Coloration — decoloration cycles

Hereafter, a and b indicate the closed- and open-ring form isomers, respectively. The photocyclization and cycloreversion quantum yields were determined to be 0.46 and 0.015, respectively.1121 In the absence of oxygen, the coloration/decoloration cycle could be repeated more than 2000 times.[13] The basic performance of diaryl-ethenes is described below. 

Even if the side reaction quantum yield, 0S, is as small as 0.001 and B converts perfectly into A (0B— a= 1), 63 % of the initial concentration of A will decompose after 1000 coloration/decoloration cycles. Thus, if the cycle is to be repeated more than 10,000 times, the quantum yield of by-product formation has to be less than 0.0001. 

A crucial point that must be addressed concerns the thermal stability and the fatigue phenomenon observed in the chromophores. It is a fact that many photo-chromic compounds are irreversibly degraded upon long exposure to light, thus limiting their use for various applications. Major advances in the preparation and performance of photochromic materials have been made in the past five years. Irie et al.11271 have recently developed new photochromic compounds, 1,2-diarylethenes, which display photochromic behavior with unchanged intensity even after 104 coloration decoloration cycles. 

The fatigue resistance was improved when the furyl group was replaced with indole or oxazole [29, 30]. The coloration/decoloration cycles of oxazolyl fulgide 26 could be repeated more than 500 times in toluene, even in the presence of oxygen. The fatigue resistance strongly depends on the aryl groups. 

Figure 7. Fatigue resistance of fulgide derivatives a) in toluene and b) in PMMA film. Aq and A were the absorbances of the first cycle and after n coloration/decoloration cycles, respectively.

The trans-Xo-ana and the reverse quantum yields of 40 were determined to be 0.60 and 0.10, respectively. The coloration/decoloration cycle could be repeated more than 500 times. The quantum yield of side product formation was also determined to be 5 X 10 which agrees with the fatigue resistance of this molecule [41]. Vari-... 

Diarylethenes with heterocyclic aryl groups are newcomers to the photochromic field. They belong to the thermally irreversible (P-type) photochromic compounds. The most striking feature of the compounds is their fatigue resistance. The coloration/decoloration cycle can be repeated more than 10 times while retaining the photochromic performance. Both properties, thermal stability of both isomers and fatigue resistance, are indispensable for application to optoelectronic devices, such as devices for memory and switches. In this chapter, recent research on diarylethene derivatives will be described. 

When we intend to apply organic molecular materials, especially photochromic dyes, to optical memory media, the indispensable condition is stability, both thermal and photochemical. The photogenerated isomers are required never to return to the initial isomers in the dark, even at elevated temperatures, e.g., 80 °C. In addition, the coloration/decoloration can be cycled many times while the photochromic performance is maintained, and the memory media are provided with nondestructive readout capability. Although several molecules which fulfill the former condition have been developed, some problems still remain to gain access to molecules and systems which fully satisfy the latter condition. 

Though cycle time plays an important role in the SBR for the decolorization process, not many reports are found in the literature. The long retention times are often applied in the anaerobic phase of the reactor studies, such as 18 and 21 h. In several studies, it was reported that there is a positive correlation between the anaerobic cycle time and the color removal [30, 31]. Indeed, in combined anaerobic-aerobic SBRs, since bacteria shifted from aerobic to anaerobic conditions, or vice versa, anaerobic azo reductase enzyme can be adversely affected by aerobic conditions, which is essential for aromatic amine removal, thereby resulting in insufficient color removal rate. To investigate the effect of cycle time on biodegradation of azo dyes, inar et al. [20] operated SBR in three different total cycle times (48-, 24- and 12-h), fed with a synthetic textile wastewater. The results indicated that with a... 

Azo dye-containing wastewaters seems to be one of the most polluted wastewaters, which require efficient decolorization and subsequent aromatic amine metabolism. On the basis of the available literature, it can be concluded that anaerobic-aerobic SBR operations are quite convenient for the complete biodegradation of both azo dyes and their breakdown products. Nevertheless, like the other methods used for biological treatment, SBRs treating colored wastewaters have some limitations. Presence of forceful alternative electron acceptors such as nitrate and oxygen, availability of an electron donor, microorganisms, and cycle times of anaerobic and aerobic reaction phases can be evaluated as quite significant. 

Decolorization and Finishing. Decolor-ization is sometimes required for certain applications, mostly as an aesthetic preference. It is always desirable to solve these issues upstream. For example, color can be minimized by choice of fermentation medium components and control of the sterilization cycle so as to lessen the Maillard reactions between nitrogen and sugars and carameliza-tion. Color can also be reduced by treatment with activated carbon, use of antioxidants, and by diafiltration with membranes. Carbon-impregnated filter pads can be used to combine polish filtration with a decolorization step. 

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