Photocatalytic Transformations Research Articles

Fully Conjugated Microporous Polymers as Metal-Free Heterogeneous Photocatalysts for Organic Transformations.

Photoactive conjugated microporous polymers (CMPs) have recently received huge attention in photocatalytic organic transformations owing to their adjustable structure and functionality. However, commonly reported CMPs are synthesized through metal catalyzed coupling reactions, which require complicated product separation and result in increased costs. In this study, two sp2 carbon-linked CMPs are constructed by organic base induced Knoevenagel reaction using 2,6-dimethylbenzo[1,2-d:4,5-d']bisoxazole and aromatic polyaldehydes as co-monomers. The new benzobisoxazole-based polymer materials feature fully π-conjugated skeleton with broad visible-light absorption, permanent porosity as well as outstanding stability. Importantly, they can effectively induce many organic reactions such as C-3 thiocyanation of indoles under visible-light illumination and show broad substrate applicability and superior recyclability.

Energy-transfer-enabled photocatalytic transformations of aryl thianthrenium salts

Aryl thianthrenium salts are valuable in photocatalysis but traditionally require external electron donors for activation. This study introduces an energy transfer (EnT) strategy for the activation of aryl thianthrenium salts using 2,3,4,5,6-penta(carbazol-9-yl)benzonitrile (5CzBN) as a metal-free photocatalyst, eliminating the need for external donors. Utilizing this EnT approach, we achieve C–H deuteration of arenes under visible light with CDCl3 as a deuterium source to synthesize various deuterated aromatic compounds, including important natural products and pharmaceuticals. Additionally, this strategy enables diverse functionalizations including borylation, arylation, cyanation, and selenylation, enhancing the applicability of aryl sulfonium salts in environmentally friendly photocatalysis.

Selective Oxidation of Alcohols to Carbonyls Under Decatungstate-Mediated Photoelectrochemical Conditions.

The oxidation of alcohols to the corresponding carbonyl derivatives has been realized under photoelectrochemical conditions in the presence of tetrabutylammonium decatungstate (TBADT) as the homogeneous photocatalyst. The protocol can be applied to both primary and secondary, benzylic and aliphatic alcohols. The desired products are obtained selectively, skipping the need for purposely added chemical oxidants. An in-depth study of photoelectrochemical conditions revealed that the protocol works best under amperostatic conditions in an undivided electrochemical cell irradiated with a 390 nm LED lamp. The comparison with analogous electrochemical and chemical oxidant-promoted photocatalytic transformations demonstrates the superior efficiency and selectivity of the hereby reported photoelectrochemical conditions.

Hole Polaron-Mediated Suppression of Electron-Hole Recombination Triggers Efficient Photocatalytic Nitrogen Fixation.

In the pursuit of successful photocatalytic transformations, challenges persist due to limitations in charge carrier utilization and transfer efficiency, which stemming from rapid recombination. Overcoming these limitations necessitates the exploration of novel mechanisms that enhance the effective separation of photogenerated electron-hole pairs. Herein, deviating from the conventional approach of enhancing carrier migration to separate photogenerated charges and extend their lifetime, the proposal is to directly prevent the recombination of photogenerated electrons and holes by forming hole polarons. Specifically, disordered pores are introduced on the surface of KTaO3 ultrathin sheets, and the clear-cut evidences in electron paramagnetic resonance, photoluminescence, and ultrafast spectroscopy unambiguously confirm the enhanced carrier-phonon coupling, which results in the formation of hole polarons to impede the recombination of photogenerated electron-hole pairs. Taking the challenging nitrogen oxidation reaction as an example, it is found that the hole polarons in atomic-disordered pore KTaO3 ultrathin nanosheets trigger outstanding photo-oxidation performance of nitrogen (N2)to nitrate, with a nitrate-producing rate of 2.1 mg g-1 h-1. This scenario is undoubtedly applicable to a wide variety of photocatalytic reactions due to the common challenge of charge carrier recombination in all photocatalytic processes, manifesting broad implications for promoting photocatalysis performance.

