The dye industry goes back several thousand years. The early dyes were obtained mainly through plants roots, vegetables, leaves, bark or berries.
The relationship between the absorbed wavelength and the chemical structure depends on the presence of double bonds connecting with single bonds.
The structure of these compounds allows the absorption of wavelengths of light from the visible part of the structure.
The variation between double and single bonds, are referred to as conjugated. Conjugation can be extended and the wavelength of absorbed light changed.
Blue dyes were especially valued because, compared with red or yellow, blue shades were not as common in plants. The blue dye was harvested from the Indigofera tinctoria.
Indican itself is colorless, but fermentation under alkaline conditions splits off the glucose unit to produce the indoxyl molecule.
OH groups are more important for producing color in these compounds than the number of rings.
This is shown as well in compounds derived from naphthoquinone.
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