Three-Motif Molecular Junction Type Covalent Organic Frameworks for Efficient Photocatalytic Aerobic Oxidation

Homologous-Targeting Porous Type I/II Nanophotosensitizers for Efficient Delivery of STING Agonists and Enhanced Photodynamic Cancer Immunotherapy

Immunotherapy as a transformative cancer treatment modality frequently struggles with the immunosuppressive tumor microenvironment, which hinders effective immune responses. In this report, we construct biomimetic tumor cell membrane-cloaked porous covalent organic framework (COF) nanophotosensitizers (CMSCOFs) to synergistically enhance photodynamic therapy (PDT) and stimulate interferon genes (STING)-mediated immunotherapy. CMSCOF is prepared from porphyrin and benzothiadiazole-based units and cloaked with 4T1 cancer cell membranes for homologous tumor targeting. The porous structure of COF enables efficient encapsulation of the non-nucleotide STING agonist SR717. Upon 660 nm light irradiation, CMSCOFs trigger both type I and II photodynamic effects by producing both superoxide (O2•-) and singlet oxygen (1O2). The tumor cell membrane-cloaked design improves the stability of the nanophotosensitizers and mimics the natural cancer cells for enhanced blood circulation, tumor accumulation, and homologous-targeting to tumors. Inside tumor tissues, this unique CMSCOF design leads to enhanced immunogenic cell death (ICD) of tumor cells upon exposure to light irradiation. Furthermore, the encapsulated STING agonist SR717 is released after cellular internalization to activate the STING pathway and elicit a potent antitumor immune response. This synergistic approach effectively reverses the immunosuppressive tumor microenvironment, enhances cytotoxic T cell infiltration, and suppresses both primary and metastatic tumors, demonstrating the potential of CMSCOF nanophotosensitizers as a promising platform for photodynamic cancer immunotherapy.

Semiconducting Polyaromatic Covalent Organic Frameworks Constructed through Self-Aldol Condensation

The construction of semiconducting covalent organic frameworks (COFs) via single-component self-polymerization is of broad interest in reticular chemistry. Herein, two semiconducting polyaromatic COFs with all-fused-ring conjugation structures were synthesized through the self-aldol condensation of indanone-based building blocks. The resulting COFs exhibit n-type semiconducting properties and exceptional stability under harsh acidic and alkaline conditions. The electrical conductivity and charge carrier mobility of the polyaromatic COFs reached up to 5.5 × 10-3 S cm-1 and 0.62 cm2 V-1 s-1, which ranked as the highest values among n-type semiconducting COFs. The high crystallinity, intrinsic porosity, excellent conductivity, and abundant five-membered rings as active sites render these COFs as effective metal-free electrocatalysts toward oxygen reduction reaction (ORR). Notably, one of these COFs shows a half-wave potential of up to 0.77 V under alkaline conditions, which constitutes one of the highest values among the reported metal-free ORR electrocatalysts. In addition, owing to the strong robustness of the polyaromatic COFs, they also exhibit long-term catalytic durability. This study not only expands the diversity of semiconducting COFs but also establishes new paradigms for the development of high-performance metal-free electrocatalysts toward the ORR process.

Designing pillar-layered metal-organic frameworks with photo-induced electron transfer interactions between ligands for enhanced photodynamic sterilization and photocatalytic degradation of dyes and antibiotics

