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

Design and structure-function interplay in covalent organic frameworks for photocatalytic CO2 reduction

The escalating global energy demands and the need to alleviate the rapid rise in greenhouse gases have led to colossal interest in designing efficient catalytic systems for photocatalytic CO2 reduction. While inorganic semiconductors have been the frontrunners for a long time, porous photocatalysts, particularly covalent organic frameworks (COFs), are gaining traction due to their atomically precise structures, enabling tuning their structural and chemical properties. Designed using the principles of reticular chemistry, the building units of COFs can be modulated to incorporate catalytically active sites periodically using robust covalent bonds to endow them with high efficiency, selectivity, and stability. Unlike the non-porous congeners, COFs, with their high porosity and precisely defined pore channels, allow for quicker diffusion of substrates and products, enabling the utilization of deeply buried photocatalytic sites. Our approach is to comprehend the significant roadblocks that must be overcome for designing state-of-the-art catalysts for photocatalytic CO2 reduction. Building upon that, we highlight the key strategies devised to design COF-based CO2RR photocatalysts. A fundamental understanding of the structure-property relationship is quintessential for utilizing the precision of COF chemistry for developing next-generation materials combining activity, selectivity, and efficiency in a single system. Throughout this review, we have taken a closer look at how the critical design aspects and molecular engineering reciprocate towards augmenting the bulk photocatalytic properties of efficiency and selectivity. Understanding molecular engineering and structure-property relationships will be conducive to developing sophisticated systems to solve global crises in this burgeoning area of research.

Expanded Synthesis of 3D Covalent Organic Frameworks via Linker Exchange for Efficient Photocatalytic Aerobic Oxidation

Despite recent progress in 3D covalent organic frameworks (3D-COFs), their design and synthesis still pose significant challenges, mainly due to a limited mechanistic understanding of their synthesis. Herein, a linker exchange approach has been utilized to synthesize a series of new 3D-COFs by first preparing an imine-linked 3D-COF followed by exchanging with selected linear diamine linkers. This approach can be widely applicable to different types of diamines, enabling rational-designed synthesis of 3D frameworks that are previously inaccessible via direct polymerization in a one-pot reaction. Mechanistic aspects associated with the improved 3D-COF synthesis via the linker exchange approach, are investigated by density functional theory calculations, in which the possibility of the departure of the leaving linker is a spontaneous process with a decrease in enthalpy. Catalytic and computational results revealed that incorporating benzoxazole moiety into the 3D-COF frameworks enables a significant increase in the capability of visible-light-driven catalysis. The overall findings of the present study will pave the way toward the development of 3D-COFs with tunable structures and functions for other promising and challenging applications.

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.

High-Connectivity 3D Covalent Organic Frameworks with pdp Net for Efficient C2H2/CO2 Separation

High-connectivity 3D covalent organic frameworks (COFs) have garnered significant attention due to their structural complexity, stability, and potential for functional applications. However, the synthesis of 3D COFs using mixed high-nodal building units remains a substantial challenge. In this work, we introduce two novel 3D COFs, JUC-661 and JUC-662, which are constructed using a combination of D2h-symmetric 8-nodal and D3h-symmetric 6-nodal building blocks. These COFs feature an unprecedented [8+6]-c pdp net with rare mesoporous polyhedral cages (~3.9 nm). Remarkably, JUC-661 and JUC-662 exhibit outstanding separation capabilities, achieving adsorption selectivities of 4.3 and 5.9, respectively, for C2H2/CO2 (1/1, v/v) mixtures. Dynamic breakthrough experiments confirm their excellent separation capability, maintaining this performance even under conditions of 100 % humidity. Monte Carlo simulations and DFT calculations indicate that the exceptional adsorption performance is attributed to the well-defined pore cavities of the COFs, with fluorination of the building unit further enhancing C2H2 selectivity through improved electrostatic and host-guest interactions. This study expands the structural diversity of COFs and highlights their potential for low-energy separation processes.

