Zinc, photo-oxidation

The possibility of photo-oxidizing zinc and magnesium porphyrins in vitreous matrices in the presence of the efficient acceptor of electrons C(N02)4 was first demonstrated by Kholmogorov and Bobrovskii [54]. The authors observed the formation of N02 free radical and the cation radical of metalloporphyrin, MP+, upon illumination of solutions in the MP absorption band. 

The reaction of diethyl zinc with water produces zinc oxide, and then zinc carbonate, as the alkaline reserve. These chemicals have antiseptic properties which may also prevent the growth of mold in paper. They may also improve the brightness of treated papers. However, it is also known that zine oxide is a photosensitizer (56) which may trigger photo-oxidation of treated papers to initiate a chemical chain reaction that will lead eventually to the formation of acidic products (57). Moreover, the interaction of zinc oxide and zinc carbonate with copper, iron and cobalt present in the paper and their subsequent effects on paper stability have not been studied. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

The photophysics and photochemistry of gaseous PuFe have been ex-amined. Studies involving zinc porphyrins have been reported and include photo-oxidations in aqueous solution, photoreductions of Zn-TPP with hydrazines, and the role of Zn-TPPSa/ethyl viologen in photoredox processes. The mechanism of the photo-oxidation of water to oxygen with silver chloride has been discussed, and the synthesis of bis(chlorosilyl)-mercury compounds described. Colloidal CdSe has been shown to sensitize the photoreduction of O2 and of methyl viologen by cysteine. ... 

The desirable properties of photovoltaic dyes have been reviewed, and porphyrins appear to be promising candidates. A model for the structure of zinc porphyrin deposited on to various metal oxides has been proposed and the use of zinc porphyrin-coated electrodes for the photo-oxidation of SO2 has been described. Further studies on carrier generation in 3-metal free phthalocyanine have been reported and a method for the deposition of Pt islands on X-phthalocyanine particles has been developed. The Pt-coated material is much more photoactive than naked phthalocyanine, and it catalyses oxidation of amines by O2. 

Photo-oxidation and photoreduction of the zinc tetraphenylporphyrin radical cation have been described. The disproportionation equilibrium [equation (1)]... 

In the absence of a solvent, aniline was photo-oxidized, and the rate of oxidation was not altered by the presence of zinc oxide or sulfide. The same was true when hydroquinone or PPD was dissolved in aniline. 

Polyurethane ether (PUR-ether) foams are thought to degrade primarily by oxidation, particularly in the presence of light, resulting in discolouration and a loss of mechanical properties. Polyurethanes synthesized from a polyether polyol and an aromatic diisocyanate such as diphenylmethane diisocyanate (MDI) are highly vulnerable to photo-oxidation, whereas polyester-based polyurethanes are more resistant to ultraviolet radiation (Kerr and Batcheller, 1993). Metal ions, particularly copper, aluminium and zinc, form chelates with some polyurethanes imparting increased sensitivity to photo-oxidation (Ranby and Rabek, 1975). 

The nickel dithiocarbamates (Vllb) are hydroperoxide decomposers [21]. Unlike the zinc analogues, they are light stable and effective UV screening agents below 340 nm [30]. They react rapidly with hydroperoxides as they are formed in the photo-oxidizing polymer to give sulphur acids which are catalysts for hydroperoxide decomposition [21] and are themselves converted to nickel sulphate (see Scheme 1.5). This chemistry leads to the eventual depletion of the metal thiolate in the polymer and at this point (the end of the induction period), the rate of photooxidation reverts to that of the polymer containing the uncomplexed transition metal ion. 

Cadmium and zinc stearates (3.71) prevent photo-oxidation of poly(vinyl chloride) to Some extent, whereas barium stearate (3.71) shows poor photostability [183, 661, 1427, 1462, 1513, 1935, 2002, 2196, 2197]. 

