Fully Conjugated Covalent Organic Framework Nanosheets for Visible-Light-Driven Organic Synthesis in Water

Covalent organic framework (COF) nanosheets have recently garnered great attention as a new class of functional materials. As one of the sustainable processes, however, the photocatalytic organic synthesis in water has not been investigated using COF nanosheets as a photocatalyst to date. Herein, we reported the synthesis of a fully conjugated COF nanosheets with carboxyl functional group through a cooperative strategy of chemical exfoliation and group transformation. The new COF nanosheets was found to be an efficient heterogeneous photocatalyst for a wide range of organic synthesis including selective oxidation of sulfides and oxidative coupling of benzylamines in water under visible-light illumination. This work contributes a new roadmap for the design and synthesis of functional COF-based nanosheets, but also further extends the application boundary of the ultrathin COF nanosheets.

Molecular surface-dependent light harvesting and photo charge separation in plant-derived carbon quantum dots for visible-light-driven OH radical generation for remediation of aromatic hydrocarbon pollutants and real wastewater

Despite the growing emphasis on eco-friendly nanomaterials as energy harvesters, scientists are actively searching for metal-free photocatalysts to be used in environmental remediation strategies. Developing renewable resource-based carbon quantum dots (CQDs) as the sole photocatalyst to harvest visible light for efficient pollutant degradation is crucial yet challenging, particularly for addressing the escalating issue of water deterioration. Moreover, the photocatalytic decomposition of H2O2 under visible light irradiation remains an arduous task. Based on this, we designed two types of CQDs, C-CQDs (carboxylic-rich) and A-CQDs (amine-rich) with distinct molecular surfaces. Owing to the higher amount of upward band bending induced by amine-rich molecular surface, A-CQDs efficiently harvest the visible light and prevent recombination kinetics resulting in prolonged lifetimes (25 ps), and augmented charge carrier density (35.7 × 1018) of photoexcited charge carriers. A-CQDs enabled rapid visible-light-driven photolysis of H2O2 (k = 0.058 min-1) and produced higher quantity of •OH radicals (0.158 μmol/sec) for the mineralization of petroleum waste, BETX (i.e. Benzene, Ethylbenzene, Toluene and Xylene) (k = 0.017-0.026 min-1) and real textile wastewater (k = 0.026 min-1). To assess comparative toxicities of both remediated and non-remediated real wastewater samples in a time and dose depended manner, Drosophila melanogaster was used as a model organism. The findings unequivocally demonstrate the potential of remediated wastewater for watering urban forestry.

Alcohol imination catalyzed by carbon nanostructures synthesized by C(sp2)-C(sp3) free radical coupling

Imines are important intermediates for synthesizing various fine chemicals, with the disadvantage of requiring the use of expensive metal-containing catalysts. We report that the dehydrogenative cross-coupling of phenylmethanol and benzylamine (or aniline) directly forms the corresponding imine with a yield of up to 98%, and water as the sole by-product, in the presence of a stoichiometric base, using carbon nanostructures as the "green" metal-free carbon catalysts with high spin concentrations, which is synthesized by C(sp2)-C(sp3) free radical coupling reactions. The catalytic mechanism is attributed to the unpaired electrons of carbon catalysts to reduce O2 to O2·-, which triggers the oxidative coupling reaction to form imines, whereas the holes in the carbon catalysts receive electrons from the amine to restore the spin states. This is supported by density functional theory calculations. This work will open up an avenue for synthesizing carbon catalysts and offer great potential for industrial applications.

Carbon Quantum Dots: Synthesis, Structure, Properties, and Catalytic Applications for Organic Synthesis

Carbon quantum dots (CQDs), also known as carbon dots (CDs), are novel zero-dimensional fluorescent carbon-based nanomaterials. CQDs have attracted enormous attention around the world because of their excellent optical properties as well as water solubility, biocompatibility, low toxicity, eco-friendliness, and simple synthesis routes. CQDs have numerous applications in bioimaging, biosensing, chemical sensing, nanomedicine, solar cells, drug delivery, and light-emitting diodes. In this review paper, the structure of CQDs, their physical and chemical properties, their synthesis approach, and their application as a catalyst in the synthesis of multisubstituted 4H pyran, in azide-alkyne cycloadditions, in the degradation of levofloxacin, in the selective oxidation of alcohols to aldehydes, in the removal of Rhodamine B, as H-bond catalysis in Aldol condensations, in cyclohexane oxidation, in intrinsic peroxidase-mimetic enzyme activity, in the selective oxidation of amines and alcohols, and in the ring opening of epoxides are discussed. Finally, we also discuss the future challenges in this research field. We hope this review paper will open a new channel for the application of CQDs as a catalyst in organic synthesis.

