Phenothiazine-based covalent organic frameworks with low exciton binding energies for photocatalysis

Programming Tetrathiafulvalene-Based Covalent Organic Frameworks for Promoted Photoinduced Molecular Oxygen Activation

Despite the pivotal role of molecular oxygen (O2) activation in artificial photosynthesis, the activation efficiency is often restricted by sluggish exciton dissociation and charge transfer kinetics within polymer photocatalysts. Herein, we propose two tetrathiafulvalene (TTF)-based imine-linked covalent organic frameworks (COFs) with tailored donor-acceptor (D-A) structures, TTF-PDI-COF and TTF-TFPP-COF, to promote O2 activation. Because of enhanced electron push-pull interactions that facilitated charge separation and transfer behavior, TTF-PDI-COF exhibited superior photocatalytic activity in electron-induced O2 activation reactions over TTF-TFPP-COF under visible light irradiation, including the photosynthesis of (E)-3-amino-2-thiocyano-α,β-unsaturated compounds and H2O2. These findings highlight the significant potential of the rational design of COFs with D-A configurations as suitable candidates for advanced photocatalytic applications.

Microwave Assisted Fast Synthesis of a Donor-Acceptor COF Towards Photooxidative Amidation Catalysis

The synthesis of covalent organic frameworks (COFs) at bulk scale require robust, straightforward, and cost-effective techniques. However, the traditional solvothermal synthetic methods of COFs suffer low scalability as well as requirement of sensitive reaction environment and multiday reaction time (2-10 days) which greatly restricts their practical application. Here, we report microwave assisted rapid and optimized synthesis of a donor-acceptor (D-A) based highly crystalline COF, TzPm-COF in second (10 sec) to minute (10 min) time scale. With increasing the reaction time from seconds to minutes crystallinity, porosity and morphological changes are observed for TzPm-COF. Owing to visible range light absorption, suitable band alignment, and low exciton binding energy (Eb=64.6 meV), TzPm-COF can efficaciously produce superoxide radical anion (O2 .-) after activating molecular oxygen (O2) which eventually drives aerobic photooxidative amidation reaction with high recyclability. This photocatalytic approach works well with a variety of substituted aromatic aldehydes having electron-withdrawing or donating groups and cyclic, acyclic, primary or secondary amines with moderate to high yield. Furthermore, catalytic mechanism was established by monitoring the real-time reaction progress through in situ diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) study.

Photochromic radical states in 3D covalent organic frameworks with zyg topology for enhanced photocatalysis

Covalent-organic frameworks (COFs) with photoinduced donor-acceptor (D-A) radical pairs show enhanced photocatalytic activity in principle. However, achieving long-lived charge separation in COFs proves challenging due to the rapid charge recombination. Here, we develop a novel strategy by combining [6 + 4] nodes to construct zyg-type 3D COFs, first reported in COF chemistry. This structure type exhibits a fused Olympic-rings-like shape, which provides a platform for stabilizing the photoinduced D-A radical pairs. The zyg-type COFs containing catalytically active moieties such as triphenylamine and phenothiazine (PTZ) show superior photocatalytic production rates of hydrogen peroxide (H2O2). Significantly, the photochromic radical states of these COFs show up to 400% enhancement in photocatalytic activity compared to the parent states, achieving a remarkable H2O2 synthesis rate of 3324 μmol g-1 h-1, which makes the PTZ-COF one of the best crystalline porous photocatalysts in H2O2 production. This work will shed light on the synthesis of efficient 3D COF photocatalysts built on topologies that can facilitate photogenerating D-A radical pairs for enhanced photocatalysis.

Identification of two-dimensional covalent organic frameworks with topology and their application in photocatalytic hydrogen evolution

Covalent organic frameworks have attracted considerable attention in recent years as a distinct class of crystalline porous organic materials. Their functional properties are inherently linked to their structural characteristics. Although hundreds of COFs have been reported so far, the types of their topologic structure are still limited. In this article, we report the identification of mcm topology for three porphyrin-based two-dimensional COFs, which are constructed from [4 + 4] imine condensation reactions. The mcm net is generated by pentagonal tiling, which has not been identified for COFs before. The structure of the COFs is elucidated by a variety of experimental characterization and structural simulations, by which their reticular frameworks exclusively composed of pentagonal pores have been confirmed. Moreover, the COFs exhibit high performance in photocatalytic hydrogen evolution from water, with the best one up to 10.0 mmol g-1 h-1 after depositing 0.76 wt% Pt as a co-catalyst. This study identifies mcm topology for COFs for the first time and highlights the potential of these COFs as promising photocatalysts for sustainable hydrogen production from water.

