Color and Constitution

The color and constitution of cyanine dyes may be understood through detailed consideration of their component parts, ie, chromophoric systems, terminal groups, and solvent sensitivity of the dyes. Resonance theories have been developed to accommodate significant trends very successfully. For an experienced dye chemist, these are useful in the design of dyes with a specified color, band shape, or solvent sensitivity. More recendy, quantitative values for reversible oxidation—reduction potentials have allowed more complete correlation of these dye properties with organic substituent constants. 

J. Griffiths, Color and Constitution of Organic Molecules, Academic Press, Inc., London, 1976. 

Daehne S (1978) Color and constitution one hundred years of research. Science 199 1163-1167... 

R. H. Sprague, L. P. Smyth and G. L. Lewis Color and constitution. I. Halo-chromism of anhydronium bases related to the cyanine dyes. J. Amer. chem. Soc. 62, 1117 (1940). 

Color and constitution, IX. Absorption of cyanines derives from 3-me-... 

Color and constitution. XII. Absorption of the phtalones. J. Amer. chem. 

From about 1930 onwards, developments in the field of naphthoquinone dyes concentrated on the use of naphthazarin and intermediates for the preparation of violet, blue, and green acid and disperse dyes [1]. More recently there has been interest in the synthesis and color and constitution properties of simple colored naphthoquinones, stimulated by the fact that such dyes have similar tinctorial properties to the anthraquinones but a smaller molecular size. The naphthoquinones provide a useful alternative to the anthraquinones for certain specialized applications, e g., as pleochroic dyes with improved solubility for liquid-crystal displays. As a result, research interest in these chromogens remains unabated, even though they have failed to make any major impact as textile dyes [2-8],... 

The color and constitution of object 7 are similar to those of the violet trailing stick, object 5. The color is caused by iron in the presence of manganese and cobalt. There is half as much copper (0.16%) in this object as in object 5 (0.34%), so that copper at this level must not influence coloring, or its effect is masked by the higher proportion of iron. 

Color and Constitution of Phenolphthalein.—We have used phenolphthalein as an example of the pMhalein dyes in order to show their relation as tri-phenyl methane derivatives. When, however, we attempt to establish a consitution for phenolphthalein which will explain its character as a dye, in harmony with the structure of related compounds, e.g.j fluorescein and other pyronine dyesj we meet with considerable trouble and it may be said that the question is one that does not seem to be cleared up. Strictly speaking phenolphthalein is not a dye. Its well known use as an indicator is associated with the following facts, (i) In neidral or acid solution it is colorless. (2) In weak alkaline solution it is red. (3) In strong alkaline solution it is again colorless. (4) On neutralizing the excess alkali of (3) with acetic acid and boiling, the red color is restored and phenolphthalein is precipitated. 

According to Wilhelm Ostwald, acid-base indicators are weak acids or bases which, when undissociated, exhibit a color different from that of their ionic forms. Hantzsch and others have shown that the color change is not due to ionization, but rather results from a structural change. The explanation of Ostwald, however, is most suitable for purposes of elucidating the behavior of indicators at various hydrogen ion concentrations. We shall return in Chapter Seven to a comparison of the views of Ostwald and Hantzsch. It will then be found advisable to revise the definition of Ostwald as follows Indicators are weak acids and bases of which the ionogenic form possesses a color and constitution different from the color and structure of the pseudo or normal form. 

Hantzsch and his students demonstrated the relationship between color and constitution most clearly with the nitroparaffins and the nitrophcnols. These substances are yellow in alkaline solution and colorless in the presence of acids. Hantzsch showed... 

The author finds himself at variance with these views. He feels that the chromophore theory fails to explain the transformation satisfactorily, and that it merely considers a phenomenon which occurs simultaneously with the color change. The fact that color and constitution vary at the same time does not necessarily indicate that constitutional rearrangements are the cause of the color change. 

Indicators are (apparent) weak acids or bases of which the iono genic (a< - or baso- respectively) form possesses a color and constitution different from the color and structure of the pseudo- or normal compound. 

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