Cooperative α-C–H activation enabled quantitative and partial photooxidation of biomass-derived 5-hydroxymethylfurfural

Photocatalytic activation of C‒H bonds is versatile but challenging for undergoing oriented conversion processes. Herein, a spatially site-isolated heterojunction (ZS-Vs/ZIS) of ZnIn2S4 with strong Lewis acidity (ZIS) and ZnS with S-vacancy (ZS-Vs) is constructed for activating α-C‒H bond and forming •O2‒ to cleave the C‒H bond, respectively. ZS-Vs/ZIS displays outstanding performance in visible-light partial photooxidation of bio-based 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in an unprecedented yield of 95.7% at 25 ºC. In-situ experiments and calculations reveal that Zn sites of ZIS serve as hole enrichment to adsorb HMF for α-C‒H activation via ligand-to-metal charge transfer. Shallow trap states introduced by S-vacancy in ZS-Vs act as an electron pool to realize directed O2 activation into •O2‒ for breaking pre-activated α-C‒H bond in HMF to exclusively give DFF. Moreover, ZS-Vs/ZIS has good recyclability and universality in the photooxidation of various alcohols to carbonyls (86.4‒95.6% yields). The synergistic C‒H activation/breaking strategy exhibits high potential in targeted photocatalytic transformations.

Carbonized Polymer Dots-Promoted Photocatalytic Activation of Molecular Oxygen for Efficient and Selective Oxidation of Thioethers to Sulfoxides.

Sulfoxides are essential in pharmaceuticals and chemicals, yet traditional thioether oxidation struggles with selectivity and sustainability. This study introduces carbonized polymer dots (CPDs) as effective photocatalysts for ecofriendly thioether to sulfoxide oxidation, using water and ethanol to enhance reaction selectivity and efficiency under 455 nm blue light. These catalysts not only show remarkable efficacy under mild conditions but also display high selectivity for sulfoxide formation, proving versatile across a broad range of substrates. We further elucidated the catalytic mechanism, confirming the predominant roles of singlet oxygen and superoxide anions through both spectroscopic evidence and quenching experiments. The method extends to the synthesis of pharmaceuticals such as oxfendazole, albendazole sulfoxide, and sulindac, highlighting its practical utility. Overall, our findings present a sustainable and efficient avenue for sulfoxide synthesis, thereby broadening the practical utility of CPDs in photocatalytic transformations.

Recent Advances in the Utilization of Chiral Covalent Organic Frameworks for Asymmetric Photocatalysis.

The use of light energy to drive asymmetric organic transformations to produce high-value-added organic compounds is attracting increasing interest as a sustainable strategy for solving environmental problems and addressing the energy crisis. Chiral covalent organic frameworks (COFs), as porous crystalline chiral materials, have become an important platform on which to explore new chiral photocatalytic materials due to their precise tunability, chiral structure, and function. This review highlights recent research progress on chiral COFs and their crystalline composites, evaluating their application as catalysts in asymmetric photocatalytic organic transformations in terms of their structure. Finally, the limitations and challenges of chiral COFs in asymmetric photocatalysis are discussed, with future opportunities for research being identified.

Carbon dot-based type I photosensitizers for photocatalytic oxidation reaction of arylboric acid and N-phenyl tetrahydroisoquinoline

Carbon dots (CDs) have emerged as promising materials for photocatalytic organic transformations due to their excellent photostability, tunable electronic properties, and environmental friendliness; However, the ability of CDs to selectively generate reactive oxygen species (ROS) and its integration with organic photocatalytic synthesis applications has always been a long-term challenge. In this work, we synthesized a new nitrogen and phosphorus co-doped carbon dots (N,P-CDs) with enhanced light absorption and notable efficiency in generating superoxide anion (O2•−) selectively. Leveraging the selective generation of superoxide anions, we achieved highly efficient photooxidation of boronic acids and N-phenyl tetrahydroisoquinolines, demonstrating the practical applicability of N,P-CDs as photocatalysts and represents good functional-group tolerance as well as a broad substrate scope. This study provides valuable insights into the design of carbon-based photocatalysts with controlled ROS generation, opening new avenues for environmentally benign organic transformations.

