using the COF as cathode catalyst, the assembled anion-exchange membrane fuel cells (AEMFCs) exhibited a high peak power density up to 493 mW cm^-2 . DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample, which therefore boosts the ORR performance. This is the first report on the application of COFs as metal-free catalysts in fuel cells, demonstrating the great potential of fully conjugated 3D COFs as promising semiconductors in energy fields.

A CO2 -Masked Carbene Functionalized Covalent Organic Framework for Highly Efficient Carbon Dioxide Conversion

Free N-heterocyclic carbenes (NHCs) are generally prepared by treatment of imidazolium precursors with strong alkali reagents, which usually produces inactive NHC dimers. This treatment would destroy porous supports and thus make supported NHC catalysts difficult to recovery and reuse. Herein, we report the first stable CO₂ -masked N-heterocyclic carbenes (NHCs) grafted on a porous crystalline covalent organic framework (COF). The stable NHC-CO₂ moieties in the COF-NHC-CO₂ could be transformed in situ into isolated NHCs by heating, which exhibit superior catalytic performances in hydrosilylation and N-formylation reactions with CO₂ . The NHC sites can reversibly form NHC-CO₂ and thus can be easily recycled and reused while maintaining excellent catalytic activity. Density functional theory calculations revealed that NHC sites can be fully exposed after removal of CO₂ -masks and rapidly react with silanes, which endows COF-NHC with high catalytic activity.

Engineering Olefin-Linked Covalent Organic Frameworks for Photoenzymatic Reduction of CO2

It is of profound significance concerning the global energy and environmental crisis to develop new techniques that can reduce and convert CO₂ . To address this challenge, we built a new type of artificial photoenzymatic system for CO₂ reduction, using a rationally designed mesoporous olefin-linked covalent organic framework (COF) as the porous solid carrier for co-immobilizing formate dehydrogenase (FDH) and Rh-based electron mediator. By adjusting the incorporating content of the Rh electronic mediator, which facilitates the regeneration of nicotinamide cofactor (NADH) from NAD⁺ , the apparent quantum yield can reach as high as 9.17±0.44 %, surpassing all reported NADH-regenerated photocatalysts constructed by crystalline framework materials. Finally, the assembled photocatalyst-enzyme coupled system can selectively convert CO₂ to formic acid with high efficiency and good reusability. This work demonstrates the first example using COFs to immobilize enzymes for artificial photosynthesis systems that utilize solar energy to produce value-added chemicals.

One-Dimensional Organic Conjugated Polymers as Recyclable Heterogeneous Photocatalysts

Organic conjugated polymers with long-range conjugation generally have strong light absorption capacity in visible light region and impressive performance in charge transfer, which endows them great application potential in the field of opto-electronic materials. But there are few reports on their use in photocatalytic reactions. At present, it has been reported that a variety of donor-acceptor (D-A) type organic dyes can be used in efficient organic photocatalytic transformations. We designed and synthesized one-dimensional organic conjugated polymers pPhCzBP-Th and pPhCzBP-DTh with D-A structure, and proved that they are good heterogeneous photo-redox catalysts, which can photocatalyze hydrodehalogenation reduction of α-bromoacetophenone and its derivatives. Due to the strong reducibility of the excited state, pPhCzBP-Th can also efficiently reduce α-chloroacetophenone. Furthermore, by simply wrapping the catalyst powder, high-efficient separation of products and catalysts recycling can be achieved.

Acetylene/Vinylene-Bridged π-Conjugated Covalent Triazine Polymers for Photocatalytic Aerobic Oxidation Reactions under Visible Light Irradiation

Solar-driven photocatalytic chemical transformation provides a sustainable strategy to produce valuable feedstock, but designing photocatalysts with high efficiency remains challenging. Herein, two acetylene- or vinylene-bridged π-conjugated covalent triazine polymers, A-CTP-DPA and V-CTP-DPE, were successfully fabricated toward metal-free photocatalytic oxidation under visible light irradiation. Compared to the one without acetylene or vinylene bridge, both resulting polymers exhibited superior activity in photocatalytic selective oxidation of sulfides and oxidative coupling of amines; in particular, A-CTP-DPA delivered an optimal photocatalytic performance. The superior activity was attributed to the broadened spectral response range, effective separation, rapid transportation of photogenerated charge carriers, and abundant active sites for photogenerated electrons due to the existence of the acetylene bridge in the framework. This work highlights the potential of acetylene and vinylene bridges in tuning catalytic efficiency of organic semiconductors, providing a guideline for the design of efficient photocatalysts.

