A Scalable General Synthetic Approach toward Ultrathin Imine-Linked Two-Dimensional Covalent Organic Framework Nanosheets for Photocatalytic CO2 Reduction

Signal-on photoelectrochemical aptasensor for ultrasensitive zearalenone detection via cation exchange reaction amplification in COF-367 NSs/CdS QDs/Ag2S heterojunction

BACKGROUND

Zearalenone (ZEN) is a non-steroidal estrogenic mycotoxin closely related to our health. It is of great significance to detect ZEN quickly and accurately in agricultural production. This study presents a novel signal-on photoelectrochemical (PEC) aptasensor for ultrasensitive detection of ZEN by integrating COF-367 nanosheets (NSs), CdS quantum dots (QDs), and Ag2S heterojunctions with cation exchange reaction (CER)-based signal amplification. The detection of ZEN was based on signal enhancement of COF-367 NSs/CdS QDs/Ag2S heterojunction and CER without any biological assembly, solving the problem of low abundance and false positives of signals.

RESULTS

In this work, the COF-367 NSs/CdS QDs composite was synthesized via in-situ growth, demonstrating a 10-fold enhancement in photocurrent. The sensing strategy employs a hairpin aptamer structure stabilized by Ag+ ions. In the presence of ZEN, the hairpin structure unfolds, releasing Ag+ ions that trigger CER with COF-367 NSs/CdS QDs, forming the COF-367 NSs/CdS QDs/Ag2S heterostructure, which exhibits a hundredfold enhancement in PEC performance compared to individual components due to improved charge separation efficiency and light-harvesting capabilities. The amount of ZEN directly correlates with the quantity of released Ag+ ions, thereby determining the proportion of Ag2S in the composite. This multiplex release of Ag + ions also significantly enhances detection sensitivity. The sensor exhibits a strong linear relationship between photocurrent and the logarithmic concentration of ZEN over a wide range (5 pM-100 nM), with an exceptionally low detection limit of 1.1 pM (S/N = 3).

SIGNIFICANCE AND NOVELTY

This strategy provides a highly sensitive, straightforward, versatile, and cost-effective method for trace analysis, eliminating the need for complex biological assembly. The results highlight the potential of this approach as a universal platform for high-performance PEC sensing and its applicability in rapid detection kits. This sensing strategy achieves multiple enhancements in photoelectrochemical signals without requiring complex assembly at sensing interface.

"Turn-on" mode fluorescence detection of amines based on a cationic covalent organic framework linked with C-C single bond

Developing methods to detect amine pollutants at trace levels is urgently needed due to their high toxicity to both human health and environment. Covalent organic frameworks (COFs) have emerged as promising candidates for amine sensing due to their exceptional stability when exposed to corrosive amines. While several COF-based sensors have recently been developed for amine detection, to the best of our knowledge, fluorescent "turn-on" sensors have been limited to imine-linked COFs. However, the relatively low stability of imine linkages may compromise structural integrity in the presence of corrosive amines. Here, for the first time, we constructed a cationic C-C single bond linked COF (CSBL-COF-4) through the reaction between cationic porphyrin TMPyP and terephthaldicarboxaldehyde. The abundant cationic sites distributing throughout the networks not only improved the dispersity of CSBL-COF-4 in aqueous solution but also provided numerous acidic sites to enhance the affinity with alkaline amines via Lewis acid-base interaction. CSBL-COF-4 exhibited an efficient response to amine solutions or vapors and was further utilized to evaluate the freshness of meat samples, highlighting its potential for practical applications. Our result would thus open up a new avenue towards constructing a broader class of COF-based sensors for the fluorescence "turn-on" detection of amines.

Two-Dimensional Covalent Organic Frameworks in Organic Electronics

Two-dimensional covalent organic frameworks (2DCOFs) are a unique class of crystalline porous materials interconnected by covalent bonds, which have attracted significant attention in recent years due to their chemical and structural diversity, as well as their applications in adsorption, separation, catalysis, and drug delivery. However, research on the electrical properties of 2DCOFs remains limited, despite their potential in organic electronics. Early studies recognized the poor electrical conductivity of 2DCOFs as a significant obstacle to their application in this field. To overcome this challenge, various strategies have been proposed to enhance conductivity. This review first introduces the concept of computational screening for 2DCOFs and explores approaches to improve their intrinsic conductivity, with a focus on four key aspects: in-plane and out-of-plane charge transport, topology, bandgap, and morphology. It then examines the application of pristine 2DCOFs in organic electronics, including applications in field-effect transistors, memristors, photodetectors, and chemiresistive gas sensors. We support these strategies with detailed statistical data, providing a comprehensive guide for the design and development of novel 2DCOFs for organic electronics. Finally, we outline future research directions, emphasizing the challenges that remain to be addressed in this emerging area.

Functionalization of Covalent Organic Frameworks with Thiazole Rings and Hydroxyl Groups for Improved Photocatalytic Water Splitting Performance

Photocatalytic water splitting for hydrogen production holds considerable potential for simultaneously addressing fuel production and carbon neutrality, although increasing photocatalyst activity and enhancing exciton dissociation continue to be very difficult tasks. Covalent organic frameworks (COFs) with predesignable structures and customizable functionalities are promising candidates for photocatalysis. In this study, we present the design and synthesis of these COFs using thiazole rings as linkage units. Three COFs (denoted as COF-S-OH-1, COF-S-OH-2, and COF-S-OH-3) were prepared based on aldehyde ligands with varying numbers of hydroxyl groups (2-hydroxybenzene-1,3,5-tricarbaldehyde, 2,4-dihydroxybenzene-1,3,5-tricarbaldehyde, and 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde, respectively). The results demonstrate that COFs constructed with thiazole linkages exhibit superior stability and conjugation compared to those linked solely by imine bonds, ultimately achieving enhanced electronic conductivity. The tight interaction between donor and acceptor groups in this donor-acceptor (D-A) system facilitates an improved photocatalytic hydrogen evolution performance. Furthermore, increasing the number of hydroxyl groups (electron-donating groups) significantly enhances the photocatalytic efficiency of the resulting COFs.

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.

