2D sp2 Carbon-Conjugated Covalent Organic Frameworks for Photocatalytic Hydrogen Production from Water

Computation-based regulation of excitonic effects in donor-acceptor covalent organic frameworks for enhanced photocatalysis

The strong excitonic effects widely exist in polymer-semiconductors and the large exciton binding energy (E_b) seriously limits their photocatalysis. Herein, density functional theory (DFT) calculations are conducted to assess band alignment and charge transfer feature of potential donor-acceptor (D-A) covalent organic frameworks (COFs), using 1,3,5-tris(4-aminophenyl)triazine (TAPT) or 1,3,5-tris(4-aminophenyl)benzene (TAPB) as acceptors and tereph-thaldehydes functionalized diverse groups as donors. Given the discernable D-A interaction strengths in the D-A pairs, their E_b can be systematically regulated with minimum E_b in TAPT-OMe. Guided by these results, the corresponding D-A COFs are synthesized, where TAPT-OMe-COF possesses the best activity in photocatalytic H₂ production and the activity trend of other COFs is associated with that of calculated E_b for the D-A pairs. In addition, further alkyne cycloaddition for the imine linkage in the COFs greatly improves the stability and the resulting TAPT-OMe-alkyne-COF with a substantially smaller E_b exhibits ~20 times higher activity than the parent COF.

Structural Regulation of Thiophene-Based Two-Dimensional Covalent Organic Frameworks toward Highly Efficient Photocatalytic Hydrogen Generation

Two imine-based 2D covalent organic frameworks (COFs) with slight differences in their core structures are presented. The COF containing benzotrithiophene moieties with better planarity and π-conjugation (BTTh-TZ-COF) shows much better photocatalytic activity than the COF with trithienylbenzene cores (TThB-TZ-COF). Further photoelectrochemical study reveals the catalytic mechanism in more detail. Since other factors such as crystallinity, porosity, and optical bandgaps are equal, the different structures of the cores in the two similar COFs are the major contributors to the significantly different photocatalytic performance. The better electron delocalization of the planar trithiophene-based core and the enhanced D-A interactions between the triazine and trithiophene units in BTTh-TZ-COF create efficient charge separation and transfer, thus leading to superior photocatalytic hydrogen evolution activity. A new strategy for preparing high-performance organic photocatalysts for solar-energy conversion is revealed by this study.

Regulating π-Conjugation in sp2 -Carbon-Linked Covalent Organic Frameworks for Efficient Metal-Free CO2 Photoreduction with H2 O

The development of sp² -carbon-linked covalent organic frameworks (sp² c-COFs) as artificial photocatalysts for solar-driven conversion of CO₂ into chemical feedstock has captured growing attention, but catalytic performance has been significantly limited by their intrinsic organic linkages. Here, a simple, yet efficient approach is reported to improve the CO₂ photoreduction on metal-free sp² c-COFs by rationally regulating their intrinsic π-conjugation. The incorporation of ethynyl groups into conjugated skeletons affords a significant improvement in π-conjugation and facilitates the photogenerated charge separation and transfer, thereby boosting the CO₂ photoreduction in a solid-gas mode with only water vapor and CO₂ . The resultant CO production rate reaches as high as 382.0 µmol g^-1  h^-1 , ranking at the top among all additive-free CO₂ photoreduction catalysts. The simple modulation approach not only enables to achieve enhanced CO₂ reduction performance but also simultaneously gives a rise to extend the understanding of structure-property relationship and offer new possibilities for the development of new π-conjugated COF-based artificial photocatalysts.

Three-Component Donor-π-Acceptor Covalent-Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

Two-dimensional covalent-organic frameworks (2D COFs) have recently emerged as great prospects for their applications as new photocatalytic platforms in solar-to-hydrogen conversion; nevertheless, their inefficient solar energy capture and fast charge recombination hinder the improvement of photocatalytic hydrogen production performance. Herein, two photoactive three-component donor-π-acceptor (TCDA) materials were constructed using a multicomponent synthesis strategy by introducing electron-deficient triazine and electron-rich benzotrithiophene moieties into frameworks through sp² carbon and imine linkages, respectively. Compared with two-component COFs, the novel TCDA-COFs are more convenient in regulating the inherent photophysical properties, thereby realizing outstanding photocatalytic activity for hydrogen evolution from water. Remarkably, the first sp² carbon-linked TCDA-COF displays an impressive hydrogen evolution rate of 70.8 ± 1.9 mmol g^-1 h^-1 with excellent reusability in the presence of 1 wt % Pt under visible-light illumination (420-780 nm). Utilizing the combination of diversified spectroscopy and theoretical prediction, we show that the full π-conjugated linkage not only effectively broadens the visible-light harvesting of COFs but also enhances charge transfer and separation efficiency.

