Stabilisation of cis-1,4-polybutadiene against photo-oxidation and singlet oxygen oxidation by hindered phenols

The hindered phenols, 1,6-di-methylphenol, 2,4,6-tri-methylphenol, 2,6-di-tert-butylphenol and 2,4,6-tri-tert-butylphenol, were examined for their stabilising effect in free radical photo-oxidative and singlet oxygen oxidative degradation of cis-1,4-polybutadiene in solution. The stabilising activity was found to be a complex function of the ability of the hindered phenols to react with free radicals and singlet oxygen. Steric hindrance has an obvious effect on the stabilising activity of phenols.     

Radical Chain Breaking Bis(ortho-organoselenium) Substituted Phenolic Antioxidants

The presence of a chalcogen atom at the ortho-position of phenols enhances their radical chain-breaking activity. Here, a copper(I)-catalyzed reaction of 2,6-dibromo- and 2,6-diiodophenols with diorganodiselenides has been studied for the introduction of two organoselenium substituents at both ortho-positions of the phenolic radical chain-breaking antioxidants, which afforded 2,6-diorganoseleno-substituted phenols in 80–92% yields having electron-donating CH₃, and electron-withdrawing CN and CHO functionalities. Additionally, 2,6-diiodophenols with electron-withdrawing CHO and CN groups also afforded novel 5,5′-selenobis(4-hydroxy-3-(phenylselanyl)benzaldehyde) and 5,5′-selenobis(4-hydroxy-3-(phenylselanyl)benzonitrile) consisting of three selenium and two phenolic moieties along with 2,6-diorganoseleno-substituted phenols has been synthesized. The electron-withdrawing CHO group has been reduced by sodium borohydride to the electron-donating alcohol CH₂OH group, which is desirable for efficient radical quenching activity of phenols. The developed copper-catalyzed reaction conditions enable the installation of two-arylselenium group ortho to phenolic radical chain-breaking antioxidants, which may not be possible by conventional organolithium-bromine exchange methods due to the sluggish reactivity of trianions (dicarba and phenoxide anion), which are generated by the reaction of organolithium with 2,6-dibromophenols, with diorganodiselenides. The antioxidant activities of the synthesized bis and tris selenophenols have been accessed by DPPH, thiol peroxides, and singlet oxygen quenching assay. The radical quenching antioxidant activity has been studied for the synthesized compounds by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The bis-selenophenols show comparable radical deactivating activity, while tris seleno-bisphenols show higher radical deactivating activity than α-tocopherol. Furthermore, the tris seleno-bisphenol shows comparable peroxide decomposing activity with ebselen molecules.     

THE REACTIVITY OF CITRONELLOL and α-THUJENE WITH SINGLET OXYGEN. RATE CONSTANTS OF CHEMICAL REACTION and PHYSICAL QUENCHING*

The quantum yields of Rose Bengal sensitized photooxidation of citronellol and α-thujene have been determined as a function of added acceptor and compared with those of furfuryl alcohol as a standard. The results permitted the calculation of the corresponding rate constants of chemical reaction (k_T) and physical quenching (K_q) of singlet oxygen. The sum (k_T+ k_q) has been verified independently by a Stern-Volmer analysis of the singlet oxygen luminescence quenching. α-Thujene reacts faster with singlet oxygen than citronellol, physical quenching being negligible in both cases.     

Dicyanoanthracene sensitized photo-oxygenation of olefins : Electron transfer and singlet oxygen mechanisms

