Photoinduced Oxidation Reactions Research Articles

UV-C-induced changes in a white wine: Evaluating the protective power of hydrolysable tannins and SO<sub>2</sub>

UV‑C treatment is a non-thermal technology that could be used for the microbiological stabilisation of must and wine. However, light - in this case UV‑C light - has an impact on wine. UV‑C is a high-energy light source and can promote photo-induced oxidation reactions. The formation of reactive oxygen species during the UV-C treatment of wine can be prevented via supplementation with antioxidants. These have the ability to neutralise reactive oxygen species or directly interact with the products of photo-induced oxidation reactions. This study investigates the impact of different UV‑C doses on the chemical and sensory characteristics of Chardonnay wine and examines the protective effect of two antioxidants: sulphur dioxide (SO2) and hydrolysable tannins. A sensory evaluation revealed that UV‑C doses higher than 1 kJ/l increased the colour intensity and the oxidised and burnt odour attributes of the wine. The peach odour attribute decreased with UV‑C treatment. On a chemical level, increasing UV‑C doses promoted the formation of compounds such as acetaldehyde and 2‑aminoacetophenone (2‑AAP), while causing the degradation of C13‑norisoprenoids, higher alcohols, monoterpenes and esters. Contrary to expectations, SO2 did not act as an antioxidant when wine was treated with UV-C. It intensified the burnt odour attribute and did not mitigate the intensity of the oxidised odour attribute, which can be explained by the formation of acetaldehyde. However, hydrolysable tannins acted as antioxidants when combined with UV‑C treatment, effectively mitigating the formation of 2‑AAP and acetaldehyde, while preventing the wine from becoming oxidised.

A covalent crosslinking strategy to construct a robust peptide-based artificial esterase.

Peptide-based artificial enzymes derived from the supramolecular assembly of short peptides have attracted growing attention in recent years. However, the stability of these artificial enzymes is still a problem since their noncovalent supramolecular structure is quite sensitive and frail under environmental conditions. In this study, we reported a covalent crosslinking strategy for the fabrication of a robust peptide-based artificial esterase. Inspired by the di-tyrosine bonds in many natural structural proteins, multi-tyrosines were designed into a peptide sequence with histidine as the catalytic residue for the ester hydrolysis reaction. Upon the photo-induced oxidation reaction, the short peptide YYHYY rapidly transferred into nanoparticle-shaped aggregates (CL-YYHYY) and displayed improved esterase-like catalytic activity than some previously reported noncovalent-based artificial esterases. Impressively, CL-YYHYY showed outstanding reusability and superior stability under high temperature, strong acid and alkaline and organic solvent conditions. This study provides a promising approach to improving the catalytic activity and stability of peptide-based artificial enzymes.

Adsorption and Oxidation of NO2 on Anatase TiO2: Concerted Nitrate Interaction and Photon-Stimulated Reaction

The catalytic and photon-induced oxidation of NO2 on anatase TiO2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO3. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO3– species, where NO2 bonds to the lattice oxygen, and freely adsorbed NO3– ions. These products can be obtained either by dissociative adsorption of HNO3 or by catalytic/photocatalytic oxidation of NO2, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO3– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N2O5) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO2 crystal. In particular, a stable surface–NO3– and NO3– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N2O during continued UV light illumination.

Mechanochemical construction of Bi1-xLaxOI solid solution with abundant oxygen vacancies for enhanced photocatalytic oxidation of inorganic/organic arsenic

Bismuth oxyiodide is one of the bismuth-based semiconductors which exhibits narrow band gaps and excellent visible light response, however modification strategies are usually desired for improving its photocatalytic reactivity. In this context, introduction of valuable components into the interlayer of BiOI is expected to work, considering its typical layered structure. In this case, mechanochemical synthesis of BiOI, with simultaneous introduction of lanthanum into its interlayer to form Bi1-xLaxOI, was conducted for the first time, and arsenic (As(III)) was selected to evaluate the photocatalytic reactivity, demonstrating excellent reaction efficiency for arsenic oxidation under a mild condition. Characterizations confirmed the incorporation of La and the presented well crystalline structure of Bi1-xLaxOI, and revealed its better visible light absorption capacity than raw BiOI. Besides the excellent electronic conductivity of Bi0.9La0.1OI, photogenerated holes (h+) and superoxide radical species (·O2-) were characterized by electron spin resonance (ESR) test, to act as predominant reactive groups during the photo-induced oxidation reaction. These findings provided a novel concept for the interlayer decoration of BiOI, and offered an alternative approach to develop its potential application for the removal of hazardous arsenic pollutants.

