Studies on the photooxidation mechanism of polymers. II. The role of quinones as sensitizers in the photooxidative degradation of polystyrene

During the quinone-sensitized photooxidative degradation of polystyrene film and its solution in benzene, an initial rapid decrease of average molecular weight has been observed by GPC and viscosity measurements. The reaction rates are strongly increased by quinones such as p-quinone, duroquinone, anthraquinone, and chloranil. It has been suggested that this photosensitized degradation of polystyrene occurs by a singlet oxygen reaction which might be related to an energy transfer mechanism from excited triplet states of quinones to molecular oxygen. The photooxidative degradation of polystyrene in solution can be diminished by addition of typical singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene.     

Die photosensibilisierte Oxydation einwertiger Phenole zu Chinonen

The photosensitized oxidation of methyl substituted phenols with free para-positions starts with an electrophilic attack of position 4 by singlet oxygen. This follows from the course of the reaction using phenols with methyl groups in different positions. The influence of the solvent shows that the hydroperoxide formation from the primary oxygen adduct proceeds via an inter-molecular hydrogen shift. The solvent is the hydrogen donator, whereas the phenoxy radicals resulting from the oxidation of the phenol by the excited sensitizer are the hydrogen acceptors. Finally, the quinones are formed from the hydroperoxides by elimination of water.     

Micro-organizational control of photochemical oxidations: Rose bengal and derivatives (XV)

we define a microheterogeneous photo-oxidation to be a photosensitized oxidation reaction whose efficiency is enhanced beyond that of diffusion control by the covalent bonding of a sensitizer to a ligand. The ligand is responsible for enhancing the local concentration of a specific substrate susceptible to reaction with an excited state derived from the proximate sensitizer. We illustrate the principle with several applications in singlet oxygen processes.     

Inhibiting the photosensitized oxidation of anthracene and tryptophan by means of natural antioxidants

It is shown that model reactions of photosensitized oxidation of anthracene and tryptophan can be used for evaluation and comparison of antioxidant activity of various classes of compounds. Inhibition of the oxidation of substrates in the presence of the familiar antioxidants tocopherol (vitamin E), ascorbic acid (vitamin C), and mixtures of these vitamins with methionine, and in the presence of reputed antioxidants dihydroquercetin and taurine, are considered. It is concluded that all of the above compounds except for taurine have antioxidant properties; i.e., they reduce the rate constants of the photosensitized oxidation of anthracene and tryptophan. It is found that the inhibition of oxidation is associated with the interaction between antioxidants and singlet oxygen. Analysis of the kinetic dependences of the photosensitized oxidation of substrates in the presence of antioxidants reveals that a mixture of vitamins inhibits the process most efficiently, and inhibition occurs at the initial stages due to more active interaction between singlet oxygen and vitamin C     

Photochemical processes of 2,6-di-tert-butyl-4-methylphenol

The photosensitized oxidation of 2,6-di-t-butyl-4-methylphenol (BHT) by anthracene (A) in CCI₄, hexane and polystyrene films with light of wavelength 365 nm has been investigated. Interestingly, BHT was found to inhibit the consumption of A in the presence of oxygen. A possible kinetic scheme for the reaction of A + BHT + CCl₄ has been devised; the rate constants for the most important processes have been evaluated.     

PHOTOCHEMICAL DECOMPOSITION OF 1, 4-BENZODIAZEPINES. NITRAZEPAM*

Abstract— The photochemical activity of nitrazepam, a drug of which phototoxic effects are known, is investigated. Nitrazepam decomposes photochemically in an oxygen-poor medium, while it is relatively stable in the presence of oxygen. It appears that this quenching of excited nitrazepam by molecular oxygen leads to the formation of singlet oxygen. This is demonstrated in three different ways: 1. In the presence of 2-methyl-2-pentene as acceptor of singlet oxygen, two characteristic products are formed: 2-methyl-1-penten-3-ol and 2-methyl-3-penten-2-ol. 2. The photosensitized oxidation of L-dopa in the presence of nitrazepam is increased when D₂O, instead of H₂O, is added to the solvent system, due to the longer lifetime of singlet oxygen in D₂O. 3. The β-value, the ratio of the rate of decay of singlet oxygen to its rate of reaction, is determined with L-dopa as acceptor of singlet oxygen. The same value is obtained with both nitrazepam and methylene blue, a known singlet oxygen generator.     