Cyanoaromatic sensitizers, in particular 9,10-dicyanoanthracene (DCA), sensitize the photo-oxygenation of olefins by two distinct mechanisms. In the case of aryl substituted olefins (OL), which react extremely slowly (if at all) with singlet oxygen, the reaction proceeds by way of electron transfer to produce discrete radical ions (DCA^-and OL⁺). In the presence of oxygen, this ionic process results, ultimately, in the cleavage of the olefin to carbonyl compounds along with production of some epoxide and other minor by-products. Aromatic ethers can interfere with this process by reducing the radical cation by electron transfer, resulting in net quenching of the reaction. With simple alkenes the DCA-sensitized reaction takes a different course, producing hydroperoxide products with distributions which are very similar to those obtained with the singlet oxygen ene reaction. Careful study has shown that this reaction does, indeed, proceed by way of singlet oxygen, which is produced by at least two mechanisms : (1) enhanced intersystem crossing, in which ¹DCA is quenched by interaction with the olefin, leading to a low yield of ³DCA, which subsequently reacts with oxygen to produce singlet oxygen; and (2) direct reaction of ¹DCA with oxygen. At limiting high oxygen concentration, this process produces 2 mol of singlet oxygen for each mol of ¹DCA quenched; the mechanism involves energy transfer to produce ³DCA and 1 mol of singlet oxygen ; the ³DCA reacts again with oxygen to produce a second mol of singlet oxygen. The complex kinetic behaviour of simple olefins in the presence of DCA can be satisfactorily rationalized by these mechanisms.     

Multi‐faceted Reactivity of Alkyltellurophenols Towards Peroxyl Radicals: Catalytic Antioxidant Versus Thiol‐Depletion Effect

Hydroxyaryl alkyl tellurides are effective antioxidants both in organic solution and aqueous biphasic systems. They react by an unconventional mechanism with ROO^. radicals with rate constants as high as 10⁷ M ^?1 s^?1 at 303 K, outperforming common phenols. The reactions proceed by oxygen atom transfer to tellurium followed by hydrogen atom transfer to the resulting RO^. radical from the phenolic OH. The reaction rates do not reflect the electronic properties of the ring substituents and, because the reactions occur in a solvent cage, quenching is more efficient when the OH and TeR groups have an ortho arrangement. In the presence of thiols, hydroxyaryl alkyl tellurides act as catalytic antioxidants towards both hydroperoxides (mimicking the glutathione peroxidases) and peroxyl radicals. The high efficiency of the quenching of the peroxyl radicals and hydroperoxides could be advantageous under normal cellular conditions, but pro‐oxidative (thiol depletion) when thiol concentrations are low.     

REACTIVITY OF SINGLET OXYGEN TOWARD AMINO ACIDS AND PEPTIDES

Quenching of singlet oxygen (¹O₂) in D₂O-ethanol by the amino acids tryptophan, tyrosine, histidine, methionine, cysteine and their derivatives was measured by exciting the sensitizers rose bengal or meso-tetra (N-methyl-4-pyridyl)porphyrin tetratosylate in the presence of oxygen and the above quenchers in solution. In our polar solvent, containing 75% D₂O on a molar basis it was found that (1) substitution of the aromatic ring in indole, phenol and imidazole by the electron-donating methyl group increases the total (i.e. nonreactive and reactive) quenching rate constant by a factor of five to eight. Free or blocked amino and carboxyl groups removed by two methylene groups from the ring counteract the above increase in the rate constant. The reactive quenching of singlet oxygen, which leads to oxidative destruction of the aromatic ring, correlates with the above substitution effects. It has been proposed that the quenching process takes place by formation of an exciplex between ¹O₂ and the quencher. Thus our results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with ¹O₂ and the more likely it is to lead to charge separation and hence to a chemical reaction. (2) Oligopeptides in solution or peptide bonds linked to the amino acid residue have only a minor effect on singlet oxygen. It can therefore be expected that the polypeptide chains per se in the protein network will not interact significantly with the single oxygen molecules present. The quenching of the latter should, to a first approximation, depend only on the presence of the above reactive amino acid residues and to their accessibility to ¹O₂ as well as on the effective dielectric constant within the protein structure.     