Photoinduced oxidation of nickel in Ni(II)EDTA with TiO2 nanoparticles

TiO2 nanoparticles are well-known for their attractive photocatalytic properties for degradation of adsorbed species. Another area of interest is the use of TiO2 nanoparticles for preparation of photoelectrochromic materials, which are those designed to reversibly change color upon illumination and change of applied potential. This work explored conditions of photo-induced reactions of Ni(II)EDTA in the presence of TiO2 nanoparticles and detected a reversible oxidization reaction to Ni(III)EDTA as a result ultraviolet and visible irradiation. Once irradiation was terminated, the Ni(III)EDTA complex reverted back to Ni(II)EDTA, demonstrating its reversibility. The progress of the reaction was monitored by UV–vis absorption spectroscopy. The photo-induced oxidation reaction was examined for the effect of irradiation time, solution pH and metal–ligand complex concentration. The reaction favors acidic conditions and can be detected for concentrations on the order to 10-3 to 10-1 M Ni(II)EDTA.

Ni-doped MoS2 modified graphitic carbon nitride layered hetero-nanostructures as highly efficient photocatalysts for environmental remediation

Highly efficient and cost-effective photocatalysts are among the most prominent targets in the field of environmental remediation and clean energy production. Here, we report that 2D/2D layer heterostructures composed of exfoliated Ni-doped MoS2 nanosheets and g-C3N4 layers can carry out photocatalytic Cr(VI) reduction in aqueous solutions with outstanding activity, exhibiting apparent QYs as high as 29.6 % and 23.7 % at 375 and 410 nm. We show that Ni doping of MoS2 markedly increases the photochemical activity, which, together with electrochemical, spectroscopic and theoretical DFT studies, arises from the enhanced carrier density and mobility at the Ni-MoS2/g-C3N4 interface. In addition to the favorable charge transport properties, delineation of the photoinduced oxidation reactions by gas monitoring techniques reveals that the high efficiency also arises from fast water oxidation kinetics. The results of this work mark an important step forward in understanding and designing low-cost and earth-abundant catalysts for detoxification of Cr(VI)-contaminated industrial effluents.

COF-based photocatalyst for energy and environment applications

Covalent organic frameworks (COFs) are promising organic materials that can be designed and synthesized from small molecules like building blocks with photo-functional properties. Photocatalysis using photo-responsive COFs is a relatively new field that has been developed in the last five years. Here, we review recent progress in COF-based photocatalysis for environmental and energy applications. The first part of this review focuses on synthesis of COFs, including basic processes for increased solubility to facilitate handling, including exfoliation methodologies, and work towards further improvement of COF-based photocatalytic processes. Time-resolved absorption measurement techniques and theoretical calculations to understand the carrier dynamics of COFs for photocatalytic process are also reviewed. The main focus of the review is to summarize the practical applications of COF-based photocatalysts in the environmental and energy fields. Important examples such as photoinduced oxidation reactions, hydrogen generation, and CO2 reduction reactions are covered. Finally, we conclude this review with prospects for COF-based photocatalysts. This review will serve as a guide for understanding recent progress in COF-based photocatalysis.

Boosting photocatalytic water oxidation by surface plasmon resonance of AgxAu1−x alloy nanoparticles