On the other hand, dithiocarbamate nickel and zinc complexes are very effective light stabilizers for polyolefins. Using a combination of iron(pro-oxidant) and nickel or zinc (stabilizers) dithiocarbamates, it is possible to control the rate of photo-oxidative degradation very accurately [1931]. For long induction periods a combination of iron and nickel dithiocarbamates is recommended, whereas for shorter times it is more usual to combine iron and zinc dithiocarbamates [1932]. The combination of different metal ions decreases the induction period and increases the rate of photo-oxidation. At higher concentrations of stabilizing (Ni, Zn) dithiocarbamates, much longer lifetimes can be achieved. 

Anodic photo currents for zinc oxide, 470 Anodic potential, oxidation of the polymer and, 395 Antimony... 

Zinc oxide, anodic photo currents for, 470 Zinc oxide layers, spotted, 471 Zinc oxide-electrolyte interfaces, electron transfer rate and its exponential increase at, 512... 

Zhao, H. and H, R.K.Y. (2006) A study on the photo-degradation of zinc oxide (ZnO) filled polypropylene nanocomposites. Polymer, 47,3207-3217. 

The electronic microscopy method on the EM-125 (fig. 1) for definition of ZnCFO particles size and characteristic of its surface was applied. Known zinc oxide was chosen as the object of comparison. The electronic photos of powders testify, that new composite and zinc oxide have external similarity under the form of particles, wide range on dispersiveness (0,4-6,0 microns for zinc oxide, fig. la 0,3-6,0 microns for ZnCFO, fig. lb) also contain as crystal as amorphous phases in their structure. 

Metal chelates of 8-hydroxyquinoline such as (111) with photoconductive properties are reported to be useful in electrophotographic systems.233 The incorporation of a tin complex into a photo-conductive zinc oxide layer is stated to reduce dark decay . In other words, the electrostatic charge applied to the photoconductor has a longer lifetime. Two of the complexes disclosed for this application are (112) and (113). These compounds are prepared from dibutyltin oxide by reaction with 2-mercaptopropionic add and thioglycolic acid, respectively 234... 

At low temperatures, the EPR signal of Fe ions and Pb ions in zinc oxide is photosensitive. We observed this signal in relatively low-ohmic single crystals of ZnO. A pronounced photo excitation band shows up. It can be related to the photo ionization of the deep donor Fe ions. The analysis of the form of the photo ionization spectrum allows to calculate the optical ionization energy, which turned out to be equal to 1.4 eV for Fe zn ions and 1.65 eV for Pb zn ions. 

Photocatalytic activation of allyl and benzyl ethers results either in carbon-carbon coupling or oxygenation [148-151]. The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt, by preformed catalysts, or by particles supported within surfactant vesicles. Radical intermediates formed by hydrogen abstraction by photogenerated hydroxyl or hy-droperoxyl radicals may also be important in the photoelectrochemically induced oxidation of hydrocarbons. In the Ti02-sensitized photooxidation of toluene to cresols, for example, a photo-Fenton (radical) type mechanism has been suggested [150]. ... 

The zinc oxide, titanium dioxide and cadmium sulphide photo-catalysed oxidation of an ethylene-propylene elastomer has been... 

Amiodarone commonly causes phototoxicity reactions (186,187). The risk of phototoxicity increases with the duration of the exposure. Window glass and sun screens do not give protection, although zinc or titanium oxide formulations and narrow band UVB photo therapy can help (188-190). For most patients this adverse effect will be no more than a nuisance, and the benefit of therapy may be worthwhile. However, in a few cases treatment may have to be withdrawn. Histological examination of skin biopsies shows intracytoplasmic inclusions of phospholipids (191). There has been a single report of a severe case of photosensitivity in conjunction with a syndrome resembling porphyria cutanea tarda, resulting in bullous lesions (192). 

Although the photosensitizing action of zinc oxide suspensions has aroused a good deal of interest over the years, notably for the production of hydrogen peroxide, very little attention has been given in academic studies to ZnO photosensitization in the dry state. The simplest photo-... 

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