ON THE MECHANISM OF QUENCHING OF SINGLET OXYGEN BY AMINES-III. EVIDENCE FOR A CHARGE-TRANSFER-LIKE COMPLEX

Abstract— A series of amines were found to quench singlet oxygen in the order tertiary > secondary > primary, with a reasonable correlation between the log of their rate constant of quenching and their ionization potential. In addition, a Hammett rho plot gave a rho value of - 1.39 for the quenching of singlet oxygen by a series of substituted N, N-dimethylanilines, in good agreement with the results obtained by a different method. It was found that some of the amines (anilines) quenched the triplet state of the dye-sensitizer (Rose Bengal) used for the production of singlet oxygen. Corrections in the results were made in the calculations of rates of quenching of singlet oxygen to allow for the triplet-state quenching. No extensive quenching of the singlet state of the dye was observed at the concentrations of the amines necessary for singlet-oxygen quenching. In one case (N, N, N', N'-tetramethylphenylenediamine) there was no observable chemical reaction between singlet oxygen and the amine. It was concluded that singlet oxygen undergoes physical quenching by the amines via partial charge-transfer intermediates.     

9, 10-DICYANOANTHRACENE-SENSITIZED PHOTOOXYGENATION OF α,α'-DIMETHYLSTILBENES. MECHANISM AND KINETICS OF THE COMPETING SINGLET OXYGEN AND ELECTRON TRANSFER PHOTOOXYGENATION REACTIONS

Abstract— The 9, lodicyanoanthracene-sensitized photooxygenation of 2-methyl-2-butene and (+)-limonene proceeds via the singlet oxygen pathway in carbon tetrachloride as well as in acetonitrile, although the fluorescence of the sensitizer in acetonitrile is quenched by these olefins in an electron transfer quenching mechanism. The 9, 10-dicyanoanthracene-sensitized photooxygenation of cis- and trans-ä, ä′-dimethylstilbenes occurs exclusively via the singlet oxygen pathway in carbon tetrachloride; in acetonitrile, however, singlet oxygen and electron transfer photooxygenation reactions compete with one another. Addition of tetra-n-butyl ammonium bromide and increasing oxygen concentrations favor the formation of the singlet oxygen product, whereas addition of anisole, increasing substrate concentrations and decreasing oxygen concentrations favor the electron transfer photooxygenation products. In carbon tetrachloride, exciplexes of the sensitizer and the dimethylstilbenes are formed which give rise to cidrrans-isomerization of the substrates. In acetonitrile, neither exciplex formation nor cisltrans-isomerization are observed. A mechanism is proposed which allows us to calculate product distributions of the competing singlet oxygen/electron transfer photooxygenation reactions and thus to determine the efficiencies with which encounters between the singlet excited sensitizer and the substrates finally result in electron transfer photooxygenation products. Using (I) these efficiencies, (2) the β-value obtained from singlet oxygen photooxygenation sensitized by rose bengal, and (3) the appropriate k-values determined from fluorescence quenching of 9, 10-dicyanoanthracene in MeCN by oxygen and the stilbene, allows the calculation of the quantum yield of oxygen consumption by this stilbene. The quantum yield thus calculated is strictly proportional to the rate of oxygen consumption experimentally obtained; this result is considered as convincing evidence for the mechanism proposed.     

Photoinduced Electron‐Transfer Oxidation of Olefins with Molecular Oxygen Sensitized by Tetrasubstituted Dimethoxybenzenes: A Non‐Singlet‐Oxygen Mechanism

α‐Methylstyrene ( 1 ) was photo‐oxidized in the presence of a series of alkylated dimethoxybenzenes as sensitizers in an oxygen‐saturated MeCN solution to afford the cleaved ketone 2 , epoxide 3 , as well as a small amount of the ene product 4 in ca. 1 : 1 : 0.04 ratio. The relative rate of conversion was well‐correlated with the fluorescence quantum yield of sensitizers. Thus, a non‐singlet‐oxygen mechanism is proposed, in which an excited sensitizer is quenched by (ground‐state) molecular oxygen to produce a sensitizer radical cation and a superoxide ion (O), the former of which oxidizes the substrate, while the latter reacts with the resulting olefin radical cation ( 1 ^{+ .}) to give the major oxidation products. Photodurability of such electron‐donating sensitizers is dramatically improved by substituting four aromatic H‐atoms in 1,4‐dimethoxybenzene with Me or fused alkyl groups, which provides us with an environmentally friendly, clean method of photochemical functionalization with molecular oxygen, alternative to the ene reaction via singlet oxygenation.     