Integrating Multifunctionalities into a 3D Covalent Organic Framework for Efficient CO2 Photoreduction

Fabrication of highly efficient photocatalysts for CO2 conversion is still challenging. Herein, integrating nitrogen-rich organic cages and the photoactive porphyrin moieties together, a 3D covalent organic framework (COF), Cage-PorCOF, is successfully synthesized. After incorporating metal ions (Co2+ and Ni2+) into the cage-based COF, Cage-PorCOF(Co) and Cage-PorCOF(Ni) are subsequently constructed for the CO2 photoreduction. Catalytic experiments show impressive performance in CO2 photoreduction with CO generation rates of up to 48 748 and 28 446 µmol g-1 h-1 in the first initiating hour for Cage-PorCOF(Co) and Cage-PorCOF(Ni), respectively, which is attributed to the synergistic effects from CO2-affinity of the porous frameworks and incorporated metal atoms, the light-absorption and charge separation ability of metalloporphyrin groups as well as the fully exposed single-atomic catalytic sites confirmed by both experimental and theoretical analyses. This study demonstrates that by the integration of multiple functionalities into 3D porous solids, highly effective photocatalysts for CO2 conversion can be achieved.

Porphyrin-Based Covalent Organic Frameworks for CO2 Photo/Electro-Reduction

Photo/electro-catalytic CO2 reduction into high-value products are promising strategies for addressing both environmental problems and energy crisis. Duo to their advantageous visible light absorption ability, adjustable optic/electronic properties, definite active center, post-modification capability, and excellent stability, porphyrin-based covalent organic frameworks (COFs) have emerged as attractive photo/electro-catalysts towards CO2 reduction. In this review, the research progress of the porphyrin-based COFs for photo/electro-catalytic CO2 reduction is summarized including the design principles, catalytic performance, and reaction mechanism. In addition, this review also presents some challenges and prospects for the application of porphyrin-based COFs in photo/electro-catalytic CO2 reduction, laying the base for both fundamental research and application efforts.

Precisely Tuning Band Gaps of Hexabenzocoronene-Based MOFs Toward Enhanced Photocatalysis

Precise adjusting the band gaps in metal-organic frameworks (MOFs) is crucial for improving their visible-light absorption capacity during photocatalysis, presenting both a formidable challenge and a charming opportunity. This present study employed a symmetry-reduction strategy to pre-design six novel 4-connected ligands with systematic substituents (-NO2, -H, -tBu, -OCH3, -OH and -NH2) and synthesized the corresponding pillared-layer Zr-MOFs (NKM-668) retaining the hexaphenylbenzene fragment. Subsequently, the NKM-668 MOFs were transformed into large-π-conjugated hexabenzocoronene-based MOFs (pNKM-668) via the Scholl reaction. These twelve MOFs exhibited broad and tunable band gaps over 1.41 eV (ranging from 3.25 eV to 1.84 eV), and the photocatalytic CO2 conversion rate raised by 33.2-fold. This study not only enriches the type of hexaphenylbenzene-based MOFs, but also paves the way for nanographene-containing MOFs in the further application of photocatalysis.

Manipulating p-π Resonance through Methoxy Group Engineering in Covalent Organic Frameworks for an Efficient Photocatalytic Hydrogen Evolution

Kinetic factors frequently emerge as the primary constraints in photocatalysis, exerting a critical influence on the efficacy of polymeric photocatalysts. The diverse conjugation systems within covalent organic frameworks (COFs) can significantly impact photon absorption, energy level structures, charge separation and migration kinetics. Consequently, these limitations often manifest as unsatisfactory kinetic behavior, which adversely affects the photocatalytic activity of COFs. To address these challenges, we propose a methoxy (-OMe) molecular engineering strategy designed to enhance charge carrier kinetics and mitigate mass transfer resistance. Through strategic modulation of the position and quantity of -OMe units, we can effectively manipulate the p-π conjugation, thereby enhancing charge separation and migration. Moreover, COFs enriched with -OMe moieties exhibit enhanced mass transfer dynamics due to the hydrophilic nature of methoxy groups, which facilitate the diffusion of reactants and products within the porous structure. This approach is hypothesized to drive an efficient photocatalytic hydrogen evolution reaction.