Celebrating Ten Years of Covalent Organic Frameworks for Solar Energy Conversion: Past, Present and Future

AbstractAccelerated anthropogenic emission of greenhouse gases due to increasing energy demands has created a negative impact on our planet. Therefore, the replacement of fossil by renewable energy resources has become of paramount interest, both societally and scientifically. It is within this setting that organic photocatalysts have emerged as a new generation of earth‐abundant catalysts for the conversion of solar radiation into chemical energy. In 2014, the first example of a covalent organic framework (COF) photocatalyst for the hydrogen evolution reaction was reported by our group, which has not only marked the beginning of COF photocatalysis for solar fuel production but also helped to accelerate research into “soft photocatalysis” based on porous polymers in general. In the last decade, significant progress has been made toward developing COFs as robust, molecularly precise platforms emulating artificial photosynthesis. This mini‐review commemorates the 10th anniversary of COF photocatalysis and gives a brief historical overview of the milestones in the field since its inception in 2014. We review milestones in the development of COFs for solar fuel production and related photocatalytic transformations, including hydrogen evolution, oxygen evolution, overall water splitting, CO2 reduction, N2 fixation, oxygen reduction, and alcohol oxidation. We discuss lessons learned for the design of structure‐property‐function relationships in COF photocatalysts, and future perspectives and challenges for the field of “soft photocatalysis” are given.

Celebrating Ten Years of Covalent Organic Frameworks for Solar Energy Conversion: Past, Present and Future.

Accelerated anthropogenic emission of greenhouse gases due to increasing energy demands has created a negative impact on our planet. Therefore, the replacement of fossil by renewable energy resources has become of paramount interest, both societally and scientifically. It is within this setting that organic photocatalysts have emerged as a new generation of earth-abundant catalysts for the conversion of solar radiation into chemical energy. In 2014, the first example of a covalent organic framework (COF) photocatalyst for the hydrogen evolution reaction was reported by our group, which has not only marked the beginning of COF photocatalysis for solar fuel production but also helped to accelerate research into "soft photocatalysis" based on porous polymers in general. In the last decade, significant progress has been made toward developing COFs as robust, molecularly precise platforms emulating artificial photosynthesis. This mini-review commemorates the 10th anniversary of COF photocatalysis and gives a brief historical overview of the milestones in the field since its inception in 2014. We review milestones in the development of COFs for solar fuel production and related photocatalytic transformations, including hydrogen evolution, oxygen evolution, overall water splitting, CO2 reduction, N2 fixation, oxygen reduction, and alcohol oxidation. We discuss lessons learned for the design of structure-property-function relationships in COF photocatalysts, and future perspectives and challenges for the field of "soft photocatalysis" are given.

Bioorthogonal Synthetic Chemistry Enabled by Visible-Light Photocatalysis.

The field of bioorthogonal chemistry has revolutionized our ability to interrogate and manipulate biological systems at the molecular level. However, the range of chemical reactions that can operate efficiently in biological environments without interfering with the native cellular machinery, remains limited. In this context, the rapidly growing area of photocatalysis offers a promising avenue for developing new type of bioorthogonal tools. The inherent mildness, tunability, chemoselectivity, and external controllability of photocatalytic transformations make them particularly well-suited for applications in biological and living systems. This minireview summarizes recent advances in bioorthogonal photocatalytic technologies, with a particular focus on their potential to enable the selective generation of designed products within biologically relevant or living settings.

Bioorthogonal Synthetic Chemistry Enabled by Visible‐Light Photocatalysis

AbstractThe field of bioorthogonal chemistry has revolutionized our ability to interrogate and manipulate biological systems at the molecular level. However, the range of chemical reactions that can operate efficiently in biological environments without interfering with the native cellular machinery, remains limited. In this context, the rapidly growing area of photocatalysis offers a promising avenue for developing new type of bioorthogonal tools. The inherent mildness, tunability, chemoselectivity, and external controllability of photocatalytic transformations make them particularly well‐suited for applications in biological and living systems. This minireview summarizes recent advances in bioorthogonal photocatalytic technologies, with a particular focus on their potential to enable the selective generation of designed products within biologically relevant or living settings.