Transformation of Covalent Organic Frameworks from N-Acylhydrazone to Oxadiazole Linkages for Smooth Electron Transfer in Photocatalysis

Covalent organic frameworks (COFs) are regarded as new platforms for solar-to-chemical energy conversion due to their tailor-made functions and pre-designable structures. Their intrinsic reversibility and high polarization of organic linkages inevitably result in poor chemical stability and weak optoelectronic properties. Herein, one N -acylhydrazone-linked COF (H-COF) was converted into stable and π-conjugated oxadiazole-linked COF via post-oxidative cyclization. Both chemical stability and π-electron delocalization throughout the reticular framework are significantly improved, leading to high hydrogen evolution amount of 13075 μmol g -1 in 5 h upon visible-light irradiation, which is over four times higher than that of H-COF. This work provides a facile protocol for the fabrication of p-conjugated COFs and the modulation of photophysical properties for photocatalytic application.

A Nanographene-Based Two-Dimensional Covalent Organic Framework as a Stable and Efficient Photocatalyst

Synthesis of covalent organic frameworks (COFs) with desirable organic units furnishes advanced materials with unique functionalities. As an emerging class of two‐dimensional (2D) COFs, sp²‐carbon‐conjugated COFs provide a facile platform to build highly stable and crystalline porous polymers. Herein, a 2D olefin‐linked COF was prepared by employing nanographene, namely, dibenzo[hi,st]ovalene (DBOV), as a building block. The DBOV‐COF exhibits unique ABC‐stacked lattices, enhanced stability, and charge‐carrier mobility of ≈0.6 cm² V⁻¹ s⁻¹ inferred from ultrafast terahertz photoconductivity measurements. The ABC‐stacking structure was revealed by the high‐resolution transmission electron microscopy and powder X‐ray diffraction. DBOV‐COF demonstrated remarkable photocatalytic activity in hydroxylation, which was attributed to the exposure of narrow‐energy‐gap DBOV cores in the COF pores, in conjunction with efficient charge transport following light absorption.

Heteroatom-Embedded Approach to Vinylene-Linked Covalent Organic Frameworks with Isoelectronic Structures for Photoredox Catalysis

Embedding heteroatoms into the main backbones of polymeric materials has become an efficient tool for tailoring their structures and improving their properties. However, owing to comparatively harsh heteroatom-doping conditions, this has rarely been explored in covalent organic frameworks (COFs). Herein, upon aldol condensation of a trimethyl-substituted pyrylium salt with a tritopic aromatic aldehyde, a two-dimensional oxonium-embedded COF with vinylene linkages was achieved, which was further converted to a neutral pyridine-cored COF by in situ replacement of oxonium ions with nitrogen atoms under ammonia treatment. The two heteroatom-embedded COFs are conceptually isoelectronic with each other, featuring similar geometric structures but different electronic structures, rendering them capable of catalyzing the visible-light-promoted multi-component synthesis of tri-substituted pyridine derivatives with good recyclability.

A Porphyrin-Based Covalent Organic Framework for Metal-Free Photocatalytic Aerobic Oxidative Coupling of Amines

Imine is an important intermediate in drug synthesis. Photocatalytic aerobic oxidative coupling of amines has been considered as a clean and promising way to produce imine and attracted great attentions. Herein, we designed and synthesized a novel two-dimensional porphyrin-based COF (Por-BC-COF) which adopts an AA stacking mode with excellent crystallinity, high Brunauer-Emmett-Teller surface areas (1200 m 2 g -1 ), wide light absorption range (200-1300 nm) and good stability in a variety of organic solvents. Por-BC-COF can be used as a metal-free heterogeneous photocatalyst for the photocatalytic oxidation of amines to imines under visible light and red light with a high yield (97%). This work presents a novel and efficient COF photocatalyst in the application of light-driven organic synthesis.

Synthesis of Ionic Vinylene-Linked Covalent Organic Frameworks through Quaternization-Activated Knoevenagel Condensation

We developed a simple approach to synthesizing ionic vinylene-linked two-dimensional covalent organic frameworks (COFs) through a quaternization-promoted Knoevenagel condensation at three aromatic methyl carbon atoms of N-ethyl-2,4,6-trimethylpyridinium halide with multitopic aromatic aldehyde derivatives. The resultant COFs exhibited a honeycomb-like structure with high crystallinity and surface areas as large as 1343 m²  g^-1 . The regular shape-persistent nanochannels and the positively charged polymeric frameworks allowed the COFs to be uniformly composited with linear polyethylene oxide and lithium salt, displaying ionic conductivity as high as 2.72×10^-3  S cm^-1 .