Photosensitized generation of singlet oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.     

Formation of radicals in the reaction of oxygen with styrene and cobalt

Conclusions A radical-forming reaction in hydrocarbon solution between styrene, oxygen, and Co^II in the form of its stearate and acetylacetonate has been established. The kinetic laws of this reaction have been studied. A mechanism has been proposed which consists in the formation of a complex between the oxygen and the Co^II by a reversible reaction, with the subsequent reaction of this complex with styrene to form free radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1095–1097, June, 1966.The authors express their thanks to I. I. Uskov and A. L. Aleksandrov for help in performing some of the experiments.     

Mechanisms of photoinitiated cleavage of DNA by 1,8-naphthalimide derivatives.

Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were < 10(5) M-1 s-1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 x 10(9) and 8.8 x 10(9) M-1 s-1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.     

高分子受阻胺对顺1,4-聚丁二烯溶液光敏降解过程的稳定作用

 本文研究了高分子受阻胺光稳定剂苯乙烯4-(甲基丙烯酸)-2,2,6,6-四甲基哌啶醇酯共聚物在用罗丹明6G光敏氧化降解顺1,4-聚丁二烯过程中的稳定作用。它具有淬灭单线态氧、分解过氧化氢以及捕获大分子自由基的能力,对其光稳定机理也进行了初步的讨论。     

SOLVENT EFFECT ON PHOTOSENSITIZED OXIDATION OF IODIDE ION BY ANTHRACENE SULPHONATES

Abstract— Iodide ion oxidation by singlet oxygen is found to be influenced by two properties of the solvent. One is the polarity of the solvent and the other is the availability of protons in the solvent. By suitable choice of solvents and solvent mixtures both kinds of effects have been studied. In water-methanol mixtures the reaction is found to be facilitated in more polar medium. In isodielectric media as obtained by mixtures of acetonitrile and methanol the reaction is repressed with increase in the aprotic component (CH₃CN) in the solvent. The same effect is observed in the DMSO-H, O mixtures. In heavy water the reaction rate is doubled and the effect fits well into the proposed kinetic scheme involving singlet oxygen for photosensitized oxidation of I^- by anthracene sulphonates.     

高分子受阻胺对顺1,4-聚丁二烯溶液光敏降解过程的稳定作用

本文研究了高分子受阻胺光稳定剂苯乙烯4-(甲基丙烯酸)-2,2,6,6-四甲基哌啶醇酯共聚物在用罗丹明6G光敏氧化降解顺1,4-聚丁二烯过程中的稳定作用。它具有淬灭单线态氧、分解过氧化氢以及捕获大分子自由基的能力,对其光稳定机理也进行了初步的讨论。     

铜、锰氧化物的表面过剩氧及其甲苯催化燃烧活性

研究了负载型Ｃｕ－Ｍｎ－Ｏ催化剂对烃类深度氧化的活性，并将ＸＲＤ分析、电子探针考察和表面过剩氧浓度的分布测定等结果与催化剂活性进行了关联．以甲苯催化燃烧为模型反应，除用常规的微反考察其动力学性质外，还用脉冲反应技术研究了表面过剩氧的氧化功能和甲苯的吸附等．Ｍｎ／Ｃｕ原子比为０．８８～０．９时，催化剂活性最好．这是由ＣｕＯ和ＣｕＭｎ２Ｏ４的功能匹配决定的，后者提供与催化作用有关的表面过剩氧，前者促进甲苯的吸附．对催化剂的作用机理也进行了初步的探讨．     