Dual Metallosalen-Based Covalent Organic Frameworks for Artificial Photosynthetic Diluted CO2 Reduction

Directly converting CO2 in flue gas using artificial photosynthetic technology represents a promising green approach for CO2 resource utilization. However, it remains a great challenge to achieve efficient reduction of CO2 from flue gas due to the decreased activity of photocatalysts in diluted CO2 atmosphere. Herein, we designed and synthesized a series of dual metallosalen-based covalent organic frameworks (MM-Salen-COFs, M: Zn, Ni, Cu) for artificial photosynthetic diluted CO2 reduction and confirmed their advantage in comparison to that of single metal M-Salen-COFs. As a results, the ZnZn-Salen-COF with dual Zn sites exhibits a prominent visible-light-driven CO2-to-CO conversion rate of 150.9 μmol g-1 h-1 under pure CO2 atmosphere, which is ~6 times higher than that of single metal Zn-Salen-COF. Notably, the dual metal ZnZn-Salen-COF still displays efficient CO2 conversion activity of 102.1 μmol g-1 h-1 under diluted CO2 atmosphere from simulated flue gas conditions (15 % CO2), which is a record high activity among COFs- and MOFs-based photocatalysts under the same reaction conditions. Further investigations and theoretical calculations suggest that the synergistic effect between the neighboring dual metal sites in the ZnZn-Salen-COF facilitates low concentration CO2 adsorption and activation, thereby lowering the energy barrier of the rate-determining step.

Exploring high-connectivity three-dimensional covalent organic frameworks: topologies, structures, and emerging applications

Covalent organic frameworks (COFs) represent a highly versatile class of crystalline porous materials, formed by the deliberate assembly of organic building units into ordered two-dimensional (2D) and three-dimensional (3D) structures. Their unique combination of topological precision and tunable micro- or mesoporous architectures offers unmatched flexibility in material design. By selecting specific building units, reactive sites, and functional groups, COFs can be engineered to achieve customized skeletal, porous, and interfacial properties, opening the door to materials with optimized performance for diverse applications. Among recent advances, high-connectivity 3D COFs have emerged as a particularly exciting development, with their intricate network structures enabling unprecedented levels of structural complexity, stability, and functionality. This review provides a comprehensive overview of the synthesis strategies, topological design principles, structural characterization techniques, and emerging applications of high-connectivity 3D COFs. We explore their potential across a broad range of cutting-edge applications, including gas adsorption and separation, macromolecule adsorption, dye removal, photocatalysis, electrocatalysis, lithium-sulfur batteries, and charge transport. By examining these key areas, we aim to deepen the understanding of the intricate relationship between structure and function, guiding the rational design of next-generation COF materials. The continued advancements in this field hold immense promise for revolutionizing sectors such as energy storage, catalysis, and molecular separation, making high-connectivity 3D COFs a cornerstone for future technological innovations.

Simultaneous Photocatalytic CO2 Reduction and H2O Oxidation Under Non-Sacrificial Ambient Conditions

The utilization of CO2, H2O, and solar energy is regarded as a sustainable route for converting CO2 into chemical feedstocks, paving the way for carbon neutrality and reclamation. However, the simultaneous photocatalytic CO2 reduction and H2O oxidation under non-sacrificial ambient conditions is still a significant challenge. Researchers have carried out extensive exploration and achieved dramatic developments in this area. In this review, we first primarily elucidate the principles of two half-reactions in the photocatalytic conversion of CO2 with H2O, i. e., CO2 reduction by the photo-generated electrons and protons, and H2O oxidation by the photo-generated holes without sacrificial agents. Subsequently, the strategies to promote two half-reactions are summarized, including the vacancy/facet/morphology design, adjacent redox site construction, and Z-scheme heterojunction development. Finally, we present the advanced in situ characterizations and future perspectives in this field. This review aims to provide fresh insights into effectively simultaneous photocatalytic CO2 reduction and H2O oxidation under non-sacrificial ambient conditions.

