Untersuchungen zum photochemischen Ausbleichverhalten von tautomeriefähigen Azofarbstoffen. VI. Zur photochemischen Oxydation von Phenylazo-acetylacetonen und Phenylazopyrazolonen durch Singulettsauerstoff

Investigations of the Photo-Fading of Tautomeric Azo Dyes. VI. The Photochemical Oxidation by Singlet Oxygen of Phenylazoacetylacetones and Phenylazopyrazolones The dye-sensitized photo-oxidation by singlet oxygen of phenylazoacetylacetones ( 1 ) and Phenylazopyrazolones ( 2 ) has been investigated. The photo-fading of dyes is accelerated in the presence of Methylene Blue, a sensitizer of singlet oxygen, in airsatured methanol An excellent relationship exists between the fading-rate of a series of dyes ( 1 ) and Hammett-σ-constants. An unexpected trend was found for photo-fading of a series of dyes ( 2 ). The Hammett plot consists of two parts, with a negative slope derived from electron-releasing substituents and a positive slope derived from electron-withdrawing substituents. The anomalous photo-fading is due to an attack of singlet oxygen on the more reactive anion of the phenylazopyrazolones in the case of electron-withdrawing substituents. In methanol containing acetic acid the dyes ( 2 ) exist only as the hydrazone tautomers. Under these conditions the fading-rate show a linear relationship with σ-constants.     

微反应器控制的化学反应的选择性

研究利用分子筛、Ｎａｆｉｏｎ薄膜、低密度聚乙烯薄膜和囊泡作为微反应器控制有机光化学反应的方向,提高反应的选择性和可能性。在ＮａＹ沸石或低密度聚乙烯薄膜中,带有长烷基链或醚链的二芳基化合物光二聚只生成分子内的加成产物,而不生成分子间的加成产物,从而在底物浓度很大的情况下,高选择性地合成了大环化合物。通过控制底物和敏化剂分子在ＺＳＭ－５沸石、Ｎａｆｉｏｎ薄膜和囊泡中的分布高选择性地控制烯烃光敏氧化反应     

Role of singlet oxygen in photo-oxidative degradation and photostabilization of polymers

Evidence is presented that singlet oxygen may have an important role in the photo-oxidation of polymers. Increased attention is being paid to singlet oxygen as a possible reactive intermediate in the initiation step of the photodegradation of polymers. Singlet oxygen may be produced by energy transfer either from impurities or internal groups after excitation, e.g. carbonyl groups, which are present in commercial polymers. As a consequence, reactions of singlet oxygen may also be important in the stabilization of polymers, e.g. leading to search for new and more effective antioxidants.     

Participation of singlet oxygen in photosensitized oxidation of 1,4-dienoic systems and photooxidation of soybean oil

Sensitized photooxidation of a model 1,4-diene, 4cis, 7cis-undecadiene, was shown to yield 4-hydroperoxy-5trans,7cis-undecadiene and 5-hydroperoxy-3trans,7cis-undecadiene as initial products. Further irradiation (in the presence of the sensitizer) caused the isomerization of 4-hydroperoxy-5trans,7cis-undecadiene to 4-hydroperoxy-5trans,7trans-undecadiene. Oxidation of 4cis,7cis-undecadiene with chemically formed singlet oxygen gave the same initial products as the photosensitized oxidation. 5-Hydroperoxy-3trans,7cis-undecadiene is, however, not formed in the radical autoxidation of the diene. It is concluded that singlet oxygen is the reactive intermediate in the photooxidation. Comparison with this model reaction suggests that the photooxidation of refined soybean oil in propanol also proceeds via singlet oxygen: the photooxidations of both 4,7-undecadiene and soybean oil are inhibited by β-carotene and by triethylamine, but unlike radical chain autoxidation, they are not inhibited by 2,6-di-t-butyl-4-methylphenol. Also soybean oil can act as a sensitizer for the photooxidation of 4,7-undecadiene. Chlorophyl-like sensitizers are probably unimportant in the well refined soybean oil used in this work. The observed photooxidation of the skipped dienoic components of soybean oil, which is probably due to some other, unidentified sensitizer(s), absorbing below 500 nm, can be avoided by using a yellow filter. Presented in part in the symposium “Oxidation Chemistry,” New Chemical Society Meeting, Manchester, April 1972.     