Benzotrithiophene-based Covalent Organic Framework Photocatalysts with Controlled Conjugation of Building Blocks for Charge Stabilization

Covalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure-property-activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)-based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT-COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts.

N,N'-octyl biphenothiazine and dibenzothiophene dioxide-based soluble porous organic polymer for biphasic photocatalytic hydrogen evolution

A donor-acceptor-based soluble porous organic polymer (PzDBS) was fabricated using a flexible core composed of N,N'-octyl biphenothiazine and a rigid building unit involving dibenzothiophene dioxide. The soluble porous organic polymer was explored for aqueous-organic biphasic photocatalytic hydrogen evolution, introducing a promising avenue in the domain of porous polymer photocatalysts.

Structural Engineering of Ionic MOF@COF Heterointerface for Exciton-Boosting Sunlight-Driven Photocatalytic Filter

Sunlight-driven photocatalytic filters against pathogenic bioaerosols have attracted a lot of interest. However, developing an efficient interception system that shows enhanced visible-light harvesting, controllable charge dynamic, and boosted ROS generation remains a grand challenge. Here, we designed an ionic ZIF-8@iCOF nanocomposite as a sunlight-driven photocatalytic filter through elaborate structural engineering of the heterointerface between ZIF-8 and cationic iCOF layers. The photoactive experiments reveal significant improvements in the visible light absorption and sunlight-driven exciton-enhanced intersystem crossing to boost the generation of singlet oxygen (220%) and also obtain antibacterial efficiency of 99.99999% after 15 min irradiation. After combining with commercial polymer, resultant ZIF-8@iCOF/polyacrylonitrile (PAN) fibrous membranes exhibited high interception efficiency for both PM₁₀ and PM_2.5 (98%), being close to the commercial N95. This fibrous membrane also possesses good biocompatibility and strong elimination of bacteria under sunlight conditions, satisfying for the long-lasting contact usage. This finding not only showcases the promise of the porous materials-based fibrous membranes for efficient photocatalytic filter against pathogenic bioaerosols but also highlights the importance of accurate structural engineering for the advancement of sunlight-driven photocatalytic systems in environment and energy-related fields.

Interface-confined synthesis of a nonplanar redox-active covalent organic framework film for synaptic memristors

The development of novel synthetic methodologies and unprecedented structures of covalent organic framework (COF) films is of great importance for exploring their potential applications in optoelectronic devices, sensors, and membrane separation. From the point of view of monomer selection, rigid building blocks are always the first choice for synthesizing crystalline COF films. However, the preparation of COF films with flexible building units remains challenging. Herein, by introducing flexible triphenylamine-based building units, a nonplanar COF film (TFPA-TAPA film) is fabricated via liquid-liquid interface-confined synthesis at room temperature and atmospheric pressure. The growth mechanism of the flexible building units at the liquid-liquid interface is related to the transformation of strip-type slices into free-standing COF films by dynamic covalent chemistry. As a proof-of-concept, the as-fabricated Al/TFPA-TAPA/ITO device shows excellent multilevel storage and history-dependent memristive switching behavior. The synaptic potentiation/depression, human learning and memorization functions, as well as the transition from short-term synaptic plasticity to long-term plasticity, are successfully emulated by using this synaptic memristor.

