Photocatalytic Organic Transformations Research Articles

Engineering Covalent Organic Frameworks as Heterogeneous Photocatalysts for Organic Transformations

AbstractCovalent organic frameworks (COFs) are an emerging category of organic polymers with highly porous crystalline structures. In the last decade, reports on the use of COFs as heterogeneous photocatalysts for organic transformations have shown significant progress. Still, comprehensive reviews on the mechanisms of the photocatalytic organic transformations using COFs are lacking. This Review provides a comprehensive and systematic overview of COF‐based photocatalysts for organic transformations. Firstly, we discuss the photophysical properties and the characterization methods of COF‐based photocatalysts. Then, the general photocatalytic mechanism, the advantages, and the strategies to improve the photocatalytic efficiency of COF‐based photocatalysts are summarized. After that, advanced examples of COF‐based photocatalysts for organic transformations are analyzed with regard to the underlying mechanisms. The Review ends with a critical perspective on the challenges and prospects.

Ultrathin Two-Dimensional ZnIn2S4/Nix-B Heterostructure for High-Performance Photocatalytic Fine Chemical Synthesis and H2 Generation.

Photocatalytic H2 evolution coupled with organic transformation provides a new avenue to cooperatively produce clean fuels and fine chemicals, enabling a more efficient conversion of solar energy. Here, a novel two-dimensional (2D) heterostructure of ultrathin ZnIn2S4 nanosheets decorated with amorphous nickel boride (Nix-B) is prepared for simultaneous photocatalytic anaerobic H2 generation and aromatic aldehydes production. This ZnIn2S4/Nix-B catalyst elaborately combines the ultrathin structure advantage of the ZnIn2S4 semiconductor and the cocatalytic function of Nix-B. A high H2 production rate of 8.9 mmol h-1 g-1 is delivered over the optimal ZnIn2S4/Nix-B with a stoichiometric production of benzaldehyde, which is about 22 times higher than ZnIn2S4. Especially, the H2 evolution rate is much higher than the value (2.8 mmol h-1 g-1) of the traditional photocatalytic half reaction of H2 production with triethanolamine as a sacrificial agent. The apparent quantum yield reaches 24% at 420 nm, representing an advanced photocatalyst system. Moreover, compared with traditional sulfide, hydroxide, and even noble metal modified ZnIn2S4/M counterparts (M = NiS, Ni(OH)2, Pt), the ZnIn2S4/Nix-B also maintains markedly higher photocatalytic activity, showing a highly efficient and economical advantage of the Nix-B cocatalyst. This work sheds light on the exploration of 2D ultrathin semiconductors decorated with novel transition metal boride cocatalyst for efficient photocatalytic organic transformation integrated with solar fuel production.

Two-Dimensional Metal-Organic Framework Nanosheets: Synthesis and Applications in Electrocatalysis and Photocatalysis.

Two-dimensional metal-organic nanosheets (2D MONs) are an emerging class of ultrathin, porous, and crystalline materials. The organic/inorganic hybrid nature offers MONs distinct advantages over other inorganic nanosheets in terms of diversity of organic ligands and metal notes. Compared to bulk three-dimensional metal-organic frameworks, 2D MONs possess merits of high density and readily accessible catalytic sites, reduced diffusion pathways for reactants/products, and fast electron transport. These features endow MONs with enhanced physical/chemical properties and are ideal for heterogeneous catalysis. In this Review, state-of-the-art synthetic methods for the fabrication of 2D MONs were summarized. The advances of 2D MONs-based materials for electrocatalysis and photocatalysis, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2 RR), and electro-/photocatalytic organic transformations were systematically discussed. Finally, the challenges and perspectives regarding future design and synthesis of 2D MONs for high-performance electrocatalysis and photocatalysis were provided.

Functionalized UiO-66(Ce) for photocatalytic organic transformation: the role of active sites modulated by ligand functionalization

The active sites in UiO-66(Ce) can be modulated by ligand functionalization, which plays an important role in photocatalytic organic transformation.

Facile construction of olefin-linked covalent organic frameworks for enhanced photocatalytic organic transformation via wall surface engineering

We present a facile strategy to regulate the absorption range, band gap, charge transfer efficiency and photocatalytic activity of olefin-linked 2D COFs via wall surface engineering.