Tris(triazolo)triazine‐Based Covalent Organic Frameworks for Efficiently Photocatalytic Hydrogen Peroxide Production

AbstractTwo‐dimensional covalent organic frameworks (2D‐COFs) have recently emerged as fascinating scaffolds for solar‐to‐chemical energy conversion because of their customizable structures and functionalities. Herein, two tris(triazolo)triazine‐based COF materials (namely COF‐JLU51 and COF‐JLU52) featuring large surface area, high crystallinity, excellent stability and photoelectric properties were designed and constructed for the first time. Remarkably, COF‐JLU51 gave an outstanding H2O2 production rate of over 4200 μmol g−1 h−1 with excellent reusability in pure water and O2 under one standard sun light, that higher than its isomorphic COF‐JLU52 and most of the reported metal‐free materials, owing to its superior generation, separation and transport of photogenerated carriers. Experimental and theoretical researches prove that the photocatalytic process undergoes a combination of indirect 2e− O2 reduction reaction (ORR) and 4e− H2O oxidation reaction (WOR). Specifically, an ultrahigh yield of 7624.7 μmol g−1 h−1 with apparent quantum yield of 18.2 % for COF‐JLU52 was achieved in a 1 : 1 ratio of benzyl alcohol and water system. This finding contributes novel, nitrogen‐rich and high‐quality tris(triazolo)triazine‐based COF materials, and also designate their bright future in photocatalytic solar transformations.

Tris(triazolo)triazine-Based Covalent Organic Frameworks for Efficiently Photocatalytic Hydrogen Peroxide Production.

Two-dimensional covalent organic frameworks (2D-COFs) have recently emerged as fascinating scaffolds for solar-to-chemical energy conversion because of their customizable structures and functionalities. Herein, two tris(triazolo)triazine-based COF materials (namely COF-JLU51 and COF-JLU52) featuring large surface area, high crystallinity, excellent stability and photoelectric properties were designed and constructed for the first time. Remarkably, COF-JLU51 gave an outstanding H2O2 production rate of over 4200 μmol g-1 h-1 with excellent reusability in pure water and O2 under one standard sun light, that higher than its isomorphic COF-JLU52 and most of the reported metal-free materials, owing to its superior generation, separation and transport of photogenerated carriers. Experimental and theoretical researches prove that the photocatalytic process undergoes a combination of indirect 2e- O2 reduction reaction (ORR) and 4e- H2O oxidation reaction (WOR). Specifically, an ultrahigh yield of 7624.7 μmol g-1 h-1 with apparent quantum yield of 18.2 % for COF-JLU52 was achieved in a 1 : 1 ratio of benzyl alcohol and water system. This finding contributes novel, nitrogen-rich and high-quality tris(triazolo)triazine-based COF materials, and also designate their bright future in photocatalytic solar transformations.

Pioneering Metal-Free Late-Stage C-H Functionalization Using Acridinium Salt Photocatalysis.

Using organic dyes as photocatalysts is an innovative approach to photocatalytic organic transformations. These dyes offer advantages such as widespread availability, adaptable absorption properties, and diverse chemical structures. Recent progress has led to the development of organic photocatalysts that can utilize visible light to modify chemically inert C-H bonds. These catalysts are sustainable, selective, and versatile, enabling mild reactions, late-stage functionalization, and various transformations in line with green chemistry principles. As catalysts in photoredox chemistry, they contribute to the development of efficient and environmentally friendly synthetic pathways. Acridinium-based organic photocatalysts have proved valuable in late-stage C-H functionalization, enabling transformative reactions under mild conditions. This review emphasizes their innovative features, such as organic frameworks, efficient light absorption properties, and their applications in modifying complex molecules. It provides an overview of recent advancements in the use of acridinium-based organic photocatalysts for late-stage C-H bond functionalization without the need for transition metals, showcasing their potential to expedite the development of new molecules and igniting excitement about the prospects of this research in the field.

