Hydrothermal Synthesis of Highly Crystalline Zwitterionic Vinylene-Linked Covalent Organic Frameworks with Exceptional Photocatalytic Properties

Neighboring Effect of Adjacent Nitrogen Sites on Vinylene Linkage in Covalent Organic Frameworks for Regulating Oxygen Reduction Reaction

Vinylene-linked covalent organic frameworks (COFs) are attractive electrocatalysts owing to their corresponding high chemical stability and excellent conjugated frameworks. In this study, for the first time, the methyl group of the pyrimidine ring was used to synthesize conjugated COFs (TB-TFT-COF and TB-TFC-COF) with vinylene linkages, which were employed as catalysts for the oxygen reduction reaction (ORR). In addition, local electronic structures of the vinylene linkages could be regulated by the adjacent nitrogen atomic sites of various functional moieties (triazine, pyridine, and pyrimidine), resulting in tunable electrocatalytic activity and selectivity of the COFs. Notably, the TB-TFT-COF attained a half-wave potential of 0.74 V relative to RHE alongside superior electrochemical stability, matching the performance of metal-free COF-based catalysts for ORR. Furthermore, as evidenced by density functional theory (DFT) calculations, the adjacent nitrogen sites of the pyrimidine unit around the vinylene linkage are crucial for enhancing the utilization of electrocatalytic active sites. This work establishes that the precise modulation of electronic coupling between neighboring active sites enables the development of efficient oxygen reduction reaction catalysts.

Enantioselective Immobilization of Nonprecious Metal Complexes on Chiral Covalent Organic Frameworks for Improved Single-Site Photocatalytic Hydrogen Evolution

The noncovalent assembly of molecular catalysts into photocatalytic systems represents a pivotal strategy for exploring single-site heterogenous catalysts, excluding the need for elaborate functionalization design. However, the reliance on weak noncovalent interactions (e.g., van der Waals forces) often leads to inefficient outer-sphere electron transfer and inferior structural stability. Herein, we report the enantioselective immobilization of cobalt-based molecular catalysts with chiral tetradentate ligands onto the surface of a β-ketoenamine-linked chiral covalent organic framework (COF) synthesized through chirality induction. The enantiomeric assembly enables axial coordination between the molecular catalysts and the chiral COF, accompanied by enamine-to-imine tautomerization. Leveraging efficient inner-sphere electron transfer, the resulting composite exhibits a significantly enhanced H2 evolution rate (5.70 mmol g-1 h-1) and sustained performance without the use of precious metals. The enantiomeric assembly strategy on a COF platform demonstrates a viable approach to improve both the stability and activity of molecular catalysts, thereby expanding the design paradigm of single-site photocatalysts.

Thiazolium-Linked Crystalline Porous Covalent Organic Frameworks for Mixed Electronic-Ionic Transport

Developing efficiently mixed electronic-ionic (MEI) conductive microcosmic pathways within a single functional material is essential yet challenging for electronic devices. Covalent organic frameworks (COFs) feature pre-designed functionalities and uniform pores, making them highly desirable platforms for transporting electrons and ions. However, for MEI conductive COFs, achieving high crystallinity when incorporating high-density ionic groups within the extensively π-electron delocalized skeletons remain a challenge due to intermolecular interactions. Herein, we reported a "pre-polymerization followed by self-ionization" approach to synthesize new thiazolium-linked COFs (MEICOFs, M═Cu, Co, Fe), where the ionic groups synthesized following the connection of building blocks. MEICOFs demonstrated broad ultraviolet-visible-near-infrared absorption bands and narrow bandgaps. As a proof of concept, the mixed electronic and hydroxide ionic conductivity of CuEICOF was determined to be 55.2 and 0.01 S m-1, respectively. Moreover, MEICOFs film could directly catalyze the oxygen reduction reaction without additional conductive agent and the rotation of the electrode.

Fully Conjugated Benzobisoxazole-Bridged Covalent Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

2D covalent organic frameworks (2D-COFs) have attracted extensive interest in solar energy to hydrogen conversion. However, insufficient light harvesting and difficult exciton dissociation severely limit the improvement of photocatalytic activity for COFs, thereby impeding the progression of this advanced field. In this work, two benzobisoxazole-bridged and fully conjugated 2D-COFs with triazine (COF-JLU44) and pyrene (COF-JLU45) units were constructed for the first time via Knoevenagel polycondensation, and they hold long-range ordered structures, largely acceptable surface area, and fascinating photoelectric properties. Significantly, COF-JLU45 exhibits an impressive hydrogen evolution rate of 272.5 mmol g-1 h-1 and superior reusability in the presence of 1.0 wt% Pt under light irradiation, coupled with a remarkable apparent quantum yield of 12.9% at a long wavelength of 600 nm. Multiple spectroscopy and theoretical simulation demonstrate the ingenious design of COF-JLU45 widen its light absorption and effectively promote the exciton dissociation. This finding contributes valuable insights for constructing metal-free photocatalysts for solar energy conversion and utilization.