Semiconducting Covalent Organic Frameworks

Semiconductors form the foundational bedrock of modern electronics and numerous cutting-edge technologies. Particularly, semiconductors crafted from organic building blocks hold immense promise as next-generation pioneers, thanks to their vast array of chemical structures, customizable frontier orbital energy levels and bandgap structures, and easily adjustable π electronic properties. Over the past 50 years, advancements in chemistry and materials science have facilitated extensive investigations into small organic π compounds, oligomers, and polymers, resulting in a rich library of organic semiconductors. However, a longstanding challenge persists: how to organize π building units or chains into well-defined π structures, which are crucial for the performance of organic semiconductors. Consequently, the pursuit of methodologies capable of synthesizing and/or fabricating organic semiconductors with ordered structures has emerged as a frontier in organic and polymeric semiconductor research. In this context, covalent organic frameworks (COFs) stand out as unique platforms allowing for the covalent integration of organic π units into periodically ordered π structures, thus facilitating the development of semiconductors with extended yet precisely defined π architectures. Since their initial report in 2008, significant strides have been made in exploring various chemistries to develop semiconducting COFs, resulting in a rich library of structures, properties, functions, and applications. This review provides a comprehensive yet focused exploration of the general structural features of semiconducting COFs, outlining the basic principles of structural design, illustrating the linkage chemistry and synthetic strategies based on typical one-pot polymerization reactions to demonstrate the growth of bulk materials, nanosheets, films, and membranes. By elucidating the interactions between COFs and various entities such as photons, phonons, electrons, holes, ions, molecules, and spins, this review categorizes semiconducting COFs into nine distinct sections: semiconductors, photoconductors, light emitters, sensors, photocatalysts, photothermal conversion materials, electrocatalysts, energy storage electrodes, and radical spin materials, focusing on disclosing structure-originated properties and functions. Furthermore, this review scrutinizes structure-function correlations and highlights the unique features, breakthroughs, and challenges associated with semiconducting COFs. Furnished with foundational knowledges and state-of-the-art insights, this review predicts the fundamental issues to be addressed and outlines future directions for semiconducting COFs, offering a comprehensive overview of this rapidly evolving and remarkable field.

Synergistic Assembly of Single-Crystal 2D Porphyrin-Based Organic Polymers via Dative B─N Bonds and Halogen Bonds

The rigorous synthetic methodologies have significantly impeded the progress in developing single-crystal extended organic polymers. Notably, the existence of macroscopic single-crystalline 2D porphyrin-based organic polymers has never been documented in the literature until now. In this study, we present a groundbreaking example of single-crystal 2D porphyrin-based organic polymers that are compatible with single-crystal X-ray diffraction (SXRD) for precise structural elucidation. Their formation is fundamentally dependent on the synergistic assembly facilitated by dative B─N bonds and halogen bonds. These crystals exhibit remarkable stability in both air and aqueous environments. Notably, the formation of the B-N Lewis pairs within these crystals significantly enhances the separation of photogenerated carriers, and their single crystals demonstrate exceptional photocatalytic activity for the production of hydrogen peroxide (H2O2) from water and oxygen, without the requirement for sacrificial agents. This pioneering discovery establishes a new approach for crystalline control within the realm of organic polymers.

Covalent Organic Framework Nanosheets for the Assembly of Efficient Membrane Bioreactors

Immobilizing fragile enzymes in porous materials holds significant potential for biocatalysis but encounters challenges such as mismatched enzyme size and pore structure of host materials, along with harsh assembly conditions that can denature enzymes. Herein, we present a versatile strategy for constructing membrane bioreactors through water-mediated, vacuum-assisted layer-by-layer assembly of covalent organic framework (COF) nanosheets with enzymes. This method effectively addresses pore size limitations, preserves enzyme activity, and promotes convective transport of reactants to active sites, while shielding enzymes from harmful by-products through rapid transport in continuous membrane catalysis. The optimized bioreactor achieves a 1018-fold increase in relative activity compared to free enzymes in batch reactions, completing substrate conversion in just 7.95 s, and demonstrating enhanced stability.

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.

A novel fully conjugated COF adorned on 3D-G to boost the "D-π-A" electron regulation in oxygen catalysis performance

Covalent organic frameworks (COFs) are promising materials for oxygen catalysis. Here, a novel, highly stable, conjugated two-dimensional poly(benzimidazole porphyrin-cobalt) (PBIPorCo) with a large delocalization energy is synthesized using meso-5,10,15,20-tetra (4-cyano-phenylporphyrin) cobalt (TCNPorCo) and 3,3'-diaminobenzidine (DAB). The decrease in energy between the HOMO and LUMO orbitals of PBIPorCo could enhance the capability for the gain and loss of electrons during the catalytic process. In a nitrogen-rich environment, a benzimidazole (BI) group can transfer electrons to the Co-N4 site and enhance the protonation process in the oxygen reduction reaction (ORR). The π-π interactions between PBIPorCo and three-dimensional graphene (3D-G) form an "electron donor-π-electron acceptor" structure to boost the bifunctional oxygen catalysis process. PBIPorCo/3D-G exhibits outstanding bifunctional oxygen catalytic performance (ΔE = 0.62 V) and outstanding performance in zinc-air batteries. It exhibits satisfactory potential for application in fuel cells (FCs) and overall water splitting (OWS). This work presents a promising strategy for the design of novel COFs as bifunctional oxygen catalysts.

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.

Molecular Hybrid Photocatalysts for CO2 Photoreduction by Hybridization of Molecular Catalysts and Photoactive Covalent Organic Frameworks - A Review

Photocatalytic reduction of CO2 to obtain storable fuels is an effective way to fix and utilize greenhouse gas CO2, ultimately achieving a carbon-free energy cycle. The core of this goal lies the development of efficient, sustainable, and economically practical catalysts and light absorbers. Currently, hybrid photocatalytic systems that immobilize molecular catalysts on covalent organic frameworks (COFs) are of intensified interest, as this strategy enables the simultaneous exploitation of the catalytic properties of high-performance molecular catalysts, together with the durability of heterogeneous semiconductors, based on the readily available synthetic tunability of both components. This review focuses on the significant progress and challenges that have been overcome for the photocatalytic CO2 reduction reaction, mediated by COFs hybridized with molecular catalysts, aiming to provide strong guidance for innovative utilization towards sustainable photocatalytic CO2 reduction in the future.

Reticular Design and Synthesis of Covalent Organic Frameworks with Irregular Hexagonal Tiling

Reticular chemistry has greatly stimulated the development of framework materials, yet covalent organic frameworks (COFs) featuring irregular tiling are rare, because of the strict constraints on irregular tile lengths and angles. Guided by reticular chemistry, we deconstructed type III irregular hexagonal tiling into a combination of D2h + D2h monomers and achieved the first successful synthesis of 2D COFs featuring such tiling with hit topology. By tuning the crystal growth conditions, we obtained a set of reticular isomers, COF-hit and COF-bex, with distinct reticular structures. Powder X-ray diffraction and high-resolution transmission electron microscopy were employed to precisely characterize their topology structures. COF-hit demonstrates outstanding water absorption capabilities under high humidity conditions. This rational design of COFs using irregular tiling opens a new avenue to diversify the structural types and topological varieties of COFs and promote the development of reticular chemistry.