Enhancement of Visible-Light-Driven Hydrogen Evolution Activity of 2D π-Conjugated Bipyridine-Based Covalent Organic Frameworks via Post-Protonation

Photocatalytic hydrogen (H₂ ) evolution represents a promising and sustainable technology. Covalent organic frameworks (COFs)-based photocatalysts have received growing attention. A 2D fully conjugated ethylene-linked COF (BTT-BPy-COF) was fabricated with a dedicated designed active site. The introduced bipyridine sites enable a facile post-protonation strategy to fine-tune the actives sites, which results in a largely improved charge-separation efficiency and increased hydrophilicity in the pore channels synergically. After modulating the degree of protonation, the optimal BTT-BPy-PCOF exhibits a remarkable H₂ evolution rate of 15.8 mmol g^-1  h^-1 under visible light, which surpasses the biphenyl-based COF 6 times. By using different types of acids, the post-protonation is proved to be a potential universal strategy for promoting photocatalytic H₂ evolution. This strategy would provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

Three-Dimensional Covalent Organic Frameworks with Ultra-Large Pores for Highly Efficient Photocatalysis

Benefiting from their unique structural merits, three-dimensional (3D) large-pore COF materials demonstrate high surface areas and interconnected large channels, which makes these materials promising in practical applications. Unfortunately, functionalization strategies and application research are still absent in these structures. To this end, a series of functional 3D stp-topologized COFs are designed based on porphyrin or metalloporphyrin moieties, named JUC-640-M (M = Co, Ni, or H). Interestingly, JUC-640-H exhibits a record-breaking low crystal density (0.106 cm³ g^-1) among all crystalline materials, along with the largest interconnected pore size (4.6 nm) in 3D COFs, high surface area (2204 m² g^-1), and abundant exposed porphyrin moieties (0.845 mmol g^-1). Inspired by the unique structural characteristics and photoelectrical performance, JUC-640-Co is utilized for the photoreduction of CO₂ to CO and demonstrates a high CO production rate (15.1 mmol g^-1 h^-1), selectivity (94.4%), and stability. It should be noted that the CO production rate of JUC-640-Co has exceeded those of all reported COF-based materials. This work not only produces a series of novel 3D COFs with large channels but also provides a new guidance for the functionalization and applications of COFs.

Linkage-Affected Donor–Acceptor Covalent Organic Frameworks for Photocatalytic Hydrogen Production

The depletion of traditional fossil energy and the resulting environmental pollution forces people to explore new energy sources. Direct use of solar energy is now a viable solution for solving these problems. Covalent organic frameworks (COFs) are a porous crystalline material; their well-defined two-dimensional or three-dimensional frameworks can ensure the orderly arrangement of photoelectric active units, giving them potential photoelectric conversion applications. The tunable structural features endow COFs many advantages in photocatalytic hydrogen production under visible light. This review comprehensively summarizes the research progress on photoelectronic donor–acceptor (D-A) COFs with tunable structure for photocatalytic hydrogen evolution and will provide a feasible guiding strategy for applying this type of COFs in photocatalytic hydrogen production.

Integrated interfacial design of covalent organic framework photocatalysts to promote hydrogen evolution from water

Attempts to develop photocatalysts for hydrogen production from water usually result in low efficiency. Here we report the finding of photocatalysts by integrated interfacial design of stable covalent organic frameworks. We predesigned and constructed different molecular interfaces by fabricating ordered or amorphous π skeletons, installing ligating or non-ligating walls and engineering hydrophobic or hydrophilic pores. This systematic interfacial control over electron transfer, active site immobilisation and water transport enables to identify their distinct roles in the photocatalytic process. The frameworks, combined ordered π skeletons, ligating walls and hydrophilic channels, work under 300-1000 nm with non-noble metal co-catalyst and achieve a hydrogen evolution rate over 11 mmol g^-1 h^-1, a quantum yield of 3.6% at 600 nm and a three-order-of-magnitude-increased turnover frequency of 18.8 h^-1 compared to those obtained with hydrophobic networks. This integrated interfacial design approach is a step towards designing solar-to-chemical energy conversion systems.