Bis-Anthracene Fused Porphyrin as an Efficient Photocatalyst: Facile Synthesis and Visible-Light-Driven Oxidative Coupling of Amines

Development of high-performance photocatalysts for the conversion of amines is of great importance, but has remained a challenging task. Here, a bis-anthracene fused porphyrin (AFP) was synthesized in a high yield by a facile synthetic protocol, which involves a Suzuki coupling for the conjugation of two anthracene groups with a porphyrin ring, followed by oxidative cyclodehydrogenation. When serving as a photocatalyst, AFP exhibits an outstanding photocatalytic performance for the visible-light-induced aerobic oxidation of amines to imines at ambient conditions. Density functional theory calculations revealed that the low energy band gap, caused by the large planar and π-extended porphyrin structure of AFP, contributed to its high photocatalytic performance.

Molecular Engineering of Fully Conjugated sp2 Carbon-Linked Polymers for High-Efficiency Photocatalytic Hydrogen Evolution

The diverse nature of organic precursors offers a versatile platform for precisely tailoring the electronic properties of semiconducting polymers. In this study, three fully conjugated sp² carbon-linked polymers have been designed and synthesized for photocatalytic hydrogen evolution under visible-light illumination, by copolymerizing different C₃ -symmetric aromatic aldehydes as knots with the 1,4-phenylene diacetonitrile (PDAN) linker through a C=C condensation reaction. The hydrogen evolution (HER) is achieved at a maximum rate of 30.2 mmol g^-1  h^-1 over a polymer based on 2,4,6-triphenyl-1,3,5-triazine units linked by cyano-substituted phenylene, with an apparent quantum yield (AQY) of 7.20 % at 420 nm. Increasing the degree of conjugation and planarity not only extends visible-light absorption, but also stabilizes the fully conjugated sp² -carbon-linked donor-acceptor (D-A) polymer. Incorporating additional electron-withdrawing triazine units into the D-A polymer to form multiple electron donors and acceptors can greatly promote exciton separation and charge transfer, thus significantly enhancing the photocatalytic activity.

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D-PdPor-COF and 3D-PdPor-COF) are synthesized. By model building and Rietveld refinement of powder X-ray diffraction, 2D-PdPor-COF crystallizes as 2D sheets while 3D-PdPor-COF adopts a five-fold interpenetrated pts topology. Interestingly, compared with 2D-PdPor-COF, 3D-PdPor-COF showed interesting properties, including 1) higher CO₂ adsorption capacity; 2) better photocatalytic performance; and 3) size-selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.

Pore Surface Engineering of Covalent Triazine Frameworks@MoS2 Electrocatalyst for the Hydrogen Evolution Reaction

Electrochemical water splitting is an important strategy for the mass production of hydrogen. Development of synthesizable catalysts has always been one of the biggest obstacles to replace platinum-group catalysts. In this work, a high quality crystal polymer covalent triazine framework [CTF; Brunauer-Emmett-Teller (BET) surface area of 1562.6 m²  g^-1 ] is synthesized and MoS₂ nanoparticles are grown in situ into/onto the 1 D channel arrays or the external surface for electrocatalysis [hydrogen evolution reaction (HER)] . The state-of-the-art CTFs@MoS₂ structure exhibits superior catalytic kinetics with an overpotential of 93 mV and Tafel slope of 43 mV dec^-1 , which is improved over most other reported analogous catalysts. The inherent π-conjugated crystal channels in CTFs provides a multifunctional support for electron transmission and mass diffusion during the hydrogen evolution process. Catalytic kinetics analysis shows that the HER performance is closely correlated to the hierarchical pore parameters and aggregated thickness of MoS₂ nanoparticles. This work provides an attractive and durable alternative to synthesize high activity and stable catalysts for HER.

Covalent Organic Frameworks: A Sustainable Photocatalyst toward Visible-Light-Accelerated C3 Arylation and Alkylation of Quinoxalin-2(1H)-ones

A practical and scalable protocol for visible-light-accelerated arylation and alkylation of quinoxalin-2(1H)-ones with hydrazines is reported. In this protocol, a hydrazone-based two-dimensional covalent organic frameworks (2D-COF-1) was employed as the heterogeneous photocatalyst (PC). Due to its excellent photocatalytic properties, good chemical stability and heterogeneous nature, the present method exhibits high efficiency, good functional group tolerance, easy scalability and remarkable catalyst reusability. More importantly, it provides an alternative way that allows rapid access to various C3 arylated or alkylated quinoxalin-2(1H)-ones in a greener and sustainable manner.