Photo-oxidation of polymers without light: Oxidation of polybutadiene and an ABS polyblend with singlet oxygen

In recent years much evidence has been accumulated to implicate electronically excited oxygen (¹Δ_g) molecules as the agent responsible in photosensitized oxidations for the formation of allylic hydroperoxides from olefins and of endoperoxides from 1,3-dienes. Little regarding the mechanistic aspects of the photo-oxidative degradation of polybutadiene (PBD) is known, however. To determine if electronically excited oxygen (¹Δ_g) molecules can oxidize PBD, the ABS polyblend and standard samples of PBD's containing high trans, high cis, and high vinyl content were treated in homogeneous solution at low temperature with chemically produced singlet oxygen in situ. The source of the singlet oxygen was the triphenylphosphite-ozone adduct. Studies by spectroscopy, elemental analysis, viscosity determinations, and gel measurements showed only the cis- and the trans-PBD were susceptible to oxidation; no chain scission was involved in the attack of cis- and trans-PBD by singlet oxygen; the oxidation of the cis PBD involved the initial formation of hydroperoxides which on thermal decomposition yielded gel. The trans-PBD was found to oxidize but apparently by a mechanism different from that of cis-PBD. Initial singlet oxygen attack of ABS proceeds by oxidation of the PBD portion of the polyblend. It was also observed that when only a small amount of the double bonds in the cis-PBD polymer had been oxidized to hydroperoxides, subsequent thermal treatment of this sample resulted in gross structural changes in the whole polymer.     

PHOTOOXIDATION OF 3, 4-DIHYDROXYPHENYLALANINE BY HEMATOPORPHYRIN IN AQUEOUS SOLUTIONS: AN ELECTRON SPIN RESONANCE STUDY USING 2, 2, 6, 6-TETRAMETHYL-4-PIPERIDONE-1-OXYL (TEMPONE)

Abstract— The oxidation of 3, 4-dihydroxyphenylalanine (dopa) photosensitized by hematoporphyrin has been studied by ESR spectroscopy. The nitroxide free radical 2, 2, 6, 6-tetramethyl-4-piperidone-I-oxyl (TEMPONE) is shown to be a convenient probe for monitoring oxygen consumption during the reaction. Isotope and azide effects on oxygen consumption suggest that photooxidation in D₂O proceeds mainly by a singler oxygen mechanism, whereas in H₂O both singlet oxygen and free radical mechanisms are important. The relative importance of these mechanisms can change as oxygen is consumed during the photoreaction. TEMPONE also can be used as an indicator of the presence of reducing radicals (in this case hematoporphyrin radical anions) in the system, but only at low oxygen levels: in aerated solutions efficient reaction of these radical ions with oxygen is proposed.     

Mineralization of 4-Chlorophenol under Visible Light Irradiation in the Presence of Aluminum and Zinc Phthalocyaninesulfonates

Photosensitized oxidation of 4-chlorophenol (4CP) by the title complexes (AIPcS and ZnPcS) in aerated aqueous solution uponvisible light irradiation(λ=450nm) has been investigated using methanol as a disassociating reagent.It is confirmed that the monomeric species of the sesitizer is more active than the corresponding dimer in singlet oxygen generation for 4CP oxidation.However,the monomer is also the main component found in the sensitlzer‘s photobleaching, In this reload, AIPcS is much more stable than ZnPcS, and the Dhotoble~hlno is observed to proceed via singlet and triplet oxygen, respectlvely.The final products of 4CP oxidation in alkaline solution are carbon dioxide and chloride ions.while at pH=7 and pH=3 the p-benzoquinone is the product.The temperature is found to have influence on both the photosensitized degradation of methyl orange and ZnPcS photobleaching,with an activation energy of 15.8 and 24.2kJ/mol,respectively.     