Singlet oxygen oxidation of lipids resulting from photochemical sensitizers in the presence of antioxidants

Singlet oxygen produced by photochemical sensitizers may play an important role in the oxidation of lipids in foods. Therefore, we studied the singlet oxygen oxidation of lipids and the scavenging ability of antioxidants. Singlet oxygen was generated using the photosensitizer rose bengal. The oxidation products of lipids and antioxidants were separated by high-performance liquid chromatography and monitored using post-column chemiluminescence and/or iodometric detection. The competitive reaction rates of various antioxidants and lipids were studied to elucidate the roles played by antioxidants in the prevention of food oxidation by singlet oxygen.     

Singlet-Oxygen Generation by Peroxidases and Peroxygenases for Chemoenzymatic Synthesis

Singlet oxygen is a reactive oxygen species undesired in living cells but a rare and valuable reagent in chemical synthesis. We present a fluorescence spectroscopic analysis of the singlet-oxygen formation activity of commercial peroxidases and novel peroxygenases. Singlet-oxygen sensor green (SOSG) is used as fluorogenic singlet oxygen trap. Establishing a kinetic model for the reaction cascade to the fluorescent SOSG endoperoxide permits a kinetic analysis of enzymatic singlet-oxygen formation. All peroxidases and peroxygenases show singlet-oxygen formation. No singlet oxygen activity could be found for any catalase under investigation. Substrate inhibition is observed for all reactive enzymes. The commercial dye-decolorizing peroxidase industrially used for dairy bleaching shows the highest singlet-oxygen activity and the lowest inhibition. This enzyme was immobilized on a textile carrier and successfully applied for a chemical synthesis. Here, ascaridole was synthesized via enzymatically produced singlet oxygen.     

Effect of α-, β-, γ-, and δ-Tocotrienol on the Singlet Oxygen Oxidation of Lard

The impacts of four different types of tocotrienol homologues on the singlet oxygen oxidation of lard were evaluated by measuring the headspace oxygen content and the peroxide value. Singlet oxygen oxidation of lard was induced by chlorophyll photosensitization. Samples of 0.100, 0.250, and 0.400 M lard in methylene chloride containing chlorophyll and α‐, β‐, γ‐, or δ‐tocotrienol were prepared and stored under light at 3,000 lux for 4 h. All tocotrienol homologues at 1.20 mM significantly prevented the singlet oxygen oxidation of lard. Chlorophyll under light produced singlet oxygen at 1.09 μmol oxygen/mL headspace/h. A steady state kinetic study showed that tocotrienols reduced the singlet oxygen oxidation of lard by quenching the singlet oxygen. Singlet oxygen reacted with lard at 6.50 × 10⁴M^?1 s^?1. α‐, β‐, γ‐, and δ‐tocotrienol quenched singlet oxygen with the rate of 2.16, 1.99, 2.05, and 0.800 × 10⁷M^?1 s^?1, respectively. Among them, α‐tocotrienol significantly prevented singlet oxygen oxidation of lard.     

Photooxidation von Arylazonaphtholen mit Singulett-Sauerstoff

Photooxidation of Arylazo-Naphtholes with Singlet Oxygen The reaction of p-substituted 4-arylazo-1-naphtholes 1 and 1-arylazo-2-naptholes 5 with singlet oxygen (produced by methylene blue-sensitized photolysis) has been studied spectroscopically in methanol as a solvent. The rate constants of dye bleaching are the same, irrespective whether the extinctions of the azo or the hydrazone forms of the tautomeric systems are measured. Both reaction series 1 and 5 afford linear Hammett plots, which is discussed in the light of the azohydrazone equilibrium of the dyes and an „ene”︁ mechanism of the reaction with singlet oxygen. It could be shown by short-time spectroscopy that the azo-hydrazone isomerization proceeds much faster than the attack of singlet oxygen on the hydrazone form, which explains all observed facts.     