Highly Efficient Photocatalytic CO2-to-CO on Ni-Based Cationic Polymer with TiO2-Assisted Exfoliation and Stabilization

Porous organic polymers have shown great potential in photocatalytic CO2 reduction due to their unique tunable structure favoring gas adsorption and metal sites integration. However, efficient photocatalysis in porous polymers is greatly limited by the low surface reactivity and electron mobility of bulk structure. Herein, we incorporate TiO2 nanoparticles and Ni(II) sites into a layered cationic imidazolium polymer (IP), in which the imidazolium moieties and free anions can stabilize the key intermediates and enhance the reaction kinetics of CO2 reduction. During the photocatalytic reaction, the layered TiO2/NiIP is in situ exfoliated to nanosheets (NSs) with more accessible active sites and shorten electron transport pathways. The formed TiO2/NiIP-NSs exhibit an impressively high CO production rate as 54.9 mmol ⋅ g-1 ⋅ h-1 with selectivity of 99.9 %. The embedding with TiO2 nanoparticles could improve the electron transport efficiency so as to facilitate the photochemical stripping process of layered polymer. Moreover, the exfoliated nanosheets with assistance of TiO2 possess excellent stability during the recycling experiments in comparison to the rapidly declined activity of NiIP-NSs. This work presents a new strategy to construct highly efficient photocatalysts for CO2 reduction.

Highly Selective CO2 Conversion to CH4 by a N-Doped HTiNbO5/NH2-UiO-66 Photocatalyst without a Sacrificial Electron Donor

Photocatalytic reduction of CO2 to value-added chemicals is a promising technology for reducing atmospheric CO2, but selectively producing a specific product still remains a great challenge. In this study, a Z-scheme heterojunction, N-doped HTiNbO5/NH2-UiO-66(Zr) (referred to as NH-NU), is developed to integrate the advantages of semiconductor photocatalysts and porous CO2 adsorbents for CO2-to-CH4 conversion. The NH-NU Z-scheme heterojunctions are fabricated via a simple one-pot solvothermal method, enabling the formation of a tight and uniform interface between the two phases, thereby facilitating the separation and transfer of the photoinduced charge carriers, as confirmed by TEM, EPR, electrochemical studies, and work functions. As a result, the as-prepared photocatalyst demonstrates a significant increase in selectivity for CH4 production through CO2 photoreduction, achieving a 10-fold enhancement compared to that of the pristine MOF, NH2-UiO-66. Moreover, there is no obvious decrease in the photocatalytic activity for CH4 production across four consecutive cycles. In situ FT-IR spectroscopy and DFT calculations reveal that charge-enriched N-doped NH-NU-3 composites stabilize various C1 intermediates in multistep elementary reactions, leading to superior selectivity in the CO2-to-CH4 conversion process. This work establishes that efficient and selective heterogeneous catalytic processes can be achieved through the stabilization of reaction intermediates by designing suitable Z-scheme heterojunctions.

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.

Assembling Giant Nanoclusters as Heterogeneous Catalysts for Effectively Converting CO2 to CO Under Visible Light

Heterometallic lanthanide-transition metal (3d-4f) nanoclusters with well-defined structures and multiple active sites are excellent vehicles for achieving efficient catalysis and studying heterometallic synergism. In this work, two closely related yet different high-nuclearity nanoclusters, 72-nuclear {Ni28RE44} (1, RE = Pr, Nd, Sm, Eu, and Gd) and 111-nuclear {Ni48La63} (2), are synthesized using a mixed-ligand strategy. Importantly, the crystal solids of these giant coordination clusters are insoluble when soaking in H2O/CH3CN and can be used as heterogeneous catalysts for visible-light-driven catalytic conversion of CO2 to CO. Cluster 2 exhibits a maximum CO production rate of 4800 µmol g-1 h-1 and a CO selectivity of 92% over H2. Furthermore, the catalytic properties are investigated of different rare earths in the cluster 1 series, found that 1-Eu exhibited superior catalytic performance under identical conditions, likely due to the lower reduction potential of the europium ions. This study represents the first report of 3d-4f heterometallic nanoclusters as heterogeneous catalysts for photocatalytic reaction and provides a reference for the study of high-nuclearity 3d-4f nanoclusters as catalysts for photocatalytic reduction of CO2 to CO.

Regulating Charge Distribution in Porphyrin-Based Polymer for Achieving Photocatalytic CO2 Conversion to CH4 or C2H6

The photocatalytic conversion of CO2 into products such as CH4 and C2H6 poses a significant challenge due to the lengthy reaction steps and the high energy barrier involved. In this study, both benzothiadiazole (BTD) and hydroxyl groups (-OH) are introduced into cobalt-based polymerized porphyrinic network (PPN) through a C-C coupling reaction. This modification of orbital energy levels that strengthens the ability of gain electrons and facilitates the charge transfer in PPN. Hydroxyl group largely enhances the ability for light response, while thiadiazole unit tunes the molecular orbital to proper energy level. By this way, BTD-DBP-PPN(Co) achieves the capability for CO2 conversion to CH4 and C2H6 under the irradiation of light. Co active site is introduced to reduce the energy barrier and facilitate the charge transfer. The reaction pathway for C2H6 production has been studied for further mechanism explanation. Overall, a series of cobalt-based porphyrin centers with a donor-acceptor (D-A) structure are designed and synthesized to enhance CO2 reduction performance and achieve the formation of C2 products under 300-W Xe lamp irradiation.

In Situ Integration of Metallic Catalytic Sites and Photosensitive Centers within Covalent Organic Frameworks for the Enhanced Photocatalytic Reduction of CO2

Covalent organic frameworks (COFs) are a promising platform for heterogeneous photocatalysis due to their stability and design diversity, but their potential is often restricted by unmanageable targeted excitation and charge transfer. Herein, a bimetallic COF integrating photosensitizers and catalytic sites is designed to facilitate locally ultrafast charge transfer, aiming to improve the photocatalytic reduction of CO2. The strategy uses a "one-pot" method to synthesize the bimetallic COF (termed PBCOFRuRe) through in situ Schiff-base condensation of Pyrene with MBpy (M = Ru, Re) units. In this structure, Ru and Re are anchored within bipyridine as the photosensitive center and catalytic site, respectively. The bimetallic architecture of PBCOFRuRe significantly boosts the photocatalytic efficiency for CO2 reduction, achieving an impressive CO yield of 8306.6 µmol g-1 h-1 with 99.8% selectivity, surpassing most reported COF materials. This improvement is attributed to the localized ultrafast charge transfer (0.23 ps) from Ru to Re, as demonstrated by femtosecond transient absorption spectroscopy (TAS). Further investigations demonstrate its heterogeneous feature, showcasing exceptional long-term stability and recyclability. This study represents a versatile approach for designing bimetallic COFs with ultrafast charge transfer, paving the pathway for advancements in artificial photosynthesis.