(Invited) Semiconductor Facet Effects Toward Photocatalysis

Photo-induced charge transfer has been found to strongly depend on the exposed semiconductor surfaces. For instance, {110}-bound Cu2O rhombic dodecahedra exhibit a high photocatalytic activity toward dye degradation, but {100}-terminated Cu2O cubes are inert. Photocatalytic behaviors of Cu2O-based semiconductor heterostructures, such as Cu2O-ZnS and Cu2O-ZnO composites, can both enhance and suppress photocatalytic activity depending on the contacting planes at the heterojunction. Significantly, Cu2O photocatalytic activity can be greatly enhanced by functionalizing with conjugated molecules such as 4-nitrophenylacetylene (4-NA) to tune the surface band structure. This can turn the inactive cubes to possess a superior photocatalytic activity. Recently, Cu2O rhombic dodecahedra with and without 4-NA modification have been employed in photocatalytic hydroxylation of boronic acids, aryl sulfide oxidation, and amine coupling. SrTiO3 truncated rhombic dodecahedra were also found to be effective in amine coupling. Oxidative cyclization of thioamides is the latest example of surface control to photocatalytic organic transformations.

Challenges in photocatalysis using covalent organic frameworks

Photocatalysis is an attractive, energy-efficient technology for organic transformations, polymer synthesis, and degradation of environmental pollutants. There is a need for new photocatalysts stable in different media and that can be tailored for specific applications. Covalent organic frameworks (COF) are crystalline, nanoporous materials with π-conjugated backbone monomers, representing versatile platforms as heterogeneous, metal-free photocatalysts. The backbone structure can be tailored to achieve desired photocatalytic properties, side-chains can mediate adsorption, and the nanoporous structure provides large surface area for molecular adsorption. While these properties make COFs attractive as photocatalysts, several fundamental questions remain regarding mechanisms for different photocatalytic transformations, reactant transport into porous COF structures, and both structural and chemical stability in various environments. In this perspective, we provide a brief overview of COF photocatalysts and identify challenges that should be addressed in future research seeking to employ COFs as photocatalysts. We close with an outlook and perspective on future research directions in the area of COF photocatalysts.

Hydrogen‐Bond‐Mediated Formation of C−N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets

AbstractReductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C−N or C=N bond readily manipulated through a solvent‐induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive‐coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C−N bonds via hydrogen‐bonding activation. Experimental and theoretical study reveals that the building of the hydrogen‐bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C−N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

Hydrogen-Bond-Mediated Formation of C-N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets.

Reductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C-N or C=N bond readily manipulated through a solvent-induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive-coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C-N bonds via hydrogen-bonding activation. Experimental and theoretical study reveals that the building of the hydrogen-bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C-N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

Regulation on photocatalytic debromination by the reconfiguration of carbonyl groups on the covalent organic frameworks

Covalent organic frameworks (COFs) as promising organic semiconductor photocatalysts have become a research hotspot in the field of photocatalytic organic synthesis. Herein, a series of imine-linked EN-COF-x were synthesized by condensation reaction of amine monomer with triformylbenzene monomer. Then, β-ketoamine-linked KE-COF-y were obtained by the reconstruction of EN-COF-x in alkaline solution. The photocatalytic performance of COFs before and after the isomerization from enols to ketones were investigated. The characterization results and density functional theory calculations clearly demonstrated that the carbonyl groups not only make the conduction band energy level more negative, but also lead to a more inhomogeneous charge distribution in the molecule, which improved the separation efficiency of the photogenerated electrons. In addition, the increase in the number of hydroxyl groups on triformylbenzene monomer contributed to the formation of more carbonyl groups in the COF frameworks, which is beneficial to improving its photocatalytic performance. KE-COF-3 synthesized from 2,4,6-Trihydroxy-1,3,5-benzenetricarbaldehyde has more carbonyl group units and more suitable redox potential, which effectively promoted the debromination reaction with 99% yield under light irradiation. Meanwhile, it can be reused at least five times without loss of catalytic activity. This work provides insights for the rational design of functionalized β-ketoamine COFs for photocatalytic organic transformations.