Pollution caused by antibiotics, bacteria, and organic dyes presents global public health challenges, posing serious risks to human health. Consequently, new, efficient, fast, and simple photocatalytic systems are urgently required. To this end, 2,7-di(pyridin-4-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI)-an electron acceptor-is introduced as a connecting column into a porphyrin-based metal-organic layer (2DTcpp) with excellent photocatalytic activity; this modification yields a three-dimensional pillar-layered metal-organic framework (MOF, 3DNDITcpp) with superior photocatalytic reactive oxygen species (ROS) generation capability. Introducing NDI enlarges the pore cavity of 3DNDITcpp creating active sites and boosting type II ROS production. The orderly arrangement of the electron donor (porphyrin layer) and acceptor (NDI) within 3DNDITcpp promotes photo-induced electron transfer (PET) interactions-as confirmed by density functional theory calculations-substantially boosting type I ROS production. Specifically, the energy levels of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the porphyrin derivative ligand are -0.122252 and -0.185307 eV, respectively. The energy levels of the LUMO and HOMO of the NDI ligand are -0.15977 and -0.221199 eV, respectively. The HOMO energy level of the porphyrin ligand is between the HOMO and LUMO of NDI, and higher than the HOMO orbital energy level of NDI, proving that the porphyrin derivative ligand can act as an electron donor and carry out an efficient PET process with the electron acceptor NDI. Various ROS indicators demonstrate the superior ROS generation ability of 3DNDITcpp under light irradiation. Using activated 2,7-dichlorodihydrofluorescein diacetate (DCFH-DA) as an indicator of total ROS, the fluorescence enhancement factors of 2DTcpp, 3DPyTcpp, and 3DNDITcpp were 42.13, 48.24 and 94.21 times, respectively. Both the degradation curve and degradation rate of 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) demonstrated that the order of 1O2 production ability was 3DNDITcpp (rate up to 0.312 min-1) > 3DpyTcpp (0.158 min-1) ≈ 2DTcpp (0.155 min-1). In addition, dihydrorhodamine 123 (DHR 123) and hydroxyphenyl fluorescein (HPF) were used as specific indicators of O2- and OH to monitor the generation of type I ROS of 2DTcpp, 3DPyTcpp, and 3DNDITcpp, respectively. The fluorescence enhancement factors of DHR 123 and HPF aqueous solutions containing 3DNDITcpp were as high as 47.70 and 192.19 times, respectively. The fluorescence enhancement factors of DHR 123 and HPF containing 2DTcpp and 3DPyTcpp were 19.65/63.07 (2DTcpp) and 27.97/134.19 times (3DPyTcpp), respectively. Photocurrent response (3DNDITcpp is 1.2 and 2.7 times better than 3DPyTcpp and 2DTcpp, respectively) and electrochemical impedance (3DNDITcpp is 1.9 and 2.9 times smaller than 3DPyTcpp and 2DTcpp, respectively) measurements confirming its excellent type I ROS production capability. Under low-power light irradiation (60 mW·cm-2, 5 min), ROS generated by 3DNDITcpp effectively inactivates Escherichia coli and Staphylococcus aureus, with an inhibition zone diameter of approximately 4.00 cm. Furthermore, 3DNDITcpp rapidly degrades various colored dyes and antibiotics within 30 min, achieving degradation rates as high as 0.095 and 0.054 min-1, outperforming most traditional photosensitizers (PSs). To our knowledge, this is the first instance when differences in the electron clouds of mixed ligands are leveraged to induce PET interactions within pillar-layered MOFs, yielding excellent porous PSs. Overall, our study offers a new approach for developing porous PSs with enhanced ROS generation capacity and advances MOFs crystal engineering based on mixed ligands.

Decoupling Interlayer Interactions Boosts Charge Separation in Covalent Organic Frameworks for High-Efficiency Photocatalytic CO2 Reduction

Covalent organic frameworks (COFs) have emerged as promising photocatalysts owing to their structural diversity, tunable bandgaps, and exceptional light-harvesting capabilities. While previous studies primarily focus on developing narrow-bandgap COFs for broad-spectrum solar energy utilization, the critical role of interlayer coupling in regulating charge transfer dynamics remains unclear. Conventional monolayer-based theoretical models inadequately address interlayer effects that potentially hindering intralayer electron transport to catalytic active sites. This work employs density functional theory (DFT) calculations to investigate the influence of interlayer interactions on intralayer charge transfer in imine-based COFs. Theoretical analyses reveal that bilayer architectures exhibit pronounced interlayer interference in intramolecular charge transfer processes which has not been observed in monolayer models. Based on these mechanistic insights, this work designs two isomeric pyrene-based COFs incorporating identical electron donor (pyrene) and acceptor (nickel bipyridine) units but with distinct interlayer coupling strengths. Strikingly, the optimized COF with weakened interlayer interactions demonstrates exceptional photocatalytic CO2 reduction performance, achieving a CO evolution rate of 553.3 µmol g-1 h-1 with 94% selectivity under visible light irradiation without additional photosensitizers or co-catalysts. These findings establish interlayer engineering as a crucial design principle for developing high-performance COF-based photocatalysts for solar energy conversion applications.

Three-Dimensional Covalent Organic Frameworks with jcg Topology Based on a Trinodal Strategy

The development of three-dimensional (3D) covalent organic frameworks (COFs) holds significant promise for various applications, but the conventional uninodal or binodal design strategies limit their structural diversity. In this work, we present a novel trinodal strategy for the synthesis of 3D COFs featuring both microporous and mesoporous nanochannels. Using powder X-ray diffraction (PXRD), computational simulations, and high-resolution transmission electron microscopy (HR-TEM), we demonstrate that employing an 8-c building block with reduced symmetry, which can be considered as 4- and 3-connected subunits, along with planar 4-c building blocks, results in an unprecedented [4 + 3 + 4]-c jcg net. This structure features rare saddle-shaped eight-membered rings and mirror-symmetrical chains. Furthermore, the incorporation of chromophore pyrene and redox-active triphenylamine components, coupled with structural conjugation, imparts tunable photophysical and electronic properties to these COFs, making them promising candidates for photocatalytic H2O2 production. This work highlights the potential of the trinodal strategy in creating intricate COF architectures and enhances their applicability in heterogeneous photocatalysis.