Kinetics and Mechanism of Cumene Oxidation Initiated by N‐Hydroxyphthalimide

This paper discusses the results of investigations on the kinetics of cumene oxidation to cumene hydroperoxide by molecular oxygen in the presence of N‐hydroxyphthalimide as a catalyst. A mathematical model has been developed on the basis of the regularities of hydroperoxide and by‐products formation. The model adequately describes changes in all reaction components in the course of the reaction. It has also been established that the role of N‐hydroxyphthalimide in the process consists in the creation of a steady‐state concentration of N‐phthalimide radicals initiating oxidation by interacting with the hydrocarbon.     

THE FLAVIN-SENSITIZED PHOTOOXIDATION OF NUCLEOTIDES—II. THE PARTICIPATION OF THE FLAVIN TRIPLET STATE

Abstract— –A study has been made of the effects of a series of nucleotides upon the electronic excited states of lumiflavin in order to determine the mechanism of their flavin-sensitized oxidation. A hydrogen-abstraction mechanism is ruled out, because if the nucleotide acts as a reducing agent for the excited dye molecules, it should increase the rate of reduction of the dye when the irradiation is carried out in the absence of oxygen. However, each of the nucleotides studied was found to reduce the rate of anaerobic photoreduction. While oxidation by an intermediate species such as the dye 'moloxide' or singlet oxygen is not entirely ruled out, our evidence suggests that the initial reaction is between the nucleotide and the flavin triplet. This results in a loss of the triplet excitation energy and is a very efficient reaction, guanosine monophosphate shewing 36 per cent of the triplet quenching efficiency of potassium iodide. The relative rates of reaction of the nucleotides with the flavin triplet exactly parallels their quantum yields of sensitized photo-oxidation. The formation of ground-state complexes between flavin and nucleotide and the participation of the singlet excited state of the flavin are not considered to be important.     

Unraveling the Degradation Mechanism of Purine Nucleotides Photosensitized by Pterins: The Role of Charge‐Transfer Steps

Photosensitized reactions contribute to the development of skin cancer and are used in many applications. Photosensitizers can act through different mechanisms. It is currently accepted that if the photosensitizer generates singlet molecular oxygen (¹O₂) upon irradiation, the target molecule can undergo oxidation by this reactive oxygen species and the reaction needs dissolved O₂ to proceed, therefore the reaction is classified as ¹O₂‐mediated oxidation (type II mechanism). However, this assumption is not always correct, and as an example, a study on the degradation of 2′‐deoxyguanosine 5′‐monophosphate photosensitized by pterin is presented. A general mechanism is proposed to explain how the degradation of biological targets, such as nucleotides, photosensitized by pterins, naturally occurring ¹O₂ photosensitizers, takes place through an electron‐transfer‐initiated process (type I mechanism), whereas the contribution of the ¹O₂‐mediated oxidation is almost negligible.     

A comprehensive analysis of the properties of light crude oil in oxidation experimental studies

The analysis of light crude oil for oxidation reaction experiments is a kind of important technological for evaluating an air injection project in a reservoir. In this study, the paper comprehensively analyzes the variations of Jilin crude oil composition comparing crude oil component’s variations before and after oxidation, and investigates the effluent gas composition and hydrocarbon, analyzes the mechanism of low temperature oxidation reaction (LTO), and rebuilds the light crude oil cracking reaction of intermediate component in a new pattern. In the early stage of the oxidation reaction, firstly, oxygen is captured by forming chemical bond in liquid hydrocarbon. And then oxygen takes part in the free radical chain reaction by forming hyperoxide and decomposes to ketones, aldehydes, alcohols, and so on. Meanwhile, chain scission reaction comes up. Research result shows that the intermediate components (C_7–17) of crude oil make great contribution to crude oil cracking. The experimental result shows that Jilin reservoir has the potential of implementing air injection project.