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

Single-atom catalysts (SACs) have been investigated and applied to energy conversion devices. However, issues of metal agglomeration, low metal loading, and substrate stability have hindered realization of the SACs' full potential. Recently, covalent organic framework (COF)-based SACs have emerged as promising materials to enable highly efficient catalytic reactions. Here, we summarize the representative COF-based SACs and their wide application in clean energy devices and conversion reactions, such as hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, and oxygen evolution reaction. Based on their catalysis conditions, these reactions are categorized into photocatalyzed and electrocatalyzed reactions. We also summarize their design strategies, including heteroatom inclusion, donor-acceptor pairs, pore engineering, interface engineering, etc. Although COF-based SACs are promising, more efforts, such as linkage engineering, functional groups, ionization, multifunctional sites for cocatalyzed systems, etc., could improve them to be the ideal SAC materials. At the end, we provide our perspectives on where the field will proceed in the next 5 years.

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.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

A reversible photochromic covalent organic framework

Covalent organic frameworks are a type of crystalline porous materials that linked through covalent bond, and they have numerous potential applications in adsorption, separation, catalysis, and more. However, there are rarely relevant reported on photochromism. Fortunately, a hydrazone-linked DBTB-DETH-COF is rapidly generated through ultrasound method. The DBTB-DETH-COF is found to exhibit reversible photochromism (at least 50 cycles) from yellow to olive in the presence of light and air, and subsequently back to the original color upon heating. In addition, the structure of DBTB-DETH-COF remains unchanged after 15 days of light illumination. Furthermore, the reason of photochromic process is discussed by electron paramagnetic resonance, X-ray photoelectron spectroscopy, electrochemistry characterizations and transient absorption measurements. The reversible photochromic DBTB-DETH-COF can be used as anti-counterfeiting ink and optical switch in the presence of air. This work expands a stable organic photochromic material and broadens the applications of COFs.

Covalent Organic Frameworks-Based Fluorescence Sensor Array and QSAR Study for Identification of Energetic Heterocyclic Compounds

The accurate identification of energetic heterocyclic compounds (EHCs) is of great significance in munition assessment, environmental monitoring, and biosafety but remains largely underexplored. Herein, a covalent organic frameworks-based fluorescence sensor array (COFx sensor array) for efficient screening of EHCs is reported. The topologies of the COFs were rationally designed by modulating the pore sizes and linkage strategies to achieve the simplified sensor array. Eighteen EHC representatives, including single-, dual-, and three-ring EHCs with multivariate substructures, were successfully discriminated ranging from 10 μM to 1 mM. The sensor array showed robust selectivity against a wide range of interferences. The quantitative structure-activity relationship (QSAR) analysis has been conducted for the mechanistic study of the sensor array. Three multiple linear regression models have been established using molecular descriptors to evaluate and predict Stern-Volmer coefficient values, achieving explicit correlation between EHC structures and the signal outputs of the sensor array. Five molecular descriptors are retained to reveal the governing factors of the sensor array resolution. The QSAR analysis facilitates the design and development of the COFx sensor array, offering a new approach for customized multivariate analysis.

Electrostatic Self-Assembly of CdS Quantum Dots with Co9S8 Hollow Nanotubes for Enhanced Visible Light Photocatalytic H2 Production

CdS quantum dots (CdS QDs) are regarded as a promising photocatalyst due to their remarkable response to visible light and suitable placement of conduction bands and valence bands. However, the problem of photocorrosion severely restricts their application. Herein, the CdS QDs-Co9S8 hollow nanotube composite photocatalyst has been successfully prepared by loading Co9S8 nanotubes onto CdS QDs through an electrostatic self-assembly method. The experimental results show that the introduction of Co9S8 cocatalyst can form a stable structure with CdS QDs, and can effectively avoid the photocorrosion of CdS QDs. Compared with blank CdS QDs, the CdS QDs-Co9S8 composite exhibits obviously better photocatalytic hydrogen evolution performance. In particular, CdS QDs loaded with 30% Co9S8 (CdS QDs-30%Co9S8) demonstrate the best photocatalytic performance, and the H2 production rate reaches 9642.7 μmol·g-1·h-1, which is 60.3 times that of the blank CdS QDs. A series of characterizations confirm that the growth of CdS QDs on Co9S8 nanotubes effectively facilitates the separation and migration of photogenerated carriers, thereby improving the photocatalytic hydrogen production properties of the composite. We expect that this work will facilitate the rational design of CdS-based photocatalysts, thereby enabling the development of more low-cost, high-efficiency and high-stability composites for photocatalysis.

