Oxidation, of leuco dyes

A method was devised to determine extinction coefficients of radicals. A flash photolysis unit with filters to confine irradiation to a spectral region where only biimidazoles absorbed was utilized to generate radicals whose initial absorbance was recorded photographically from an oscilloscope. In the same cell another solution was then photolyzed as above. This second solution contained an identical concentration of biimidazole to the first plus leuco Crystal Violet dye, tris-(4-dimethylamino-phenyl) methane, and toluenesulfonic acid. The absorbance of dye produced by oxidation of leuco dye by was observed at an unobstructed wavelength. The extinction of the dye at this wavelength in this medium was determined experimentally. The stoichiometry of radical attack on dye was and the quantum yield of this reaction under... 

The leuco dyes of this new class are peculiar in that they are capped at their 10-position by another dye moiety. They are in fact colored leuco dyes which on oxidation are split into two different dye fragments one of the dyes is destined for the receptor sheet while the oxidized leuco hopefully remains on the donor sheet. The capping dye is selected among sublimable dyes whereas the azine dyes being cationic in nature would be less mobile. They have been developed to address some problems encountered in thermographic and photothermographic systems.24 Representative examples of this class of leuco dye developers are compounds 65, 66, and 67. 

The kinetics for the oxidation of leuco bases using oxygen has been studied.19 The oxidation involves complex formation between the proto-nated leuco base and the peroxy radical formed by air oxidation of the solvent. Addition of a radical initiator (AIBN) facilitates the reaction, while radical inhibitors retard the dye formation. In addition, oxidation reactions employing 2,3-dichloro-5,6-dicyanoquinone have shown large isotope effects in acetonitrile.20... 

Hexa(hydroxyethyl)pararosaniline nitrile has been used in a chemical radiochromic dosimeter.130 Ferricyanide oxidation of leuco Crystal Violet to Crystal Violet dye finds use in detection of various heavy metals131 at trace quantities. Oxidation of leuco triphenylmethanes by chloramine-T is catalyzed by iodide and therefore is used for detection of iodide.132 On the other hand, the inhibition of the catalytic effect of iodide by some ions can be used for determining traces of Ag(I), Hg(II), Pd(II). In addition, the triphenylmethane leuco dyes, phenolphthalein or phenol red are used extensively as indicators in calorimetric and titrimetric determinations. 

Tetrazolium salts are an important class of leuco dyes that have found a wide range of applications. They are unique among the synthetic leuco dyes in that the colored form of the dye is the reduced form rather than the oxidized form. There are at least three extensive reviews on tetrazolium salts and the corresponding formazan dyes1-3 and one textbook.96 This chapter will describe some recent developments as well as some of the material covered in these reviews. 

The book is divided into seven chapters. Chapter 1 describes photo-chromic materials which have critical applications in memory technology. These compounds generally are activated by light. Chapter 2 covers leuco quinones which, in many cases, when oxidized, have their absorption maxima in the near-infrared region. Chapter 3 describes leuco dyes of a common group of compounds—oxazine, thiazine, and phenazines—that have found applications in color photography. Chapters 4-6 describe arylmethine-type compounds that can be triggered to dyes by common chemistry. Chapter 7 describes a special class of leuco dyes, namely, tetra-... 

The use of stable derivatives of leuco dyes has been proposed. These have the advantage over dye developers in that because they are colourless, they can be coated in the appropriate emulsion layers. Examples are the pyrazole (79) and the pyrazolotriazole (80) (75BRP1415537). Processing in alkali results in loss of the acetyl groups from the oxichromic developers and transfer to the mordant from areas where no exposed silver halide is present. The leuco dye can be oxidized in this mordant layer by an incorporated oxidant such as the nitroxyl radical (81) (77BRP1478995). 

Another initiator system that can operate using electron transfer pathways involvers the HABI s discussed above in connection with color formation systems (see Sect. 6). Lophyl radicals were shown to be able to oxidize various leuco dye species to their colored forms. Similarly, such radicals can extract an electron from an arylamine containing hydrogens on a carbon alpha to N to form a-amino radicals capable of initiation [Eqs. (21,22)] of... 