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.

Redox-Mediated TEMPO-Based Donor-Acceptor Covalent Organic Framework for Efficient Photo-Induced Hydrogen Peroxide Generation

Molecular engineering of covalent organic frameworks (COFs) offers an alternative approach to conventional anthraquinone oxidation via photo-induced H2O2 production from O2 reduction. Despite their potential, reported photocatalysts suffer limited proton mobility, low selectivity, and insufficient charge separation and utilization. Herein, we report a nitroxyl radical (TEMPO) decorated two-dimensional (2D) donor-acceptor (D-A)-COF photocatalyst via a one-pot strategy. Under visible light irradiation, highly crystalline TAPP-TPDA-TEMPO-COF (TT-T-COF) exhibits a remarkable photocatalytic H2O2 yield of 10066 μmol g-1 h-1 in two-phase water-benzyl alcohol (10 % BA) system through direct two-electron (2e-) pathway. The mechanistic study by DFT calculations and in situ DRIFT spectra suggests Yeager-type adsorption of *O2⋅- intermediate on the nitroxyl radical site (N-O⋅). The efficient photocatalytic performance and stability of TT-T-COF are attributed to the involvement of the nitroxyl radical, which enhances selective O2 adsorption, establishes a distinct electron density distribution, and facilitates photogenerated charge separation compared to TT-HT-COF and TT-COF counterparts. This study uncovers a new perspective for constructing metal-free, redox-mediated radical-based COFs for sustainable energy conversion, storage, and biomedical applications.

Terpyridine- and Quarterpyridine-Based Cationic Covalent Organic Frameworks for Visible-Light-Catalytic H2O2 Synthesis

This paper presents multipyridine-containing covalent organic frameworks (COFs) with precisely defined position and number of pyridinium cationic groups. Specifically, three terpyridine- and quarterpyridine-based trialdehydes were synthesized, and utilized as the starting monomers to polymerize with trimethylpyridinium bromide to yield vinylene-linked iTPy-COF, iTPPy-COF and iQPPy-COF, respectively. Thus constructed donor-acceptor cationic COFs exhibit considerably high visible-light catalytic efficiency for hydrogen peroxide (H2O2) synthesis by the dual-channel mechanisms of oxygen reduction reaction (ORR) and water oxidation reaction (WOR). In pure water and O2 atmosphere, the H2O2 production rate (HPR) of iTPPy-COF after 1 h reaction is as high as 7955 μmol g-1 h-1. Even though in air, its HPR value still reaches 6249 μmol g-1 h-1. Moreover, it is found that changing the arm lengths and ratios of pyridine to benzene ring in the frameworks significantly affects the photocatalytic capability. The structure-property relationship is investigated in terms of the variations of electronic structures through the theoretical simulations and measurements of photophysical parameters such as fluorescence lifetimes, photocurrent intensities, and impedances of charge transfer, which offers new insights into the engineering of multipyridine-based cationic COFs for highly efficient H2O2 photosynthesis.

Effective Strategies in Covalent Organic Frameworks for Enhanced Photocatalytic Hydrogen Production

Hydrogen as a significant green energy source, has emerged as one of the most promising candidates to solve serious environmental and energy problems. Photocatalytic water splitting is a prospective route to sustainable hydrogen production. Covalent organic frameworks (COFs) are considered as efficient photocatalysts due to their substantial specific surface areas, extended π-conjugated backbones, and robust chemical stability. This review summarizes the recent advances of COF-based materials in the field of photocatalytic hydrogen production, including the construction of donor-acceptor (D-A) structure, protonation of the N site, synthesis of zwitterionic COFs, introduction of co-catalysts, use of metal-containing monomers, and compositing COFs with other catalysts. The properties of the catalysts are meticulously adjusted through those structural and system design strategies, thereby significantly enhancing the hydrogen production performance of the COFs. Finally, the challenges and potential opportunities for future developments are discussed in terms of the current research status and practical applications of photocatalytic hydrogen production from COFs.

Ultrathin zwitterionic COF membranes from colloidal 2D-COF towards precise molecular sieving

Membrane technology is an important component of resource recovery. Covalent organic frameworks (COFs) with inherent long-range ordered structure and permanent porosity are ideal materials for fabricating advanced membrane. Zwitterionic COFs have unique features beyond single ionic COFs containing anions or cations. Here, a zwitterionic colloidal 2D-COF (TpPa-Py) is synthesized via a single-phase method. ultrathin zwitterionic COF membranes are fabricated via a facile blade-coating method. Experimental and molecular dynamics simulation results showed that due to the unique amphiphilic nature of the TpPa-Py, the TpPa1-Py1 membrane exhibits high level permeance and rejection of both positively and negatively charged dyes. Moreover, the TpPa1-Py1 membrane exhibits excellent dye/dye and dye/salt separation performance. The selectivity factors were 89 for the separation of acid blue and rhodamine B, and 47.8 for the separation of methyl blue and NaCl. This work provides a promising solution for the development of high-performance membranes tailored for resource recovery of dye wastewater, addressing a critical need in water treatment.