Effect of singlet oxygen sensitizers and quenchers on the photo-oxidation of some unsaturated polymers

The photo-oxidation of cis-polyisoprene was studied in the absence and in the presence of both singlet oxygen sensitizers (benzophenone, anthracene, rubrene) and quenchers (benzidine, diphenylamine, β-carotene). The amount of hydroperoxide which was formed at constant flow rate of oxygen during the u.v. irradiation of cis-polyisoprene increased by increasing the concentration of sensitizer as well as the number of quanta absorbed. The efficiency of the sensitizers used for the formation of hydroperoxide increased in the order: benzophenone < anthracene < rubrene. This effect was attributed to the fact that anthracene and rubrene can sensitize the photo-oxidation reaction by both physical and chemical generation of singlet oxygen while the generation of singlet oxygen is a purely physical process in case of benzophenone. In presence of quencher compounds β-carotene was more efficient than diphenylamine and the latter was more efficient than benzidine. The effect of a singlet oxygen oxidation on the different organic polymers was in the sequence: cis-polyisoprene > polybutadiene > styrene-polybutadiene > polychloroprene.     

Die Charakterisierung der strahlungslosen Desaktivierung des ersten Singulett-Anregungszustandes von N-Methylacridon mittels Triplett-sensibilisierter (E) → (Z)-Photoisomerisierung von (E)-Stilben

Characterization of the Radiationless Deactivation of the Lowest Excited Singlet State of N-Methylacridone by Means of the Triplet Sensitized (E) → (Z) Photoisomerization of (E)-Stilbene The (E) → (Z) isomerization of (E)-stilbene photosensitized by N-methylacridone (A) has been studied under anaerobic conditions at 298 K in dependence on the solvent as well as the concentration of iodoethane as heavy atomquencher. For the experimental conditions the quantum yield of the triplet-triplet energy transfer ³A* → ³E* is unity and the photoreaction is homogenous. The ISC quantum yield of the sensitizer N-methylacridone was determined from the initial slopes of the plotted concentration-time functions in the solvents methanol, N-methylformamide, acetonitrile, dichloromethane, tetrahydrofuran, benzene and tetrachloromethane, respectively. The comparison of the ISC quantum yields ψ_isc and the quantum yields of fluorescence ψ_f indicate that ψ_f + ψ_isc = 1 is true for the used solvents; the IC process is negligible.     

Quenching by oxygen of excited states of dyes on cotton investigated with diffuse reflectance laser flash photolysis and singlet oxygen detection

Diffuse reflectance laser flash photolysis was successfully employed to study the triplet states of dyes adsorbed on or chemically attached to cotton. This technique, together with the detection of singlet oxygen from dyes on cotton, made it possible to investigate the diffusion of oxygen in cotton fibres and the mobility of dyes adsorbed onto cotton. Absorption by the triplet states of aluminium phthalocyanine chloride and eosin and the phosphorescence of singlet oxygen produced by sensitisation with rose bengal was used to monitor oxygen diffusion and dye mobility by measuring differences in triplet lifetimes, singlet oxygen lifetimes and in signal intensities. It was found that swollen cotton allows diffusion of oxygen in the fibres. However, a noticeable effect on the triplet state of adsorbed aluminium phthalocyanine chloride is only observed if oxygen is removed or added by thorough evacuation or oxygenation of the samples over several days. Singlet oxygen was quenched dynamically by adsorbed dyes, which were found to be immobilised on the fibres, demonstrating that oxygen is mobile within the fibres at the molecular level.     

