Bleaching reaction

Metal-ion catalysis of hydrogen peroxide decomposition can generate perhydroxyl and hydroxyl free radicals as in Scheme 10.26 [235]. The catalytic effects of Fe2+ and Fe3+ ions are found to be similar [235]. It is not necessary for the active catalyst to be dissolved [237], as rust particles can be a prime cause of local damage. The degradative free-radical reaction competes with the bleaching reaction, as illustrated in Scheme 10.27 [237]. Two adverse consequences arise from the presence of free radicals ... 

Experiments in which a silica disk was the only filter used, so that the film was photolyzed with the whole visible and ultraviolet output of the lamp,showed that under these conditions the bleaching reaction was accompanied by a background photo-... 

We often clean away the grime and dirt in a kitchen with bleach, the active ingredient of which is the hypochlorite ion CIO-. The cleaning process we see by eye ( the bleaching reaction ) occurs between an aqueous solution of CIO- ion and coloured species stuck to the kitchen surfaces, which explains why the dirt or grease, etc., appears to vanish during the reaction. The reaction proceeds concurrently with colour loss in this example. 

We will consider the chemical reaction between the hypochlorite ion and coloured grease to form a colourless product P (the bleaching reaction) as having the following stoichiometry ... 

A central aspect of the PIE concept is that the dye and resist exposures should be completely separated (9,15) any departure from this condition makes the system more like CEL and lowers its performance. Further bleaching of the anthracene is of course eliminated simply by removing oxygen the bleaching reactions described above (e.g. Figure 8) are much lower in efficiency and therefore irrelevant insofar as pattern transfer is concerned. However, "antibleaching" due to the loss of oxygen from the anthracene peroxide upon DU V irradiation (47) is a concern it constitutes a dose-dependent loss of contrast. 

The mechanism in Scheme 15 suggests the rapid formation of an intermediate that is cis about the central (3-methine bond, which leads eventually to the ring-closed form but which can also re-form the state as the equilibrium concentration depletes. Scheme 16 is based on the fact that two possible conformers for the merocyanine have been identified (TTC and TTT) [36] and these may have different ground-state recovery times, but TTC and TTT generate the closed form with equal efficiency. Both of these schemes have merit and both have possible flaws however, what is clear is that there is no triplet-state involvement and that the bleaching reaction generates the spiropyran closed form essentially within 1.5 nsec. 

The silver dye bleach reaction is very slow (65MI11401), and an additional agent is needed to promote the reaction between the immobile silver image and the immobile dye. 1,4-Diazines have been used as catalysts for this reaction, 2,3-dimethylquinoxaline (65), 2,3-diphenylpyrazine (66) and 2-hydroxy-3-aminophenazine (67) being specific examples. 

While bleaching reactions are outside the usual scope of catalytic research, they are particularly important i.a. in the context of laundry washing and effluent cleaning [20]. For textile cleaning, activation of the oxidant (H2O2, domestic bleach) at temperatures below 40 °C is critical. Some Mn compounds seem to have potential for this application [21]. For... 

Clark and Lothian (30) reported an energy of activation of 0.06 ev. for the photoinduced reaction and 0.83 ev. for the bleaching reaction for the crystalline photochrome. The dark reaction, at 25°C., requires about 10 hr. for complete reversion. Although, at room temperature, solutions of 2-(2, 4 -dinitrobenzyl)pyridine do not appear photochromic, Hardwick et al. (31) observed that in low temperature (—30°C.) solutions the same color changes could be induced as in the crystals. 

Specific research, including with radical scavengers, showed that it is not the H02 particle that participates in the bleaching reaction, but a radical oxygen anion 0214. It is presumed that this particle is formed from H02 which, owing to the localised charge on the terminal oxygen, is unstable and further decomposes ... 

The chemistry of the bleaching reaction is not known in detail, but some evidence indicates that the main reaction is the rearrangement ... 

During the acid esterification and the acid work-up, the remaining phospholipids, proteins and pigments are removed by a degumming and bleaching reaction in this medium, and are discarded with the acidic glycerol and water layers. 

A highly efficient photochromic process is essential for organic photochromic compounds used as optical recording materials.39,40 That means that the quantum yields for the photocoloring and bleaching reactions should be high, but the quantum yield for the side reactions should be as low as possible. In order to solve the problems mentioned earlier, extensive studies have been carried out, and some promising results have been obtained so far. 

These results demonstrated that it is possible to design fulgide systems that have high quantum yields for both coloring and bleaching reactions and so far fulgide 41b is the best one for such aims. 

In fulgide 45, the low E > c and high d> c e values cannot be attributed to the steric hindrance effect of the acetyl group. They are due to the electronic effect, i.e., the electron-withdrawing ability of the 4-acetyl group. Therefore, not only the steric hindrance effect, but also the electronic effect of the substituents can influence the reactivity of both coloring and bleaching reactions. 

The third possibility is to design photochromic materials in which the photochemical bleaching reaction of the colored form has an activity energy barrier that could act as a threshold. Picosecond laser photolysis studies on furyl fulgides12 demonstrated that the transformation from the excited state of the colored form to the S o potential surface of furyl fulgides has an activity energy barrier. 

The thermal decay rates of the colored forms, which correspond to the amount of time required for a UV-activated photochromic to lose its color, typically range from 0.02 to 1.51 mol"1 s"1. However, this decay rate is highly influenced by temperature and the nature of the polymeric host. For instance, at 0°C a SO possesses a thermal fade rate of 2.67 x 10 2, and at 50°C the same molecule demonstrates a thermal decay rate of 3.3221. In fact, the thermal bleach reaction of some SOs ceases at -60°C. 

According to Griffing and West, the main requirements for CEL materials are as followsd) (1) the molar absorption coefficient for the photobleachable dye must be as high as possible, (2) the Ta>/Tb ratio must be as large as possible, and (3) the quantun yield of the bleaching reaction must be as high as possible. 

Fig. 9A.2 Anthocyanin equilibrium forms and bleaching reaction with bisulfite...

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