Visible-light harvesting 2D copper-cluster-based MOFs as efficient ROS generators for selective oxidation of amines

We have designed and synthesized an aesthetically appealing two-dimensional copper-cluster-based organic framework material named Cu-BPYC. This material exhibits superior charge separation and transfer efficiency, as well as reactive oxygen species (ROS) generation capability under visible-light irradiation. Through the synergistic mechanisms of photo-induced energy and charge transfer, it effectively promotes the oxidation of amines to imines. Additionally, Cu-BPYC demonstrates excellent structural stability and reusability in heterogeneous catalytic systems.

Donor-acceptor conjugated porous polymers from truxene and triazine: Effect of connecting units on photocatalytic activity for selective oxidation of amines and sulfides

Donor-acceptor (D-A) conjugated polymers have been widely reported as promising photocatalysts for organic conversion. However, achieving excellent photocatalytic performance still relies on the rational design of molecular structures and the careful selection of appropriate building blocks. In this study, we designed two D-A type conjugated porous polymers (CPPs) using 2,7,12-tribromo-5,5,10,10,15,15-hexamethyl-10,15-dihydro-5H-diindeno[1,2-a:1',2'-c]fluorene (Tx) as the donor unit and two 1,3,5-triazine-based derivatives, namely 2,4,6-tri(thiophen-2-yl)-1,3,5-triazine (TTT) and 2,4,6-triphenyl-1,3,5-triazine (TPT), as the acceptor units. The resulting CPPs are named ThTx-CPP and PhTx-CPP, respectively. The research findings emphasize the profound impact of minute structural changes in the triazine peripheral groups on the photocatalytic activity of the polymers. Compared to PhTx-CPP, ThTx-CPP exhibits superior light-harvesting capabilities, narrower bandgaps, and improved efficiency in charge separation. Specifically, ThTx-CPP demonstrates outstanding activity and selectivity in both amine coupling and sulfide oxidation reactions, surpassing PhTx-CPP by a significant margin. Furthermore, the catalyst retains its consistent activity even after five cycles of reuse, showcasing its high stability and excellent reusability.

Programmed Charge Transfer in Conjugated Polymers with Pendant Benzothiadiazole Acceptor for Simultaneous Photocatalytic H2O2 Production and Organic Synthesis

While being important candidate for heterogeneous photocatalyst, conjugated polymer typically exhibits random charge transfers between the alternating donor and acceptor units, which severely limits its catalytic efficiency. Herein, inspired by natural photosystem, the concept of guiding the charge migration to specific reaction sites is employed to significantly boost photocatalytic performance of linear conjugated polymers (LCPs) with pendant functional groups via creating programmed charge-transfer channels from the backbone to its pendant moiety. The pendant benzothiadiazole, as revealed in both in situ X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations, can act as electron "reservoir" that aggregates electrons at the active sites. Moreover, the presence of charge-transfer channels, evidenced by transient absorption spectroscopy (TAS), accelerates the electron transfer, preventing the recombination of electrons and holes. As a result, in this elaborately-designed architecture, the photogenerated electron can move smoothly towards the reduction sites, facilitating the reduction of O2 into H2O2, while remaining holes are directed to oxidation centers, simultaneously oxidizing furfuryl alcohol to furoic acid. The optimized photocatalyst LCP-BT demonstrates a competitive catalytic performance with H2O2 productivity of 868.3 μmol L-1 h-1 (9.8 times higher than conventional random charge-transfer polymer LCP-1) and furfuryl alcohol conversion over 95 % after 6 h.

Constructing Donor-Acceptor Covalent Organic Frameworks for Highly Efficient H2O2 Photosynthesis Coupled with Oxidative Organic Transformations

The development of photocatalytic systems that enable the simultaneous production of H2O2 and value-added organic chemicals presents a dual advantage: generating valuable products while maximizing the utilization of solar energy. Despite the potential, there are relatively few reports on photocatalysts capable of such dual functions. In this study, we synthesized a series of donor-acceptor covalent organic frameworks (COFs), designated as JUC-675 to JUC-677, to explore their photocatalytic efficiency in the co-production of H2O2 and N-benzylbenzaldimine (BBAD). Among them, JUC-675 exhibited exceptional performance, achieving a H2O2 production rate of 22.8 mmol g-1 h-1 with an apparent quantum yield of 15.7 %, and its solar-to-chemical conversion efficiency was calculated to be 1.09 %, marking it as the most effective COF-based photocatalyst reported to date. Additionally, JUC-675 demonstrated a high selectivity (99.9 %) and yield (96 %) for BBAD in the oxidative coupling of benzylamine. The underlying reaction mechanism was thoroughly investigated through validation experiments and density functional theory (DFT) calculations. This work represents a significant advancement in the design of COF-based photocatalysts and the development of efficient dual-function photocatalytic platforms, offering new insights and methodologies for enhanced solar energy utilization and the synthesis of value-added products.