Surface plasmon resonance (SPR) induced photocatalysis with multi-metallic nanoparticles stays indistinct, especially in photo-induced water oxidation reaction. Herein, we report a strategy of engineering band structure by noble metal alloying (Ag x Au 1−x ). The modulated composition of AgAu alloy nanoparticles loaded on rutile TiO 2 to selectively tune SPR wavelengths and d-band position. We found that SPR-induced water oxidation activity varies as a function of the ratio of Au and Ag in Ag x Au 1−x /TiO 2 photocatalysts, exhibiting“volcano-like” relationship between the oxygen evolution production rate and the metal-composition. The Ag 0.6 Au 0.4 /TiO 2 photocatalyst shows superior water oxidation performance about 3-folds of individual Ag and Au. Photoemission spectra (PES) of alloy are in correlation with density of states, revealing the sensitivity of d-band onset energies and SPR excitation wavelengths. Furthermore, surface-photovoltage (SPV) response of alloy signifies the superiority of alloy in interfacial charge separation of hot carriers. Density functional theory (DFT) calculations validate the experimental results by revealing complex AgAu alloy-semiconductor (Ti-O-Au-Ag) for oxygen evolution along with electrodynamic simulations to trace distribution of electric-field components. This work demonstrates critical role of d-band tuning across alloy composition for boosting catalytic fraction through the integrated effects of light absorption, Schottky barrier height and d-band position of the plasmonic photocatalysts. In surface plasmon induced photocatalyst, both optical and electronic properties are important for eventual photocatalytic efficiency. For photocatalytic water oxidation reaction, the stability of plasmonic metal is also a critical issue. In this work, an alloyed Ag x Au 1−x /TiO 2 plasmon-induced water oxidation photocatalyst is employed as a model system to study the structure-function relationship. We demonstrated that a three-in-one photocatalyst, i.e. efficient visible-light absorption, suitable electronic structure and high-stability, could be achieved through alloy components adjustment. • Simple chemical approach for compositional tuning of Ag x Au 1−x alloy across SPR wavelengths of electromagnetic spectrum. • Enhancement in plasmon-induced water oxidation activity achieved by tuning of d-band structure and Schottky barrier height. • SPV confirms efficient charge separation across Ag 0.6 Au 0.4 /TiO 2 photocatalyst. • DFT calculation confirms that noble metal as an active may increase photocatalytic OER performance.

A complex role for chlorine radicals

Organic Chemistry Radicals are atoms or molecules that are highly reactive because they have an unpaired electron. A common means of investigating whether they are involved in a particular reaction is to try to trap them with an acceptor compound. Yang et al. reinvestigated a photoinduced alkane oxidation reaction for which a trapping study had previously implicated alkoxy radicals. Their spectroscopic, kinetic, and isotopic labeling studies revealed that chlorine, rather than alkoxy, was the key radical intermediate; the prior trapping results had stemmed from its complexation with alcohols. Science , abd8408, this issue p. [847][1] [1]: /lookup/doi/10.1126/science.abd8408

Photocatalytic C-H activation and the subtle role of chlorine radical complexation in reactivity.

The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [CeIVCl5(OR)]2- [CeIV, cerium(IV); OR, -OCH3 or -OCCl2CH3] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by CeIV-OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C-H) activation of alkanes is instead mediated by the photocatalyst [NEt4]2[CeCl6] (NEt4 +, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

Photoinduced Oxidation Reactions at the Air-Water Interface.

Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.

Recent Advances in Heterogeneous Photo-Driven Oxidation of Organic Molecules by Reactive Oxygen Species.

The photo-driven oxidation of organic molecules into corresponding high-value-added products has become a promising method in chemical synthesis. This strategy can drive thermodynamically non-spontaneous reactions and achieve challenging thermocatalytic processes under ambient conditions. Reactive oxygen species (ROS) are not only significant intermediates for producing target products via photoinduced oxidation reactions but also contribute to the creation of sustainable chemical processes. Here, the latest advances in heterogeneous photo-driven oxidation reactions involving ROS are summarized. The major types of ROS and their generation are introduced, and the behaviors of various ROS involved in photo-driven processes are reviewed in terms of the formation of different bonds. Emphasis is placed on unraveling the reaction mechanisms of ROS and establishing strategies for their regulation, and the remaining challenges and perspectives are summarized and analyzed. This Review is expected to provide an in-depth understanding of the mechanisms of ROS involved in photo-driven oxidation processes as an important foundation for the design of efficient catalysts. Clarifying the role of ROS in oxidation reactions has important scientific significance for improving the atomic and energy efficiency of reactions in practical applications.

Benzo- and Thieno-Annulated Tetracenes: A One-Pot Synthesis via Cross-Dehydrogenative Annulation.

A facile method for the direct cross-annulation of unfunctionalized tetracene is reported. The one-pot oxidative cross-dehydrogenative coupling (CDC) between tetracene and aromatic compounds, such as benzene or 2-methylthiophene, furnished annulated products with an extended π-network. Moreover, relative to the benzo-annulated tetracenes, thieno-annulated tetracenes exhibited notably improved photooxidative stability. This behavior stands in sharp contrast with that of tetracene and its derivatives, such as rubrene, which readily engage in photoinduced oxidation reactions.

