Dual donor-acceptor covalent organic frameworks for hydrogen peroxide photosynthesis

Enhancing the crystallinity of covalent organic frameworks to achieve improved photocatalytic hydrogen peroxide production under ambient conditions

Photocatalytic production of hydrogen peroxide (H2O2) presents a promising strategy for environmental remediation and energy production. However, achieving clean and efficient H2O2 production under ambient conditions without organic sacrificial agents remains challenging. Enhancing the low crystallinity of covalent organic frameworks (COFs) can promote the separation and transmission of photo-generated carriers, thereby boosting their photocatalytic performance. Herein, we introduce a novel synthetic approach by substituting traditional acetic acid catalysts with organic base catalysts to enhance the crystallinity of β-ketoenamine-linked COF, TpBD-COF. Compared to TpBD-COF-A synthesized using acetic acid catalysts, TpBD-COF-B, synthesized with organic base catalysts, exhibited advancements including increased absorption intensity in the visible spectrum, reduced photoluminescence intensity, enhanced photo-generated carrier separation performance, and a 2.1-fold increase in photocatalytic H2O2 production. Under visible light irradiation, TpBD-COF-B achieved a photocatalytic H2O2 production rate of 533 µmol/h/g using only air and water, without the need for organic sacrificial agents. Furthermore, TpBD-COF-B also exhibited good performance in long-term catalytic production experiments, tests with actual water bodies, and cyclic usage experiments. This study offers a strategy for enhancing the crystallinity of COFs to improve their photocatalytic activity, with promising applications in clean energy production and environmental remediation.

The role of branching in the ultrafast dynamics and two-photon absorption of two pyrimidine push-pull molecules

The dynamics and two-photon absorption (2PA) properties of two pyrimidine chromophores are studied using femtosecond time-resolved fluorescence and two-photon excited fluorescence techniques. The pyrimidine is used as an electron withdrawing group and is substituted at the C2 position with a phenylacridan fragment, while diphenylaministyryl donor moieties are appended at positions C4/6 to afford the pseudo-dipolar and pseudo-quadrupolar molecules 1 and 2, respectively. Chromophore 2 shows more efficient fluorescence emission, while 1 exhibits larger Stokes shifts. Their decay pathways are discussed through an emission from a Franck-Condon charge transfer (FC-CT) and a relaxed charge transfer (R-CT) state. Ultrafast dynamics in tetrahydrofuran show population of the R-CT state for 1 that is faster than solvation, while for 2, due to its pseudo-quadrupolar nature, R-CT population is slower and occurs from the solvated FC-CT state. Finally, molecule 2 shows better 2PA properties with cross sections reaching 560 GM at 820 nm.

Revealing the Mechanism of Exciton Spontaneous Separation at Room Temperature for Efficient Photocatalytic Hydrogen Peroxide Synthesis

The photocatalytic synthesis of hydrogen peroxide (H2O2) at room temperature has garnered significant attention as an environmentally friendly alternative to traditional anthraquinone oxidation processes. However, the low exciton dissociation efficiency at room temperature often hinders photocatalytic performance. In this study, it is demonstrated that tuning the substitution sites of electron donors in Donor-Acceptor (D-A) conjugated polymers can significantly enhance exciton dissociation by reducing exciton activation energy, which facilitates the spontaneous separation of excitons at room temperature. For comparison, materials with exciton separation energies ≈89 meV exhibit a hydrogen peroxide production rate of 2692 µmol·g-1·h-1. In contrast, the main material developed in this work, O-PTAQ, demonstrates a substantially lower exciton separation energy of 22 meV, resulting in a hydrogen peroxide production rate of 4989 µmol·g-1·h-1 under ambient conditions, outperforming most reported organic semiconductors. This enhancement is attributed to the increased electron delocalization in the electron donors, which lowers exciton activation energy to promote efficient exciton separation. The findings highlight the critical role of molecular-level structural tuning in enhancing exciton dissociation, providing a promising strategy for the development of high-efficiency photocatalysts for sustainable H2O2 production.

π-Bridge-Linked Ionic Covalent Organic Framework with Fast Reaction Kinetics for High-Rate-Capacity Lithium-Ion Batteries

Covalent organic frameworks (COFs) have emerged as promising cathode materials for high-performance lithium-ion batteries (LIBs) due to their well-defined topologies and tunable pore architectures. However, their practical application is often limited by intrinsically sluggish charge transfer and inferior reaction kinetics. To address these challenges, we develop an ionic quinoline-linked COF (iQCOF) cathode via a one-pot Povarov reaction with triazole ionic liquid. The iQCOF architecture achieves a synergistic enhancement by integrating π-bridge-induced charge delocalization to facilitate charge transport, the specific adsorption effect to gain fast ionic atmosphere dissociation rate, and polar triazine units to enable uniform ion flux for stable interfaces. As a result, iQCOF delivers a high specific capacity of 407 mAh g-1 with 701 Wh kg-1, and exceptional rate capability (121 mAh g-1 at 10 A g-1) with 0.0027% per cycle over 10 000 cycles, further highlighting its potential as a high-performance organic cathode. This work provides a convenient strategy for advanced COF-based cathodes with fast reaction kinetics to achieve high-rate performance, paving the way for next-generation energy storage technologies.

Nitrogen Modified Linear Polythiophene Derivatives with Polarized Charge Distribution for Red Light-Induced Photocatalysis

Elevating the long-wavelength activation of photocatalysts represents a formidable approach to optimizing sunlight utilization. Polythiophene (PTh), although renowned for its robust light absorption and excellent conductivity, is largely overlooked for its potential as a photocatalyst due to the swift recombination of photogenerated charge carriers. Herein, we unveil that the strategic introduction of an aromatic ring containing varying nitrogen content into PTh instigates polarized charge distribution and facilitates the narrowing of the band gap, thereby achieving efficient photocatalytic activities for both hydrogen and hydrogen peroxide generation. Notably, the best sample, PTh-N2, even demonstrates photocatalytic activity in the red light region (600-700 nm). This study offers a promising avenue for the development of polymer photocatalysts with efficient photocatalytic performance for red light-induced photocatalysis.

π-Stacked organic heterojunction enabled efficient hydrogen peroxide photoproduction

Hydrogen peroxide (H2O2) finds wide application in disinfection, chemical production, and bleaching owing to its sustainability and ease of storage compared to hydrogen. However, the current industrial engineering practices for producing photocatalysts used in H2O2 generation face challenges such as low yields with organic catalysts and high costs associated with inorganic catalysts. In light of these challenges, we investigate a viable method utilizing a simple condensation reflux technique to synthesize pure organic heterojunction photocatalysts featuring a π-π stacked structure derived from graphitized carbon nitride (melem) and meso-tetrakis (4-carboxyphenyl) porphyrin, enabling efficient H2O2 production without the necessity of sacrificial agents. The synthesized catalyst demonstrates an impressive H2O2 production rate of 106 mmol/h.g.L under seawater conditions and 77.67 mmol/h.g.L under pure water conditions in ambient air. Coupling to a solar evaporator, an H2O2 concentration reaches 0.88 wt%. Transient absorption spectroscopy revealed ultrafast charge separation which enabled efficient photocatalysis.