Carbon-Carbon Linked Organic Frameworks: An Explicit Summary and Analysis

Organic frameworks with carbon-carbon (CC) linkage are an important class of materials owing to their outstanding chemical stability and extended π-electron delocalization resulting in unique optoelectronic properties. In the first part of this review article, the design principles for the bottom-up synthesis of 2D and 3D sp/sp² CC linked organic frameworks are summarized. Representative reaction methodologies, such as Knoevenagel condensation, Aldol condensation, Horner-Wadsworth-Emmons reaction, Wittig reaction, and coupling reactions (Ullmann, Suzuki, Heck, Yamamoto, etc.) are included. This is discussed in the context of their reaction mechanism, reaction dynamics, and whether and why resulting in an amorphous or crystalline product. This is followed by a discussion of different state-of-the art bottom-up synthesis methodologies, like solvothermal, interfacial, and solid-state synthesis. In the second part, the structure-property relationships in CC linked organic frameworks with representative examples of organocatalysis, photo(electro)catalysis, energy storage and conversion, magnetism, and molecular storage and separation are analyzed. The importance of linkage type, building blocks, topology, and crystallinity of the framework material in connection with the structure-property relationship is highlighted. Finally, brief concluding remarks are presented based on the key development of bottom-up synthetic methods and provide perspectives for future development in this field.

Highly crystalline and water-wettable benzobisthiazole-based covalent organic frameworks for enhanced photocatalytic hydrogen production

Two-dimensional covalent organic frameworks are promising for photocatalysis by virtue of their structural and functional diversity, but generally suffer from low activities relative to their inorganic competitors. To fulfill their full potential requires a rational tailoring of their structures at different scales as well as their surface properties. Herein, we demonstrate benzobisthiazole-based covalent organic frameworks as a superior photocatalyst for hydrogen production. The product features high crystallinity with ordered 2.5-nm-wide cylindrical mesopores and great water wettability. These structural advantages afford our polymeric photocatalyst with fast charge carrier dynamics as evidenced by a range of spectroscopic characterizations and excellent catalytic performances when suspended in solution or supported on melamine foams. Under visible-light irradiation, it enables efficient and stable hydrogen evolution with a production rate of 487 μmol h^-1 (or a mass-specific rate of 48.7 mmol g^-1 h^-1)-far superior to the previous state of the art. We also demonstrate that hydrogen production can be stoichiometrically coupled with the oxidation conversion of biomass as exemplified by the conversion of furfuryl alcohol to 2-furaldehyde.

An ionic vinylene-linked three-dimensional covalent organic framework for selective and efficient trapping of ReO4- or 99TcO4. Nat Commun 2022;13:7621. [PMID: 36494388 PMCID: PMC9734744 DOI: 10.1038/s41467-022-35435-7]

The synthesis of ionic olefin linked three-dimensional covalent organic frameworks (3D COFs) is greatly challenging given the hardness of the formation of stable carbon-carbon double bonds (-C = C-). Herein, we report a general strategy for designing porous positively charged sp2 carbon-linked 3D COFs through the Aldol condensation promoted by quaternization. The obtained 3D COFs, namely TFPM-PZI and TAPM-PZI, showed impressive chemical stability. Furthermore, the positively charged frameworks with regular porosity endow 3D ionic COFs with selective capture radioactive ReO4-/TcO4- and great removal efficiency in simulated Hanford waste. This research not only broadens the category of 3D COFs but also promotes the application of COFs as efficient functional materials.

Impact of Interfaces, and Nanostructure on the Performance of Conjugated Polymer Photocatalysts for Hydrogen Production from Water

The direct conversion of sunlight into hydrogen through water splitting, and by converting carbon dioxide into useful chemical building blocks and fuels, has been an active area of research since early reports in the 1970s. Most of the semiconductors that drive these photocatalytic processes have been inorganic semiconductors, but since the first report of carbon nitride organic semiconductors have also been considered. Conjugated materials have been relatively extensively studied as photocatalysts for solar fuels generation over the last 5 years due to the synthetic control over composition and properties. The understanding of materials' properties, its impact on performance and underlying factors is still in its infancy. Here, we focus on the impact of interfaces, and nanostructure on fundamental processes which significantly contribute to performance in these organic photocatalysts. In particular, we focus on presenting explicit examples in understanding the interface of polymer photocatalysts with water and how it affects performance. Wetting has been shown to be a clear factor and we present strategies for increased wettability in conjugated polymer photocatalysts through modifications of the material. Furthermore, the limited exciton diffusion length in organic polymers has also been identified to affect the performance of these materials. Addressing this, we also discuss how increased internal and external surface areas increase the activity of organic polymer photocatalysts for hydrogen production from water.