Proton-Mediated Topological Interlayer Shift in 2D Covalent Organic Frameworks for Efficient Photocatalysis

The interlayer carriers dynamics are of significance in optoelectronic applications of 2D donor-acceptor (D-A) covalent organic frameworks (COFs), while are challenged by the delicate control over the inherently variable and sensitive interlayer interaction. Present work demonstrates an efficient proton-mediation strategy that allows for the precise regulation of interlayer shift of 2D D-A COFs for facilitated charge transfer and exciton dissociation. Exemplified by three imine-linked D-A COFs (IMDA), mild proton-mediation generates an eclipsed AA stacking (IMDA-AA) featuring in-plane D-A pairs and fully overlapping D-A π-conjugations, while excessive proton-mediation disrupts these conjugations, resulting in a slipped AA stacking (IMDA-SAA) with out-of-plane D-A pairs. Further analysis reveals that the interlayer topology of eclipsed AA stacking of IMDA favors for the synergistically optimized charge transfer dynamics, including enhanced intralayer charge transport with reduced exciton binding energy, and boosted interlayer exciton dissociation. IMDA-AA COF delivers an improved hydrogen evolution rate up to 171.2 mmol g-1h-1 under visible light illumination in the presence of 1.5 wt.% Pt co-catalysts, which is as far as is known the highest value among the reports of 2D COFs based photocatalysis. Present work will provide an important avenue of addressing the topology-governed charge transfer dynamics within COFs for solar energy conversion.

Overcoming Boundaries: Towards the Ambient Aqueous Synthesis of Covalent Organic Frameworks

The synthesis of covalent organic frameworks (COFs) has traditionally been carried out under strict solvothermal and anaerobic conditions. The utilization of organic solvents in such reactions not only carries significant costs but also imposes a great burden on the environment. The fabrication of COFs using alternative synthetic pathways has, therefore, seen rapid development in recent years and much attention has been placed on green and sustainable methods in particular. The synthesis of COFs in purely aqueous media, however, remains challenging due to the delicate nature of the chemical reactions and the crystallization process in water. This mini-review discusses different synthetic strategies for the construction of crystalline COFs in aqueous media and offers a perspective on the future development of facile COF synthesis in ambient conditions.

Crystalline, Porous Figure-Eight-Noded Covalent Organic Frameworks

Figure-eight macrocycles represent a fascinating class of π-conjugated units characterized by unique aesthetics and non-contact molecular crossing at the center. Despite progress in synthesis over the past century, research into inorganic, organic, and polymeric figure-eight materials remains in its infancy. Here we report the first examples of figure-eight covalent organic frameworks by condensing figure-eight knots to create extended porous figure-eight π architectures. A distinct feature is that polymerization interweaves figure-eight knots into double-decker layers, which upon supramolecular polymerization organize well-defined layer frameworks. The figure-eight frameworks exhibit a band gap of 2.3 eV and emit bright orange florescence with benchmark quantum yields. Remarkably, the donor-acceptor figure-eight skeletons convert the figure-eight knots into reduction centers and the linkers into oxidation sites upon light irradiation, enable charge transport and accumulation through π columns, while the built-in hydrophilic micropores allow rapid water and oxygen delivery via capillary effect. With these distinct features, the figure-eight frameworks function as a photocatalyst to produce hydrogen peroxide at high rate and efficiency with water/saltwater, oxygen/air, and light as sole inputs. This work paves a way to a new class of molecular frameworks, underpinning the study of well-defined figure-eight materials to explore unprecedented structures and functions so far we untouched.

sp2 Carbon-Conjugated Covalent Organic Frameworks (sp2c-COFs): Synthesis and Application in Photocatalytic Water Splitting

Preparation of irreversible sp2 carbon-conjugated covalent organic frameworks (sp2c-COFs) with specific porosity, easy structural functionalization, high chemical stability, and unique π-electron conjugation structure (especially the combination of π-π stacking interactions and conjugation system), can remove the barrier of electron transfer and provide a unique advantage for photocatalytic water splitting. Herein, based on three kinds of reactions (Aldol condensation reaction, Knoevenagel condensation reaction, and Horner-Wadsworth-Emmons reaction) and guided by the precise modulation of ligand structure and topology, this review summarizes the synthesis of sp2c-COFs and their applications in photoelectrocatalytic water splitting (hydrogen evolution and oxygen evolution reactions). Furthermore, challenges and possible research directions for sp2c-COFs in photocatalytic water splitting are also provided.