Linkage Engineering of Triazine-Based Covalent Organic Frameworks for Selective Photocatalytic Oxidation of Amines

Covalent organic frameworks (COFs) have garnered significant attention as versatile photocatalysts due to their tunable structure and activity. In this work, linkage engineering is applied to two triazine-based COFs, yielding TFPT-sp2c-COF, with a C═C linkage and TFPT-IM-COF, with a C═N linkage. TFPT-sp2c-COF exhibits better thermal stability and a significantly higher specific surface area than TFPT-IM-COF. More importantly, the C═C linkage in TFPT-sp2c-COF leads to better optoelectronic properties than the C═N linkage in TFPT-IM-COF, as it promotes efficient charge separation and transfer and π-delocalization, as evidenced by experiments and density functional theory calculations. Consequently, TFPT-sp2c-COF demonstrates higher activities than TFPT-IM-COF for selective photocatalytic oxidation of amines. Notably, a diverse range of amines achieve high conversions to corresponding imines with high selectivities. Superoxide, generated through photoexcited electron transfer to O2, is identified as the predominant reactive oxygen species. This work underscores the pivotal role of linkage engineering in optimizing COFs for selective reactions.

Singlet-Oxygen-Driven Cooperative Photocatalytic Coupling of Biomass Valorization and Hydrogen Peroxide Production Using Covalent Organic Frameworks

Traditional H2O2 photocatalysis primarily depends on photoexcited electrons and holes to drive oxygen reduction and water oxidation, respectively. However, singlet oxygen (1O2), often underappreciated, plays a pivotal role in H2O2 production. Meanwhile, photocatalytic biomass conversion has attracted attention, yet studies combining H2O2 synthesis with biomass valorization remain rare and typically yield low-value products. Herein, a strategy of photocatalytic valorization of furfuryl alcohol (FFA) coupled with the efficient co-production of H2O2 is reported, enabled by covalent organic frameworks (COFs) induced, 1O2-participated Achmatowicz rearrangement. This study introduces polyimide-based COF-N0-3 with tailored nitrogen content, representing an unprecedently efficient platform for 1O2 production. Remarkably, reducing the nitrogen content of the COF enhances 1O2 production, significantly boosting the H2O2 generation rate. In FFA, the primary pathway for H2O2 production is Achmatowicz rearrangement, achieving a rate ten times higher than that reliant on oxygen reduction reaction in pure water, reaching 4549 µmol g⁻¹ h⁻¹. Mechanism studies revealed 1O2 selectively engaged FFA, bypassing hole oxidation to trigger the Achmatowicz rearrangement, producing valuable 6-hydroxy-(2H)-pyranone with 99% conversion and 92% selectivity. This work establishes a coupling strategy for simultaneous synthesis of H2O2 and biochemicals, offering a transformative approach to sustainable photocatalysis.

Regulating Electron Distribution in Regioisomeric Covalent Organic Frameworks for Efficient Solar-Driven Hydrogen Peroxide Production

Covalent organic frameworks (COFs) are emerging as a transformative platform for photocatalytic hydrogen peroxide (H2O2) production due to their highly ordered structures, intrinsic porosity, and molecular tunability. Despite their potential, the inefficient utilization of photogenerated charge carriers in COFs significantly restrains their photocatalytic efficiency. This study presents two regioisomeric COFs, α-TT-TDAN COF and β-TT-TDAN COF, both incorporating thieno[3,2-b]thiophene moieties, to investigate the influence of regioisomerism on the excited electron distribution and its impact on photocatalytic performance. The β-TT-TDAN COF demonstrates a remarkable solar-to-chemical conversion efficiency of 1.35%, outperforming its α-isomeric counterpart, which is merely 0.44%. Comprehensive spectroscopic and computational investigations reveal the critical role of subtle substitution change in COFs on their electronic properties. This structural adjustment intricately connects molecular structure to charge dynamics, enabling precise regulation of electron distribution, efficient charge separation and transport, and localization of excited electrons at active sites. Moreover, this finely tuned interplay significantly enhances the efficiency of the oxygen reduction reaction. These findings establish a new paradigm in COF design, offering a molecular-level strategy to advance COFs and reticular materials toward highly efficient solar-to-chemical energy conversion.

Efficient Photosynthesis of Hydrogen Peroxide from Water and Air Over Water-Dispersible Anthraquinone-Based Porous Aromatic Frameworks

Photosynthesis of hydrogen peroxide from earth-abundant water and air over organo-based semiconducting materials is a promising alternative to the traditional anthraquinone (AQ) method. However, the generally hydrophobic nature of organic semiconductors has led to their poor dispersibilities in aqueous systems, which built huge barriers for photon capture and reactant contact in water-based photocatalysis. Aiming at this issue, this study reports the facile synthesis of AQ-based porous aromatic frameworks (AQ-PAFs) by coupling AQ fragments with thiophene-derived linkers via robust carbon-carbon bonds. Remarkably, the interfacial hydrogen bonding interactions between water molecules and AQ sites on the surface improve the general hydrophilicity of AQ-PAFs, which can be well-dispersed in water-only systems with uniform particle size distributions. Moreover, the AQ moieties also function as mediators of photoinduced electrons, and the protons produced from water oxidation reaction (WOR), which would kinetically favor the charge separation and subsequent electron transfer reactions. The mono-dispersed AQ-PAF photocatalyst promotes hydrogen peroxide (H2O2) photosynthesis from water and air under visible light achieving a high productivity of 7124 µmol g-1 h-1 in the absence of any organic alcohol reagents among organic semiconductor photocatalysts. Furthermore, a continuous H2O2 photosynthesis for 190 h is also achieved in a flow system.

Benzotrithiophene-based covalent organic frameworks for sensitive fluorescence detection and efficient removal of Ag+ from drinking water

The simultaneous detection and removal of Ag+ from drinking water was crucial for preventing human health, while it was also extremely challenging due to bifunctional materials that combine both Ag+ adsorption and detection functions rarely being explored. In this study, a benzotrithiophene-based covalent organic framework (TAPA-BTT) was synthesized and applied to detect and remove Ag+. TAPA-BTT exhibited high crystallinity, a large specific surface area, and good thermal stability. As a fluorescent probe, TAPA-BTT had a low detection limit (0.14 μg/L), wide linear range (0.2-700 μg/L), and good linearity (R2 > 0.9948). It was also successfully applied to identify Ag+ in drinking water including tap, pure, and mineral water with satisfactory detection performance. Moreover, TAPA-BTT had a high efficiency in removing Ag+ from water, offering a high capacity for adsorption (344.83 mg/g) and a removal rate of 99.45 %. The adsorption of TAPA-BTT towards Ag+ can be well explained by the quasi-second-order kinetic model and the Langmuir isotherm model. In addition, experimental and theoretical studies revealed the interaction mechanism between TAPA-BTT and Ag+. The specific Ag+ detection by TAPA-BTT was assumed to be caused by the electron transfer from thiophene-S to Ag+, which enhanced the fluorescence of TAPA-BTT. The effective removal of Ag+ was attributed to the co-chelation of imine-N and thiophene-S on TAPA-BTT. These novel findings revealed the great potential of benzotrithiophene-based COFs in the detection and removal of Ag+, providing a new strategy and alternative material for monitoring and controlling Ag+ in drinking water.