Evidence for the Participation of Singlet Oxygen in the Vat–dye Photosensitized Degradation of Cellulose

A study has been made of the vat–dye photosensitized degradation of cellulose. Whilst the part played by the dyes in direct hydrogen abstraction from a cellulose substrate cannot be overlooked, it is apparent that the role played by singlet oxygen is important. The ability of a dye to produce singlet oxygen has been studied by measuring the amount of tetraphenylcyclopentadienone the dye can photo–oxidize. It would appear that the singlet oxygen is initially formed in its high energy g state by energy transfer from the triplet state of the dye to ground state molecular oxygen. Luminescence spectroscopy has been employed in order to gain information about the excited state energies of the dye molecules, and an electron spin resonance study of the dyes in rigid media has been carried out in order to obtain information about the triplet state.     

Analysis of autoxidized fats by gas chromatography-mass spectrometry: V. Photosensitized oxidation

The role of singlet oxygen in oxidation was studied by analyzing hydroperoxide isomers in unsaturated fats and esters by gas chromatography-mass spectrometry (GC-MS). On oxidation photosensitized with methylene blue at 0 C, methyl oleate produced a 50–50% mixture of 9- and 10-hydroperoxides, linoleate a mixture of 66% conjugated (9+13) and 34% unconjugated (10+12) hydroperoxides, and linolenate a mixture of 75% conjugated (9+12+13+16) and 25% unconjugated (10+15) hydroperoxides. Cottonseed, safflower, and corn oil esters showed, as in soybean esters, the presence of varying amounts of 12-hydroxy esters derived from the corresponding hydroperoxide at low peroxide values. Since these oils do not contain linolenic acid, a likely source of the 12-hydroperoxide is linoleic acid by photosensitized oxidation. Several lines of evidence support the conclusion that singlet oxygen may contribute to the unique hydroperoxide composition of vegetable oil esters at low levels of oxidation. In the presence of photosensitizers such as methylene blue and chlorophyll, the unique hydroperoxide composition (high levels of 10- and 12-hydroperoxides) obtained in soybean esters was similar to that produced by oxidation at low peroxide values. In contrast, a normal hydroperoxide composition was produced, as expected from the fatty acid composition of soybean oil esters, when singlet oxygen quenchers such as β-carotene and α-tocopherol were used and when the esters were treated with carbon black to remove natural photosensitizers. GC-MS analyses of the derived unsaturated alcohols provided indirect evidence for 12-hydroperoxy-9,13-diene in soybean esters as expected by photosensitized oxidation of linoleate. Presented at the AOCS Meeting, San Francisco, California, April 29–May 3, 1979. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned.     

The contribution of singlet oxygen to the photofading of triphenylmethane and related dyes

The contribution of singlet oxygen (¹Δ_gO₂) to the photofading of Crystal Violet in some solvents is examined. The rates of photofading in dichloromethane or acetone are accelerated in the presence of singlet oxygen sensitizers, e.g. Methylene Blue. The rates are retarded by adding effective singlet oxygen quenchers such as β-carotene or nickel dimethyldithiocarbamate. It seem shtat Crystal Violet is mainly photooxidized to give Michler's ketone and p-dimethylaminophenol via the reaction with singlet oxygen, which can be generated by the dye itself or by an added sensitizer. The effect of various singlet oxygen quenchers on the photostability of coloured materials derived from colour formers, such as Crystal Violet Lactone and 3-diethylamino-6-methyl-7-anilinofluoran, is examined also on silica gel.     

An e.s.c.a. investigation of the surface oxidation of bisphenol A polycarbonate films induced by reactive oxygen species

The changes in surface chemistry of bisphenol A polycarbonate films on exposure to various reactive oxygen species have been examined using e.s.c.a. Oxygen uptake, as a result of ozonation involves reactions of both the gem dimethyl and phenyl moieties. Singlet oxygen does not react with polycarbonate. The results presented here indicate that the interpretation of oxidation mechanisms due to singlet oxygen sould be treated with caution when generating this species by the microwave discharge technique.     