Chemically modified covalent organic frameworks for a healthy and sustainable environment: First-principles study

Pure water is a key element for a sustainable and healthy environment of human inhabitation. Since major sources of water contamination are industrially generated heavy metal cations there is great demand for efficient methods of their treatment. Here, using density functional theory, we investigate the covalent organic framework's electronic and optical properties and their interaction with the most dangerous heavy metal pollutants, namely Hg⁺², Pb^+2, and Cd⁺². We consider biphenyl boroxine covalent organic frameworks before and after chemical modification with CN, COOH, NH₂, and NO₂ groups. In addition to the molecular geometries, such parameters as the dipole moment, chemical potential, electronegativity, chemical hardness, and binding energy are calculated. It is found that CN, COOH, and NO₂ functional groups are favorable for intermolecular bonding with harmful transition metals. The functionalization with the mentioned groups reduces the band gap of the pristine covalent organic frameworks and increases their reactivity. As a result, strong complexes with Cd⁺², Hg⁺², and Pb⁺² can form, which, as follows from our calculations, can be detected by the red shift in their optical absorption spectra.

Zn (II) Porphyrin Built-in D-A Covalent Organic Framework for Efficient Photocatalytic H2 Evolution

Covalent organic frameworks (COFs) with donor-acceptor (D-A) units are credible photocatalysts for their per-designed structure, inherent porosity, large surface area, splendid stability and so forth. Developing COFs with an excellent photocatalytic efficiency for hydrogen evolution is of a great significance in alleviating the energy crisis. Herein, a D-A type imine-linked crystalline Zn-Por-TT COF was fabricated successfully via the co-polymerization of electron-deficient Zinc (II) 5,10,15,20-tetrakis(para-aminophenyl) porphyrin (Zn-TAPP), and electron-rich thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TT). Profiting from the D-A complex structure, the obtained Zn-Por-TT COF showcases an excellent photocatalytic activity with a hydrogen evolution rate of 8200 μmol/g/h, while the Zn-TAPP monomer presents practically no capacity for the generation of hydrogen under identical conditions. In addition, the counterparts Por-TT COF and COF-366-Zn were employed to illustrate the enhancement of the photocatalytic performance by metal catalytic sites and D-A structures. In addition, the counterparts Por-TT COF and COF-366-Zn were employed to illustrate the enhancement of metal catalytic sites and D-A structures for the photocatalytic performance.

Tale of COF-on-MOF Composites with Structural Regulation and Stepwise Luminescence Enhancement

Integrating metal-organic framework (MOF)-covalent organic framework (COF) allows versatile engineering of hybrid materials with properties superior to pristine components, especially COFs suffered from aggregation-caused quenching (ACQ), unlocking more possibilities to improve the luminescence of COFs. In this work, we prepared various MOF@COF composites with different COF layer thicknesses, in which stable UiO-66-NH₂ served as the inner substrate and 1,3,5-benzenetricarboxaldehyde (BT), and 3,3'-dihydroxybenzidine (DH) were used to construct a COF layer. In addition to the conventional preparation method, we increased the ratio of BT and DH to be 1:2.5, and impressively, the morphologies of acquired UC (1:2.5) materials were quite different from the previous reticular structure and gradually extended from the spherical structure to the prickly structure with the increase of COF monomers. Remarkably, all of the UC materials possessed better luminescence properties than individual COF due to the limited COF layers. Meanwhile, UC-1 materials with an optimal COF layer displayed the strongest emission. In comparison with a single COF, the quantum yields of UC-1 and UC-1 (1:2.5) were increased nearly 7 times and 5 times, respectively. Moreover, the fluorescence of UC-1 materials was progressively enhanced via selective F^- sensing. This work is expected to shed light on the potential hybridization of MOF-COF with structural adjustment, morphological design, and luminescence enhancement.

Spatial Regulation of Acceptor Units in Olefin-Linked COFs toward Highly Efficient Photocatalytic H2 Evolution

Covalent organic frameworks (COFs)-based photocatalysts have received growing attention for photocatalytic hydrogen (H₂ ) production. One of the big challenges in the field is to find ways to promote energy/electron transfer and exciton dissociation. Addressing this challenge, herein, a series of olefin-linked 2D COFs is fabricated with high crystallinity, porosity, and robustness using a melt polymerization method without adding volatile organic solvents. It is found that regulation of the spatial distances between the acceptor units (triazine and 2, 2'-bipyridine) of COFs to match the charge carrier diffusion length can dramatically promote the exciton dissociation, hence leading to outstanding photocatalytic H₂ evolution performance. The COF with the appropriate acceptor distance achieves exceptional photocatalytic H₂ evolution with an apparent quantum yield of 56.2% at 475 nm, the second highest value among all COF photocatalysts and 70 times higher than the well-studied polymer carbon nitride. Various experimental and computation studies are then conducted to in-depth unveil the mechanism behind the enhanced performance. This study will provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