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.

Advances in single-molecule electrical transport studies of peptides

The charge transport between peptide molecules is one of the crucial factors in sustaining various biochemical processes within biological organisms. Understanding the charge transport processes between peptide molecules is of great significance for further investigating life reaction processes. Through single-molecule electronic characterization techniques, we have reviewed the effects of peptide molecular stuctures and external experimental factors on charge transport. Additionally, we have summarized the latest research on supramolecular interactions between peptide chains, particularly focusing on the even-odd effect. This not only enhances our understanding of the charge transport mechanisms between peptide molecules but also lays a solid theoretical foundation for the widespread application of peptide molecules in fields such as biological devices.

Remarkable increase in electrochemiluminescence of isomeric bipyridine-based covalent organic frameworks via regulating the direction of imine linkage for sensing application

Covalent organic frameworks (COFs) with highly ordered structures and predictable optoelectronic properties provide an ideal platform to investigate the electrochemiluminescence (ECL) performance based on organic materials by atomically varying the molecular construction. Herein, the effect of imine-bond orientation on the ECL performance of COFs is investigated. We report two COFs (NC-COFPy-Bpy and CN-COFPy-Bpy) with different orientations of imine bonds using pyrene donor units (D) and bipyridine acceptor motifs (A) monomers. The direction of the imine linkage (carbon of imine bonds as the D unit and nitrogen as the A unit) in NC-COFPy-Bpy oppose to the direction of donor and acceptor charge transfer between the molecular motifs. In contrast, CN-COFPy-Bpy is in the same direction (D-A type imine COFs). However, the NC-COFPy-Bpy demonstrates a superior ECL performance with 22.4-fold enhancement in ECL intensity compared to CN-COFPy-Bpy, due to the effective separation of the highest and lowest occupied molecular orbitals to facilitate intramolecular charge transfer (IRCT) for strong ECL. Moreover, the ECL intensity of NC-COFPy-Bpy remains higher stability than CN-COFPy-Bpy at low-excited positive potential with tri-n-propylamine (TPrA) as the coreactant. The experimental and modeling investigation indicate that the bimodal ECL patterns of NC-COFPy-Bpy were derived from the competitive oxidation mechanism of IRCT regulated: the co-reactant-mediated oxidation at lower potential and the direct oxidation at higher potential, but the CN-COFPy-Bpy only has a co-reactant pathway. Notably, the NC-COFPy-Bpy can be used to construct an ECL sensor for sensitive detection of doxorubicin (DOX). This research provides a protocol for the molecular design of COFs as ECL emitters with atomically regulated charge transfer direction in D-A systems.

Ether-Embedded Covalent Organic Frameworks Enable Efficient Photocatalytic CO2 Reduction

The photocatalytic conversion of carbon dioxide (CO2) into valuable solar fuels is a promising strategy for addressing energy crises and mitigating the greenhouse effect. However, the challenge of efficiently regulating photogenerated electrons to CO2 active sites remains a key hurdle for high-performance CO2 reduction. Herein, an embedded functional group, ether group is introduced into porphyrin-triazine COFs to regulate the transfer behavior of photogenerated electrons. The ether-embedded COFs (TOT-TAPP, BOD-TAPP and QOB-TAPP) demonstrate significantly faster charge transport and higher photoactivity compared with the corresponding non-ether-embedded counterpart COFs. The theoretical calculations and in situ characterizations reveal that the ether group could not only accelerate the separation of photogenerated charge carriers, but also lead to a more substantial accumulation of electrons at the CO2 adsorption region (C=N imine bond), thus greatly promoting the efficiency of CO2 photoreduction.

Building a Confluence Charge Transfer Pathway in COFs for Highly Efficient Photosynthesis of Hydrogen Peroxide from Water and Air

Sunlight-driven photosynthesis by covalent organic frameworks (COFs) from water and air without using sacrificial reagents is a promising H2O2 fabrication approach but is still restricted by the insufficient charge separation and sluggish 2e- water oxidation process. Herein, we provide a facile strategy to simultaneously improve charge separation and water oxidation in COFs via confining the charge transfer pathways from two diversion ones to a confluence one through regulating the site of nitrogen in bipyridine. Combining in-situ characterization with computational calculations, we reveal that compared to COF-BD1 containing two diversion charge transfer pathways, the charge transfer pathway in COF-BD2 is confined to a confluence due to the electron-deficiency effect of nitrogen, which greatly accelerates the intermolecular and out-of-plane charge transfer. Via effectively reducing the energy barrier of rate-determining water oxidation reaction (WOR), the subsequent water oxidation process to produce the key *OH intermediate in COF-BD2 is also greatly facilitated, boosting the yield of H2O2 (5211 µmol g-1 h-1) from water, oxygen, and light without sacrificial agents or additional energy consumption. We further demonstrate that H2O2 can be efficiently produced by COF-BD2 in a broad pH range, in real water, and in an enlarged reactor using natural sunlight for water decontamination.

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.

Linkages Chemistry of Covalent Organic Frameworks in Photocatalysis and Electrocatalysis

Covalent organic frameworks (COFs) have emerged as promising candidates for electrocatalysis and photocatalysis applications due to their structurally ordered architectures and tunable physicochemical properties. In COFs, organic building blocks are linked via covalent bonds, and the structural and electronic characteristics of COFs are critically governed by their linkage chemistry. These linkages influence essential material attributes including surface area, crystallinity, hydrophobicity, chemical stability, and the optoelectronic behavior (e.g., photoelectron separation efficiency, electron conductivity, and reductive activity), which collectively determine catalytic performance in energy conversion systems. A systematic understanding of linkage engineering in COFs not only advances synthetic methodologies but also provides innovative solutions to global energy and environmental challenges, thereby accelerating the development of sustainable technologies for clean energy production and environmental remediation.

Engineered covalent organic frameworks (COFs) for adsorption-based metal separation technologies: A critical review

Porous covalent organic frameworks (COFs) are promising materials used for separation and purification during environmental remediation. This critical review focuses on two key aspects. First, it critically examines strategies to improve COF design and structure and evaluates their impact on separation performance. Second, engineering approaches for enhancing the interactions between COF-based adsorbents and metals for enhanced separation and capture are elucidated. The latest body of research on separating metals (e.g., Li, K, Sr, Hg, Cd, Pb, Cr, Au, Ag, Pd, and U) using COF-based adsorbents is discussed to understand the factors that influence their performance. However, it is to be noted that COF-based adsorbents are still in their infancy and remain largley unexplored, mainly hindered by synthetic complexities and suboptimal crystalline structures. This highlights the need for further research and development to fully unlock the excellent potential of COFs for metal separation applications, particularly in environmental and energy applications.

