Singlet oxygen and polymer photooxidations. I. Sensitizers, quenchers, and reactants

The photooxidative deterioration of unsaturated food oils by near UV and visible light was found to be retarded by nickel (II) chelates well known to quench singlet oxygen (1O2). The rates of hydroperoxide formation during light exposure were unaffected by efficient peroxy radical scavengers such as hindered phenols. α-Tocopherol (vitamin E) underwent a relatively rapid oxidation itself when exposed to1O2 in oil solutions, despite being able to deactivate1O2 quite efficiently. From a comparison of the rates of scavenging by stabilizers and attack on allyl groups,1O2 initiated deterioration is appreciably more difficult to retard than peroxy radical processes.

Photooxidation of unsaturated oils: Effects of singlet oxygen quenchers

The triplet state properties of 1,4-dihydroxy-9,10-anthraquinone (quinizarin, QNZ), 1-amino-4-hydroxy-9,10-anthraquinone (AHAQ) and 1,4-diamino-9,10-anthraquinone (DAAQ) were investigated in cyclohexane and isopropanol solutions using nanosecond laser flash photolysis. TT absorption maxima, extinction coefficients, triplet quantum yields and kinetic parameters were measured. The corresponding properties of the neutral semiquinone radicals were also determined in isopropanol. Pulse radiolysis data were used to supplement the measurements. The possibility of dimerization in solution and its effect on the photophysics of the triplet state are discussed for these quinones.

Triplet excited states and semiquinone radicals of 1,4-disubstituted anthraquinones

The effect of sensitising dyes on the photochemical degradation of textile fibres is discussed. The degradation is dependent upon the presence of oxygen, and is accelerated by water vapour and rise in temperature. The study of simple model photochemical systems intended to simulate the dyed fibre system indicates that there are significant differences between the two systems. The sensitising effect of dyes in the textile systems seems to be due to the formation of excited singlet oxygen and hydrogen peroxide. The latter is not formed in the absence of water vapour.

The Photochemistry of Dyes. IV—The Role of Singlet Oxygen and Hydrogen Peroxide in Photosensitised Degradation of Polymers

Experimental evidence and published results show that when dyes are faded they are usually oxidised, probably by atmospheric oxygen. In certain systems reduction may occur, e.g. when the substrate is more readily oxidised than the dye or under anaerobic conditions in the presence of a hydrogen donor.

Oxidation and Reduction in Light Fading of Dyes

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)2-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time viaoperando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·gcat–1·h–1) under ambient conditions. These performance indicators make UiO-66(COOH)2-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)2-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)2-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)2-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

https://hal.science/hal-03767207/document

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal–Organic Framework under Visible Light

Studies on Aminoanthraquinone Compounds: VII—Validity of the Relation between Photosensitisation and Photoreduction with Piperidino Derivatives

The effect of (a) short-wave ultraviolet radiation (mainly 253-7 mμ) and (b) near-ultraviolet and visible radiation on polymer films dyed with the two disperse dyes, 1-methylaminoanthraquinone and 1,4-bismethylaminoanthraquinone, has been studied. Dyes on nylon films undergo reduction when exposed in dry nitrogen to 253-7-mμ radiation. The effect of near-ultraviolet and visible radiation is different; in dry nitrogen, 1-methylaminoanthraquinone on N-methoxymethylnylon film is reduced to the same extent as with 253-7-mμ radiation, but on other nylon films (nylon 6, nylon 6.6, and nylon 11) the reduction observed with this dye is much smaller. There is no evidence of reduction of 1,4-bismethylaminoanthraquinone on any of the nylon films. On cellulose acetate film, neither dye is reduced on exposure in dry nitrogen, by either source of radiation. The significance of these results in relation to the photo-sensitised degradation of textile fibres is discussed.

Studies on Aminoanthraquinone Compounds VI—Photochemistry of N-Methyl Derivatives on Polymer Films

The action of (a) near-ultraviolet and (b) short-wave ultraviolet radiation (mainly 253.7 nm) on nylon 6 and cellulose films dyed with one non-sensitising and two sensitising vat dyes has been studied. Photoreduction (in nitrogen) to the acid leuco derivative of the dye is observed only on exposure to 253.7-nm radiation. Vat dyes desensitise the degradation of cotton by 253.7-nm radiation, but there is no simple relation between this desensitising effect and their sensitising effect with near-ultraviolet and visible radiation. Degradation of vat-dyed cotton by 253.7-nm radiation is not increased by the reduction of the dye to the acid leuco form. Radical formation is unrelated to fibre degradation. The experimental evidence favours the concept of Kautsky's activated oxygen as an intermediary in the photosensitised degradation of cotton by near-ultraviolet and visible radiation, rather than that of hydrogen abstraction.

Photochemical Behaviour of Vat Dyes I—Reaction with the Polymer Substrate

A study has been made of the action of near-ultraviolet and visible radiation on anaerobic solutions of 1-methylaminoanthraquinone and 1,4-bismethylaminoanthraquinone in different organic solvents. Fading in hydrogen-containing solvents occurs by reduction, but substantial fading is also observed in carbon tetrachloride solution.

Studies on Aminoanthraquinone Compounds V—Photochemistry of N‐Methyl Derivatives in Solution

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