Carbon Quantum Dots from Amino Acids Revisited: Survey of Renewable Precursors toward High Quantum-Yield Blue and Green Fluorescence

Carbon quantum dots (CQDs) were synthesized via a green, one-step hydrothermal method. As CQD precursors, nine amino acids of different structural descriptors (negatively/positively charged in water, polar, hydrophobic, sulfur-containing, and other/complex ones) were surveyed: Asp, Cys, Gly, His, Leu, Lys, Phe, Pro, and Ser. The reactions were performed in an autoclave in the presence of citric acid at 180 °C for 24 h and yielded core-shell CQDs. CQDs were comprehensively characterized by transmission electron microscopy, dynamic light scattering, Raman, UV/Vis, infrared, X-ray photoelectron spectroscopy, and fluorescence spectroscopy. At the excitation wavelength of λ_ex = 350 nm, Cys-, Phe-, Leu-, and Lys-based CQDs displayed the highest quantum yield blue fluorescence-90 ± 5, 90 ± 4, 87 ± 5, and 67 ± 3%, respectively-superior to the conventional fluorescent dyes. Strikingly, for Lys- and Phe-CQDs, dissimilar trends in the excitation-emission wavelength relationships were identified, that is, constantly strong red shifts versus excitation wavelength-independent emission. Cys- and Lys-CQDs were water-dispersible toward the narrow unimodal distribution of hydrodynamic diameters-0.6 and 2.5 nm, respectively. Additionally, Lys- and Cys-CQDs, with high absolute zeta potential values, formed stable aqueous colloids in a broad range of pH (2, 7, and 12). The results constitute important premises for water-based applications of CQDs, such as bioimaging or photocatalysis.

Light-Induced Hypoxia in Carbon Quantum Dots and Ultrahigh Photocatalytic Efficiency

Carbon quantum dots (CQDs) represent a class of carbon materials exhibiting photoresponse and many potential applications. Here, we present a unique property that dissolved CQDs capture large amounts of molecular oxygen from the air, the quantity of which can be controlled by light irradiation. The O₂ content can be varied between a remarkable 1 wt % of the CQDs in the dark to nearly half of it under illumination, in a reversible manner. Moreover, O₂ depletion enhances away from the air-solution interface as the nearby CQDs quickly regain them from the air, creating a pronounced concentration gradient in the solution. We elucidate the role of the CQD functional groups and show that excitons generated under light are responsible for their tunable adsorbed-oxygen content. Because of O₂ enrichment, the photocatalytic efficiency of the CQDs toward oxidation of benzylamines in the air is the same as under oxygen flow and far higher than the existing photocatalysts. The findings should encourage the development of a new class of oxygen-enricher materials and air as a sustainable oxidant in chemical transformations.

Visible Light-Induced Aerobic Oxidative Dehydrogenation of C-N/C-O to C=N/C=O Bonds Using Metal-Free Photocatalysts: Recent Developments

Performing synthetic transformation using visible light as energy source, in the presence of a photocatalyst as a promoter, is currently of high interest, and oxidation reactions carried out under these conditions using oxygen as the final oxidant are particularly convenient from an environmental point of view. This review summarizes the recent developments achieved in the oxidative dehydrogenation of C-N and C-O bonds, leading to C=N and C=O bonds, respectively, using air or pure oxygen as oxidant and metal-free homogeneous or recyclable heterogeneous photocatalysts under visible light irradiation.