Recent advances in room-temperature synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) have become a promising class of highly-crystalline polymers with layered stacking structures, ordered porous channels, and highly-tailorable structures. To date, most COFs have been synthesized via high-temperature solvothermal methods, which require complicated optimization of factors including temperature, solvent ratio, catalyst, and reaction time. Additionally, solvothermal conditions with high temperature and high pressure restrict the facile and large-scale synthesis of COFs for practical applications. In addition, the insolubility and lack of processability of the COF powders obtained via solvothermal methods hinder their potential application in film-related fields. Energy-efficient and environmentally benign synthetic methods to resolve these problems are highly desired. In this review, we provide an overview of the recent progress in room-temperature synthetic strategies for constructing COF powders or COF films. We first discuss in situ characterization technologies for exploring the COF growth mechanism. Then, we present representative room-temperature synthesis methods for COFs, including solid-liquid interfacial synthesis, liquid-liquid interfacial synthesis, on-water surface synthesis, water-phase synthesis, electrosynthesis, sonochemical synthesis, single-solution phase synthesis, mechanochemical synthesis, high-energy ionizing radiation synthesis, and photochemical synthesis. Finally, perspectives on room-temperature synthesis are proposed in the areas of single-crystal domains, novel room-temperature reaction types, crystallization mechanism, the design of chemical structures and green synthesis.

Bioinspired Photo-Thermal Catalytic System Using Covalent Organic Framework-Based Aerogel for Synchronous Seawater Desalination and H2O2 Production

Efficient utilization of solar energy is widely regarded as a crucial solution to addressing the energy crisis and reducing reliance on fossil fuels. Coupling photothermal and photochemical conversion can effectively improve solar energy utilization yet remains challenging. Here, inspired by the photosynthesis system in green plants, we report herein an artificial solar energy converter (ASEC) composed of light-harvesting units as solar collector and oriented ionic hydrophilic channels as reactors and transporters. Based on such architecture, the obtained ASEC (namely ASEC-NJFU-1) can efficiently realize parallel production of freshwater and H2O2 from natural seawater under natural light. The total solar energy conversion (SEC) of ASEC-NJFU-1 reaches up to 8047 kJ m-2 h-1, corresponding to production rates of freshwater and H2O2 are 3.56 kg m-2-1 h-1 and 19 mM m-2 h-1, respectively, which is a record-high value among all photothermal-photocatalytic systems reported to date. Mechanism investigation of combining spectrum and experimental studies indicated that the high SEC performance for ASEC-NJFU-1 was attributed to the presence of plant bioinspired architecture with carbon nanotubes as solar-harvestor and COF-based oriented aerogel as reactors and transporters. Our work thus establishes a novel artificial photosynthesis system for highly efficient solar energy utilization.

Ionic porous materials: from synthetic strategies to applications in gas separation and catalysis

Ionic porous materials possess a unique combination of tunable pore sizes and task-specific interactions between guest molecules and the charged frameworks, which endow them with versatility across diverse domains in chemistry and materials science. Significant advancements in their applications for gas separation and catalysis have been achieved in recent years due to the incorporation of ionic functionalities and ultra-microporous structures that enable molecular-scale recognition of guest molecules. This review summarizes recent advancements in the synthetic strategies of ionic porous materials, establishing design guidelines for the incorporation of ionic moieties into the backbone to fine-tune pore sizes and chemistry. It highlights the synergistic interplay of task-specific interactions with custom-designed pore structures in key applications, including adsorption separation, membrane separation, and gas conversion. Additionally, it examines structure-property relationships, offering deeper insights into enhancing performance. The report also addresses the current challenges in the practical application of these materials. Finally, the review provides future perspectives on ionic porous materials from both scientific and industrial viewpoints. Overall, this review aims to provide insights into pore structure and chemistry, supporting the precise placement of ionic functionalities.

Precise Regulation of Intra-Nanopore Charge Microenvironment in Covalent Organic Frameworks for Efficient Monovalent Cation Transport

Charged channels are considered an effective design for achieving efficient monovalent cation transport; however, it remains challenging to establish a direct relationship between charge microenvironments and ionic conductivity within the pores. Herein, we report a series of crystalline covalent organic frameworks (COFs) with identical skeletons but different charge microenvironments and explore their intra-pore charge-driven ion transport performance and mechanism differences. We found that the charged nature determines ion-pair action sites, modes, host-guest interaction, thereby influencing the dissociation efficiency of ion pairs, the hopping ability of cations, and the effective carrier concentration. The order of transport efficiency for Li+, Na+, and H+ follows anion > zwitterion > cation > neutrality. Ionic COFs exhibit up to 11-fold higher ionic conductivity than neutral COFs. Notably, the ionic conductivity of anionic COF achieves 2.0 × 10-4 S cm-1 for Li+ at 30 °C and 3.8 × 10-2 S cm-1 for H+ at 160 °C, surpassing most COF-based ionic conductors. This COF platform for efficient ion migration and stable battery cycling in lithium-metal quasi-solid-state batteries has also been verified as proof of concept. This work offers new insights into the development and structure-activity relationship studies of the next generation of solid-state ionic conductors.