2D Undulated Metal Hydrogen-Bonded Organic Frameworks with Self-Adaption Interlayered Sites for Highly Efficient C-C Coupling in the Electrocatalytic CO2 Reduction

The hydrogen-bonded organic frameworks (HOFs) as a new type of porous framework materials have been widely studied in various areas. However, the lack of appropriate active sites, low intrinsic conductivity, and poor stability limited their performance in the field of electrocatalysis. Herein, we designed two 2D metal hydrogen-bonded organic frameworks (2D-M-HOF, M = Cu2+ or Ni2+) with coordination compounds based on 2,3,6,7,14,15-hexahydroxyl cyclotricatechylene and transition metal ions (Cu2+ and Ni2+), respectively. The crystal structure of 2D-Cu-HOF is determined by continuous rotation electron diffraction, indicating an undulated 2D hydrogen-bond network with interlayered π-π stacking. The flexible structure of 2D-M-HOF leads to the formation of self-adaption interlayered sites, resulting in superior activity and selectivity in the electrocatalytic conversion of CO2 to C2 products, achieving a total Faradaic efficiency exceeding 80% due to the high-efficiency C-C coupling. The experimental results and density functional calculations verify that the undulated 2D-M-HOF enables the energetically favorable formation of *OCCHO intermediate. This work provides a promising strategy for designing HOF catalysts in electrocatalysis and related processes.

Tailoring the Electrocatalytic Properties of Porphyrin Covalent Organic Frameworks for Highly Selective Oxygen Sensing In Vivo

In vivo selective sensing of oxygen (O2) dynamics in the central nervous system could provide insights into energy metabolism and neural activities. Although the electrocatalytic four-electron oxygen reduction reaction (ORR) paves an effective way to the electrochemical sensing of O2 in vivo, the concurrent hydrogen peroxide reduction reaction (HPRR) within the potential windows for four-electron ORR unfortunately poses a great challenge to the conventional mechanism employed for selective electrochemical O2 sensing. In this work, we find that regulation of the linkers within the skeleton of porphyrin-based covalent organic frameworks (COFs) could improve the selectivity of the O2 sensor against hydrogen peroxide (H2O2). The electrochemical results reveal that the Co porphyrin active sites facilitate the direct four-electron pathway for ORR and that the Co porphyrin-based COF, enriched with pyrene units, shows enhanced four-electron ORR kinetics and better tolerance to HPRR. The theoretical calculation suggests that introducing pyrene units essentially weakens the adsorption of H2O2, leading to suppression of the HPRR. The microsensor fabricated with the Co porphyrin-based COF as the electrocatalyst features a high selectivity for real-time monitoring of O2 in a living rat brain.

Photo-Driven Ammonia Synthesis via a Proton-Mediated Photoelectrochemical Device

N2 reduction reaction (NRR) by light is an energy-saving and sustainable ammonia (NH3) synthesis technology. However, it faces significant challenges, including high energy barriers of N2 activation and unclear catalytic active sites. Herein, we propose a strategy of photo-driven ammonia synthesis via a proton-mediated photoelectrochemical device. We used redox-catalysis covalent organic framework (COF), with a redox site (-C=O) for H+ reversible storage and a catalytic site (porphyrin Au) for NRR. In the proton-mediated photoelectrochemical device, the COF can successfully store e- and H+ generated by hydrogen oxidation reaction, forming COF-H. Then, these stored e- and H+ can be used for photo-driven NRR (108.97 umol g-1) under low proton concentration promoted by the H-bond network formed between -OH in COF-H and N2 on Au, which enabled N2 hydrogenation and NH3 production, establishing basis for advancing artificial photosynthesis and enhancing ammonia synthesis technology.

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.

3D N-heterocyclic covalent organic frameworks for urea photosynthesis from NH3 and CO2

Artificial photosynthesis of urea from NH3 and CO2 seems to remain still essentially unexplored. Herein, three isomorphic three-dimensional covalent organic frameworks with twofold interpenetrated ffc topology are functionalized by benzene, pyrazine, and tetrazine active moieties, respectively. A series of experiment results disclose the gradually enhanced conductivity, light-harvesting capacity, photogenerated carrier separation efficiency, and co-adsorption capacity towards NH3 and CO2 in the order of benzene-, pyrazine-, and tetrazine-containing framework. This in turn endows tetrazine-containing framework with superior photocatalytic activity towards urea production from NH3 and CO2 with the yield of 523 μmol g-1 h-1, 40 and 4 times higher than that for benzene- and pyrazine-containing framework, respectively, indicating the heterocyclic N microenvironment-dependent catalytic performance for these three photocatalysts. This is further confirmed by in-situ spectroscopic characterization and density functional theory calculations. This work lays a way towards sustainable photosynthesis of urea.

