Structural color changes

The partially reduced form of niobium accounts for the color change of samples that underwent thermal treatment in vacuum or inert atmospheres. Whereas the thermal treatment of the mixture in air leads to the simultaneous oxidation of Nb4+ by oxygen, this is actually equivalent to the replacement of fluorine ions by oxygen ions in the complex structure of oxyfluoroniobate. Extended thermal treatment of systems containing LiNbOF4 and LiF yields a mixture of LiF and LiNbOs as the final thermal decomposition product. 

This color change was first observed by Duffin and Kendall (1954). A first structural interpretation (Baigrie et al., 1979) was later refuted by Gorelik et al. (1980). 

An acid—base indicator changes color with pH because it is a weak acid that has one color in its acid form (HIn, where In stands for indicator) and another color in its conjugate base form (In-). The color change results because the proton in HIn changes the structure of the molecule in such a way that the light absorption characteristics of HIn are different from those of In. When the concentration of HIn is much greater than that of In, the solution has the color of the acid form of the indicator. When the concentration of In is much greater than that of HIn, the solution has the color of the base form of the indicator. 

As frequently mentioned in the literature, anthocyanins co-exist in equilibrium in four different forms. The pH conditions shift this equilibrium toward a variety of structural forms, with the direct consequences of color changes of these pigments. As a rule, at pH above 4, yellow compounds (chalcone form), blue compounds (quinoid base), or colorless compounds (methanol form) are produced. Anthocyanins have the highest stabilities at a pH between 1 and 2 since the flavylium cation is the most stable predominant form. 

Research into the copigmentation of anthocyanins started as early as 1913 when Willstatter and Everest determined the chemical structure of cyanidin 3-glucoside isolated from blue cornflowers and red roses, and attributed the color changes to different pH levels in cell saps. This theory, however, was questioned and in 1916, Willstatter and Zollinger,revising the previous work, proposed a new theory according to which the colors of the anthocyanins varied significantly by the effects... 

Color Change. The development of color change within a photopolymer structure has been utilized in a number of commercial products. Specifically, the light induced change of a colorless leuco dye into a colored image forms the basis of several monotone positional pre-press proofing products. Other color change schemes (e.g. photochromic chemistry) have been used in a variety of products. 

Riva, M., Corteflino, G., Maestrelli, A., and Torreggiani, D. 2001. Structure collapse and color changes in osmo-air-dehydrated peach cubes. Food Sci. Biotechnol. 10, 598-601. 

Electrochromism is observed in reversible redox systems, which exhibit significant color changes in different oxidation states. Violenes, whose general structure is represented in Figure 1, are typical examples that exhibit electrochromism (1). 

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