Hollow S-Doped ZnFe2O4 Microcubes with Magnetic Separability for Photocatalytic Removal of Uranium(VI) under Different Light Intensity

Under xenon lamps, ZnFe2O4 (ZFO) has been shown to be effective in removing uranium through photocatalysis. However, its performance is still inadequate in low-light environments due to low photon utilization and high electron-hole complexation. Herein, S-doped hollow ZnFe2O4 microcubes (Sx-H-ZFO, x = 1, 3, 6, 9) were synthesized using the MOF precursor template method. The hollow morphology improves the utilization of visible light by refracting and reflecting the incident light multiple times within the confined domain. S doping narrows the band gap and shifts the conduction band position negatively, which enhances the separation, migration, and accumulation of photogenerated charges. Additionally, S doping increases the number of adsorption sites, ultimately promoting efficient surface reactions. Consequently, Sx-H-ZFO is capable of removing U(VI) in low-light environments. Under cloudy and rainy weather conditions, the photocatalytic rate of S3-H-ZFO was 100.31 μmol/(g·h), while under LED lamps (5000 Lux) it was 72.70 μmol/(g·h). More interestingly, a systematic mechanistic investigation has revealed that S doping replaces some of the oxygen atoms to enhance electron transfers and adsorption of O2. This process initiates the formation of hydrogen peroxide, which reacts directly with UO22+ to form solid studtite (UO2)O2·2H2O. Additionally, the promising magnetic separation capability of Sx-H-ZFO facilitates the recycling and reusability of the material. This work demonstrates the potential of ZnFe2O4 extraction uranium from nuclear wastewater.

Temperature-dependent excitonic emission characteristics of highly crystallized carbon nitride nanosheets

Highly-crystallized carbon nitride (HCCN) nanosheets exhibit significant potential for advancements in the field of photoelectric conversion. However, to fully exploit their potential, a thorough understanding of the fundamental excitonic photophysical processes is crucial. Here, the temperature-dependent excitonic photoluminescence (PL) of HCCN nanosheets and amorphous polymeric carbon nitride (PCN) is investigated using steady-state and time-resolved PL spectroscopy. The exciton binding energy of HCCN is determined to be 109.26 meV, lower than that of PCN (207.39 meV), which is attributed to the ordered stacking structure of HCCN with a weaker Coulomb interaction between electrons and holes. As the temperature increases, a noticeable reduction in PL lifetime is observed on both the HCCN and PCN, which is ascribed to the thermal activation of carrier trapping by the enhanced electron-phonon coupling effect. The thermal activation energy of HCCN is determined to be 102.9 meV, close to the value of PCN, due to their same band structures. Through wavelength-dependent PL dynamics analysis, we have identified the PL emission of HCCN as deriving from the transitions:σ*-LP,π*-π, andπ*-LP, where theπ*-LP transition dominants the emission because of the high excited state density of the LP state. These results demonstrate the impact of high-crystallinity on the excitonic emission of HCCN materials, thereby expanding their potential applications in the field of photoelectric conversion.

Modulating Inorganic Dimensionality of Ultrastable Lead Halide Coordination Polymers for Photocatalytic CO2 Reduction to Ethanol

Lead halide hybrids have shown great potentials in CO2 photoreduction, but challenging to afford C2+ reduced products, especially using H2O as the reductant. This is largely due to the trade-off problem between instability of the benchmark 3D structures and low carrier mobility of quasi-2D analogues. Herein, the lead halide dimensionality of robust coordination polymers (CP) was modulated by organic ligands differing in a single-atom change (NH vs. CH2), in which the NH groups coordinate with interlamellar [PbI2] clusters to achieve the important 2D→3D transition. This first CP based on 3D cationic lead iodide sublattice possesses both high aqueous stability and a low exciton binding energy of 25 meV that is on the level of ambient thermal energy, achieving artificial photosynthesis of C2H5OH. Photophysical studies combined with theoretical calculations suggest the bridging [PbI2] clusters in the 3D structure not only results in enhanced carrier transport, but also promotes the intrinsic charge polarization to facilitate the C-C coupling. With trace loading of Rh cocatalyst, the apparent quantum efficiency of the 3D CP reaches 1.4 % at 400 nm with a high C2H5OH selectivity of 89.4 % (product basis), which presents one of the best photocatalysts for C2 products to date.