Novel Inorganic-Organic Dual-Photosensitizing Dinuclear-Metal Self-Assembly System for Efficient Artificial Photosynthesis without Sacrificial Electron Donors

Owing to the unique synergistic effect, dinuclear-metal-molecule-catalysts (DMCs) show excellent performance in catalytic fields. However, for overall photocatalytic CO2 reaction (CO2RR), it is still a challenge to construct well-matched photosensitizer (PS) components for DMCs-based photocatalysts. Inorganic-quantum-dot PS possesses capacities of multiple exciton generation and catalyzing water oxidation but is incompatible with DMCs. In contrast, organic PS can be covalently linked with DMCs but inescapable of using sacrificial electron donors. For overall photocatalytic CO2RR, organic-inorganic dual-photosensitizing system might be a promising candidate. Herein, we employed covalent-linking and electrostatic-driven approaches to construct the self-assembly of pyrene-sensitized Co2L DMCs (Py-Co2L) and perovskite (PVK) quantum dots, i.e., PVK@[Py-Co2L]. Using H2O as an electron donor, PVK@[Py-Co2L] realized 105.24 μmol ⋅ g-1⋅h-1 CO yield in photocatalytic CO2RR, much higher than PVK (15.44 μmol ⋅ g-1⋅h-1) and PVK@Co2L (32.30 μmol ⋅ g-1 ⋅ h-1), ascribing to the efficient photogenerated charge separation and transfer. The experimental results and theoretical investigations demonstrated that the pyrene linked on Co2L boosted the electron delivery from PVK to DMCs. Besides, this strategy could also be extended to the photocatalytic H2 evolution coupled with alcohol oxidation. As a proof-of-concept, our work lightens the integration of DMCs, organic and inorganic PS components, promoting the development of photocatalysis without sacrificial electron donors.

Vertically Expanded Covalent Organic Frameworks for Photocatalytic Water Oxidation into Oxygen

Covalent organic frameworks with unique π architectures and pores could be developed as photocatalysts for transformations. However, they usually form π-stacking layers, so that only surface layers function in photocatalysis. Here we report a strategy for developing vertically expanded frameworks to expose originally inaccessible active sites hidden in layers to catalysis. We designed covalently linked two-dimensional cobalt(II) porphyrin layers and explored coordination bonds to connect the cobalt(II) porphyrin layers with bidentate ligands via a three-component one-pot polymerization. The resultant frameworks expand the interlayer space greatly, where both the up and down faces of each cobalt(II) porphyrin layer are exposed to reactants. Unexpectedly, the vertically expanded frameworks increase skeleton oxidation potentials, decrease exciton dissociation energy, improve pore hydrophilicity and affinity to water, and facilitate water delivery. Remarkably, these positive effects work collectively in the photocatalysis of water oxidation into oxygen, with an oxygen production rate of 1155 μmol g-1 h-1, a quantum efficiency of 1.24 % at 450 nm, and a turnover frequency of 1.39 h-1, which is even 5.1-fold as high as that of the π-stacked frameworks and ranks them the most effective photocatalysts. This strategy offers a new platform for designing layer frameworks to build various catalytic systems for chemical transformations.

Anion-Regulated Ionic Covalent Organic Frameworks for Highly Selective Recovery of Gold from E-Waste

The existing electronic waste (e-waste) and leaching solutions generated by industries accumulate significant amounts of gold (Au), even in excess of those in natural minerals. Therefore, the recycling of Au is extremely significant for the potential sustainability of chemical industry. By designing ionic covalent organic frameworks (COFs), here we synthesize a series of Ionic-COF-X (X=Cl-, Br-, AcO-, and SO4 2-) by anion regulation strategy and further explore their adsorption performance towards Au recovery. All these ionic COFs exhibit ultrahigh gold adsorption efficiency and excellent regeneration. Moreover, anion regulation could indeed affect the Au capture performance. In particular, when Cl- ions serve as counter ions, the Au capacity of Ionic-COF-Cl could reach 1270.8 mg g-1. Moreover, in the actual CPU leaching solution test, the selectivity of Ionic-COF-Cl towards Au3+ ion hits 39000 and 4600 times higher than that of Cu2+ and Ni2+ ions, respectively, suggesting that the Ionic-COF-Cl is a promising material for highly selective recovering gold from actual e-waste. DFT calculations further reveal that counter ions can regulate the adsorption affinity of ionic COF framework toward Au. In short, this work provides a useful anion regulation strategy to design ionic COFs as a promising platform for gold selective recovery from actual e-waste.