Solvent Isotope Effect on the Rate Constants of Singlet-Oxygen Quenching by edta and Its Metal Complexes

The reactivity of singlet oxygen (O₂(¹Δ_g)) with edta and its metal complexes with Al³⁺, Cu²⁺, Fe³⁺, and Mn²⁺ was investigated. The emission of singlet oxygen at 1270 nm in D₂O was measured in order to determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constant (k_r + k_q) of the chemical reaction between singlet oxygen and the acceptor (k_r) and of the physical quenching of singlet oxygen by the acceptor (k_q) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H₂O were used to determine quantum yields of the sensitized photooxidation, and the combined results of these experiments allowed the determination of k_r and k_q separately. A strong isotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical procedure used. The isotope effect, as well as the reactivity of edta and its metal complexes, depend markedly on the complexed metal ion.     

Photosensitization mechanisms in photopolymer coating film containing photoinitiators sensitized by aminochalcone‐type dye for computer‐to‐photopolymer plate

Photosensitization mechanisms in photopolymer coating film containing an aminochalcone‐type dye sensitizer and a radical generating reagent, sensitizer dyes, (E)‐3‐(9‐julolidinyl)‐1‐phenyl‐2‐propen‐1‐one (A), (E)‐2‐(9‐julolidinyl)‐methylene‐1‐indanone (B), 9‐benzoyl‐2,3,6,7‐tetrahydro‐1H,5H‐benzo[i,j]‐furano‐[3,2‐g]quinolizine (C), 4‐(dimethylamino) chalcone (D) and a radical‐generating reagent, 2,4,6‐tris (trichloromethyl)‐1,3,5‐triazine (TCT), were investigated by laser flash photolysis using a total reflection cell. Weak fluorescence and strong broad triplet absorption were detected. The fluorescence was statically quenched by TCT at quenching distances (R_f) of 15, 14, 20 and 14 Å for A, B, C and D as well as the triplet initial absorption, at quenching distances (R_t) of 16, 16, 16 and 14 for A, B, C and D, similar to the fluorescence quenching distances. The triplet decay time of the dyes was inefficiently quenched by TCT with the rate constants (k _q) of 1.9, 3.1, 0.7 and 1.0×10⁵ mol⁻¹/dm³/s for A, B, C and D. The sensitivity of photopolymers containing a sensitizer dye and a TCT was obtained at an excitation of 488 nm corresponding to the emission peaks of argon ion laser of 1.1, 0.2, 0.54 and 9.1 mJ cm² for A, B, C and D. The results indicated that the static sensitization process from the fluorescent singlet excited state of the dyes to the ground state of TCT was predominant, and the high sensitivity for A and B was caused by the high absorbance at 488 nm and that for C by the high fluorescent quenching distance. Copyright © 1999 John Wiley & Sons, Ltd.     

Energy transfer processes involving ultraviolet stabilizers. Quenching of singlet oxygen

The role of electronically excited singlet (¹Δ_g) oxygen molecules in the photooxidation of polymers has received increased attention in recent years. Little information regarding the interaction of ultraviolet stabilizers with singlet oxygen is known, however. In this study, singlet oxygen was produced by a microwave discharge in a flow system and rate constants were obtained for quenching by ultraviolet stabilizers. It was found that some nickel chelate stabilizers are effective quenchers of singlet oxygen and their quenching behaviors can be correlated with ultraviolet stabilization effectiveness in thin polypropylene and polyethylene films. The best oxygen quenchers of those examined are nickel chelates with sulfur donor ligands.     