Enhanced Charge Transfer in Poly(Heptazine Imide) Synergistically Induced by Donor-Acceptor Motifs and Ohmic Junctions for Efficient Photocatalytic CO2 Reduction

Poly(heptazine imide) (PHI) has received widely interest in the photocatalytic CO2 reduction due to its good crystallinity and complete in-plane structure. However, its poor photo-induced carrier separation and migration efficiency and insufficient active sites results in undesirable photocatalytic CO2 reduction performance. Herein, we designed and constructed a novel ohmic junction photocatalyst by integrating melamine edge-modified PHI (mel-PHI) with extended π-conjugated system with TiN (TiN/mel-PHI) for enhancing the photocatalytic CO2 reduction activity. Strikingly, the photocatalytic CO2 reduction yield of the optimal TiN/mel-PHI is 62.64 μmol g-1 h-1, which is 5.6 and 2.8 times higher than PHI (11.26 μmol g-1 h-1) and mel-PHI (22.32 μmol g-1 h-1), respectively. The superior photocatalytic CO2 reduction activity is attributed not only to the formation of D-A structure by the introduction of melamine, which extends the π-conjugation system, alters the electronic structure of PHI, and accelerates the charge separation and migration, but also to the induced internal electric field by ohmic junction further enhances the charge separation and migration efficiency. Meanwhile, the synergistic effect of mel-PHI and TiN enriched the electron number of TiN, reducing the CO2 reduction potential. This work highlights the synergistic enhancement of charge transfer between D-A motifs and ohmic junctions, confirming their potential in optimizing photocatalysts.

Protonation of Nitrogen-Containing Covalent Organic Frameworks for Enhanced Catalysis

Covalent organic frameworks (COFs) are a class of porous crystalline materials with ordered structures and tunable properties, which have been widely explored in catalysis, sensing, gas storage, and separation. Among various post-synthetic modifications, protonation emerges as a simple yet effective strategy to fine-tune the properties of nitrogen-containing COFs, thereby enhancing their catalytic performance. This concept article highlights the contribution of protonation on the mass transfer kinetics, charge distribution, photo-response, charge transfer, and other properties related to photocatalysis and electrocatalysis. The applications of protonated COFs are explored in catalytic processes including hydrogen evolution, CO2 reduction, H2O2 synthesis, and singlet oxygen generation. We also emphasize the necessity of considering the protonation process when nitrogen-containing COFs are applied in acidic environments to accurately reveal the structure-activity relationship. By analyzing recent advancements in protonated COFs, this article underscores the potential and challenges of protonation as a powerful tool for advancing COF-based catalytic systems.

Functional Groups-Regulated Organic Semiconductors for Efficient Artificial Photosynthesis of Hydrogen Peroxide

Hydrogen peroxide (H2O2) is an environmentally friendly reagent, and organic semiconductors (OSCs) are ideal photocatalysts for the synthesis of H2O2 due to their well-defined molecular structure, strong donor-acceptor interactions, and efficient charge separation. This review discusses the regulatory mechanisms of functional group modifications in tuning the photocatalytic performance of OSCs, highlighting the relationship between functional group structure and catalytic performance. For example, electron-regulating groups, such as cyano and halogen, induce molecular dipoles, facilitating the migration of photogenerated electrons. Fluorine groups optimize the band structure and prolong carrier lifetime due to their high electronegativity. π-Conjugated extension groups, like anthraquinone and thiophene, expand conjugation, improve visible light capture, and stabilize intermediates through redox cycles. Hydroxyl groups enhance surface hydrophilicity and promote H2O activation, while imine bond protonation adjusts charge distribution and improves selectivity and cycle stability. Multi-active site functional groups, such as sulfonic acid and amide, accelerate reaction kinetics and inhibit H2O2 decomposition. Functional groups enhance light absorption, charge separation, and surface reactions through electronic structure regulation, intermediate adsorption optimization, and proton-electron transfer. Future work should integrate machine learning to identify optimal functional group combinations and develop green functionalization strategies for efficient H2O2 photocatalyst synthesis.

p-π Conjugated Covalent Organic Frameworks Expedite Molecular Triplet Excitons for H2O2 Production Coupled with Biomass Upgrading

High-efficiency production of triplet states in covalent organic framework photocatalysts is crucial for high-selectivity oxygen (O2) reduction to hydrogen peroxide (H2O2). Herein, fluorine and partial fluorine atoms are incorporated into an olefin-linked triazine covalent organic framework (F-ol-COF and HF-ol-COF), in which the adjacent fluorine (F) atoms-olefinic bond forms p-π conjugation that induces spin-polarization under irradiation, thus expediting triplet excitons for activating O2 to singlet oxygen (1O2) and contributing to a high H2O2 selectivity (91%). Additionally, the feasibility of coupling H2O2 production with the valorization of 5-hydroxymethylfurfural (HMF) is exhibited. The F-ol-COF demonstrates a highly stable H2O2 yield rate of 12558 µmol g-1 h-1 with the HMF-to-functionalized furan conversion yield of 95%, much higher than the partially fluorinated COF (HF-ol-COF) and the non-fluorinated COF (H-ol-COF). Mechanistic studies reveal that F-incorporation promotes charge separation, intensifies the Lewis acidity of the carbon atoms on the olefinic bond as active sites for O2 adsorption, and provides highly concentrated holes at the triazine unit for HMF oxidation upgrading. This study suggests the attractive potential of rational design of porous-crystalline photocatalysts for high-efficiency photocatalytic O2 reduction to H2O2 and biomass upgrading.

Efficient Photocatalytic Synthesis of Hydrogen Peroxide Facilitated by Triptycene-Based 3D Covalent Organic Frameworks

Covalent organic frameworks (COFs) are widely studied for hydrogen peroxide (H₂O₂) photosynthesis, with 3D COFs standing out for their porous structures and chemical stability. However, the difficult preparation of 3D COFs and the low efficiency in separating photo-generated electrons and holes (e- and h+) limits the efficient production of H2O2. In this study, two kinds of [6+3] 3D COFs (XJU-1, XJU-2) with significant charge separation, achieving record-breaking H₂O₂ photocatalysis rates of 34 777 and 11 922 µmol g⁻¹ h⁻¹, respectively. XJU-1's superior efficiency stems from its larger pores, enhancing material transport and oxygen (O2) activation. Experimental and theoretical studies have demonstrated that triptycene monomers achieve significant charge separation toward triazine via imine bonds. Moreover, the dimer's smaller singlet-triplet energy gap (∆ES-T) and triptycene's orthogonal configuration enhance singlet oxygen (1O2) production, enabling multiple H2O2 generation pathways. Ultimately, through the oxygen reduction reaction (ORR) pathway, rapid generation of H2O2 can be achieved at multiple catalytic sites. XJU-1 mainly follows a mixed pathway involving 1e--ORR and 2e--ORR, and XJU-2 primarily follows the 2e--ORR pathway, respectively. These open the door of triptycene-based 3D COFs applied in continuous, efficient, and stable photosynthesis of H2O2.