Hydrogen-Bond-Regulated Mechanochemical Synthesis of Covalent Organic Frameworks: Cocrystal Precursor Strategy for Confined Assembly

Two-dimensional imine covalent organic frameworks (2D imine-COFs) are crystalline porous materials with broad application prospects. Despite the efforts into their design and synthesis, the mechanisms of their formation are still not fully understood. Herein, a one-pot two-step mechanochemical cocrystal precursor synthetic strategy is developed for efficient construction of 2D imine-COFs. The mechanistic investigation demonstrated that the cocrystal precursors of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) and p-toluenesulphonic acid (PTSA) sufficiently regulate the crystalline structure of COF. Evidenced by characterizations and theoretical studies, a helical hydrogen-bond network was constructed by the N-H⋅⋅⋅O supramolecular synthons between amino and sulfonic groups in TAPT-PTSA, demonstrating the role of cocrystals in promoting the organized stacking of interlayer π-π interactions, layer arrangement, and interlayer spacing, thus facilitating the orderly assembly of COFs. Moreover, the protonation degree of TAPT amines, which tuned nucleophilic directionality, enabled the sequential progression of intra- and interlayer imine condensation reactions, inhibiting the formation of amorphous polymers. The transformation from cocrystal precursors to COFs was achieved through comprehensive control of hydrogen bond and covalent bond sites. This work significantly advances the concept of hydrogen-bond-regulated COF assembly and its mechanochemical method in the design and synthesis of 2D imine-COFs, further elucidating the mechanistic aspects of their mechanochemical synthesis.

Opportunity and Challenge of Advanced Porous Sorbents for PFAS Removal

Per- and polyfluoroalkyl substances (PFASs), comprising over 9,000 persistent synthetic organic contaminants, are extensively found in the environment and pose significant risks to both human and ecological health. Among the strategies for addressing PFAS contamination, adsorption processes have proven to be cost-effective. Traditional sorbents such as ion-exchange resins and activated carbon have been found to exhibit low adsorption capacities and slow equilibration times. Recent innovations in porous materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), however, offer significant improvements in the efficiency of PFAS adsorption. This review thoroughly examines the latest advancements in these materials, analyzing their mechanisms of adsorption, and concludes by suggesting directions for future research that could further enhance their effectiveness in PFAS management.

Staggered-Stacking Two-Dimensional Covalent Organic Framework Membranes for Molecular and Ionic Sieving

Two-dimensional covalent organic frameworks (2D COFs), a family of crystalline materials with abundant porous structures offering nanochannels for molecular transport, have enormous potential in the applications of separation, energy storage, and catalysis. However, 2D COFs remain limited by relatively large pore sizes (>1 nm) and weak interlayer interactions between 2D nanosheets, making it difficult to achieve efficient membranes to meet the selective sieving requirements for water molecules (0.3 nm) and hydrated salt ions (>0.7 nm). Here, we report a high-performance 2D COF membrane with narrowed channels (0.7 × 0.4 nm2) and excellent mechanical performance constructed by the staggered stacking of cationic and anionic 2D COF nanosheets for selectively sieving of water molecules and hydrated salt ions. The mechanical performance has been improved by two times than that of single-phase 2D COF membranes due to the enhanced interlayer interactions between nanosheets. The stacked 2D COF membranes exhibit significantly improved monovalent salt ions rejection ratio (up to 77.9%) compared with single-phase COF membranes (∼49.2%), while maintaining comparable water permeability. The design of stacked 2D COF membranes provides a potential strategy for constructing high-performance nanoporous membranes to achieve precise molecular and ionic sieving.

Noble metal-free porphyrin covalent organic framework layer for CO2 photoreduction to CO

The presence of greenhouse CO2 gas in the atmosphere causes serious environmental issues. Consequently, the development of photocatalysts for reducing CO2 is essential for mimicking artificial photosynthesis. In this study, we prepared a 5,15-di(4-aminophenty)-10,20-diphentyporphyrin copper (CuDAPP)-1,3,5-triformylphloroglucinol (TP)-covalent organic framework (COF) layer on a glass sheet via a layer-by-layer (LBL) method. The 2D CuDAPP-TP-COF layer was used as a photocatalyst for CO2 reduction, and it demonstrated excellent photocatalytic activity under gas-solid conditions without sacrificial reagents, noble metals, or photosensitisers. The CO production yield was 282.6 μmol g-1 under visible-light irradiation for 6 h, outperforming the raw material CuDAPP and a mixture of CuDAPP and TP, indicating a high application potential of the 2D porphyrin COF layer material in photocatalytic CO2 reduction.

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.

Molecular Heterogeneous Photocatalysts for Visible-Light-Driven CO2 Reduction

Photoreduction of CO2 to high-value chemical fuels presents an effective strategy to reduce reliance on fossil fuels and mitigate climate change. The development of a photocatalyst characterized by superior activity, high selectivity, and good stability is a critical issue for PCR. Molecular heterogeneous photocatalytic systems integrate the advantages of both homogeneous and heterogeneous catalysts, creating a synergistic enhancement effect that increases photocatalytic performance. This review summarizes recent advancements in molecular heterogeneous photocatalysts for CO2 reduction. Much of the discussion focuses on the types of molecular heterogeneous photocatalysts, and their photocatalytic performance in CO2 reduction is summarized. The synthesis strategies for molecular heterogeneous photocatalysts are thoroughly discussed. Finally, the challenges and future prospects of molecular heterogeneous photocatalysts for PCR are addressed.

Safety Landscape of Therapeutic Nanozymes and Future Research Directions

Oxidative stress and inflammation are at the root of a multitude of diseases. Treatment of these conditions is often necessary but current standard therapies to fight excessive reactive oxygen species (ROS) and inflammation are often ineffective or complicated by substantial safety concerns. Nanozymes are emerging nanomaterials with intrinsic enzyme-like properties that hold great promise for effective cancer treatment, bacterial elimination, and anti-inflammatory/anti-oxidant therapy. While there is rapid progress in tailoring their catalytic activities as evidenced by the recent integration of single-atom catalysts (SACs) to create next-generation nanozymes with superior activity, selectivity, and stability, a better understanding and tuning of their safety profile is imperative for successful clinical translation. This review outlines the current applied safety assessment approaches and provides a comprehensive summary of the safety knowledge of therapeutic nanozymes. Overall, nanozymes so far show good in vitro and in vivo biocompatibility despite considerable differences in their composition and enzymatic activities. However, current safety investigations mostly cover a limited set of basic toxicological endpoints, which do not allow for a thorough and deep assessment. Ultimately, remaining research gaps that should be carefully addressed in future studies are highlighted, to optimize the safety profile of therapeutic nanozymes early in their pre-clinical development.