Toward Pore Size-Selective Photoredox Catalysis Using Bifunctional Microporous 2D Triazine-Based Covalent Organic Frameworks

The design and synthesis of photoactive metal-free 2D materials for selective heterogeneous photoredox catalysis continue to be challenging due to issues related to nonrecyclability, and limited photo- and chemical stability. Herein, we report the photocatalytic properties of a triazine-based porous COF, TRIPTA, which is found to be capable of facilitating both SET (single electron transfer) for photocatalytic reductive debromination of phenacyl bromide in absence of oxygen and generation of reactive oxygen species (ROS) for benzylamine photo-oxidation in the presence of oxygen, respectively, under visible light irradiation. Inspired by the latter results, we further systematically investigated different-sized benzylamine substrates in this single-component reaction and compared the results with an analogous COF (Micro-COF-2) exhibiting a larger pore size. We observed a marked improvement in the conversion of larger-sized substrates with the latter COF, thereby demonstrating angstrom-level pore size-selective photocatalytic activity of COFs.

Three-Dimensional Mesoporous Covalent Organic Framework for Photocatalytic Oxidative Dehydrogenation to Quinoline

Developing precious metal-free catalysts for organic reactions under mild conditions is urgent. Herein, we report a three-dimensional covalent organic framework (3D-COF) with high crystallinity and permanent pores, termed 3D-TABPA-COF, for the oxidation of tetrahydroquinoline to quinoline. The 3D-TABPA-COF assembled based on N4,N4-bis(4'-amino-[1,1'-biphenyl]-4-yl)-[1,1'-biphenyl]-4,4'-diamine (TABPA) is the catalytic active center for the conversion of tetrahydroquinoline. The triphenylamine in the structure is an effective photosensitizer, which not only enhances the light absorption capacity but also facilitates the rapid transfer of photogenerated electrons and ensures effective carrier separation. The obtained 3D-TABPA-COF has a high specific surface area (2745.06 m2 g-1) and mesopores of 3.57 nm. This is attributed to the fact that the bor topology is not easy to interpenetrate. It can oxidize tetrahydroquinoline to obtain quinoline efficiently under visible light irradiation. In addition, we also performed various photochemical characterizations combined with density functional theory calculations to elucidate the reaction mechanism from tetrahydroquinoline to quinoline. This work provides a feasible strategy for constructing 3D-COF to achieve efficient photocatalytic organic reactions.

Reticular Materials for Photocatalysis

Photocatalysis leverages solar energy to overcome the thermodynamic barrier, enabling efficient chemical reactions under mild conditions. It can greatly reduce reliance on traditional energy sources and has attracted significant research interest. Reticular materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), represent a class of crystalline materials constructed from molecular building blocks linked by coordination and covalent bonds, respectively. Reticular materials function as heterogeneous catalysts, combining well-defined structures and high tailorability akin to homogeneous catalysts. In this review, the regulation of light absorption, charge separation, and surface reactions in the photocatalytic process through precise molecular-level design based on the features of reticular materials is elaborated. Notably, for MOFsmicroenvironment modulation around catalytic sites affects photocatalytic performance is delved, with emphasis on their unique dynamic and flexible microenvironments. For COFs, the inherent excitonic effects due to their fully organic nature is discussed and highlight the strategies to regulate excitonic effects for charge- and/or energy-transfer-mediated photocatalysis. Finally, the current challenges and future directions in this field, aiming to provide a comprehensive understanding of how reticular materials can be optimized for enhanced photocatalysis is discussed.

In situ Construction of Single-Atom Electronic Bridge on COF to Enhance Photocatalytic H2 Production

Photocatalytic hydrogen production is one of the most valuable technologies in the future energy system. Here, we designed a metal-covalent organic frameworks (MCOFs) with both small-sized metal clusters and nitrogen-rich ligands, named COF-Cu3TG. Based on our design, small-sized metal clusters were selected to increase the density of active sites and shorten the distance of electron transport to active sites. While another building block containing nitrogen-rich organic ligands acted as a node that could in situ anchor metal atoms during photocatalysis and form interlayer single-atom electron bridges (SAEB) to accelerate electron transport. Together, they promoted photocatalytic performance. This represented the further utilization of Ru atoms and was an additional application of the photosensitizer. N2-Ru-N2 electron bridge (Ru-SAEB) was created in situ between the layers, resulting in a considerable enhancement in the hydrogen production rate of the photocatalyst to 10.47 mmol g-1 h-1. Through theoretical calculation and EXAFS, the existence position and action mechanism of Ru-SAEB were reasonably inferred, further confirming the rationality of the Ru-SAEB configuration. A sufficiently proximity between the small-sized Cu3 cluster and the Ru-SAEB was found to expedite electron transfer. This work demonstrated the synergistic impact of small molecular clusters with Ru-SAEB for efficient photocatalytic hydrogen production.