Solar-to-H2 O2 Catalyzed by Covalent Organic Frameworks

Benefiting from the excellent structural tunability, robust framework, ultrahigh porosity, and rich active sites, covalent organic frameworks (COFs) are widely recognized as promising photocatalysts in chemical conversions, and emerged in the hydrogen peroxide (H2 O2 ) photosynthesis in 2020. H2 O2 , serving as an environmental-friendly oxidant and a promising liquid fuel, has attracted increasing researchers to explore its potential. Over the past few years, numerous COFs-based photocatalysts are developed with encouraging achievements in H2 O2 production, whereas no comprehensive review articles exist to summarize this specific and significant area. Herein we provide a systematic overview of the advances and challenges of COFs in photocatalytic H2 O2 production. We first introduce the priorities of COFs in H2 O2 photosynthesis. Then, various strategies to improve COFs photocatalytic efficiency are discussed. The perspective and outlook for future advances of COFs in this emerging field are finally offered. This timely review will pave the way for the development of highly efficient COFs photocatalysts for practical production of value-added chemicals not limited to H2 O2 .

Optoelectronic materials as emerging photocatalysts: opportunities in sustainable organic synthesis

To overcome the energy and environmental crisis, the development of efficient, sustainable photocatalysts to convert inexhaustible solar energy into chemical energy is of great significance. Due to their unique optoelectronic properties, organic electronic materials have been translated into the photocatalytic field. These emerging photocatalysts are attractive because of their metal-free nature, chemical stability, and structural diversity. However, as many small molecules fail to absorb visible light solely, incorporating them into crosslinked frameworks is found to be an effective strategy to extend the conjugation and enhance visible-light absorption. In addition, the photophysical properties of these heterogeneous materials can be adjusted through structural modification and linkage engineering. Finally, these insoluble photocatalysts exhibit good recyclability and reusability. As a representative illustration, this feature article describes recent examples of the use of two types of organic electronic materials including phenothiazine and truxene in heterogeneous photocatalytic organic transformations. The synthesis and key photophysical properties of both organic electronic material-based photocatalysts are discussed combined with specific synthetic applications. We anticipate this feature article will stimulate the implementation of more diverse organic electronic materials in the field of photocatalysis, which may lead to unprecedented synthetic applications.

Rational Design of Vinylene-Linked Covalent Organic Frameworks for Modulating Photocatalytic H2 Evolution

Vinylene-linked covalent organic frameworks (COFs) have attracted enormous attention for photocatalytic H2 evolution from water because of their fully conjugated structures, high chemical stabilities, and enhanced charge-carrier mobilities. In this work, two novel vinylene-linked COFs with tuned cyano contents were successfully synthesized and then employed as photocatalysts for H2 generation. Notably, the photocatalytic H2 production rate of the COF with the higher cyano content reached 73 μmol h-1 under visible light irradiation, which is 2.4 times higher than that with the lower content (30 μmol h-1 ). Both the experimental and computational results demonstrated that the rational design incorporating cyano groups into COF skeletons could precisely tune the corresponding energy levels, expand the visible-light absorption, and improve the photoinduced charge separation. This work not only provides a simple method for modulating the photocatalytic activities of COFs at the molecular level, but also affords interesting insights into the relationship between their structures and photocatalytic performance.

Linker Engineering of Sandwich-Structured Metal-Organic Framework Composites for Optimized Photocatalytic H2 Production

While the microenvironment around catalytic sites is recognized to be crucial in thermocatalysis, its roles in photocatalysis remain subtle. In this work, a series of sandwich-structured metal-organic framework (MOF) composites, UiO-66-NH2 @Pt@UiO-66-X (X means functional groups), is rationally constructed for visible-light photocatalytic H2 production. By varying the ─X groups of the UiO-66-X shell, the microenvironment of the Pt sites and photosensitive UiO-66-NH2 core can be simultaneously modulated. Significantly, the MOF composites with identical light absorption and Pt loading present distinctly different photocatalytic H2 production rates, following the ─X group sequence of ─H > ─Br > ─NA (naphthalene) > ─OCH3  > ─Cl > ─NO2 . UiO-66-NH2 @Pt@UiO-66-H demonstrates H2 production rate up to 2708.2 µmol g-1  h-1 , ≈222 times that of UiO-66-NH2 @Pt@UiO-66-NO2 . Mechanism investigations suggest that the variation of the ─X group can balance the charge separation of the UiO-66-NH2 core and the proton reduction ability of Pt, leading to an optimal activity of UiO-66-NH2 @Pt@UiO-66-H at the equilibrium point.

Influence laws of air gap structure manipulation of covalent organic frameworks on dielectric properties and exciton effects for photopolymerization

Boosting the dissociation of excitons is essential to enhance the photocatalytic efficiency. However, the relationship between the structure of the catalyst and the exciton effect on the photocatalytic activity is still unclear as the main problem. Here, it is proposed that as a descriptive factor, an experimentally measurable dielectric constant (εr) is available to quantitatively describe its relationship with exciton binding energy (Eb) and photocatalytic activity. With tuning the linker of covalent organic frameworks (COFs), the "air gap" structure is oriented to shrink, leading to an increased εr of COFs and a lower Eb to facilitate exciton dissociation. Meanwhile, taking "water-/oxygen-fueled" photo-induced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization as a demonstration platform, it can be seen that COFs with a small "air gap" structure have relatively superior photocatalytic activity. This provides important implications for the evolution of efficient photocatalysts.