Modulating electronic structure of triazine-based covalent organic frameworks for photocatalytic organic transformations

By virtue of visible light absorption ability, the energy band structure and electronic affinity of covalent organic frameworks can be easily tuned at the molecular level, making them attractive photocatalysts for metal-free organic transformation.

Stabilizing atomically precise metal nanoclusters as simultaneous charge relay mediators and photosensitizers

Atomically precise metal nanoclusters (NCs) are judiciously stabilized as simultaneous charge relay mediators and photosensitizers for photocatalytic organic transformation.

Fabricating a hollow cuboctahedral structure for N-doped carbon coated p-n heterojunctions towards high-performance photocatalytic organic transformation

Fabricating a hollow cuboctahedral structure for N-doped carbon coated p-n heterojunctions towards high-performance photocatalytic organic transformation

Tuning product selectivity and visible-light-driven activity in oxidative coupling of amines to imines: A case study of BiOIxCl1−x photocatalyst

BiOCl has shown a promising photocatalytic activity in non-selective oxidation reactions, however its application in selective photocatalytic organic transformations is often limited by the strong oxidizing ability of photogenerated holes along with inefficient visible-light absorption. Herein, we showed that the poor visible-light-harvesting ability and low product selectivity of BiOCl in the selective oxidation of primary amines to corresponding imines can be alleviated by band energy level modification using a solid solution strategy. We combined an efficient visible light absorption performance of BiOI with a strong oxidizing ability of BiOCl to achieve BiOIxCl1−x solid solution catalysts with substantial improvements in imine yield. Among the BiOIxCl1−x catalysts, BiOI0.2Cl0.8 delivers the highest benzylamine conversion of ~84% with a selectivity of ~96% towards the imine, while pure BiOCl shows much lower conversion (~65%) and product selectivity (~81%). Such excellent performance could be attributed to electronic structure modifications induced by iodine atom incorporation into BiOCl structure as supported by UV–vis DRS, Mott-Schottky, and VB-XPS studies. Based on photoelectrochemical studies and material characterizations, band energy diagram of the BiOI0.2Cl0.8 is proposed and compared with that of pristine BiOCl and BiOI. Radical scavenging study, EPR spin trapping result, and Hammett plot suggest that the imine formation mechanism may occur via both 1O2- and O2•–-mediated pathways. This work highlights a rational catalyst design for which the benefits from each individual components are used to maximize photocatalytic performance toward the selective synthesis of value-added organic compounds.

Spatially Separated Photoinduced Charge Carriers for the Enhanced Photocatalysis Over the One-Dimensional Yolk–Shell In2Se3@N-C Nanoreactor

Charge separation is crucial for applications of semiconductors in photocatalysis, especially in the field of photocatalytic organic transformation involving both the electrons and holes. Herein, we have spatially separated the photoinduced charge carriers on a nanoscale by engineering both the composition and morphology of a semiconductor-based photocatalyst. A yolk–shell nanorod structure (In2Se3@N-C YSR), which is composed of interpenetrated In2Se3 nanosheets as the core and an N-doped carbon layer as the shell, has been fabricated. This yolk–shell structure with confined cavities can provide a high surface area and multiple light reflections. The spatial distribution of the In2Se3 nanosheets and N-doped carbon layer can provide spatially separated redox-active sites, due to the existence of a controlled transfer pathway from In2Se3 to N-doped carbon layer for electrons, which can be revealed by photocurrent measurements, time-resolved photoluminescence spectra, and density functional theory (DFT) calculations. Therefore, on the basis of the above merits, the obtained In2Se3@N-C YSR has exhibited highly enhanced photocatalytic activity toward photocatalytic organic transformations, including the selective oxidation of amines to imines and the cross-dehydrogenative coupling reaction. In addition, DFT calculations have also indicated that the effective adsorption of the substrate on In2Se3@N-C also has a good influence on the photocatalytic process of organic transformation.

Photocatalytic C−F Bond Activation of Fluoroarenes, gem‐Difluoroalkenes and Trifluoromethylarenes

AbstractFluorine‐containing compounds have been widely applied on pharmaceuticals, agrochemicals, and materials. The synthetic methodology for the introduction of fluorine into organic motifs has witnessed dramatically development recently. Selective C−F bond activation and functionalization of the easily accessible fluorinated compounds has provided an efficient way to synthesize novel fluorinated products and degradate the extremely stable fluorinated pollutants. As the development of photocatalytic organic transformations, the light induced direct C−F bond activation under mild conditions leading to the production of various functionalities has become one of the most advanced and efficient methodologies available. This minireview summarizes the recent development of the photocatalytic C−F bond activations of fluorinated compounds, including fluoroarenes, gem‐difluoroalkenes and trifluoromethylarenes, and intends to illustrate the readers a comprehensive introduction of this research topic via demonstrating the representative examples and detailed mechanism according to reaction types for the C−F bond transformations.