A Covalent Organic Framework for Cooperative Water Oxidation

The future of water-derived hydrogen as the “sustainable energy source” straightaway bets on the success of the sluggish oxygen-generating half-reaction. The endeavor to emulate the natural photosystem II for efficient water oxidation has been extended across the spectrum of organic and inorganic combinations. However, the achievement has so far been restricted to homogeneous catalysts rather than their pristine heterogeneous forms. The poor structural understanding and control over the mechanistic pathway often impede the overall development. Herein, we have synthesized a highly crystalline covalent organic framework (COF) for chemical and photochemical water oxidation. The interpenetrated structure assures the catalyst stability, as the catalyst’s performance remains unaltered after several cycles. This COF exhibits the highest ever accomplished catalytic activity for such an organometallic crystalline solid-state material where the rate of oxygen evolution is as high as ∼26,000 μmol L^–1 s^–1 (second-order rate constant k ≈ 1650 μmol L s^–1 g^–2). The catalyst also proves its exceptional activity (k ≈ 1600 μmol L s^–1 g^–2) during light-driven water oxidation under very dilute conditions. The cooperative interaction between metal centers in the crystalline network offers 20–30-fold superior activity during chemical as well as photocatalytic water oxidation as compared to its amorphous polymeric counterpart.

Engineering Single-Atom Active Sites on Covalent Organic Frameworks for Boosting CO2 Photoreduction

Solar carbon dioxide (CO₂) conversion is an emerging solution to meet the challenges of sustainable energy systems and environmental/climate concerns. However, the construction of isolated active sites not only influences catalytic activity but also limits the understanding of the structure-catalyst relationship of CO₂ reduction. Herein, we develop a universal synthetic protocol to fabricate different single-atom metal sites (e.g., Fe, Co, Ni, Zn, Cu, Mn, and Ru) anchored on the triazine-based covalent organic framework (SAS/Tr-COF) backbone with the bridging structure of metal-nitrogen-chlorine for high-performance catalytic CO₂ reduction. Remarkably, the as-synthesized Fe SAS/Tr-COF as a representative catalyst achieved an impressive CO generation rate as high as 980.3 μmol g^-1 h^-1 and a selectivity of 96.4%, over approximately 26 times higher than that of the pristine Tr-COF under visible light irradiation. From X-ray absorption fine structure analysis and density functional theory calculations, the superior photocatalytic performance is attributed to the synergic effect of atomically dispersed metal sites and Tr-COF host, decreasing the reaction energy barriers for the formation of *COOH intermediates and promoting CO₂ adsorption and activation as well as CO desorption. This work not only affords rational design of state-of-the-art catalysts at the molecular level but also provides in-depth insights for efficient CO₂ conversion.

Suppressing the Excitonic Effect in Covalent Organic Frameworks for Metal-Free Hydrogen Generation

Photocatalytic hydrogen generation is a promising solution for renewable energy production and plays a role in achieving carbon neutrality. Covalent organic frameworks (COFs) with highly designable backbones and inherent pores have emerged as novel photocatalysts, yet the strong excitonic effect in COFs can impede the promotion of energy conversion efficiency. Here, we propose a facile approach to suppress the excitonic effect in COFs, which is by narrowing the band gap and increasing the dielectric screening via a rational backbone design and chemical modifications. Based on the GW-BSE method, we uncover a linear relationship between the electronic dielectric constant and the inverse square of the optical band gap of COFs of the Lieb lattice. We further demonstrate that both reduced exciton binding energy and enhanced sunlight absorption can be simultaneously realized in COFs with a narrow band gap. Specifically, we show that one of our designed COFs whose exciton binding energy is nearly half that of g-C₃N₄ is capable of metal-free hydrogen production under near-infrared light irradiation. Our results showcase an effective method to suppress the excitonic effect in COFs and also pave the way for their applications in photocatalytic, photovoltaic, and other related solar energy conversions.