One-Dimensional Covalent Organic Framework as High-Performance Cathode Materials for Lithium-Ion Batteries

Covalent organic frameworks (COFs) have emerged as a new class of cathode materials for energy storage in recent years. However, they are limited to two-dimensional (2D) or three-dimensional (3D) framework structures. Herein, this work reports designed synthesis of a redox-active one-dimensional (1D) COF and its composites with 1D carbon nanotubes (CNTs) via in situ growth. Used as cathode materials for Li-ion batteries, the 1D COF@CNT composites with unique dendritic core-shell structure can provide abundant and easily accessible redox-active sites, which contribute to improve diffusion rate of lithium ions and the corresponding specific capacity. This synergistic structural design enables excellent electrochemical performance of the cathodes, giving rise to 95% utilization of redox-active sites, high rate capability (81% capacity retention at 10 C), and long cycling stability (86% retention after 600 cycles at 5 C). As the first example to explore the application of 1D COFs in the field of energy storage, this study demonstrates the great potential of this novel type of linear crystalline porous polymers in battery technologies.

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.

Metal-Free Carbon-Based Covalent Organic Frameworks with Heteroatom-Free Units Boost Efficient Oxygen Reduction

Accurate identification of carbon-based metal-free electrocatalyst (CMFE) activity and enhancing their catalytic efficiency for O₂ conversion is an urgent and challenging task. This study reports a promising strategy to simultaneously develop a series of covalent organic frameworks (COFs) with well-defined heterocyclic-free biphenyl or fluorenyl units. Unlike heteroatom doping, the developed method not only supplies methyl-induced molecular configuration to promote activity, but also provides a direct opportunity to identify heteroatom-free carbon active centers. The introduction of methyl groups (MGs) with reversible valence bonds into a pristine biphenyl-based COF results in an excellent performance with a half-wave potential of 0.74 V versus the reversible hydrogen electrode (RHE), which is among the highest values for CMFE-COFs as oxygen reduction reaction (ORR) electrocatalysts. Combined with in situ Raman spectra and theoretical calculations, the MG-bound skeleton (DAF-COF) is found to produce ortho activation, confirming the ortho carbon (site-5) adjacent to MGs as active centers. This may be attributed to the opening and binding of MGs, which effectively regulate the molecular configuration and charge redistribution, as well as improve charge transfer and reduce the energy barrier. This study provides insight into the design of highly efficient metal-free organic electrocatalysts via the regulation of valence bonds.

Structure-Property Relationship of Cyano-Functionalized Conjugated Polymers for Photocatalytic Hydrogen Production

Conjugated polymers (CPs) have garnered increasing attention in the field of photocatalysis due to their stability and molecular tunability. Understanding the structure-property relationship in CPs and addressing appropriate molecular design strategies are pivotal to improving the photocatalytic performance of CPs. Herein, a new efficient cyano (CN) engineering approach was proposed to promote the photocatalytic performance of CPs, and three representative CP-based photocatalysts with different CN contents were tailor-made to investigate the relationship between CN functionalization and photocatalytic activity. A series of systematically experimental and theoretical studies reveal that CN functionalization contributes to strengthening the donor-acceptor (D-A) interaction, enhancing the light absorption ability, charge separation/transfer efficiency, and hydrophilicity of CPs, and also facilitating the output of separated photoinduced electrons from CPs to Pt cocatalyst. Thus, the dicyano-functionalized polymer (P-2CN) manifests an attractive photocatalytic performance in hydrogen production. This study provides a facile strategy to develop excellent CP-based photocatalysts for solar fuel production.

Facile, Direct, De Novo Synthesis of an Alkyl Phosphoric Acid-Decorated Covalent Organic Framework

The development and understanding of proton conductors based on phosphoric acid are critical for the field of chemistry, biology, and energy. Covalent organic frameworks (COFs), featuring highly crystalline structures and controllable pore sizes, are suitable for constructing phosphoric acid-based proton conductors. However, because of tedious and intricate synthesis, how to develop COFs based on phosphoric acid remains a substantial challenge. Herein, we contributed a side-chain decorated strategy to construct a phosphoric acid-functionalized, imine-linked COF by de novo synthesis. The phosphoric acid side chains with vigorous motion integrating with 1D nanochannels endow the resulting COF with intrinsic proton conductivity. This work expectantly provides a competitive alternative for producing phosphoric acid-functionalized COFs with high intrinsic proton conductivity. This article is protected by copyright. All rights reserved.