Today Post::Dye and Colors

If white light is spread out by a prism, we can see that it is composed of different colors. Each color corresponds to a different wavelength. The color of a compound depends on the wavelength of light which it absorbs. If a compound does not absorb any visible light it will be colorless. If a compound absorbs light we will perceive the complementary color, because the light which reaches our eyes is missing the wavelengths which have been absorbed.
Color and wavelength
Wavelength of light, nm Color
Complementary color
400-430 Violet green-yellow430-480
blue
yellow
480-49o green-blue
orange
490-510
blue-green
red
510-530
green
purple
530-570
yellow-green
violet
570-580
yellow
blue
580-600
orange
green-blue
600-680
red
blue-green
The ability of a compound to absorb light depends on the presence of certain kinds of structural features called chromophores. “Chromophore” is one of those useful but sloppy words whose meaning depends somewhat on the context.
If we are talking about color, then a chromophore will be an extended conjugated system, particularly one where there are different resonance contributing forms differing in charge distribution.
If there is simply a conjugated system, with nothing but C=C, it takes quite a long chain of conjugated bonds to give a colored system:
H-(CH=CH)n-H
n =
wavelength (nm)
1
180
2
217
3
268
4
310
5
335

However if the chain of double bonds is long enough quite intense colors can result; the colors of carrots and tomatoes result from carotenoids, which have such chromophores.
Similarly, although benzene is colorless (it does not absorb visible light) compounds with fused rings absorb at longer and longer wavelengths. The ultimate is graphite, which contains large sheets of fused rings and is black because it absorbs all wavelengths of visible light.
Much smaller molecules can be colored if they have polar functional groups.
An example is indigo, the dye used in blue jeans.

If you write the various resonance contributing forms you will see that there can easily be separation of charge, with the charge spread over the molecule. Ancient Mediterranean civilizations prized a variant of indigo, obtained from a species of snail. 6,6′-Dibromoindigo or Tyrian purple was so expensive that only royalty could afford it.

Many natural pigments are based on the quinone chromophore,
The parent quinone shown, benzoquinone, is yellow; with larger conjugated systems, and especially with hydroxyl groups to modify the chromophore, they can be bright red.
Cochineal is a pigment obtained from dried female insects which live on a cactus. The pigment is used for food and cosmetics; the principal ingredient is carminic acid, which is a C-glucoside
of a polyhydroxyanthraquinone.

Before the invention of synthetic dyes, it was an important dye for wool and silk. The natural dye consists of the dried bodies of the insects. The insect, and the cactus it feeds on were brought back to the Old World from Mexico, and extensively cultivated. Since cactus species are only found in the New World, both the insect and its food had to be brought back.

The colors of many flowers are based on chemicals with the flavone chromophore or variations of it.
A complicating feature is that the color changes with pH, and the same chemical can be, and is, responsible for the red color of roses and the blue of cornflower; the pigment is cyanidin. This was once attributed to different cytoplasmic pH for the different flowers, but this difference is nowhere near large enough to cause the color change.
Only recently has it been discovered that the actual pigments are aggregates of several cyanin type molecules with metal ions and other pigments. A complication which should be mentioned is that the actual species in the flowers is a glycoside, with a sugar attached to at least one of the hydroxyls. For some of the hydroxyl groups, loss of a proton would give a molecule for which at least some resonance contributing forms would not have a charge on oxygen, although there would still be contributing forms with charge separation. Similar behavior is responsible for the change in color of morning glories as the bright blue of morning becomes purple or pink in the evening as the flower dies and becomes acidic.