Metal Halide Perovskites for Solar Fuel Production and Photoreactions.

Photocatalysis is an easily configurable and cost-effective technology for the conversion of solar energy into chemical energy. Recently, increasing attention has been given to metal halide perovskite (MHP) photocatalysts because of the development of stabilization strategies for MHPs under reaction conditions. From this perspective, we first describe several substantial breakthroughs in the photocatalytic application of MHPs. Performance trends in the solar fuel production applications of MHPs, including photocatalytic H2 generation and photocatalytic CO2 reduction reactions, are then described. Recent developments to extend the use of MHPs to various photocatalytic organic transformations are also highlighted. Finally, we propose several scientific challenges for the practical implications of MHPs for solar fuel production and various photoreactions.

Advances in Photocatalytic Organic Synthetic Transformations in Water and Aqueous Media

Advances in Photocatalytic Organic Synthetic Transformations in Water and Aqueous Media

Covalent organic frameworks: Design, synthesis, and performance for photocatalytic applications

To resolve the issues of environmental pollution and seek for sustainable clean fuels, many photocatalytic systems based on metal compounds and amorphous polymers have been studied over the past few years. However, applications of these photocatalytic systems were limited due to intrinsic drawbacks of the structures, which make the exploitation of new class of materials necessary. Recently, covalent organic frameworks (COFs) comprised of light elements, have attracted tremendous attention due to their excellent physiochemical properties. Specially, high crystallinity, good stability, intrinsic porosity and unique covalent linkages make COFs suitable for the application under harsh photocatalytic conditions. Predesigned structure of COFs provides a good platform for constructing diverse photocatalytic systems. Up to now, though the research on COFs and COFs-based photocatalysts is still in its infancy, great progress has been made especially in the H2 or O2 evolution, CO2 reduction, photocatalytic organic transformations, and elimination of organic pollutants. In this review, we focus on the developments in COFs-based photocatalytic materials and analyze the relationship between the predesigned COFs structures and their photocatalytic properties. Moreover, we elaborate the strategies for enhancing catalytic activities of COFs-based photocatalysts. Finally, the perspectives on new opportunities and challenges in the field are discussed.

Synergy of surface fluorine and oxygen vacancy of TiO2 nanosheets for O2 activation in selective photocatalytic organic transformations

The improved photocatalytic activity can be mainly attributed to the synergy between FS and adjacent VO, forming the frustrated Lewis pairs (FLP) that boosts the activation of O2.

Heterogeneous Photocatalytic Organic Transformation Reactions Using Conjugated Polymers-Based Materials

Photocatalytic heterogeneous organic transformation is considered as an efficient, clean atomic economy, and low-energy consumption strategy for organic synthesis. Conjugated polymers (CPs)-based materials have recently shown great potential for diverse photocatalytic applications because of their unique properties, such as structural designability, recyclability, high chemical stability, and low cost, and they have emerged as promising alternatives to traditional molecular or inorganic photocatalyst in photoredox reactions. Immense efforts have been made toward the construction of CPs-based materials for versatile photocatalytic chemical transformations. In this Review, we aim to summarize the recent progress in CPs-based photocatalysts for heterogeneous photocatalytic organic transformations including oxidation, reduction, coupling, and cycloaddition reaction. This Review discusses the influence of molecular, electronic, and channel structure of CPs-based materials on light absorption, charge separation, and mass transfer in different photocatalytic photoredox reactions. The regulated synthesis, mechanistic discussions, and future challenges for heterogeneous photocatalytic organic transformation in CPs-based materials systems are also considered. It is expected that this Review could not only deepen the comprehension of photocatalytic organic transformation but also open up inspirations and feasibilities for the applications of CPs-based materials.