Non-emissive RuII Polypyridyl Complexes as Efficient and Selective Photosensitizers for the Photooxidation of Benzylamines

Five new Ru^II polypyridyl complexes bearing N-(arylsulfonyl)-8-amidoquinolate ligands and three of their biscyclometalated Ir^III congeners have been prepared and employed as photocatalysts (PCs) in the photooxidation of benzylamines with O₂ . In particular, the new Ru^II complexes do not exhibit photoluminescence, rather they harvest visible light efficiently and are very stable in solution under irradiation with blue light. Their non-emissive behavior has been related to the low electrochemical energy gaps and rationalized on the basis of theoretical calculations (DFT analysis) that predict low S₀ ←T₁ energy values. Moreover, the Ru^II complexes, despite being non-emissive, display excellent activities in the selective photocatalytic transformation of benzylamines into the corresponding imines. The presence of an electron-withdrawing group (-CF3) on the arene ring of the N-(arylsulfonyl)-8-amidoquinolate ligand improves the photocatalytic activity of the corresponding photocatalyst. Furthermore, all the experimental evidence, including transient absorption spectroscopy measurements suggest that singlet oxygen is the actual oxidant. The Ir^III analogues are considerably more photosensitive and consequently less efficient photosensitizers (PSs).

Nanocrystalline Ag3PO4 for Sunlight- and Ambient Air-Driven Oxidation of Amines: High Photocatalytic Efficiency and a Facile Catalyst Regeneration Strategy

Selective oxidation of amines to imines using sunlight as clean and renewable energy source is an important but challenging chemical transformation because of high reactivity of the generated imines and lack of visible light-responsive materials with high conversion rates. In addition, oxygen gas has to be purged in the reaction mixture in order to increase the reaction efficiency which, in itself, is an energy-consuming process. Herein, we report, for the first time, the use of Ag₃PO₄ as an excellent photocatalyst for the oxidative coupling of benzyl amines induced by ambient air in the absence of any external source of molecular oxygen at room temperature. The conversion efficiency for the selective oxidation of benzyl amine was found to be greater than 95% with a selectivity of >99% after 40 min of light irradiation indicating an exceptionally high conversion efficiency with a rate constant of 0.002 min^-1, a turnover frequency of 57 h^-1, and a quantum yield of 19%, considering all of the absorbed photons. Ag₃PO₄, however, is known for its poor photostability owing to a positive conduction band position and a favorable reduction potential to metallic silver. Therefore, we further employed a simple catalyst regeneration strategy and showed that the catalyst can be recycled with negligible loss of activity and selectivity.

Targeted Development of Sustainable Green Catalysts for Oxidation of Alcohols via Tungstate-Decorated Multifunctional Amphiphilic Carbon Quantum Dots

Achieving green and sustainable chemical processes by replacing organic solvents with water has always been one of the green chemistry goals and a challenging topic for chemists. However, the poor solubility of organic materials is a major limitation to achieving this goal, especially in alcohol oxidation. In this contribution, the development and design of amphiphilic catalysts via abundant, safe, cheaper, and more biocompatible sources have received notable attention. To this purpose, herein, our group successfully synthesized a new multifunctional amphiphilic carbon quantum dot (CQD) composed of 1-aminopropyl-3-methyl-imidazolium chloride ([APMim][Cl]), dodecylamine (DDA), and citric acid (CA) (denoted as CQDs@DDA-IL/Cl) using a one-pot hydrothermal route. The CQDs@DDA-IL/Cl was then utilized as an amphiphilic stabilizer for anchoring tungsten ions using an anion-exchange method (marked as CQDs@DDA-IL/W). The CQDs@DDA-IL/W as a reusable catalyst selectivity mediated the oxidation of alcoholic substrates with stoichiometric H₂O₂ in water solvent. The extraordinary performance of our catalyst was attributable to the coexistence of ionic liquid (IL) and DDA upon the surface of the CQDs@DDA-IL/W, which plays a main duty in the hydrophobic/hydrophilic balance, and significantly increase the catalyst compatibility in the aqueous medium with the purpose of removing organic solvents. As a result, the great mass transfer occurs in the two-phase medium using this amphiphilic nanocatalyst without any phase transfer catalyst (PTC) or other additives. The 100% selectivity, excellent turnover number (TON) and turnover frequency (TOF), high yield, almost complete and fast conversion of alcohol to the desired aldehydes and ketones without more oxidation, and easy and no-trouble isolation of product and catalyst are outstanding features of this catalytic system.