Geomimetic Interfacial Hydrothermal Synthesis of Crystalline Ionic Vinylene-Linked Covalent Organic Frameworks

Ionic vinylene-linked covalent organic frameworks (ivCOFs) with ionic characteristics and highly conjugated structures have been promising functional materials. However, both the number of reported ionic vinylene-linked COFs and its synthetic methodologies still be limited. Herein, two new kinds of ionic vinylene-linked covalent organic frameworks (named as COF-NUC-1 and COF-NUC-2) are synthesized by novel geomimetic interfacial hydrothermal synthesis for the first time. Due to the insolubility of conjugated aldehyde monomers in water, a molecule/water interface is created, where the water-soluble N-ethyl-2,4,6-trimethylpyridinium bromide (ETMP-Br) can react with aldehyde monomers via the interface-confined Knoevenagel condensation reaction. The resultant ionic COFs show high crystallinity, high chemical stability, and hydrophilic nature. Benefiting from electron-withdrawing and redox properties of pyridinium salts, the as-fabricated memristor based on COF-NUC-1 film shows stable nonvolatile memory effects, featuring a high ON/OFF current ratio of 0.66 × 103 and a small switch-on voltage of -0.68 V. This work expands the realm of ivCOFs and deepens the understanding of hydrothermal synthesis.

CRISPR/Cas13a-Programmed Cu NCs and Z-Scheme T-COF/Ag2S for Photoelectrochemical Biosensing of circRNA

Circular RNAs (circRNAs), as a class of noncoding RNA molecules with a circular structure exhibit high stability and spatiotemporal-specific expression, making them ideal cancer biomarkers for liquid biopsy. Herein, a new photoelectrochemical (PEC) biosensor for a highly sensitive circRNA assay in the whole blood of lung cancer patients was designed based on CRISPR/Cas13a-programmed Cu nanoclusters (Cu NCs) and a Z-scheme covalent organic framework/silver sulfide (T-COF/Ag2S) composite. This Z-scheme T-COF/Ag2S composite accelerates electron transfer and produces an excellent initial photocurrent. When CRISPR/Cas13a precisely targets circRNA, it nonspecifically cleaves the triple-helix molecular structure to release DNA fragments (C'/C"). After the C'/C" opens the DNA hairpin probe (HP) modified on the electrode, hybridization chain reactions are performed to produce abundant AT-rich double-stranded DNA with the addition of H1 and H2 probes. Upon the incubation of Cu2+, Cu NCs are in situ formed via the A-Cu2+-T bonds and can effectively quench the photocurrent of the Z-scheme T-COF/Ag2S due to the energy transfer process. This developed PEC biosensor for the circRNA assay shows a low limit of detection of 0.5 fM, and the reusability of DNA-modified magnetic beads (MB-DNA) reduces the detection cost. Moreover, the PEC biosensor can accurately quantify the circRNA level and distinguish the circRNA expression in whole blood from healthy controls and lung cancer patients, offering strong potential in clinical diagnosis.

Industrialization of Covalent Organic Frameworks

Covalent organic frameworks (COFs) have attracted broad interest because of their well-defined, customizable, highly stable, and porous structures. COFs have shown significant potential for various practical applications, such as gas storage/purification, drug purification, water treatment, catalysis, and battery applications. Scaling up COFs is highly desirable to meet industrial application demands but is hindered by the limitations of synthesis methods and the high cost of reactants. Recently, emerging green synthesis methods, such as mechanochemical synthesis and flux synthesis, have offered promising solutions to these challenges (e.g., ton-scale production of COFs has been achieved by companies recently). This Perspective provides an overview of the state of the art with respect to the industrial production of COFs and discusses factors influencing the large-scale production of COFs. Directions and opportunities for improving the performance and sustainability of COFs toward industrial applications are also emphasized.

Diketopyrrolopyrrole-Activated Dynamic Condensation Approach to Narrow-Band Gap Vinylene-Linked Covalent Organic Frameworks

Vinyl units intrinsically featuring less steric, nonpolarity, and unsaturated character, are well-known π-bridge used in the synthesis of high-performance semiconducting materials. Two-dimensional (2D) vinylene-linked covalent organic frameworks (COFs) represent a promising class of π-conjugated structures, however, the range of available monomers for the reversible formation of carbon-carbon double bonds remains limited. In this study, a new class of 2D vinylene-linked COFs were synthesized using dimethyldiketopyrrolopyrrole (DM-DPP) as the key monomer. The strong electron deficiency of diketopyrrolopyrrole (DPP) makes its methyl substituents readily activated upon the cocatalysis of L-proline and 4-dimethylaminopyridine in aqueous solution to conduct dynamic condensation with tritopic aromatic aldehydes. The resulting COFs crystallized in an eclipsed AA stacking arrangement and featured abundant, regular nanochannels. Their robust vinyl DPP-linking mode enhanced donor-π-acceptor conjugation and promoted π-stacked alignment along the vertical direction. Consequently, the synthesized COFs exhibited band gaps as narrow as 1.02 eV and demonstrated excellent light-harvesting capability across the visible to near-infrared I (NIR-I) regions. Furthermore, the COFs could be converted into free-standing thin pellets through simple pressure casting, and show excellent photothermal response and cycling stability under different light sources.