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.

Precision-engineered metal and metal-oxide nanoparticles for biomedical imaging and healthcare applications

The growing field of nanotechnology has witnessed numerous advancements over the past few years, particularly in the development of engineered nanoparticles. Compared with bulk materials, metal nanoparticles possess more favorable properties, such as increased chemical activity and toxicity, owing to their smaller size and larger surface area. Metal nanoparticles exhibit exceptional stability, specificity, sensitivity, and effectiveness, making them highly useful in the biomedical field. Metal nanoparticles are in high demand in biomedical nanotechnology, including Au, Ag, Pt, Cu, Zn, Co, Gd, Eu, and Er. These particles exhibit excellent physicochemical properties, including amenable functionalization, non-corrosiveness, and varying optical and electronic properties based on their size and shape. Metal nanoparticles can be modified with different targeting agents such as antibodies, liposomes, transferrin, folic acid, and carbohydrates. Thus, metal nanoparticles hold great promise for various biomedical applications such as photoacoustic imaging, magnetic resonance imaging, computed tomography (CT), photothermal, and photodynamic therapy (PDT). Despite their potential, safety considerations, and regulatory hurdles must be addressed for safe clinical applications. This review highlights advancements in metal nanoparticle surface engineering and explores their integration with emerging technologies such as bioimaging, cancer therapeutics and nanomedicine. By offering valuable insights, this comprehensive review offers a deep understanding of the potential of metal nanoparticles in biomedical research.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation

The dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130-140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF-mediated dehydrogenative synthesis. The TTT-DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT-DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well-suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates' functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

A cobalt porphyrin-bridged covalent triazine polymer-derived electrode for efficient hydrogen production

Pronounced compositional regulation and microstructure evolution have a significant influence on hydrogen electrocatalysis. Herein, for the first time, we demonstrate that N,Co-codoped carbon supported Co5.47N nanoparticles (Co5.47N/N,Co-C-800) derived from a nitrogen-rich porphyrin-bridged covalent triazine polymer (CoTAPPCC) are an effective electrocatalyst for the HER in 1.0 M KOH when compared to CoCo2O4/N,Co-C-900 (pyrolysis at 900 °C) and CoO/N,Co-C-1000 (pyrolysis at 1000 °C). The structural and morphological variations of CoTAPPCC at different heat treatment temperatures were investigated through various spectroscopic techniques. We reveal that electrocatalytic HER activity is temperature- and component-dependent. The overpotentials for Co5.47N/N,Co-C-800 to reach current densities of 10 and 100 mA cm-2 were determined to be 76 and 229 mV, respectively, outperforming many other state-of-the-art HER electrocatalysts. This work also sheds light on the influence of calcination temperature on the electrocatalytic HER of final samples.

A Hydrogen-Bonded Organic Framework Based on Triphenylamine for Photocatalytic Silane Hydroxylation

Employing hydrogen-bonded organic frameworks (HOFs) as mild photocatalysts for organic conversions is still considerably challenging. In this work, we synthesized a hydrogen-bonded organic framework (HOF-16) and achieved the photocatalytic oxidation of silanes to generate silanols. Considering the constraints imposed by the framework structure, a significant improvement in the efficacy of singlet oxygen (1O2) generation is observed. HOF-16 exhibits remarkable photocatalytic performance when it comes to silane hydroxylation, displaying high efficiency, low catalyst loading, and good recyclability. This research highlights the immense potential of HOFs in the realm of organic photocatalysis.

Covalent Organic Frameworks for Photocatalytic Reduction of Carbon Dioxide: A Review

Covalent organic frameworks (COFs) are crystalline networks with extended backbones cross-linked by covalent bonds. Due to the semiconductive properties and variable metal coordinating sites, along with the rapid development in linkage chemistry, the utilization of COFs in photocatalytic CO2RR has attracted many scientists' interests. In this Review, we summarize the latest research progress on variable COFs for photocatalytic CO2 reduction. In the first part, we present the development of COF linkages that have been used in CO2RR, and we discuss four mechanisms including COFs as intrinsic photocatalysts, COFs with photosensitive motifs as photocatalysts, metalated COF photocatalysts, and COFs with semiconductors as heterojunction photocatalysts. Then, we summarize the principles of structural designs including functional building units and stacking mode exchange. Finally, the outlook and challenges have been provided. This Review is intended to give some guidance on the design and synthesis of diverse COFs with different linkages, various structures, and divergent stacking modes for the efficient photoreduction of CO2.

Non-Metal Sulfur Doping of Indium Hydroxide Nanocube for Selectively Photocatalytic Reduction of CO2 to CH4: A "One Stone Three Birds" Strategy

Photocatalytic CO2 reduction is considered as a promising strategy for CO2 utilization and producing renewable energy, however, it remains challenge in the improvement of photocatalytic performance for wide-band-gap photocatalyst with controllable product selectivity. Herein, the sulfur-doped In(OH)3 (In(OH)xSy-z) nanocubes are developed for selective photocatalytic reduction of CO2 to CH4 under simulated light irradiation. The CH4 yield of the optimal In(OH)xSy-1.0 can be enhanced up to 39 times and the CH4 selectivity can be regulated as high as 80.75% compared to that of pristine In(OH)3. The substitution of sulfur atoms for hydroxyl groups in In(OH)3 enhances the visible light absorption capability, and further improves the hydrophilicity behavior, which promotes the H2O dissociation into protons (H*) and accelerates the dynamic proton-feeding CO2 hydrogenation. In situ DRIFTs and DFT calculation confirm that the non-metal sulfur sites significantly weaken the over-potential of the H2O oxidation and prevent the formation of ·OH radicals, enabling the stabilization of *CHO intermediates and thus facilitating CH4 production. This work highlights the promotion effect of the non-metal doping engineering on wide-band-gap photocatalysts for tailoring the product selectivity in photocatalytic CO2 reduction.