Integrating Suitable Linkage of Covalent Organic Frameworks into Covalently Bridged Inorganic/Organic Hybrids toward Efficient Photocatalysis.

Covalent organic frameworks (COFs) are excellent platforms with tailored functionalities in photocatalysis. There are still challenges in increasing the photochemical performance of COFs. Therefore, we designed and prepared a series of COFs for photocatalytic hydrogen generation. Varying different ratios of β-ketoenamine to imine moieties in the linkages could differ the ordered structure, visible light harvesting, and bandgap. Overall, β-ketoenamine-linked COFs exhibited much better photocatalytic activity than those COFs having both β-ketoenamine and imine moieties on account of a nonquenched excited state and more favorable HOMO level in the photoinduced oxidation reaction from the former. Specifically, after in situ growth of β-ketoenamine-linked COFs onto NH2-Ti3C2TxMXene via covalent connection, the heterohybrid showed an obvious improvement in photocatalytic H2 evolution because of strong covalent coupling, electrical conductivity, and efficient charge transfer. This integrated linkage evolution and covalent hybridization approach advances the development of COF-based photocatalysts.

Light-driven water oxidation using hybrid photosensitizer-decorated Co3O4 nanoparticles

Cobalt nanoparticles (NPs) have been prepared by hydrogenation of the organometallic complex [Co(η3-C8H13)(η4-C8H12)] in 1-heptanol in the absence of any other stabilizer and then transformed into Co3O4 NPs using mild oxidative reaction conditions. After deposition onto glassy carbon rotating disk electrodes, the electrocatalytic performance of the Co3O4 NPs in water oxidation has been tested in 1M NaOH. The activity has been benchmarked with that of state-of-the-art Co3O4 NPs through electrochemically-active surface area (ECSA) and specific current density measurements. Furthermore, the covalent grafting of photosensitive polypyridyl-based RuII complexes onto the surface of Co3O4 NPs afforded hybrid nanostructured materials able to photo-oxidize water into O2, while steady-state and time-resolved spectroscopic measurements gave some further insight into kinetics and pertinent reaction steps following excitation. These first-row transition metal oxide hybrid nanocatalysts display better catalytic performance than simple mixtures of non-grafted photosensitizers and Co3O4 NPs, thus evidencing the advantage of the direct coupling between the two entities for the photo-induced water oxidation reaction.

Polymerization-Enhanced Photosensitization

Summary Effective photosensitizers are highly desirable in many applications, such as photodynamic therapy, photocatalytic organic waste decomposition, and synthetic chemistry. Herein, we demonstrate polymerization-enhanced photosensitization, in which conjugated-polymer-based photosensitizers show much higher efficiency in singlet oxygen (1O2) production than their small-molecule analogs. Based on small-molecule photosensitizers SM1–SM4 with long wavelength emission, four conjugated polymer photosensitizers, CP1–CP4, were prepared. Interestingly, the conjugated polymer photosensitizers CP1–CP4 showed 5.06-, 5.07-, 1.73-, and 3.42-fold higher 1O2 generation efficiency than SM1–SM4, respectively. The improved intersystem crossing process from the singlet excitation states to the triplet excitation states and improved light-harvesting ability are essential to the enhancement of 1O2 generation efficiency. To illustrate the advantages of conjugated polymer photosensitizers on various applications, we demonstrate the superior performance of CP1 by using photoinduced organic waste decomposition, photoinduced organic oxidation reaction, and photodynamic cancer therapy as examples.

Photoinduced oxidation reactions of the pendent macrocyclic complexes: Al(III) phthalocyanine and Ni(II) annulene mechanistic considerations

The goal of the work was to expand the understanding of the photoinduced redox reactions of the Al(III) sulfonated phthalocyanine, HOAlIIItspc, and Ni(II) tertramethyldibenzotetraazaannnulene, [NiII(tmdbTAA)], as components of polymeric structures in which poly(ethyleneimino) and poly(isobutyl-alt-maleate) are the respective backbones of poly(HOAlIIItspc) and poly([NiII(tmdbTAA)]). Saccharide and flavonoid glycosides form adducts that quench the photo-generated oxidizing radicals in the case of poly(HOAlIIItspc) with rate constants≈1×104s−1, largely independent of reductant structure. The charge-separated intermediate, CS, in the case of poly([NiII(tmdbTAA)]) shows greater reactivity with the saccharides and flavonoid glycosides, particularly with the flavonoid glycoside rutin where the rate constant is 2.7×106s−1. While phthalocyanine-centered radicals in poly(HOAlIIItspc) were reduced by dAMP (adenosine-5-monophosphate) and CT-DNA, energy transfer with these reagents competed with the formation of the CS intermediate in poly([NiII(tmdbTAA)]). A study of the intensity of the upper conversion luminescence (λem<500nm), modulated by the concentration of dAMP suggests that this is attributed to the formation of adducts of poly([NiII(tmdbTAA)]) with dAMP that affect the deactivation of the electronically excited Ni(II) tetraazaannnulene pendants before the formation of the CS intermediate.