Floating Photothermal Hydrogen Production

Solar-to-hydrogen (STH) is emerging as a promising approach for energy storage and conversion to contribute to carbon neutrality. The lack of efficient catalysts and sustainable reaction systems is stimulating the fast development of photothermal hydrogen production based on floating carriers to achieve unprecedented STH efficiency. This technology involves three major components: floating carriers with hierarchically porous structures, photothermal materials for solar-to-heat conversion and photocatalysts for hydrogen production. Under solar irradiation, the floating photothermal system realizes steam generation which quickly diffuses to the active site for sustainable hydrogen generation with the assistance of a hierarchically porous structure. Additionally, this technology is endowed with advantages in the high utilization of solar energy and catalyst retention, making it suitable for various scenarios, including domestic water supply, wastewater treatment, and desalination. A comprehensive overview of the photothermal hydrogen production system is present due to the economic feasibility for industrial application. The in-depth mechanism of a floating photothermal system, including the solar-to-heat effect, steam diffusion, and triple-phase interaction are highlighted by elucidating the logical relationship among buoyant carriers, photothermal materials, and catalysts for hydrogen production. Finally, the challenges and new opportunities facing current photothermal catalytic hydrogen production systems are analyzed.

Intrareticular Exciton Effects Regulate Photocatalytic Activity in Donor-Acceptor Olefin-Linked Covalent Organic Frameworks

Olefin-linked covalent organic frameworks (OL-COFs) show great promise for visible-light-driven photocatalysis. Manipulating atomic-level donor-acceptor interactions in OL-COFs is key to understanding their exciton effects in this system. Here, three OL-COFs are presented with orthorhombic lattice structures, synthesized via Knoevenagel polycondensation reaction of terephthalaldehyde and tetratopic monomers featuring phenyl, benzo[c][1,2,5]oxadiazole, and benzo[c][1,2,5]thiadiazole moieties. These OL-COFs feature tunable donor-acceptor interactions, making them ideal for studying exciton effects in olefin-linked systems. Comprehensive analyses, including temperature-dependent photoluminescence spectra, ultrafast spectroscopy, and theoretical calculations, reveal that stronger donor-acceptor interactions lead to reduced exciton binding energy (Eb), accelerated exciton dissociation, and longer-lived photogenerated charges, thereby enhancing photocatalytic performance. Notably, The TMO-BDA COF, with the lowest Eb, demonstrates superior photocatalytic activity in one-pot sequential organic transformation and excellent catalytic performance in gram-scale reactions, highlighting its potential for practical applications. This work provides valuable insights into regulating the exciton effect at the molecular level in OL-COFs, offering pathways to enhance photocatalytic efficiency.

Impact of Interfaces on the Performance of Covalent Organic Frameworks for Photocatalytic Hydrogen Production

The rise in global temperatures and environmental contamination resulting from traditional fossil fuel usage has prompted a search for alternative energy sources. Utilizing solar energy to drive the direct splitting of water for hydrogen production has emerged as a promising solution to these challenges. Covalent organic frameworks (COFs) are ordered, crystalline materials made up of organic molecules linked by covalent bonds, featuring permanent porosity and a wide range of structural topologies. COFs serve as suitable platforms for solar-driven water splitting to produce hydrogen, as their building blocks can be tailored to possess adjustable band gaps, charge separation capabilities, porosity, wettability, and chemical stability. Here, the impact of the interface in the context of the photocatalytic reaction is focused and propose strategies to enhance the hydrogen production performance of COFs photocatalysis. In particular, how hybrid photocatalytic interfaces affect photocatalytic performance is focused.

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.

Construction of cationic covalent organic framework for efficient gold extraction

In this work, we report a rare ionic covalent organic framework (named as ECUT-iCOF-5) synthesized by the Knoevenagel reaction using an ionic liquid monomer. ECUT-iCOF-5 is found to show a promising potential for gold extraction with a high extraction ability towards gold, including a high extraction capacity (3260 mg g-1), good selectivity over both cations and anions, and good recycle use (at least six times).