Dynamic Cleavage-Remodeling of Covalent Organic Networks into Multidimensional Superstructures

Superstructures with complex hierarchical spatial configurations exhibit broader structural depth than single hierarchical structures and the associated broader application prospects. However, current preparation methods are greatly constrained by cumbersome steps and harsh conditions. Here, for the first time, a concise and efficient thermally responsive dynamic synthesis strategy for the preparation of multidimensional complex superstructures within soluble covalent organic networks (SCONs) with tunable morphology from 0D hollow supraparticles to 2D films is presented. Mechanism study reveals the thermally responsive dynamic "cleavage-remodeling" characteristics of SCONs, synthesized based on the unique bilayer structure of (2.2)paracyclophane, and the temperature control facilitates the process from reversible solubility to reorganization and construction of superstructures. Specifically, during the process, the oil-water-emulsion two-phase interface can be generated through droplet jetting, leading to the preparation of 0D hollow supraparticles and other bowl-like complex superstructures with high yield. Additionally, by modulating the volatility and solubility of exogenous solvents, defect-free 2D films are prepared relying on an air-liquid interface. Expanded experiments further confirm the generalizability and scalability of the proposed dynamic "cleavage-remodeling" strategy. Research on the enrichment mechanism of guest iodine highlights the superior kinetic mass transfer performance of superstructural products compared to single-hierarchical materials.

The development of catalysts and auxiliaries for the synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) have recently seen significant advancements. Large quantities of structurally & functionally oriented COFs with a wide range of applications, such as gas adsorption, catalysis, separation, and drug delivery, have been explored. Recent achievements in this field are primarily focused on advancing synthetic methodologies, with catalysts playing a crucial role in achieving highly crystalline COF materials, particularly those featuring novel linkages and chemistry. A series of reviews have already been published over the last decade, covering the fundamentals, synthesis, and applications of COFs. However, despite the pivotal role that catalysts and auxiliaries play in forming COF materials and adjusting their properties (e.g., crystallinity, porosity, stability, and morphology), limited attention has been devoted to these essential components. In this Critical Review, we mainly focus on the state-of-the-art progress of catalysts and auxiliaries applied to the synthesis of COFs. The catalysts include four categories: acid catalysts, base catalysts, transition-metal catalysts, and other catalysts. The auxiliaries, such as modulators, oxygen, and surfactants, are discussed as well. This is then followed by the description of several specific applications derived from the utilization of catalysts and auxiliaries. Lastly, a perspective on the major challenges and opportunities associated with catalysts and auxiliaries is provided.

Constructing 2D Phthalocyanine Covalent Organic Framework with Enhanced Stability and Conductivity via Interlayer Hydrogen Bonding as Electrocatalyst for CO2 Reduction

Fabricating COFs-based electrocatalysts with high stability and conductivity still remains a great challenge. Herein, 2D polyimide-linked phthalocyanine COF (denoted as NiPc-OH-COF) is constructed via solvothermal reaction between tetraanhydrides of 2,3,9,10,16,17,23,24-octacarboxyphthalocyaninato nickel(II) and 2,5-diamino-1,4-benzenediol (DB) with other two analogous 2D COFs (denoted as NiPc-OMe-COF and NiPc-H-COF) synthesized for reference. In comparison with NiPc-OMe-COF and NiPc-H-COF, NiPc-OH-COF exhibits enhanced stability, particularly in strong NaOH solvent and high conductivity of 1.5 × 10-3 S m-1 due to the incorporation of additional strong interlayer hydrogen bonding interaction between the O-H of DB and the hydroxy "O" atom of DB in adjacent layers. This in turn endows the NiPc-OH-COF electrode with ultrahigh CO2-to-CO faradaic efficiency (almost 100%) in a wide potential range from -0.7 to -1.1 V versus reversible hydrogen electrode (RHE), a large partial CO current density of -39.2 mA cm-2 at -1.1 V versus RHE, and high turnover number as well as turnover frequency, amounting to 45 000 and 0.76 S-1 at -0.80 V versus RHE during 12 h lasting measurement.

Accelerating Charge Kinetics in Photocatalytic CO2 Reduction by Modulating the Cobalt Coordination in Heterostructures of Cadmium Sulfide/Metal-Organic Layer

Artificial photocatalytic CO2 reduction (CO2R) holds great promise to directly store solar energy into chemical bonds. The slow charge and mass transfer kinetics at the triphasic solid-liquid-gas interface calls for the rational design of heterogeneous photocatalysts concertedly boosting interfacial charge transfer, local CO2 concentration, and exposure of active sites. To meet these requirements, in this study heterostructures of CdS/MOL (MOL = metal-organic layer) furnishing different redox Co sites are fabricated for CO2R photocatalysts. It is found that the coordination environment of Co is key to photocatalytic activity. The best catalyst ensemble comprising ligand-chelated Co2+ with the bipyridine electron mediator demonstrates a high CO yield rate of 1523 µmol h-1 gcat -1, selectivity of 95.8% and TON of 1462.4, which are ranked among the best seen in literature. Comprehensive photochemical and electroanalytical characterizations attribute the high CO2R performance to the improved photocarrier separation and charge kinetics originated from the proper energy band alignment and coordination chemistry. This work highlights the construction of 2D heterostructures and modulation of transition metal coordination to expedite the charge kinetics in photocatalytic CO2 reduction.

Tailoring β-ketoenamine covalent organic framework with azo for blue light-driven selective oxidation of amines with oxygen

Covalent organic frameworks (COFs) present bright prospects in visible light photocatalysis with abundant active sites and exceptional stability. Tailoring an established COF with photoactive group is a prudent strategy to extend visible light absorption toward broad photocatalysis. Here, a β-ketoenamine COF, TpBD-COF, constructed with 1,3,5-triformylphloroglucinol (Tp) and 4,4'-biphenyldiamine (BD), is tailored with azo to validate this strategy. The insertion of azo into BD affords 4,4'-azodianiline (Azo); TpAzo-COF is successfully constructed with Tp and Azo. Intriguingly, the insertion of azo enhances π-conjugation, thereby facilitating visible light absorption and intramolecular electron transfer. Moreover, TpAzo-COF, with an appropriate electronic structure and impressive specific surface area of 1855 m2 g-1, offers substantial active sites conducive to the reduction of oxygen (O2) to superoxide. Compared with TpBD-COF, TpAzo-COF exhibits superior performance for blue light-driven oxidation of amines with O2. Superoxide controls the selective formation of product imines. This work foreshadows the remarkable capacity of tailoring COFs with photoactive group toward broad visible light photocatalysis.