Nitrogen-Site Engineering in Covalent Organic Frameworks for H2O2 Photogeneration via Dual Channels of Indirect Two-Electron O2 Reduction

Photocatalytic H2O2 production is a green and sustainable route, but far from meeting the increasing demands of industrialization due to the rapid recombination of the photogenerated charge carriers and the sluggish reaction kinetics. Effective strategies for precisely regulating the photogenerated carrier behavior and catalytic activity to construct high-performance photocatalysts are urgently needed. Herein, a nitrogen-site engineering strategy, implying elaborately tuning the species and densities of nitrogen atoms, is applied for H2O2 photogeneration performance regulation. Different nitrogen heterocycles, such as pyridine, pyrimidine, and triazine units, are polymerized with trithiophene units, and five covalent organic frameworks (COFs) with distinct nitrogen species and densities on the skeletons are obtained. Fascinatingly, they photocatalyzed H2O2 production via dominated two-electron O2 reduction processes, including O2-O2 •‒-H2O2 and O2-O2 •‒-O2 1-H2O2 dual pathways. Just in the air and pure water, the multicomponent TTA-TF-COF with the maximum nitrogen densities triazine nitrogen densities exhibited the highest H2O2 production rate of 3343 µmol g-1 h-1, higher than most of other reported COFs. The theoretical calculation revealed the higher activity is due to the easy formation of O2 •‒ and O2 1 in different catalytic process. This study gives a new insight into designing photocatalysis at atomic level.

Two-Dimensional Covalent Organic Frameworks with Carbazole-Embedded Frameworks Facilitate Photocatalytic and Electrocatalytic Processes

Catalytic technologies are pivotal in enhancing energy efficiency, promoting clean energy production, and reducing energy consumption in the chemical industry. The pursuit of novel catalysts for renewable energy is a long-term goal for researchers. In this work, we synthesized three two-dimensional covalent organic frameworks (COFs) featuring electron-rich carbazole-based architectures and evaluated their catalytic performance in photocatalytic organic reactions and electrocatalytic oxygen reduction reactions (ORRs). Pyrene-functionalized COF, termed as FCTD-TAPy, demonstrated excellent photocatalytic performance for amino oxidation coupling and showed a remarkable preference for substrates with electron-withdrawing groups (up to >99% Conv. and >99% Sel). Furthermore, FCTD-TAPy favored a four-electron transfer pathway during the ORR and exhibited favorable reaction kinetics (51.07 mV/dec) and a high turnover frequency (0.011 s-1). In contrast, the ORR of benzothiadiazole-based FCTD-TABT favored a two-electron transfer pathway, which exhibited a maximum double-layer capacitance of 14.26 mF cm-2, a Tafel slope of 53.01 mV/dec, and a hydrogen peroxide generation rate of 70.3 mmol g-1 h-1. This work underscores the potential of carbazole-based COFs as advanced catalytic materials and offers new insights into the design of metal-free COFs for enhanced catalytic performance.

Porous Bimetallic Ti-MOFs for Photocatalytic Oxidation of Amines in Air

A family of microporous titanium-containing metal-organic frameworks (denoted as M2Ti-CPCDC, M = Mn, Co, Ni) has been synthesized by using a bimetallic [M2Ti(μ3-O)(COO)6] cluster and a tritopic carbazole-based organic ligand H3CPCDC. M2Ti-CPCDC are stable and display permanent porosity for N2 and CO2 uptake, ranking among the most porous titanium-based metal-organic frameworks. M2Ti-CPCDC crystals exhibit n-type semiconductor behavior. Further catalytic studies demonstrate that all M2Ti-CPCDC materials are applicable for triggering photo-oxidative reactions of amines in air. More specifically, amines with electron-donating groups afford the aldehydes as the main products, while amines bearing electron-withdrawing groups give rise to imines as the main product. Among them, Mn2Ti-CPCDC exhibit the best photocatalytic activity, with conversion of benzylamine up to 99% and selectivity of 99%. Mn2Ti-CPCDC could be recycled in at least three runs while retaining crystallinity and catalytic activity. The reaction mechanism indicates that photoinduced hole (h+), superoxide radical anion (O2·-), and singlet oxygen (1O2) are the main active species involved in the photo-oxidation process.

Integrating Multipolar Structures and Carboxyl Groups in sp2-Carbon Conjugated Covalent Organic Frameworks for Overall Photocatalytic Hydrogen Peroxide Production

The direct production of hydrogen peroxide (H2O2) through photocatalytic reaction via H2O and O2 is considered as an ideal approach. However, the efficiency of H2O2 generation is generally limited by insufficient charge and mass transfer. Covalent organic framework (COFs) offer a promising platform as metal-free photocatalyst for H2O2 production due to their potential for rational design at the molecular level. Herein, we integrated the multipolar structures and carboxyl groups into COFs to enhance the efficiency of photocatalytic H2O2 production in pure water without any sacrificial agents. The introduction of octupolar and quadrupolar structures, along with an increase of molecular planarity, created efficient oxygen reduction reaction (ORR) sites. Meanwhile, carboxyl groups could not only boost O2 and H2O2 movement via enhancement of pore hydrophilicity, but also promote proton conduction, enabling the conversion to H2O2 from ⋅O2 -, which is the crucial intermediate product in H2O2 photocatalysis. Overall, we demonstrate that TACOF-1-COOH, consisting of optimal octupolar and quadrupolar structures, along with enrichment sites (carboxyl groups), exhibited a H2O2 yield rate of 3542 μmol h- 1 g-1 and a solar-to-chemical (SCC) efficiency of 0.55 %. This work provides valuable insights for designing metal-free photocatalysts for efficient H2O2 production.