Enhancing photocatalytic performance of covalent organic frameworks via ionic polarization

Covalent organic frameworks have emerged as a thriving family in the realm of photocatalysis recently, yet with concerns about their high exciton dissociation energy and sluggish charge transfer. Herein, a strategy to enhance the built-in electric field of series β-keto-enamine-based covalent organic frameworks by ionic polarization method is proposed. The ionic polarization is achieved through a distinctive post-synthetic quaternization reaction which can endow the covalent organic frameworks with separated charge centers comprising cationic skeleton and iodide counter-anions. The stronger built-in electric field generated between their cationic framework and iodide anions promotes charge transfer and exciton dissociation efficiency. Moreover, the introduced iodide anions not only serve as reaction centers with lowered H* formation energy barrier, but also act as electron extractant suppressing the recombination of electron-hole pairs. Therefore, the photocatalytic performance of the covalent organic frameworks shows notable improvement, among which the CH3I-TpPa-1 can deliver an high H2 production rate up to 9.21 mmol g-1 h-1 without any co-catalysts, representing a 42-fold increase compared to TpPa-1, being comparable to or possibly exceeding the current covalent organic framework photocatalysts with the addition of Pt co-catalysts.

Mesoporous Vinylene-Linked Covalent Organic Frameworks with Heteroatom-Tuned Crystallinity and Photocatalytic Behaviors

Owing to its prominent π-delocalization and stability, vinylene linkage holds great merits in the construction of covalent organic frameworks (COFs) with promising semiconducting properties. However, carbon-carbon double bond formation reaction always exhibits relatively low reversibility, unfavorable for the formation of high crystalline frameworks through self-error correction and assembling processes. In this work, we report a heteroatom-tuned strategy to build up a series of two-dimensional (2D) vinylene-linked COFs by Knoevenagel condensation of an electron-deficient methylthiazolyl-based monomer with different triformyl substituted (hetero-)aromatic derivatives. The resulting COFs show high-quality periodic mesoporous structures with high surface areas. Embedding heteroatoms into the backbones enables significantly improving their crystallinity, and finely tailoring their semiconducting structures. Upon visible light stimulation, one of the as-prepared COFs with donor-π-acceptor structure could deliver a nearly seven-fold increase in the catalytic activity of hydrogen generation as compared with the other two. Meanwhile, in combination with high crystallinity and the matched conduction band energy level, such kind of COFs can be able to selectively generate singlet oxygen and superoxide radicals in a high ratio of up to 30 : 1, allowing for catalyzing aerobic thioanisole oxidation in distinctly tunable activities through the substituent electronic effect of the substrates.

Ultrafine Spatial Modulation of Diazapyrene-Based Two-Dimensional Conjugated Covalent Organic Frameworks

Tailormade bottom-up synthesis of covalent organic frameworks (COFs) from various functional building blocks offer not only tunable topology and pore size but also multidimensional properties. High crystallinity is one of the prerequisites for their structures and associated physicochemical properties. Among different π-conjugated motifs for constructing COFs, pyrene-based tetragonal structures are effective in achieving highly ordered and crystalline states. In the present research, we demonstrated that the substitution of pyrene with 2,7-diazapyrene produces nearly "flat" structures of two-dimensional (2D) COF layers by controlling the torsional angle of linker molecules. Featuring finite pore diameters and excellent thermodynamic stability of ∼500 °C, ordered face-to-face (slipped AA) stacking arrangements were produced. Extended electrical conjugation spanning 2D frames with modest optical bandgaps (Eg) of ∼2.1 eV shows the planar character of diazapyrene-based COFs. The stacking of the conjugated 2D frames with small Eg values is also beneficial for the formation of highly stable conducting pathways in the crystalline state, which was confirmed by the results of the microwave conductivity measurements. Nitrogen centers in diazapyrene units also play a key role as the active sites for proton transfer, and the maximum proton conductivity of σ = 10-2 S cm-1 was achieved along the cocontinuous nanopore structures surrounded by the active sites. Results show that tetragonal COFs based on diazapyrene can be used as a highly crystalline two-dimensional material with special electrical and proton-conducting capabilities.

Processing polymer photocatalysts for photocatalytic hydrogen evolution

Conjugated materials have emerged as competitive photocatalysts for the production of sustainable hydrogen from water over the last decade. Interest in these polymer photocatalysts stems from the relative ease to tune their electronic properties through molecular engineering, and their potentially low cost. However, most polymer photocatalysts have only been utilised in rudimentary suspension-based photocatalytic reactors, which are not scalable as these systems can suffer from significant optical losses and often require constant agitation to maintain the suspension. Here, we will explore research performed to utilise polymeric photocatalysts in more sophisticated systems, such as films or as nanoparticulate suspensions, which can enhance photocatalytic performance or act as a demonstration of how the polymer can be scaled for real-world applications. We will also discuss how the systems were prepared and consider both the benefits and drawbacks of each system before concluding with an outlook on the field of processable polymer photocatalysts.