Cobalt quantum dots as electron collectors in ultra-narrow bandgap dioxin linked covalent organic frameworks for boosting photocatalytic solar-to-fuel conversion

Photocatalysis offers a sustainable paradigm for solar-to-fuel conversion because it conflates the merits of renewable solar energy and reusable catalysts. However, the seek for robust photocatalysts that can utilize the full visible light spectrum remains challenging. Herein, cobalt quantum dots (Co QDs) were integrated into ultra-narrow bandgap dioxin linked covalent organic frameworks (COF-318) for photocatalytic solar-to-fuel conversion under full spectrum of visible light irradiation. The optimal Co₁₀-COF exhibited superior photocatalytic CO₂ reduction performance, affording a CO yield of 4232 µmol∙g^-1∙h^-1 and H₂ evolution of 6611 µmol∙g^-1∙h^-1. Specifically, Co QDs played a crucial role in boosting the photocatalytic performance, which acted as electron collectors to capture the photoinduced electrons and then conveyed them to CO₂ molecules. Moreover, the Co QDs modification significantly improved the CO₂ adsorption and activation capacity, as well as prolonging the lifetime of photogenerated carriers. This work reveals an operable pathway for fabricating promising photocatalyst for visible-light-driven solar-to-fuel generation and provides insight into the impact of the integration of Co QDs on COF-based photocatalysts.

2D Covalent Organic Frameworks Toward Efficient Photocatalytic Hydrogen Evolution

Efficiently producing clean energy is of great importance for sustainable development of the environment. Solar-driven water splitting for H₂ evolution has an important role among the renewable energy technologies. Developing high-performance and cost-effective photocatalysts is still a critical task before practical application. 2D Covalent organic frameworks (COFs) as photocatalysts have recently attracted widespread interest thanks to their tunable optical bandgaps, tailor-made functionality, excellent crystallinity, high specific surface area, and good photo- and chemical stability. This Review focuses on the representative progress and remaining challenges in 2D COF-based photocatalysts for hydrogen evolution.

Modulating the Band Structure of Metal Coordinated Salen COFs and an In Situ Constructed Charge Transfer Heterostructure for Electrocatalysis Hydrogen Evolution

A series of crystalline, stable Metal (Metal = Zn, Cu, Ni, Co, Fe, and Mn)-Salen covalent organic framework (COF)EDA complex are prepared to continuously tune the band structure of Metal-Salen COFEDA , with the purpose of optimizing the free energy intermediate species during the hydrogen evolution reaction (HER) process. The conductive macromolecular poly(3,4-ethylenedioxythiophene) (PEDOT) is subsequently integrated into the one-dimensional (1D) channel arrays of Metal-Salen COFEDA to form heterostructure PEDOT@Metal-Salen COFEDA via the in situ solid-state polymerization method. Among the Metal-Salen COFEDA and PEDOT@Metal-Salen COFEDA complexes, the optimized PEDOT@Mn-Salen COFEDA displays prominent electrochemical activity with an overpotential of 150 mV and a Tafel slope of 43 mV dec-1 . The experimental results and density of states data show that the continuous energy band structure modulation in Metal-Salen COFEDA has the ability to make the metal d-orbital interact better with the s-orbital of H, which is conducive to electron transport in the HER process. Moreover, the calculated charge density difference indicates that the heterostructures composed of PEDOT and Metal-Salen COFEDA induce an intramolecular charge transfer and construct highly active interfacial sites.

Rapid chromium reduction by metal-free organic polymer photocatalysis via molecular engineering

The conversion of hexavalent chromium (Cr(VI)), a highly poisonous heavy metal found in natural environment, to less poisonous trivalent chromium (Cr(III)) has attracted a lot of interest. However, little interest has been paid to the development of metal-free catalysts. Here, we demonstrate for the first time a molecular engineering strategy to synthesize a range of donor-acceptor conjugated polymer photocatalysts, which can significantly increase the reduction efficiency of Cr(VI) by a factor of 5.2, corresponding to a significant change in the reduction reaction rate constant (from 0.0337 to 0.1740 min^-1). In addition, the apparent quantum efficiency (AQE) of Cr(VI) removal was obtained, and the optimized photocatalyst (Py-SO₁) could achieve the highest apparent quantum efficiency at wavelength of 420 nm in those samples. Despite the narrow light absorption of Py-SO₁ polymer, its excellent exciton separation efficiency and efficient electron output enabled it to achieve excellent performance in photoreduction of Cr(VI), surpassing that of the reported metal-free photocatalysts. The results show that the present work provides a new perspective for designing suitable environmental remediation catalysts based on molecular engineering strategies.