Ahlquist, D.A. and Schwartz, S., Use of leuco-dyes in tiie quantitative colorimetric microdetermination of hemoglobin and otiierheme compounds, Clin. Chem. 21, 362—369, 1975 Josephy, P.D., Benzidine mechanisms of oxidative activation and mutagensis. Fed. Proc. 45, 2465-2470, 1986 Choudhary, G., Human health perspectives on environmental exposure to benzidine areview, Chemosphere 32,267—291,1996 Madeira, R, Nunes, M.R., Borges, C. et al., Benzidine photodegradation a mass spectrometry and UV spectroscopy combined study. Rapid Commun. Mass Spectrom. 19, 2015—2020, 2005 Saitoh, T, Yoshida, S., and Ichikawa, J., N htiialene-l,8-diylbis(diphenylmethylium) as an organic two-electron oxidant benzidine syntiiesis via oxidative self-coupling of A,iV-dialkylanilines, J. Org. Chem. 71, 6414-6419, 2006. 

Kautsky s three-phase test indicated that the excited yellow dye generated diffusible excited 02 species that were able to reach the colourless gel particles and to oxidize the leuco dye. In spite of this convincing evidence, his hypothesis was strongly challenged and... 

In fact, the earliest application of kinetic methods was to determine trace levels of substances exerting catalytic activity in oxidation-reduction reactions involving multiple electron transfers (1885-trace level V on its catalysis of the oxidation of aniline). For example, the reduced form of many triphenylmethane dyes is colorless , and loses two electrons on oxidation to the dye. The rate of reaction with such oxidants as 104 is relatively slow, but can be catalyzed by trace levels of transition metal ions which involve single electron transfer in their own redox steps. Thus, trace levels of manganese can be determined by the proportionality of the rate of oxidation of leuco-malachite green by iodate at less than micromolar concentrations. Similarly, trace levels of Cu ", < 10 M, can be determined from the catalytic effect on the atmospheric oxidation of ascorbic acid. Such systems can be written as a generalized redox reaction... 

Both the m- and -phenylenediamines are used to manufacture sulfur dyes, either by refluxing in aqueous sodium polysulfide, or heating with elementary sulfur at 330°C to give the leuco form of the dye. These dyes are polymeric, high molecular weight compounds, and soluble in base. The color is developed by oxidation on the fabric. 2,4-Toluenediamine and sulfur give Sulfur Orange 1 (14). 

The preparation of triaryknethane dyes proceeds through several stages formation of the colorless leuco base in acid media, conversion to the colorless carbinol base by using an oxidising agent, eg, lead dioxide, manganese dioxide, or alkah dichromates, and formation of the dye by treatment with acid (Fig. 1). The oxidation of the leuco base can also be accompHshed with atmospheric oxygen in the presence of catalysts. 

Fig. 11. Reaction of ionized coupler and oxidized developer (Dev ) to produce the intermediate leuco dye. If X is a good leaving group, the reaction proceeds spontaneously to dye, and the coupler is said to be two-equivalent. If oxidation by a second molecule of oxidized developer is required, the...

For two-equivalent couplers where the conversion of the leuco dye to image dye is rapid, the experimentally observed second-order rate constant, k, can be equated with kj, the rate of nucleophilic attack of coupler anion on oxidized developer. Thus when the pH of the process is specified, two parameters, piC and k, can be convenientiy used to characterize the molecular reactivity of a large variety of photographically weU-behaved couplers (40,54). 

Fig. 6. Color-shifting dye developers (a) yeUow dye developer [16044-30-7] that becomes magenta upon hydrolysis (b) cyan dye [50695-79-9] that becomes colorless upon protonation and (c) leuco form of a dye developer [50481-86-2] that becomes yellow upon hydrolysis and oxidation.

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