Modulating N-Heterocyclic Microenvironment in β-Ketoenamine Covalent Organic Frameworks to Boost Overall Photosynthesis of H2O2

Covalent Organic Frameworks (COFs) have emerged as promising platforms for photocatalytic synthesis of hydrogen peroxide (H2O2) due to their tunable chemical compositions and efficient catalytic functionalities. Inspired by the role of the microenvironment in enzyme catalysis, this study introduces various N-heterocyclic species into β-ketoenamine COFs (Nx-COFs, where Nx represents the number of nitrogen atoms in the N-heterocycle) to regulate the microenvironment around catalytic sites on acceptor-donor-acceptor (A-D-A) COFs foroverall H2O2 photosynthesis in pure water. The Nx-COFs exhibit distinct H2O2 photosynthetic rates following the number of nitrogen atoms sequence of N3-COF > N2-COF > N1-COF > N0-COF, with N3-COF with triazine structure showing the highest H2O2 generation rate (4881 µmol h-1 g-1) and the decent solar-to-chemical conversion (SCC) efficiency (0.413%), surpassing many existing COF-based catalysts. In situ characterization and theoretical calculations support the experimental results, revealing that N-heterocyclic species promote the photosynthesis of H2O2 through both an indirect stepwise single-electron oxygen reduction reaction (1e- ORR) mechanism and a direct two-electron water oxidation (2e- WOR) pathway. This study advances the design paradigm of photocatalysts by modulating the microenvironment within A-D-A COFs, paving the way for the development of more efficient and robust photocatalytic systems for the overall photosynthesis of H2O2.

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

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

Efficient Photocatalytic Water Purification Through Novel Janus-Nanomicelles with Long-Lived Charge Separation Properties

Although the design of photocatalysts incorporating donor-acceptor units has garnered significant attention for its potential to enhance the efficiency of the photocatalysis process, the primary bottleneck lies in the challenge of generating long-lived charge separation states during exciton separation. Therefore, a novel Janus-nanomicelles photocatalyst is developed using carbazole (Cz) as the donor unit, perylene-3,4,9,10-tetracarboxydiimide (PDI) with long-excited state as the acceptor unit and polyethylene glycol (PEG) as the hydrophilic segment through ROMP polymerization. After optimizing the ratio, Cz19-PDI18-PEG10 rapidly adsorbs bisphenol A (BPA) within 10 s through π-π interaction, hydrogen-bonding interaction, and hydrophobic interaction between BPA and hydrophobic blocks when exposed to aqueous humor and efficiently photodegrades BPA (50 ppm) within 120 min for water purification purposes due to its long-lived charge separation state and achieving the highest reported efficiency so far. Mechanistic studies have shown that this excellent performance of Cz19-PDI18-PEG10 can be attributed to synergistic interactions between highly efficient adsorption capacity and long-lived charge separation states during photocatalysis. This novel Janus-nanomicelles design strategy holds promise as an effective candidate for water purification.

Engineering fluorene-based covalent organic framework photocatalysts toward efficient and selective aerobic oxidation of amines

Covalent organic frameworks (COFs) have attracted significant interest due to diverse applications, relying on their versatile molecular building blocks like fluorenes. However, the twisted structures of fluorenes pose substantial challenges for the construction of porous crystalline materials like COFs. Here, the couplings of 1,3,5-triformylphloroglucinol (Tp) with 9H-fluorene-2,7-diamine (DAF), 9,9-dimethyl-9H-fluorene-2,7-diamine (MFC) and 9,9-difluoro-9H-fluorene-2,7-diamine (FFC) with a pyrrolidine catalyst afford three fluorene-based COFs, TpDAF-COF, TpMFC-COF and TpFFC-COF, respectively. The resulting COFs, with distinct functional groups, exhibit high crystallinity and porosity. Optoelectronic tests reveal that TpFFC-COF demonstrates the most intense photocurrent density and the lowest interfacial charge transfer resistance. When applied to the selective aerobic oxidation of amines to imines, the efficiency follows the order of TpFFC-COF > TpMFC-COF > TpDAF-COF, consistent with the observed optoelectronic properties. Additionally, the TpFFC-COF photocatalyst showcases excellent reusability and broad applicability. This work illuminates the potential of engineering COFs with functional groups toward efficient photocatalysts.

Vinylene-Linking of Polycyclic Aromatic Hydrocarbons to π-Extended Two-Dimensional Covalent Organic Framework Photocatalyst for H2O2 Synthesis

Polycyclic aromatic hydrocarbons (PAHs) hold the predominant role either as individual molecules or building blocks in the field of organic semiconductors or nanocarbons. Connecting PAHs via sp2-carbon bridges to form high-crystalline π-extended structures is highly desired not only for enlarging the regimes of two-dimensional materials but also for achieving exceptional properties/functions. In this work, we developed 5,10-dimethyl-4,9-diazapyrene as a key monomer, whose two methyl groups at the positions adjacent to nitrogen atoms, can helpfully increase the solubility, and serve as the active connection sites. In the presence of organic acids, this monomer enables smoothly conducting Knoevenagel condensation to form two vinylene-linked PAH-cored COFs, which show high-crystalline honeycomb structures with large surface areas up to 1238 m2 g-1. Owing to the direct connection mode of PAH building blocks with vinylene, the as-prepared COFs possess spatially extended π-conjugation and substantial semiconducting properties. Consequently, their visible-light photocatalysis with exceptional activity and durability was manifested to generate H2O2 up to 3820 μmol g-1 h-1 in pure water, and even 17080 μmol g-1 h-1 using benzyl alcohol as a hole sacrificial agent.

Urea/Thiourea Imine Linkages Provide Accessible Holes in Flexible Covalent Organic Frameworks and Dominates Self-Adaptivity and Exciton Dissociation

Unraveling the robust self-adaptivity and minimal energy-dissipation of soft reticular materials for environmental catalysis presents a compelling yet unexplored avenue. Herein, a top-down strategy, tailoring from the unique linkage basis, flexibility degree, skeleton electronics to trace-guest adaptability, is proposed to fill the understanding gap between micro-soft covalent organic frameworks (COFs) and photocatalytic performance. The thio(urea)-basis-dominated linkage within benzotrithiophene-based COFs induce the framework contraction/swelling (intralayer micro-flexibility) in response to tetrahydrofuran or water. Adaptability of micro-flexible thiourea-COF with pore hydrophilicity not only contributes to the favorable mass transfer, but also enhances the accessible redox active sites, culminating in nearly 100 % removal of micropollutant with low-energy dissipation in wastewater. The incorporating urea/thiourea into imine linkage facilitates polarization reduction and exciton dissociation within skeleton wall, inducing strong localization for holes. This transformation facilitates interchain charge transport and unbalanced distribution conducive to oxidative holes-mediated micropollutant decomposition.