Encapsulation engineering of porous crystalline frameworks for delayed luminescence and circularly polarized luminescence

Delayed luminescence (DF), including phosphorescence and thermally activated delayed fluorescence (TADF), and circularly polarized luminescence (CPL) exhibit common and broad application prospects in optoelectronic displays, biological imaging, and encryption. Thus, the combination of delayed luminescence and circularly polarized luminescence is attracting increasing attention. The encapsulation of guest emitters in various host matrices to form host-guest systems has been demonstrated to be an appealing strategy to further enhance and/or modulate their delayed luminescence and circularly polarized luminescence. Compared with conventional liquid crystals, polymers, and supramolecular matrices, porous crystalline frameworks (PCFs) including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolites and hydrogen-bonded organic frameworks (HOFs) can not only overcome shortcomings such as flexibility and disorder but also achieve the ordered encapsulation of guests and long-term stability of chiral structures, providing new promising host platforms for the development of DF and CPL. In this review, we provide a comprehensive and critical summary of the recent progress in host-guest photochemistry via the encapsulation engineering of guest emitters in PCFs, particularly focusing on delayed luminescence and circularly polarized luminescence. Initially, the general principle of phosphorescence, TADF and CPL, the combination of DF and CPL, and energy transfer processes between host and guests are introduced. Subsequently, we comprehensively discuss the critical factors affecting the encapsulation engineering of guest emitters in PCFs, such as pore structures, the confinement effect, charge and energy transfer between the host and guest, conformational dynamics, and aggregation model of guest emitters. Thereafter, we summarize the effective methods for the preparation of host-guest systems, especially single-crystal-to-single-crystal (SC-SC) transformation and epitaxial growth, which are distinct from conventional methods based on amorphous materials. Then, the recent advancements in host-guest systems based on PCFs for delayed luminescence and circularly polarized luminescence are highlighted. Finally, we present our personal insights into the challenges and future opportunities in this promising field.

Porphyrin-based porous organic polymers synthesized using the Alder-Longo method: the most traditional synthetic strategy with exceptional capacity

Porphyrin is a typical tetrapyrrole chromophore-based pigment with a special electronic structure and functionalities, which is frequently introduced into various porous organic polymers (POPs). Porphyrin-based POPs are widely used in various fields ranging from environmental and energy to biomedicine-related fields. Currently, most porphyrin-based POPs are prepared via the copolymerization of specific-group-functionalized porphyrins with other building blocks, in which the tedious and inefficient synthesis procedure for the porphyrin greatly hinders the development of such materials. This review aimed to summarize information on porphyrin-based POPs synthesized using the Alder-Longo method, thereby skipping the complex synthesis of porphyrin-bearing monomers, in which the porphyrin macrocycles are formed directly via the cyclic tetramerization of pyrrole with monomers containing multiple aldehyde groups during the polymerization process. The representative applications of porphyrin-based POPs derived using the Alder-Longo method are finally introduced, which pinpoints a clear relationship between the structure and function from the aspect of the building blocks used and porous structures. This review is therefore valuable for the rational design of efficient porphyrin-based porous organic polymer systems that may be utilized in various fields from energy-related conversion/storage technologies to biomedical science.

Cobalt-based Polymerized Porphyrinic Network for Visible-light-driven CO2 Reduction

Visible-light-driven conversion of carbon dioxide to valuable compounds and fuels is an important but challenging task due to the inherent stability of the CO2 molecules. Herein, we report a series of cobalt-based polymerized porphyrinic network (PPN) photocatalysts for CO2 reduction with high activity. The introduction of organic groups results in the addition of more conjugated electrons to the networks, thereby altering the molecular orbital levels within the networks. This integration of functional groups effectively adjusts the levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). The PPN(Co)-NO2 exhibits outstanding performance, with a CO evolution rate of 12 268 μmol/g/h and 85.8% selectivity, surpassing most similar photocatalyst systems. The performance of PPN(Co)-NO2 is also excellent in terms of apparent quantum yield (AQY) for CO production (5.7% at 420 nm). Density functional theory (DFT) calculations, time-resolved photoluminescence (TRPL), and electrochemical tests reveal that the introduction of methyl and nitro groups leads to a narrower energy gap, facilitating a faster charge transfer. The coupling reaction in this study enables the formation of stable C-C bonds, enhancing the structural regulation, active site diversity, and stability of the catalysts for photocatalytic CO2 reduction. This work offers a facile strategy to develop reliable catalysts for efficient CO2 conversion.

Photosensitizing metal covalent organic framework with fast charge transfer dynamics for efficient CO2 photoreduction

Designing artificial photocatalysts for CO2 reduction is challenging, mainly due to the intrinsic difficulty of making multiple functional units cooperate efficiently. Herein, three-dimensional metal covalent organic frameworks (3D MCOFs) were employed as an innovative platform to integrate a strong Ru(ii) light-harvesting unit, an active Re(i) catalytic center, and an efficient charge separation configuration for photocatalysis. The photosensitive moiety was precisely stabilized into the covalent skeleton by using a rational-designed Ru(ii) complex as one of the building units, while the Re(i) center was linked via a shared bridging ligand with an Ru(ii) center, opening an effective pathway for their electronic interaction. Remarkably, the as-synthesized MCOF exhibited impressive CO2 photoreduction activity with a CO generation rate as high as 1840 μmol g-1 h-1 and 97.7% selectivity. The femtosecond transient absorption spectroscopy combined with theoretical calculations uncovered the fast charge-transfer dynamics occurring between the photoactive and catalytic centers, providing a comprehensive understanding of the photocatalytic mechanism. This work offers in-depth insight into the design of MCOF-based photocatalysts for solar energy utilization.

Bipolaronic Motifs Induced Spatially Separated Catalytic Sites for Tunable Syngas Photosynthesis From CO2

Photocatalytic reduction of CO2 into syngas is a promising way to tackle the energy and environmental challenges; however, it remains a challenge to achieve reaction decoupling of CO2 reduction and water splitting. Therefore, efficient production of syngas with a suitable CO/H2 ratio for Fischer-Tropsch synthesis can hardly be achieved. Herein, bipolaronic motifs including Co(II)-pyridine N motifs and Co(II)-imine N motifs are rationally designed into a crystalline imine-linked 1,10-phenanthroline-5,6-dione-based covalent organic framework (bp-Co-COF) with a triazine core. These featured structures with spatially separated active sites exhibit efficient photocatalytic performance toward CO2-to-syngas conversion with a suitable CO/H2 ratio (1:1-1:3). The bipolaronic motifs enable a highly separated electron-hole state, whereby the Co(II)-pyridine N motifs tend to be the active sites for CO2 activation and accelerate the hydrogenation to form *COOH intermediates; whilst, the Co(II)-imine N motifs increase surface hydrophilicity for H2 evolution. The photocatalytic reductions of CO2 and H2O thus decouple and proceed via a concerted way on the bipolaronic motifs of bp-Co-COF. The optimal bp-Co-COF photocatalyst achieves a high syngas evolution rate of 15.8 mmol g-1 h-1 with CO/H2 ratio of 1:2, outperforming previously reported COF-based photocatalysts.