Oxidative cyclization of morusin

Oxidative cyclization of morusin (I) by using one-electron transfer oxidizing agents (manganese dioxide, silver oxide) afforded morusin hydroperoxide (II). A similar reaction was carried out in the presence of 2,4,6-tri-t-butylphenol, a radical quencher, to give compounds (IV, V, VI and VII) coupled with the 2,4,6-tri-t-butylphenoxy radical. On the basis of above results, the possible mechanism of this oxidative cyclization was discussed. In addition, morusin hydroperoxide (II) was also obtained by photo-sensitized oxidation of morusin (I) in the presence of sensitizers (Rose Bengal, hematoporphyrin). To elucidate the reaction mechanism similar reactions were carried out in the presence of radical quencher (2,4,6-tri-t-butylphenol) or singlet oxygen quencher (triethylenediamine). From these results, the possible mechanism of the formation of morusin hydroperoxide (II) from morusin (I) was discussed.     

Environmental Effects on Cellular Photosensitization: Correlation of Phototoxicity Mechanism with Transient Absorption Spectroscopy Measurements

Abstract— Little is directly known about the influence of the local environment experienced by a photosensitizer in a biological system on its photophysics and photochemistry. In this paper, we have addressed this issue by correlating mechanistic studies using laser flash photolysis with cellular phototoxicity data, obtained under the same experimental conditions. In particular, we have focused on the interaction between local concentrations of photosensitizer (deuteroporphyrin) and oxygen in determining the mechanism of phototoxicity in L1210 cells. In cells, as well as in models such as liposomes and red blood cell ghosts, hypochromicity and a reduction in fluorescence and intersystem crossing yields are observed on increasing the photosensitizer concentration between 0.5 and 20 μM, which illustrates the onset of a self-association. In aerated cellular preparations, the phototoxicity is predominantly type II (singlet oxygen) for all concentrations studied but an oxygen-independent mechanism occurs at the higher concentrations in deaerated samples. These observations are readily explained by consideration of triplet state kinetics as a function of oxygen and photosensitizer concentrations in cells. The rate constant for quenching of the photosensitizer triplet state by oxygen in cells was measured as 6.6 × 10⁸ M^?1 s^?1 and by photosensitizer ground state as -10⁶M^?1s^?1 (in terms of local concentration). The latter reaction gave rise to a long-lived species that is presumably responsible for the oxygen-independent phototoxicity observed at the higher photosensitizer concentrations used. This self-quenching of the triplet state is postulated to arise from electron transfer resulting in radical ion formation. Under conditions where no self-quenching contributes, the phototoxicity measured as a function of oxygen concentration correlates well with a model based on the determined kinetic parameters, thus, unambiguously proving the intermediacy of singlet oxygen. These effects should be borne in mind when interpreting phototoxicity mechanisms from in vitro cell studies. The excellent correlation achieved between laser flash photolysis data and measured phototoxicity gives credence to the direct use of photophysical techniques to elucidate photochemical mechanisms in biological media.     

Photooxidation of Troglitazone,a New Antidiabetic Drug *

Troglitazone (CS-045) is a new oral antidiabetic drug reported to be effective in insulin-resistant diabetes and to show antihypertensive effects. Photooxidation of troglitazone gave the quinone and quinone epoxide as the major final stable products. An intermediate observed by NMR spectroscopy was shown to be the hydroperoxydi-enone, which is moderately stable at room temperature. The rate constant of singlet oxygen quenching by troglitazone is 2.14 × 10⁸M^?1s^?1 and the reaction rate constant in acetone-d, is 8.64 × 10, M^?1 s^?1. Only the chroman ring of troglitazone reacts with and quenches singlet oxygen significantly, and its reactivity and products are analogous to those of a-tocopherol. The reactivity of CS-045 toward singlet oxygen is much larger than that of the related compounds lacking the chroman ring.     

MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY†

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV²⁺) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH⁺). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH⁺ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φ_s of AOH⁺ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φ_s is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH⁺ is quenched by MV²⁺ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 10¹⁰M^?1 s^?1. The quenching of the AOH⁺ excited singlet state by MV²⁺ almost certainly returns the AOH⁺ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 10⁸M^?1s^?1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV²⁺, flash irradiation of AOH⁺ does result in the reversible formation of the semireduced MV? which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH⁺ with MV²⁺. The initial quantum yield for formation of MV? (φ_MV:)₀ was found to be constant at 0.10 ± 0.05 for [MV²⁺] from 5 × 10^?5 to 1.0 × 10^?3 with [AOH⁺] = 8 × 10^?6M. Previous workers had found that (φ_MV:)₀ appears to decrease with decreasing [AOH⁺]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH⁺ by trace amounts of oxygen. When EDTA is added to a mixture of AOH ⁺ and MV²⁺ at pH 7, the photochemical formation of MV? becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV? exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH⁺ to produce the semi-reduced AOH- which then reacts with MV²⁺ to produce MV?. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO₂ was added to the optimal mixture [EDTA] = 3.4 × 10^?2M; [MV²⁺] = 5 × 10^?4M; [AOH⁺] = 4 × 10^?5M; pH6 H₂ gas was produced at a rate of 0.2μmol H₂h^?1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.     

Thioxanthone–anthracene‐9‐carboxylic acid as radical photoinitiator in the presence of atmospheric air

Thioxanthone–anthracene‐9‐carboxylic acid (TX‐ANCA) namely 14‐oxo‐14H‐naphthol [2,3‐b]thioxanten‐12‐carboxylic acid, is synthesized and characterized as part of our continuing interest for syntheses of polyaromatic initiators. Photoinitiator, TX‐ANCA have good absorption properties in the UV and visible region of the electromagnetic spectrum (ɛ₃₇₀: 9080 M⁻¹cm⁻¹, ɛ₄₃₀: 6151 M⁻¹ cm⁻¹). The fluorescence quantum yield is calculated as 0.1 which is slightly higher than of the parent thioxanthone compound (φ_f: 0.07). The phosphorescence lifetime is found to be 39 ms. The possible initiating mechanism of TX‐ANCA is based on photoexcitation of TX‐ANCA and quenching of triplet excited states of TX‐ANCA by molecular oxygen generates singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX‐ANCA possibly forms an endoperoxide. The endoperoxides undergoes photochemical or thermal decomposition to form radicals which are able to initiate free radical polymerization. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1878–1883     

CHEMISTRY OF SINGLET OXYGEN—XXVIII*. STERIC AND ELECTRONIC EFFECTS ON THE REACTIVITY OF SULFIDES WITH SINGLET OXYGEN

Abstract—The rates of reaction of singlet oxygen with a number of alkyl and aryl sulfides have been determined in CH₃OH. The rate for diethylsulfide is 1.71 ± 0.06 × 10⁷M^-1 s^-1. The addition of methyl groups α to the sulfur causes an approximate tenfold decrease in rate for each symmetrical pair. A similar effect is caused by replacement of alkyl by phenyl groups. The rates of substituted thioanisoles correlate well with σ(ρ= -1.6). but poorly with E_P/2 (half-peak oxidation potentials). A mechanism involving nucleophilic reaction of the sulfide with oxygen, rather than charge transfer is suggested.     

Quenching of singlet oxygen by phenols

The rate constants for the quenching of singlet oxygen by sterically hindered phenols were determined. It was observed that the rate constant for the quenching increases with a decrease in the ionization potential of phenols.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2132–2134, December, 1993.This work was supported by the Russian Foundation for Basic Research (Grant 93-03-5231).     

Chemistry of singlet oxygen. XVIII. Rates of reaction and quenching of alpha-tocopherol and singlet oxygen

Abstract —α-Tocopherol scavenges singlet oxygen (produced by methylene blue photosensitization in methanol) by a combination of chemical reaction (4.6 times 10⁷M^-1s^-1) and quenching (6.2 times 10⁸M^-1 s^-1). The total rate of scavenging (6.7 times 10⁸ M^-1s^-1) makes it an effective protective agent against photooxidation mediated by singlet oxygen.