Local photo-oxidation of individual single walled carbon nanotubes probed by femtosecond four wave mixing imaging.

Photo-oxidation of individual, air-suspended single walled carbon nanotubes (SWCNTs) is studied by femtosecond laser spectroscopy and imaging. Individual SWCNTs are imaged by four wave mixing (FWM) microscopy under an inert gas (Ar or N2) atmosphere. When imaging is performed in an ambient air atmosphere, the decay of the FWM signal takes place. Electron microscopy shows that SWCNTs are not destroyed and the process is attributed to photoinduced oxidation reactions which proceed via a non-linear excitation mechanism, when irradiation is performed with ∼30 fs pulses in the visible spectral region (500-600 nm). Photo-oxidation can be localized in specific regions of SWCNTs within optical resolution (∼300 nm). The effect of photo-oxidation on Raman spectra was studied by irradiating a local spot on an individual SWCNT and comparing the spectra of irradiated and non-irradiated regions of the same tube. It is shown at an individual nanotube level that oxidation leads to a decrease of the intensity of the Raman signal and an upshift of the G-band.

Effect of photodegradation of lignocellulosic fibers transesterified with vegetable oil

This study presents the assessment of resistance of raw and vegetable oil treated transesterified lignocellulosic fibers (LCFs) such as jute, and sisal against photo-induced oxidation and subsequent hydrolysis reaction by subjecting them to alternate cycles of irradiation of ultraviolet light (UV) of wavelength between 300 to 400 nm and spray of deionized water in an artificial accelerated weathering (AAW) test chamber over a duration of 500 hours. The extent of UV and moisture induced degradation has been evaluated by measuring the residual tensile strength and water absorption before and after AAW. The results indicate that at the end of the 500 h of exposure transesterified LCFs retained about 53 to 60 % of its initial tensile strength, whereas the corresponding retention for raw fibers was only about 14 to 20 %. Average water absorption for transesterified LCFs was increased to about 100–110 % (by weight) following AAW from a pre-exposure value of about 65–80 %. The corresponding average increase for raw LCFs was 300 % from 220 %, respectively. Although both raw and transesterified LCFs exhibited susceptibility to oxidation and hydrolysis following AAW, the resulting impact was found to be more pronounced for raw fibers.

Mechanism of the Quenching of the Tris(bipyridine)ruthenium(II) Emission by Persulfate: Implications for Photoinduced Oxidation Reactions

A revised mechanism for the oxidation of the excited state of Ru(bpy)3(2+) with the persulfate anion is described in this work. The formation of the precursor complex in the electron transfer reaction involves ion pairing between the metal complex in ground and excited states and S2O8(2-). The equilibrium constant for the ion-pair formation (K(IP) = 2.7 M(-1)) was determined from electrochemical measurements and analysis of thermal reaction between Ru(bpy)3(2+) and persulfate. It was found to be consistent with the calculated value estimated from the Debye-Hückel model. The analysis of rate constants for reactions between persulfate and various metal complexes indicates that thermal and photochemical reactions most likely proceed through a common pathway. Extremely high reorganization energy (ca. 3.54 eV) for the electron transfer obtained from fitting experimental data with the Marcus equation is indicative of significant nuclear reorganization during the electron transfer step. In view of these results the electron transfer can be described as dissociative probably involving substantial elongation or complete scission of the O-O bond. The proposed model accurately describes experimental results for the quenching of *Ru(bpy)3(2+) over a wide range of persulfate concentrations and resolves some discrepancies between the values of K(IP) and k(et) previously reported. The implications of various factors such as the ionic strength and dielectric constant of the medium are discussed in relation to measurements of the quantum yields in photodriven oxidation reactions employing the Ru(bpy)3(2+)/persulfate couple.