Modulating the D-π-A Interactions in Metal-Covalent Organic Frameworks for Efficient Electroreduction of CO2 into Formate

Crystalline porous framework materials have attracted tremendous interest in electrocatalytic CO2 reduction owing to their ordered structures and high specific surface areas as well as rich designability, however, still suffer from a lack of accuracy in regulating the binding strength between the catalytic sites and intermediates, which is crucial for optimizing the electrocatalytic activity and expanding the product types. Herein, we report three new kinds of vinylene-linked metal-covalent organic frameworks (TMT-CH3-MCOF, TMP-CH3-MCOF and TMP-MCOF) with continuously tunable D-π-A interactions by adjusting the structure of the monomers at the molecular level for realizing efficient electroreduction of CO2 to formate for the first time. Interestingly, compared with TMT-CH3-MCOF and TMP-MCOF, the TMP-CH3-MCOF exhibited the highest HCOO- Faradaic efficiency (FEHCOO-) of 95.6 % at -1.0 V vs RHE and displayed the FEHCOO- above 90 % at the voltage range of -1.0 to -1.2 V vs. RHE, which is one of the highest among various kinds of reported electrocatalysts. Theoretical calculations further reveal that the catalytic sites in TMP-CH3-MCOF with unique moderate D-π-A interactions have suitable binding ability towards the reaction intermediate, which is beneficial for the formation of *HCOO and desorption of *HCOOH, thus effectively promoting the electroreduction of CO2 to formate.

Linkage-Mediated Electronic Structure Modulation in Multicomponent Covalent Organic Frameworks for Dramatically Promoted Photocatalytic Hydrogen Evolution

Linkage chemistry is an essential aspect to covalent organic framework (COF) applications; it is highly desirable to precisely modulate electronic structure mediated directly by linkage for efficient COF-based photocatalytic hydrogen evolution, which however, remains substantially challenging. Herein, as a proof of concept, a collection of robust multicomponent pyrene-based COFs with abundant donor-acceptor (D-A) interactions has been judiciously designed and synthesized through molecularly engineering linkage for photogeneration of hydrogen. Controlled locking and conversion of linkage critically contribute to continuously regulating COFs' electronic structures further to optimize photocatalytic activities. Remarkably, the well-modulated optoelectronic properties turn on the average hydrogen evolution rate from zero to 15.67 mmol g-1 h-1 by the protonated quinoline-linked COF decorated with the trifluoromethyl group (TT-PQCOF-CF3). Using diversified spectroscopy and theoretical calculations, we show that multiple modifications toward linkage synergistically lead to the redistribution of charge on COFs with extended π-conjugation and reinforced D-A effect, making TT-PQCOF-CF3 a promising material with significantly boosted carrier separation and migration. This study provides important guidance for the design of high-performance COF photocatalysts based on the strategy of linkage-mediated electronic structure modulation in COFs.

Molecular Engineering for Modulating Photocatalytic Hydrogen Evolution of Fully Conjugated 3D Covalent Organic Frameworks

Covalent organic frameworks (COFs) have recently shown great potential for photocatalytic hydrogen production. Currently almost all reports are focused on two-dimensional (2D) COFs, while the 3D counterparts are rarely explored due to their non-conjugated frameworks derived from the sp3 carbon based tetrahedral building blocks. Here, we rationally designed and synthesized a series of fully conjugated 3D COFs by using the saddle-shaped cyclooctatetrathiophene derivative as the building block. Through molecular engineering strategies, we thoroughly discussed the influences of key factors including the donor-acceptor structure, hydrophilicity, specific surface areas, as well as the conjugated/non-conjugated structures on their photocatalytic hydrogen evolution properties. The as-synthesized fully conjugated 3D COFs could generate the hydrogen up to 40.36 mmol h-1 g-1. This is the first report on intrinsic metal-free 3D COFs in photocatalytic hydrogen evolution application. Our work provides insight on the structure design of 3D COFs for highly-efficient photocatalysis, and also reveals that the semiconducting fully conjugated 3D COFs could be a useful platform in clear energy-related fields.

Interlayer Interactions and Macroscopic Property Calculations of Squaric-Acid-Linked Zwitterionic Covalent Organic Frameworks: Structures, Photocatalytic Carrier Transport, and a DFT Study

Squaric-acid-linked zwitterionic covalent organic frameworks (Z-COFs), assembled through interlayer interactions, are emerging as potential materials in the field of photocatalysis. However, the study of their interlayer interactions has been largely overlooked. To address this, this work systematically calculated interlayer interactions via density functional theory (DFT) and analyzed the differences in interlayer interactions of different structures of Z-COFs through interlayer slippage, planarity, and an independent gradient model based on the Hirshfeld partition (IGMH). Furthermore, it revealed the relationship between the interactions and the macroscopic photocatalytic carrier transport performance of the material. The results indicated that both preventing interlayer slippage and enhancing planarity can enhance the interlayer interactions of Z-COFs, thereby improving their macroscopic carrier transport performance in photocatalysis.

Ionic Covalent Organic Frameworks in Adsorption and Catalysis

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

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.