Orientation-Dependent Photocatalysis of Imine-Linked Covalent Organic Frameworks Based on Thienothiophenes for Oxidation of Amines to Imines

Toward visible light photocatalysis, covalent organic frameworks (COFs) have recently garnered growing attention. The effect of different orientations of imine of imine-linked COFs on photocatalysis should be elucidated. Here, two COFs are developed with 2,5-diphenylthieno[3,2-b]thiophene (DPTT) and 1,3,6,8-tetraphenylpyrene (Py) linked by imine, affording DPTT-Py-COF and Py-DPTT-COF, respectively. Distinctly, DPTT-Py-COF and Py-DPTT-COF have high crystallinity and porosity, paving the way to highly efficient photocatalysis. Theoretical calculations demonstrate that both DPTT-Py-COF and Py-DPTT-COF are of similar bandgaps but of varied energy positions due to the different orientations of imine. Besides, characterizations disclose that DPTT-Py-COF delivers more enhanced charge separation and transfer than Py-DPTT-COF. Probed by the oxidation of amine to imine, DPTT-Py-COF exhibits a blue light photocatalytic performance superior to that of Py-DPTT-COF. DPTT-Py-COF, a highly recyclable photocatalyst, enables the oxidation of various amines to imines with oxygen. This work highlights that tuning the microenvironment of COFs unravels tenable performances in photocatalysis.

Anisotropic Dual S-Scheme Heterojunctions Mimic Natural Photosynthetic System for Boosting Photoelectric Response

The design of heterojunctions that mimic natural photosynthetic systems holds great promise for enhancing photoelectric response. However, the limited interfacial space charge layer (SCL) often fails to provide sufficient driving force for the directional migration of inner charge carriers. Drawing inspiration from the electron transport chain (ETC) in natural photosynthesis system, we developed a novel anisotropic dual S-scheme heterojunction artificial photosynthetic system composed of Bi2O3-BiOBr-AgI for the first time, with Bi2O3 and AgI selectively distributed along the bicrystal facets of BiOBr. Compared to traditional semiconductors, the anisotropic carrier migration in BiOBr overcomes the recombination resulting from thermodynamic diffusion, thereby establishing a potential ETC for the directional migration of inner charge carriers. Importantly, this pioneering bioinspired design overcomes the limitations imposed by the limited distribution of SCL in heterojunctions, resulting in a remarkable 55-fold enhancement in photoelectric performance. Leveraging the etching of thiols on Ag-based materials, this dual S-scheme heterojunction is further employed in the construction of photoelectrochemical sensors for the detection of acetylcholinesterase and organophosphorus pesticides.

Two-Dimensional Covalent Organic Frameworks with Pentagonal Pores

The pore shapes of two-dimensional covalent organic frameworks (2D COFs) significantly limit their practical applications in separation and catalysis. Although various 2D COFs with polygonal pores have been well developed, constructing COFs with pentagonal pores remains an enormous challenge. In this work, we developed one kind of pentagonal COFs with the mcm topological structure for the first time, through the rational combination of C4 and C2 symmetric building blocks. The resulting pentagonal COFs exhibit high crystallinity, excellent porosity, and strong robustness. Moreover, the inbuilt porphyrin units render these COFs as efficient electrocatalytic catalysts toward oxygen reduction reaction with a half-wave potential of up to 0.81 V, which ranks as one of the highest values among COFs-based electrocatalysts. This work not only verified the possibility of constructing 2D COFs with pentagonal pores but also developed a strategy for the construction of functional 2D COFs for interesting applications.

Ionic Covalent Organic Frameworks in Adsorption and Catalysis

The ion extraction and electro/photo catalysis are promising methods to address environmental and energy issues. Covalent organic frameworks (COFs) are a class of promising template to construct absorbents and catalysts because of their stable frameworks, high surface areas, controllable pore environments, and well-defined catalytic sites. Among them, ionic COFs as unique class of crystalline porous materials, with charges in the frameworks or along the pore walls, have shown different properties and resulting performance in these applications with those from charge-neutral COFs. In this review, current research progress based on the ionic COFs for ion extraction and energy conversion, including cationic/anionic materials and electro/photo catalysis is reviewed in terms of the synthesis strategy, modification methods, mechanisms of adsorption and catalysis, as well as applications. Finally, we demonstrated the current challenges and future development of ionic COFs in design strategies and applications.