Stepwise Protonation of Three-Dimensional Covalent Organic Frameworks for Enhancing Hydrogen Peroxide Photosynthesis

Three-dimensional covalent organic frameworks (3D COFs), recognized for their tailorable structures and accessible active sites, offer a promising platform for developing advanced photocatalysts. However, the difficulty in the synthesis and functionalization of 3D COFs hinders their further development. In this study, we present a series of 3D-bcu-COFs with 8 connected porphyrin units linked by linear linkers through imine bonds as a versatile platform for photocatalyst design. The photoresponse of 3D-bcu-COFs was initially modulated by functionalizing linear linkers with benzo-thiadiazole or benzo-selenadiazole groups. Furthermore, taking advantage of the well-exposed porphyrin and imine sites in 3D-bcu-COFs, their photocatalytic activity was optimized by stepwise protonation of imine bonds and porphyrin centers. The dual protonated COF with benzo-selenadiazole groups exhibited enhanced charge separation, leading to an increased photocatalytic H2O2 production under visible light. This enhancement demonstrates the combined benefits of linker functionalization and stepwise protonation on photocatalytic efficiency.

Site-Selective Zn-Metalation in Poly-Triphenyl Amine-based Porous Organic Polymer for Solid-Gas Phase CO2 Photoreduction

Harvesting solar energy to produce value-added chemicals from carbon dioxide (CO2) presents a promising route for addressing the complexities of sustainable energy systems and environmental issues. In this context, the development of metal-coordinated porous organic polymers (POPs) offers a vital avenue for improving the photocatalytic performance of organic motifs. The current study presents a metal-integrated photocatalytic system (namely, Zn@BP-POP) developed via a one-pot Friedel-Crafts (F.C.) acylation strategy, for solid-gas phase photochemical CO2 reduction to CO (CO2RR). The postsynthetic incorporation of metal (Zn) active sites on the host polymeric backbone of BP-POP significantly influences the catalytic activity. Notably, Zn@BP-POP demonstrates good photocatalytic performance in the absence of any cocatalyst and photosensitizer yielding CO while impeding the competitive hydrogen evolution reaction (HER) from water. The experimental findings collectively propose that the observed catalytic activity and selectivity arise from the synergistic interplay between the singular zinc catalytic centers and the light-harvesting capacity of the highly conjugated polymeric backbone. Further, X-ray absorption spectroscopy (XAS) analysis has significantly highlighted the prominent role played by the ZnN2O4 single sites in the polymeric framework for activating the gaseous CO2 molecules. Further, time-dependent density functional theory (DFT) analysis also reveals the thermodynamic feasibility of CO2RR over HER under optimized reaction conditions. This work cumulatively presents an effective strategy to demonstrate the importance of metal-active sites and effectively establish their structure-activity relationship during photocatalysis.

Heteropolyaromatic Covalent Organic Frameworks via One-Pot Multicomponent Reactions

Multicomponent reactions (MCRs) offer a platform to create different chemical structures and linkages for highly stable covalent organic frameworks (COFs). As an illustrative example, the multicomponent Povarov reaction generates 2,4-phenylquinoline from aldehydes and amines in the presence of electron-rich alkenes. In this study, we introduce a new domino reaction to generate unprecedented 2,3-phenylquinoline COFs in the presence of epoxystyrene. This work thus presents, for the first time, structural isomeric COFs produced by multicomponent domino and Povarov reactions. Furthermore, 2,3-phenylquinolines can undergo a Scholl reaction to form extended aromatic linkages. With this approach, we synthesize two thermally and chemically stable MCR-COFs and two heteropolyaromatic COFs using both domino and in situ domino and Scholl reactions. The structure and properties of these COFs are compared with the corresponding 2,4-phenylquinoline-linked COF and imine-COF, and their activity toward benzene and cyclohexane sorption and separation is investigated. The position of the pendant phenyl groups within the COF pore plays a crucial role in facilitating the industrially important sorption and separation of benzene over cyclohexane. This study opens a new avenue to construct heteropolyaromatic COFs via MCR reactions.