Covalent Organic Frameworks for Boosting H2O2 Photosynthesis via the Synergy of Multiple Charge Transfer Channels and Polarized Field

Covalent organic frameworks (COFs) serve as one of the most promising candidates for hydrogen peroxide (H2O2) photosynthesis, while attaining high-performance COFs remains a formidable challenge due to the insufficient separation of photogenerated charges. Here, through the rational design of bicarbazole-based COFs (Cz-COFs), we showcase the first achievement in piezo-photocatalytic synthesis of H2O2 using COFs. Noteworthily, the ethenyl group-modified Cz-COFs (COF-DH-Eth) demonstrates a record-high yield of H2O2 (9212 μmol g-1 h-1) from air and pure water through piezo-photocatalysis, which is ca. 2.5 times higher than that of pristine Cz-COFs without ethenyl groups (COF-DH-H) under identical condition and COF-DH-Eth without ultrasonic treatment. The H2O2 production rate originates from the synergistic effect between an ultrasonication-induced polarized electric field and the spatially separated multiple charge transfer channels, which significantly promote the utilization of photogenerated electrons by directional transfer from bicarbazole groups to the ethenyl group-modified benzene rings. Several Cz-COFs and bifluorenylidene-based COFs (COF-BFTB-H) with similar twisted monomers exhibit obvious piezoelectric performance for promoting H2O2 generation, signifying that organic ligands with a twistable structure play a crucial role in creating broken symmetry structures, thereby establishing piezoelectric properties in COFs.

Induced Charge-Compensation Effect for Boosting Photocatalytic Water Splitting in Covalent Organic Frameworks

Imine-based covalent organic frameworks (COFs) are promising for photocatalytic water splitting, but their performance is often constrained by inefficient charge separation due to the high electron localization nature of polar imine bonds. In this study, we have optimized the electron delocalization across the imine linkage within a COF by implementing a charge compensation effect. This effect is achieved when a strong electron-donating thieno[3,2-b]thiophene linker is directly attached to the iminic carbon of a zinc-porphyrinic COF. This modification significantly reduces the electron binding effect within the imine bonds of the COF, facilitating both in-plane charge separation and out-plane charge transfer to the catalytic site. Conversely, the use of strong electron-withdrawing pyrizine linker aggravates the electron localization at the imine linkage in the ZnP-Pz variant. Consequently, ZnP-Tt shows a substantially improved photocatalytic water-splitting activity under visible light irradiation, with a hydrogen evolution of 44288±2280 μmol g-1 in 4 h, which exceeds the ZnP-Pz counterpart by a factor of 10. These results offer fresh perspectives for the design of imine-based COFs to overcome their limitations in charge separation efficiency.

Donor-Acceptor Type Carbon Nitride Photocatalysts in Photocatalysis: Current Understanding, Applications and Challenges

The exploration of photocatalytic materials with efficient charge separation has always been a prominent area of research in photocatalysis. In the preceding years, the strategy of constructing donor-acceptor (D-A) structured materials has gradually been developed in photocatalytic systems, becoming a new research crossroads and attracting extensive interdisciplinary focus. Polymeric carbon nitride (PCN) has gradually been recognized as the primary photocatalytic material for constructing D-A structures due to its attractive exceptional physicochemical stability, electronic band structure, and cost-effectiveness. However, few reports have summarized the research on D-A type PCN at the molecular level. This review focuses on the molecular level design strategies and the performance enhancement mechanisms of D-A type PCN are emphasized from two main aspects: intramolecular D-A and intermolecular D-A. In addition, the concept of dual D-A type PCN is introduced, proposing a new direction for advancing the efficacy of photocatalysts. Subsequently, this review summarizes the application scenarios of D-A type PCN in energy transformation and environmental amelioration. Finally, by elaborating on the main difficulties and opportunities in this hot field, this review can provide inspiration and innovative strategies for developing D-A type PCN and resolving energy and environmental issues.

Customized Design of Covalent Organic Framework with Asymmetric Dual-Function Hybrid Linkages for Promoting H2O2 Photosynthesis in Air and Water

Efficient sacrificial-agent-free photosynthesis of H2O2 from air and water represents the greenest, lowest-cost, most real-time avenue for H2O2 production but remains a challenging issue. Here, we show a general and effective approach through a structural design on covalent organic frameworks (COFs) with asymmetric dual-function hybrid linkages for boosting the H2O2 photosynthesis of the COFs. Through such design we can equip a COF with not only a catalytic active center but also a special function for isolating the D-A motif, which consequently endows the COF (CI-COF) built on asymmetric dual-function hybrid linkages with a significantly enhanced efficiency in the generation, transmission, and separation of photogenerated carriers, relative to the COF (II-COF and CC-COF) built on symmetric single-function single linkages. Correspondingly, the performance of H2O2 photosynthesis is enhanced by three or five times. Accompanied is the largely promoted O2 utilization and conversion efficiency from 36.6% to 99.9%. A rare dual-channel H2O2 photosynthesis is suggested for the CI-COF.

Electron Push-Pull Effect of Benzotrithiophene-Based Covalent Organic Frameworks on the Photocatalytic Degradation of Pharmaceuticals and Personal Care Products

A covalent organic framework (COF) has emerged as a promising photocatalyst for the removal of pharmaceutical and personal care product (PPCP) contaminants; however, high-performance COF photocatalysts are still scarce. In this study, three COF photocatalysts were successfully synthesized by the condensation of benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-tricarbaldehyde (BTT) with 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT), 1,3,5-Tris(4-aminophenyl)benzene (TAPB), and 4,4',4''-nitrilotris(benzenamine) (TAPA), namely, BTT-TAPA, BTT-TAPB, and BTT-TAPT, respectively. The surface areas of BTT-TAPA, BTT-TAPB, and BTT-TAPT were found to be 800.46, 1203.60, and 1413.58 cm2∙g-1, respectively, providing abundant active sites for photocatalytic reactions. Under visible-light irradiation, BTT-TAPT exhibited the highest removal rate of tetracycline (TC), reaching 82.7% after 240 min. The superior photocatalytic performance of BTT-TAPT was attributed to its large specific surface area and the strong electron-acceptor properties of the triazine group. Electron paramagnetic resonance capture experiments and liquid chromatograph mass spectrometer analysis confirmed that superoxide radicals played a pivotal role in the degradation of TC and ciprofloxacin. Moreover, BTT-TAPT exhibited high stability and reproducibility during the photocatalytic degradation process. This study confirms that BTT-based COFs are a class of promising photocatalysts for the degradation of PPCPs in water, and their performance can be further optimized by tuning the structure and composition of the frameworks.