Biomineralization-Inspired Synthesis of Hybrid COF Nanosheets toward Efficient Desalination Membranes

Synthesis of covalent organic framework nanosheets (CONs) with high aspect ratio is crucial to their assembly into advanced membranes. Nonetheless, the π-π stacking between covalent organic framework (COF) layers often leads to thick CONs. Herein, inspired by biomineralization process, a series of aspect ratio CONs >15 000 is synthesized by multifunctional polyelectrolytes which not only provide the nucleation sites for pre-assembly with COF monomer, but also suppress π-π interaction for anisotropic growth through protonation. The membrane assembled from CONs exhibited water permeance of 341 kg m-2 h-1 and salt rejection of 99.5% in desalination, outperforming ever-reported membranes. This method establishes a platform for the synthesis of crystalline nanosheets.

General and Scalable Synthesis of Mesoporous 2D MZrO2 (M = Co, Mn, Ni, Cu, Fe) Nanocatalysts by Amorphous-to-Crystalline Transformation

In modern heterogeneous catalysis, it remains highly challenging to create stable, low-cost, mesoporous 2D photo-/electro-catalysts that carry atomically dispersed active sites. In this work, a general shape-preserving amorphous-to-crystalline transformation (ACT) strategy is developed to dope various transition metal (TM) heteroatoms in ZrO2, which enabled the scalable synthesis of TMs/oxide with a mesoporous 2D structure and rich defects. During the ACT process, the amorphous MZrO2 nanoparticles (M = Fe, Ni, Cu, Co, Mn) are deposited within a confined space created by the NaCl template, and they transform to crystalline 2D ACT-MZrO2 nanosheets in a shape-preserving manner. The interconnected crystalline ACT-MZrO2 nanoparticles thus inherit the same structure as the original MZrO2 precursor. Owing to its rich active sites on the surface and abundant oxygen vacancies (OVs), ACT-CoZrO2 gives superior performance in catalyzing the CO2-to-syngas conversion as demonstrated by experiments and theoretical calculations. The ACT chemistry opens a general route for the scalable synthesis of advanced catalysts with precise microstructure by reconciliating the control of crystalline morphologies and the dispersion of heteroatoms.

Hydrogen-Bonded Cocrystals Encapsulating CsPbBr3 Perovskite Nanocrystals with Enhancement of Charge Transport for Photocatalytic Reduction of Uranium

At present, poor stability and carrier transfer efficiency are the main problems that limit the development of perovskite-based photoelectric technologies. In this work, hydrogen-bonded cocrystal-coated perovskite composite (PeNCs@NHS-M) is easily obtained by inducing rapid crystallization of melamine (M) and N-hydroxysuccinimide (NHS) with PeNCs as the nuclei. The outer NHS-M cocrystal passivates the undercoordinated lead atoms by forming covalent bonds, thereby greatly reducing the trap density while maintaining good structure stability for perovskite nanocrystals. Moreover, benefiting from the interfacial covalent band linkage and long-range ordered structures of cocrystals, the charge transfer efficiency is effectively enhanced and PeNCs@NHS-M displays superior photoelectric performance. Based on the excellent photoelectric performance and abundant active sites of PeNCs@NHS-M, photocatalytic reduction of uranium is realized. PeNCs@NHS-M exhibits U(VI) reduction removal capability of up to 810.1 mg g-1 in the presence of light. The strategy of cocrystals trapping perovskite nanocrystals provides a simple synthesis method for composites and opens up a new idea for simultaneously improving the stability and photovoltaic performance of perovskite.

Asymmetric Potential Model of Two-Dimensional Imine Covalent Organic Frameworks with Enhanced Quantum Efficiency for Photocatalytic Water Splitting

Two-dimensional (2D) covalent organic frameworks (COFs) assembled using building blocks have been widely employed in photocatalysis due to their customizable optoelectronic characteristics and porous structure, which facilitate charge carrier and mass movement. Nevertheless, the development of COF photocatalysts encounters a continuous obstacle in enhancing the efficiency of photocatalysis, impeded by a limited comprehension of the orbital interaction between molecular fragments and linkers. In this study, we present a model that examines the interaction between molecular fragments in an imine-based COF at the frontier molecular orbital level, enabling us to comprehend the impact of manipulating linkers on light adsorption, exciton efficiency, and catalytic activity. Our findings demonstrate that altering the connecting orientation of 14 R-C=N-R imine linkers in 2D COFs can enhance solar-to-hydrogen (STH) efficiency under visible light from 2.76% to 4.24%. This research has the potential to provide a valuable model for refining photocatalysts with enhanced photocatalytic performance.

Crystalline nanosheets of three-dimensional supramolecular frameworks with uniform thickness and high stability

Fabricating three dimensional (3D) supramolecular frameworks (SMFs) into stable crystalline nanosheets remains a great challenge due to the homogeneous and weak inter-building block interactions along 3D directions. Herein, crystalline nanosheets of a 3D SMF with a uniform thickness of 4.8 ± 0.1 nm immobilized with Pt nanocrystals on the surface (Q[8]/Pt NSs) were fabricated via the solid-liquid reaction between cucurbit[8]uril/H2PtCl6 single crystals and hydrazine hydrate with the help of gas and heat yielded during the reaction process. A series of experiments and theoretical calculations reveal the ultrahigh stability of Q[8]/Pt NSs due to the high density hydrogen bonding interaction among neighboring Q[8] molecules. This in turn endows Q[8]/Pt NSs with excellent photocatalytic and continuous thermocatalytic CO oxidation performance, representing the thus-far reported best Pt nano-material-based catalysts.

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.

π-Sticked Metal‒Organic Monolayers for Single-Metal-Site Dependent CO2 Photoreduction and Hydrogen Evolution Reaction

Hierarchical self-assembly of 2D metal‒organic layers (MOLs) for the construction of advanced functional materials have witnessed considerable interest, due to the increasing atomic utilizations and well-defined atom‒property relationship. However, the construction of atomically precise MOLs with mono-/few-layered thickness through hierarchical self-assembly process remains a challenge, mostly because the elaborate long-range order is difficult to control via conventional noncovalent interaction. Herein, a quadruple π-sticked metal‒organic layer (πMOL) is reported with checkerboard-like lattice in ≈1.0 nanometre thickness, on which the catalytic selectivity can be manipulated for highly efficient CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER) over a single metal site. In saturated CO2 aqueous acetonitrile, Fe-πMOL achieves a highly effective CO2RR with the yield of ≈3.98 mmol g‒1 h‒1 and 91.7% selectivity. In contrast, the isostructural Co-πMOL as well as mixed metallic FeCo-πMOL exhibits a high activity toward HER under similar conditions. DFT calculations reveal that single metal site exhibits the significant difference in CO2 adsorption energy and activation barrier, which triggers highly selective CO2RR for Fe site and HER for Co site, respectively. This work highlights the potential of supramolecular π…π interaction for constructing monolayer MOL materials to uniformly distribute the single metal sites for artificial photosynthesis.