Synthetic Strategies to Extended Aromatic Covalent Organic Frameworks

π-Conjugated materials are highly attractive owing to their unique optical and electronic properties. Covalent organic frameworks (COFs) offer a great opportunity for precise arrangement of building units in a π-conjugated crystalline matrix and tuning of the properties through choice of functionalities or post-synthetic modification. With this review, we aim at summarizing both the most representative as well as emerging strategies for the synthesis of π-conjugated COFs. We give examples of direct synthesis using large, π-extended building blocks. COFs featuring fully conjugated linkages such as vinylene, pyrazine, and azole are discussed. Then, post-synthetic modification methods that result in the extension of the COF π-system are reviewed. Throughout, mechanistic insights are presented when available. In the context of their utilization as film devices, we conduct a concise survey of the prominent COF layer deposition techniques reported and their aptness for the deposition of fused aromatic systems.

Synthetic Leaves Based on Crystalline Olefin-Linked Covalent Organic Frameworks for Efficient CO2 Photoreduction with Water

We report, for the first time, a new synthetic strategy for the preparation of crystalline two-dimensional olefin-linked covalent organic frameworks (COFs) based on aldol condensation between benzodifurandione and aromatic aldehydes. Olefin-linked COFs can be facilely crystallized through either a pyridine-promoted solvothermal process or a benzoic anhydride-mediated organic flux synthesis. The resultant COF leaf with high in-plane π-conjugation exhibits efficient visible-light-driven photoreduction of carbon dioxide (CO2) with water (H2O) in the absence of any photosensitizer, sacrificial agents, or cocatalysts. The production rate of carbon monoxide (CO) reaches as high as 158.1 μmol g-1 h-1 with near 100% CO selectivity, which is accompanied by the oxidation of H2O to oxygen. Both theoretical and experimental results confirm that the key lies in achieving exceptional photoinduced charge separation and low exciton binding. We anticipate that our findings will facilitate new possibilities for the development of semiconducting COFs with structural diversity and functional variability.

12 Connecting Sites Linked Three-dimensional Covalent Organic Frameworks with Intrinsic Non-interpenetrated shp Topology for Photocatalytic H2O2 Synthesis

Developing high connectivity (>8) three-dimensional (3D) covalent organic frameworks (COFs) towards new topologies and functions remains a great challenge owing to the difficulty in getting high connectivity organic building blocks. This however represents the most important step towards promoting the diversity of COFs due to the still limited dynamic covalent bonds available for constructing COFs at this stage. Herein, highly connected phthalocyanine-based (Pc-based) 3D COFs MPc-THHI-COFs (M=H2, Ni) were afforded from the reaction between 2,3,9,10,16,17,23,24-octacarboxyphthalocyanine M(TAPc) (M=H2, Ni) and 5,5',5'',5''',5'''',5'''''-(triphenylene-2,3,6,7,10,11-hexayl)hexa(isophthalohydrazide) (THHI) with 12 connecting sites. Powder X-ray diffraction analysis together with theoretical simulations and transmission electron microscopy reveals their crystalline nature with an unprecedented non-interpenetrated shp topology. Experimental and theoretical investigations disclose the broadened visible light absorption range and narrow optical band gap of MPc-THHI-COFs. This in combination with their 3D nanochannels endows them with efficient photocatalysis performance for H2O2 generation from O2 and H2O via 2e- oxygen reduction reaction and 2e- water oxidation reaction under visible-light irradiation (λ >400 nm). This work provides valuable result for the development of high connectivity functional COFs towards diverse application potentials.