Boosting Exciton Dissociation and Charge Transfer in Fluorine-Substituted Covalent Organic Frameworks with Biomimetic Electron Pumps for Remarkable Photocatalytic Extraction of Uranium

Visible-light-driven photocatalytic uranium extraction based covalent organic frameworks (COFs) are green and sustainable, but their performance is severely restricted by a strong exciton effect. Herein, inspired by the physiology of cardiac pacing, a novel fluorine-based COF (PyF-DaS-COF) with a biomimetic electronic pump has been fabricated and used for the photocatalytic extraction of uranium. Both experimental and theoretical calculations confirm that strongly electronegative fluorine plays a crucial role in exciton dissociation and charge transfer. The enhanced electron push-pull effect increases the intrinsic separation driving force of charge separation. Furthermore, fluorine substitution thermodynamically favors the generation of the crucial *OOH intermediate in the uranium reduction reaction. As a result, the PyF-DaS-COF achieves a record k value (T = 293 K) of 0.082 min-1 with an extraction capacity of 991.5 mg g-1. Importantly, PyF-DaS-COF achieves a removal rate of over 99% in real uranium-containing wastewater. The current work provides unique insights into designing novel and effective COFs for controlling radioactive contamination.

Rational Construction of Cyanide-Functionalized D-A-π-D Covalent Organic Framework for Highly Efficient Overall H2O2 Photosynthesis from Air and Water

Sacrificial-agent-free overall photosynthesis of H2O2 from water and air represents currently a promising route to reform the industrial anthraquinone production manner, but, still blocks by the requirement of pure O2 feedstock, due to the insufficient oxygen supply from water under air. Herein, we report a rational molecule design on COFs (covalent organic frameworks) equiped with cyanide-functionalized D-A-π-D system for highly efficient overall H2O2 production from air and water through photocatalytic oxygen reduction reactions (ORR) and water oxidation reaction (WOR). Without using any sacrificial agent, the as-synthesized D-A-π-D COF is found to enable a H2O2 production rate as high as 4742 μmol h-1 g-1 from water and air and an O2 utilization and conversion rate up to 88 %, exceeding the other D-A-π-A COF by respectively 1.9- and 1.3-fold. Such high performance is attributed to the tuned electronic structure and prolonged charge lifetime facilitated by the unique D-A-π-D structure and cyanide groups. This work highlights a fundamental molecule design on advanced photocatalytic COFs with complicated D-A system for low-cost and massive H2O2 production.

Enhancing photocatalytic hydrogen peroxide generation by tuning hydrazone linkage density in covalent organic frameworks

The conversion of solar energy into chemical energy or high-value chemicals has attracted considerable research interest in the context of the global energy crisis. Hydrogen peroxide (H2O2) is a versatile and powerful oxidizing agent widely used in chemical synthesis and medical disinfection. H2O2 also serves as a clean energy source in fuel cells, generating electricity with zero-carbon emissions. Recently, the sustainable production of H2O2 from water and oxygen using covalent organic frameworks (COFs) as photocatalysts has attracted considerable attention; however, systematic studies highlighting the role of linkages in determining photocatalytic performance are scarce. Under these circumstances, herein, we demonstrate that varying the imine and hydrazone linkages within the framework significantly influences photocatalytic H2O2 production. COFs with high-density hydrazone linkages, providing optimal docking sites for water and oxygen, enhance H2O2 generation activity (1588 μmol g-1 h-1 from pure water in the air), leading to highly efficient solar-to-chemical energy conversion.

Designed Synthesis of Mesoporous sp2 Carbon-Conjugated Benzothiadiazole Covalent Organic Frameworks for Artificial Photosynthesis of Hydrogen Peroxide

Artificial photosynthesis of hydrogen peroxide (H2O2) from ambient air, water, and sunlight has attracted considerable attention recently. Despite being extremely challenging to synthesis, sp2 carbon-conjugated covalent organic frameworks (COFs) can be powerful and efficient materials for the photosynthesis of H2O2 due to desirable properties. Herein, we report the designed synthesis of an sp2 carbon-conjugated COF, BTD-sp2c-COF, from benzothiadiazole and triazine units with high crystallinity and ultralarge mesopores (∼4 nm). The sp2 carbon-conjugated skeletons guarantee BTD-sp2c-COF superior optoelectronic properties and chemical stability. BTD-sp2c-COF exhibits an exceptional efficiency of 3066 μmol g-1 h-1 from pure water and air, much better than that of BTD-imine-COF. In contrast, the resilience of BTD-imine-COF is compromised due to the participation of imine linkages in the oxygen reduction reaction. Importantly, in situ characterization and theoretical calculation results reveal that both benzothiadiazole and triazine units serve as oxygen reduction reaction centers for H2O2 photosynthesis through a sequential electron transfer pathway, while the vinylene bridged phenyls serve as water oxidation reaction centers. The sp2 carbon-conjugated COFs pave the way for potent artificial photosynthesis.

A Review on Photocatalytic Hydrogen Peroxide Production from Oxygen: Material Design, Mechanisms, and Applications

Hydrogen peroxide (H2O2) finds extensive applications in various industries, particularly in the environmental field. The photocatalytic production of H2O2 through the oxygen reduction reaction (ORR) or the water oxidation reaction (WOR) offers a promising approach. However, several challenges hinder effective on-site production, such as the rapid electron-hole pair recombination, inefficient visible light utilization, and limited selectivity in H2O2 formation. Thus, developing efficient photocatalysts to overcome these challenges is crucial. This review comprehensively outlines the development of photocatalysts and their modification techniques. It also summarizes and compares the H2O2 yield and apparent quantum yield among various photocatalysts with and without the use of organic sacrificial reagents. Density functional theory (DFT) calculations propose the band structure of photocatalysts and the mechanisms underlying oxygen reduction to H2O2. Finally, this review explores the potential environmental applications of photocatalytically produced H2O2. This review guides the design and optimization of photocatalysts, facilitating the continued advancement and application of photocatalysts in environmental contexts.

Isomerization of Covalent Organic Frameworks for Efficiently Activating Molecular Oxygen and Promoting Hydrogen Peroxide Photosynthesis

Constitutional-isomerized covalent organic frameworks (COFs), constructed by swapping monomers around imine bonds, have attracted attention for their distinct optoelectronic properties, which significantly impact photocatalytic performance. However, limited research has delved into the inherent relationship between isomerization and the enhancement of H2O2 photosynthesis. Herein, a pair of isomeric COFs linked by imine bonds (PB-PT-COF and PT-PB-COF) is synthesized, and it is proved that isomeric COFs exhibit different rate-determining steps in the generation process of H2O2, resulting in a twofold increase in photocatalytic efficiency. Specifically, PT-PB-COF demonstrates effective adsorption and activation of molecular oxygen (O2 + e- → •O2 - + e- → H2O2), leading to a significant improvement in H2O2 photocatalytic efficiency. In contrast, PB-PT-COF exhibits robust interaction with H2O, enabling direct oxidation of H2O (H2O + h+ → H2O2). This study provides a thorough understanding of the intrinsic mechanism underlying the constitutional-isomerized COFs in the photocatalytic H2O2 generation, offering insights for further optimizing building units.

Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Evolution

Covalent organic frameworks (COFs) are considered advanced class materials due to their exotic structural and optical properties. The abundance of starting monomers with variable linkage motifs may give rise to multiple conformations in either 2D or 3D fashion. Tailoring of the abovementioned properties has facilitated the application of COFs in a wide range of applications, which are strongly correlated with energy conversion schemes. Having a crystalline porous character and a large set of donor-acceptor combinations, COFs are expected to make huge impact in photocatalytic processes. In this Review, we present the recent advances in the development of semiconducting COF-based systems towards the photocatalytic hydrogen peroxide evolution. An overview is given about the effect of various parameters on the photocatalytic performance, such as charge transfer tuning, wettability by chemical functionalization, topology, porosity and crystallinity. Various challenges are discussed, and constructive insights are given for the development of highly functional COF-based photocatalysts for H2O2 evolution.

Pairing Oxygen Reduction and Water Oxidation for Dual-Pathway H2O2 Production

Hydrogen peroxide (H2O2) is a crucial chemical applied in various industry sectors. However, the current industrial anthraquinone process for H2O2 synthesis is carbon-intensive. With sunlight and renewable electricity as energy inputs, photocatalysis and electrocatalysis have great potential for green H2O2 production from oxygen (O2) and water (H2O). Herein, we review the advances in pairing two-electron O2 reduction and two-electron H2O oxidation reactions for dual-pathway H2O2 synthesis. The basic principles, paired redox reactions, and catalytic device configurations are introduced initially. Aligning with the energy input, the latest photocatalysts, electrocatalysts, and photo-electrocatalysts for dual-pathway H2O2 production are discussed afterward. Finally, we outlook the research opportunities in the future. This minireview aims to provide insights and guidelines for the broad community who are interested in catalyst design and innovative technology for on-site H2O2 synthesis.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Double enhancement of protonation and conjugation in donor-imine-donor covalent organic frameworks for photocatalytic hydrogen evolution

Covalent organic frameworks (COFs) have emerged as highly promising platforms for photocatalytic water splitting. However, exploring the structure-activity relationships in different COF systems remains challenging. In this study, three donor-imine-donor (D-I-D) COFs as relatively pure model materials were carefully selected to investigate the effect of protonation and conjugation on the mechanism of photocatalytic H2 evolution. Unlike widely reported donor-acceptor (D-A) COF systems, these three ideal COFs have short electronic channels and lack chemical bond isomerism and heteroatoms in building blocks. These aspects are beneficial for a comprehensive investigation of the underlying mechanisms at the active sites of the imine bond. Both the calculation and experimental results indicate that increasing the conjugation intensity can enhance the efficiency of exciton dissociation and charge transfer rates. Protonation can also dominantly enhance the light absorption capacity and electron transport efficiency of D-I-D COFs. After protonation, the Py-hCOF with optimal conjugation intensity exhibits a remarkable H2 evolution rate of 44.2 mmol g-1 h-1 under visible light, which is 88.4 times higher than that of Tpe-hCOF. This result highlights the crucial roles of simultaneous enhancement of the protonation and conjugation in improving photocatalytic hydrogen evolution of COFs, providing valuable insights for the design of COF materials to achieve the superior electronic functions in photocatalysis.

Engineering Polyheptazine and Polytriazine Imides for Photocatalysis

As organic semiconductor materials gain increasing prominence in the realm of photocatalysis, two carbon-nitrogen materials, poly (heptazine imide) (PHI) and poly (triazine imide) (PTI), have garnered extensive attention and applications owing to their unique structure properties. This review elaborates on the distinctive physical and chemical features of PHI and PTI, emphasizing their formation mechanisms and the ensuing properties. Furthermore, it elucidates the intricate correlation between the energy band structures and various photocatalytic reactions. Additionally, the review outlines the primary synthetic strategies for constructing PHI and PTI, along with characterization techniques for their identification. It also summarizes the primary strategies for enhancing the photocatalytic performance of PHI and PTI, whose advantages in various photocatalytic applications are discussed. Finally, it highlights the promising prospects and challenges of PHI and PTI as photocatalysts.

Leveraging Soft Acid-Base Interactions Alters the Pathway for Electrochemical Nitrogen Oxidation to Nitrate with High Faradaic Efficiency

Electrocatalytic nitrogen oxidation reaction (N2OR) offers a sustainable alternative to the conventional methods such as the Haber-Bosch and Ostwald oxidation processes for converting nitrogen (N2) into high-value-added nitrate (NO3 -) under mild conditions. However, the concurrent oxygen evolution reaction (OER) and inefficient N2 absorption/activation led to slow N2OR kinetics, resulting in low Faradaic efficiencies and NO3 - yield rates. This study explored oxygen-vacancy induced tin oxide (SnO2-Ov) as an efficient N2OR electrocatalyst, achieving an impressive Faradaic efficiency (FE) of 54.2% and a notable NO3 - yield rate (22.05 µg h-1 mgcat -1) at 1.7 V versus reversible hydrogen electrode (RHE) in 0.1 m Na2SO4. Experimental results indicate that SnO2-Ov possesses substantially more oxygen vacancies than SnO2, correlating with enhanced N2OR performance. Computational findings suggest that the superior performance of SnO2-Ov at a relatively low overpotential is due to reduced thermodynamic barrier for the oxidation of *N2 to *N2OH during the rate-determining step, making this step energetically favorable than the oxygen adsorption step for OER. This work demonstrates the feasibility of ambient nitrate synthesis on the soft acidic Sn active site and introduces a new approach for rational catalyst design.

Impact of Imine Bond Orientations and Acceptor Groups on Photocatalytic Hydrogen Generation of Donor-Acceptor Covalent Organic Frameworks

Covalent organic frameworks (COFs) have emerged as one of the most studied photocatalysts owing to their adjustable structure and bandgaps. However, there is limited research on regulating the light-harvesting capabilities of acceptor building blocks in donor-acceptor (D-A) isomer COFs with different bond orientations. This investigation is crucial for elucidating the structure-property-performance relationship of COF photocatalysts. Herein, a series of D-A isostructural COFs are synthesized with different imine bond orientations using benzothiadiazole and its derivatives-based organic building units. Extended light absorption is achieved in COFs with acceptor groups that have strong electron-withdrawing capacities, although this resulted a decreased hydrogen generation efficiency. Photocatalytic experiments indicated that dialdehyde benzothiadiazole-based COFs, HIAM-0015, exhibit the highest hydrogen generation rate (17.99 mmol g-1 h-1), which is 15 times higher than its isomer. The excellent photocatalytic performance of HIAM-0015 can be attributed to its fast charge separation and migration. This work provides insights into the rational design and synthesis of D-A COFs to achieve efficient photocatalytic activity.

Reticular Materials for Photocatalysis

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