Metal–Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) Applied to Photocatalytic Organic Transformations

Among the different alternatives for catalysis using metal–organic frameworks (MOFs) or covalent organic frameworks (COFs), photocatalysis has remarkably evolved during the last decade. Photocatalytic reticular materials allowed recyclability and easy separation of catalyst from the product, also reaching the activity and selectivity commonly observed for molecular systems. Recently, photocatalytic MOFs and COFs have been applied to synthetic applications in order to obtain organic molecules of different complexity. However, although a good number of works have been devoted to this issue, an updated comprehensive revision on this field is still needed. The aim of this review was to fill this gap covering the following three general aspects: (1) common strategies on the design of reticular photocatalytic materials, (2) a comprehensive discussion of the photocatalytic organic reactions achieved by the use of COFs and MOFs, and (3) some critical considerations highlighting directions that should be considered in order to make advances in the study of photocatalytic COFs and MOFs.

Vinylene-Bridged Two-Dimensional Covalent Organic Frameworks via Knoevenagel Condensation of Tricyanomesitylene.

Vinylene-bridged covalent organic frameworks (COFs) have shown great potential for advanced applications because of their high chemical stability and intriguing semiconducting properties. Exploring new functional monomers available for the reticulation of vinylene-bridged COFs and establishing effective reaction conditions are extremely desired for enlarging the realm of this kind of material. In this work, a series of vinylene-bridged two-dimensional (2D) COFs are synthesized by Knoevenagel condensation of tricyanomesitylene with ditopic or tritopic aromatic aldehydes. With use of appropriate secondary amines as catalysts, high-crystalline vinylene-bridged COFs were achieved, exhibiting long-range ordered structures, well-defined nanochannels, high surface areas (up to 1231 m2 g-1), and excellent photophysical properties. Under a low loading amount and short reaction time, they enable aerobic photocatalytic transformation of arylboronic acids to phenols with high efficiency and excellent recyclability. This work demonstrates a new functional monomer, tricyanomesitylene, feasible for the general synthesis of vinylene-bridged COFs with potential application in photocatalytic organic transformation, which instigates further research on such kind of material.

Structural Engineering of Two-Dimensional Covalent Organic Frameworks for Visible-Light-Driven Organic Transformations.

Covalent organic frameworks (COFs) emerging as a novel kind of visible light-responsive organic semiconductor have attracted extensive research attention in the field of photocatalytic organic transformations. However, the key parameters affecting their photocatalytic properties are still not clear. In this work, a series of [3 + 3] COFs with similar two-dimensional hexagonal structure but different compositions are synthesized and employed as model materials for investigating the key factors affecting the photocatalytic properties in the visible-light-driven reductive dehalogenation reaction and the aerobic cross-dehydrogenative coupling reaction. In comparison with -H and -CF3, the -OH substituent in the aromatic ring could narrow the band gap of the COFs. The COFs with a triazine skeleton in the framework usually boost the photocatalytic activity, possibly because of the enhanced charge separation efficiency by the formation of a donor-acceptor domain. As a combined result of the narrow band gap, efficient charge separation, and high conductivity, the COF possessing both a -OH group and triazine skeleton shows the highest activity in the photocatalytic reductive dehalogenation reaction. Notably, COFs could be easily recovered and reused several times without the loss of crystallinity. Our primary results may shed light on the design of efficient COF-based semiconductors for photocatalytic organic transformations.

Boosting photocatalytic oxidative coupling of amines by a Ru-complex-sensitized metal-organic framework

Visible-light-driven selective photocatalytic organic synthesis has recently become a topic of great interest due to its environmental friendliness and sustainability. It is demanding for photocatalysis to utilize the wider range of light, such as visible light, and its performance is often plagued by the sluggish separation of photogenerated charge carriers. An approach is now reported to address these issues by incorporating light harvesting RuII-polypyridyl complexes into a semiconductor-type metal-organic framework (MIL-125). Delightedly, the obtained Ru(bpy)3@MIL-125 photocatalyst presents a remarkably stable and high photoactivity toward the selective oxidative coupling of amines under ambient air with visible light irradiation (λ > 440 nm). The mechanistic investigation unveiled that both effectively photoexcited electrons transfer from [Ru(bpy)3]Cl2 to MIL-125 and the interaction of CH bonds with superoxide radical (O2·−) play a critical role in photo-catalyzing selective aerobic oxidative coupling of amines. This work highlights a significant role of MOFs as heterogeneous photocatalysts in photocatalytic organic transformations.