Hetero-Motif Molecular Junction Photocatalysts: A New Frontier in Artificial Photosynthesis

ConspectusTo cope with the increasingly global greenhouse effect and energy shortage, it is urgent to develop a feasible means to convert anthropogenic excess carbon dioxide (CO2) into energy resources. The photocatalytic CO2 reduction reaction (CO2RR) coupled with the water oxidation reaction (WOR), known as artificial photosynthesis, is a green, clean, and promoting strategy to deal with the above issues. Among the reported photocatalytic systems for CO2 reduction, the main challenge is to achieve WOR simultaneously due to the limited charge separation efficiency and complicated dynamic process. To address the problem, scientists have assembled two nanosemiconductor motifs for CO2RR and WOR into a heterojunction photocatalyst to realize artificial photosynthesis. However, it is difficult to clearly explore the corresponding catalytic mechanism and establish an accurate structure-activity relationship at the molecular level for their aperiodic distribution and complicated structural information. Standing on the shoulders of the heterojunction photocatalysts, a new-generation material, hetero-motif molecular junction (HMMJ) photocatalysts, has been developed and studied by our laboratory. A hetero-motif molecular junction is a class of crystalline materials with a well-defined and periodic structure, adjustable assembly mode, and semiconductor-like properties, which is composed of two predesigned motifs with oxidation and reduction, respectively, by coordination or covalent bonds. The intrinsic properties make these catalysts susceptible to functional modifications to improve light absorption and electrical conductivity. The small size and short distance of the motifs can greatly promote the efficiency of photogenerated electron-hole separation and migration. Based on these advantages, they can be used as potential excellent photocatalysts for artificial photosynthesis. Notably, the explicit structural information determined by single-crystal or powder X-ray diffraction can provide a visual platform to explore the reaction mechanism. More importantly, the connection number, spatial distance, interaction, and arrangement mode of the structural motifs can be well-designed to explore the detailed structure-activity relationship that can be hardly studied in nanoheterojunction photocatalyst systems. In this regard, HMMJ photocatalysts can be a new frontier in artificial photosynthesis and serve as an important bridge between molecular photocatalysts and solid photocatalysts. Thus, it is very important to summarize the state-of-the-art of the HMMJ photocatalysts used for artificial photosynthesis and to give in-depth insight to promote future development.In this Account, we have summarized the recent advances in artificial photosynthesis using HMMJ photocatalysts, mainly focusing on the results in our lab. We present an overview of current knowledge about developed photocatalytic systems for artificial photosynthesis, introduce the design schemes of the HMMJ photocatalysts and their unique advantages as compared to other photocatalysts, summarize the construction strategies of HMMJ photocatalysts and their application in artificial photosynthesis, and explain why hetero-motif molecular junctions can be promising photocatalysts and show that they provide a powerful platform for studying photocatalysis. The structure-activity relationship and charge separation dynamics are illustrated. Finally, we bring our outlook on present challenges and future development of HMMJ photocatalysts and their potential application prospects on other photocatalytic reaction systems. We believe that this Account will afford important insights for the construction of high-efficiency photocatalysts and guidance for the development of more photocatalytic systems in an atom-economic, environmentally friendly, and sustainable way.

Preparation of Well-Constructed and Metal-Modified Covalent Organic Framework Nanoparticles for Biosensing Design with Cascade Catalytic Capability

Uniform covalent organic framework nanoparticles (COF NPs) with a well-defined pore structure may provide a robust platform for scaffolding enzymes. Herein, bipyridine-based spherical COF NPs have been successfully prepared in this work through the Schiff base condensation reaction. Moreover, they are functionalized by metal modification and are further used for biosensor fabrication. Experimental results reveal that the metal-modified COF NPs also display impressive peroxidase-like catalytic activities, while they can load enzymes, such as glucose oxidase (GOx) and sarcosine oxidase (SOx), to develop a cascade catalysis system for design of various kinds of biosensors with very well performance. For example, the optimized GOx@Fe-COFs can achieve a sensitive detection of glucose with a low limit of detection (LOD) of 12.8 μM. Meanwhile, the enzymes also exhibit a commendable preservation of 80% enzymatic activity over a span of 14 days under ambient conditions. This work may pave the way for advancing cascade catalysis and the analysis of different kinds of biological molecules based on COF NPs.