Donor-Acceptor Hybrid Heterostructures: An Emerging Class of Photoactive Materials with Inorganic and Organic Semiconductive Components

Just as the heterojunctions in physics, donor-acceptor (D-A) heterostructures are an emerging class of photoactive materials fabricated from two semiconductive components at the molecular level. Among them, D-A hybrid heterostructures from organic and inorganic semiconductive components have attracted extensive attention in the past decades due to their combined advantages of high stability for the inorganic semiconductors and modifiability for the organic semiconductors, which are particularly beneficial to efficiently achieve photoinduced charge separation and transfer upon irradiations. In this review, by analogy with the heterojunctions in physics, a definition of the D-A heterostructures and their general design and synthetic strategies are given. Meanwhile, the D-A hybrid heterostructures are focused on and their recent advances in potential applications of photochromism, photomodulated luminescence, and photocatalysis summarized.

A yolk-shell structure construction for metal-organic frameworks toward an enhanced electrochemical water splitting catalysis

NiFe-based transition metal catalysts are widely used in electrocatalysis, especially in the field of water splitting, due to their excellent electrochemical performance. Herein, a simple method was designed to synthesize a Ni MOF based on nickel foam and it was modified with Fe. After the introduction of Fe, the resulting material exhibits an obvious yolk-shell structure, which greatly increases the specific surface area and facilitates the construction of active sites. At the same time, the synergy between Ni and Fe is conducive to optimizing the electronic structure and effectively improving the poor stability of the MOF. As a result, the synthesized Ni MOF-Fe-2 only needs an overpotential of 229 mV to achieve the OER at a current density of 10 mA cm^-2, which is better than most reported transition metal-based electrocatalysts. To our surprise, it showed extraordinary stability under the voltage used for water splitting.

Structural and Electronic Modulations of Imidazolium Covalent Organic Framework-Derived Electrocatalysts for Oxygen Redox Reactions in Rechargeable Zn-Air Batteries

Covalent organic frameworks (COFs) are promising candidates for the controllable design of electrocatalysts. However, bifunctional electrocatalytic activities for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remain challenging in COFs. In this study, imidazolium-rich COFs (IMCOFs) with well-defined active sites and characteristic three-dimensional assembly structures were readily prepared, and their electronic structures were tuned by Co incorporation to elicit bifunctional electrocatalytic activities for the ORR and OER. The Co nanoparticle-incorporated spherical IMCOF-derived electrocatalyst (CoNP-s-IMCOF) exhibited lower overpotentials for the ORR and OER compared with the atomic Co-incorporated planar IMCOF-derived electrocatalyst (Co-p-IMCOF). Computational simulations revealed that the imidazole carbon sites of CoNP-s-IMCOF rather than the triazine carbons were the active sites for the ORR and OER, and its p-band center downshifted via charge transfer, facilitating the chemisorption of oxygen intermediates during the reactions. A Zn-air battery with CoNP-s-IMCOF exhibited a small voltage gap of 1.3 V with excellent durability for 935 cycles. This approach for control over the three-dimensional assembly and electronic structures of IMCOFs can be extended to the development of diverse catalytic nanomaterials for applications of interest.

Photocatalytic hydrogen evolution based on carbon nitride and organic semiconductors

Photocatalytic hydrogen evolution (PHE) presents a promising way to solve the global energy crisis. Metal-free carbon nitride (CN) and organic semiconductors photocatalysts have drawn intense interests due to their fascinating properties such as tunable molecular structure, electronic states, strong visible-light absorption, low-cost etc. In this paper, the recent progresses of photocatalytic hydrogen production based on organic photocatalysts, including CN, linear polymers, conjugated porous polymers and small molecules, are reviewed, with emphasis on the various strategies to improve PHE efficiency. Finally, the possible future research trends in the organic photocatalysts are prospected.