Products formed by photosensitized oxidation of unsaturated fatty acid esters

Photosensitized oxidation of unsaturated fatty acid methyl ester was carried out using methylene blue as a sensitizer. Oxidation products, monohydro-peroxides, were identified as trimethylsilyl derivatives. Methyl oleate gave the 9- and 10-isomers; methyl linoleate, the 9-, 10-, 12-, and 13-isomers; and methyl linolenate, the 9-, 10-, 12-, 13-, 15-, and 16-isomers, respectively. The double bond to which the hydroperoxide group attached was shifted to the adjacent position in each isomer. Thus, both conjugated and nonconjugated isomers were present in methyl linoleate monohydroperoxides and methyl linolenate monohydroperoxides. By the inhibition experiment, it was ascertained that the above reaction proceeded via singlet oxygen. The relative rates of methyl oleate, methyl linoleate, and methyl linolenate were 1.0∶1.7∶2.3, respectively. These results obtained from the methyl esters were applied to the photosensitized oxidation of triglycerides purified from vegetable oils, and the reaction mechanism on triglycerides was proposed.     

Christopher Foote's discovery of the role of singlet oxygen [1O2 (1Delta g)] in photosensitized oxidation reactions

The chemistry of singlet molecular oxygen [1O2 (1Delta g)], its importance in atmospheric, biological, and therapeutic processes, and its use as a reagent in organic synthesis have been of considerable interest. Many aspects of singlet oxygen chemistry have emanated from the work of Christopher S. Foote and co-workers. Singlet oxygen is a historically interesting molecule with an unusual story connected with its discovery. Foote and Wexler conducted experiments in the 1960s where evidence was obtained supporting 1O2 generation via two independent routes: (1) a photochemical reaction (dye-sensitized photooxidation) and (2) a chemical reaction (NaOCl with H2O2). An important factor in the discovery of 1O2 as the critical reaction intermediate in dye-sensitized photooxygenations was Foote's reassessment of the chemical literature of the 1930s, when 1O2 was suggested to be a viable intermediate in dye-sensitized photooxidation reactions. Experiments that used silica gel beads provided evidence for a volatile diffusible oxidant such as 1O2. However, a contemporaneous quarrel surrounded this early work, and the possible existence of solution-phase 1O2 was ignored for over 2 decades. Not long after Foote's initial studies were published in 1964, the idea of singlet oxygen as an intermediate in photooxidation chemistry gained increasing recognition and verification in organic, gas phase, and biological processes. There are many documented impacts that 1O2 has had and continues to have on biology and medicine, for example, photodynamic therapy and plant defenses.     

竹红菌素自敏光氧化反应机制的研究

本文对竹红菌素自敏光氧化反应的机制作了较详细的研究,竹红菌素通过自敏光氧化反应生成不稳定的过氧化物,它可以放出~1O_2回到母体化合物,也可以转化为稳定的氧化产物,我们用活泼的单重态氧的接受体捕获到了体系中放出的~1O_2,用吸收光谱的变化证明过氧化物回到了母体化合物。文中还用猝灭实验证实此自敏光氧化反应除涉及~1O_2机制外,还有其它机制起着作用。     

用核磁共振氢谱法研究胸腺嘧啶与过氧化钒配合物的光照射反应

利用核磁共振氢谱的方法,研究在单线态氧的作用下,胸腺嘧啶残基(包括胸腺嘧啶(T)、胸腺嘧啶脱氧核苷(dT)和胸腺嘧啶脱氧核苷酸(TMP))的分解产物。胸腺嘧啶残基与过氧化钒配合物的光照射反应,由过氧化钒配合物光分解所产生的单线态氧使胸腺嘧啶碱基发生了光氧化反应,可能的反应产物通过核磁共振氢谱进行了测定。     

几种萘酞菁硅配合物的光动力效应的研究

研究了7种轴向配位的萘酞菁硅配合物的激发态性质、产生单重态氧的能力及对几种氨基酸的光敏氧化能力.研究表明,对于相同浓度的不同配合物来说,随着轴向配位基团推电子能力的逐渐增强,激发三重态寿命降低,产生单重态氧的能力逐渐下降,而产生单重态氧的能力是光敏氧化氨基酸反应速率的决定因素,因此,其光敏氧化氨基酸的一级反应速率常数逐渐减小.