Dyes: The chemistry (colour and constitution)

 Dyes are colored substances which are soluble or go into solution during the application process and impart color by selective absorption of light. The absorption of electromagnetic radiations in the UV and visible regions by a molecule causes to the electronic excitation and an electron moves to higher electronic energy level from a lower. A covalently unsaturated group responsible for absorption in the UV or visible region is known as a chromophore. For example, C=C, C≡C, C=O, C≡N, N=N, NO2 etc. If a compound absorbs light in the visible region (400–800 nm), only then it appears colored. Thus, a chromophore may or may not impart color to a compound depending on whether the chromophore absorbs radiation in the visible or UV region.

A covalently saturated group which, when attached to a chromophore, changes both the wavelength and the intensity of the absorption maximum is known as auxochrome, e.g., NH2, OH, SH, halogens etc. Auxochromes generally increase the value of λmax as well as εmax by extending the conjugation through resonance. These are also called colour enhancing groups. An auxochrome itself does not show absorption above 200 nm. Actually, the combination of chromophore and auxochrome behaves as a new chromophore having different values of λmax and εmax.

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When two or more chromophoric groups are conjugated, the absorption maximum is shifted to a longer wavelength (lower energy) and usually to a greater intensity compared to the simple unconjugated chromophore. It is known that as the energy of electronic transitions decreases, their probability usually increases and so εmax also increases. Since conjugation lowers the energy required for electronic transitions, it increases the value of λmax and usually that of εmax also. In general, the longer the conjugated system, the higher are the values of λmax and εmax. Thus, a compound with sufficient conjugation absorbs in the visible region (400–800 nm) and becomes colored.

In short we can say, dye possess color because they:

v absorb light in the visible spectrum (400–700 nm),

v have at least one chromophore (color-bearing group),

v have a conjugated system, i.e. a structure with alternating double and single

v bonds, and

v exhibit resonance of electrons, which is a stabilizing force in organic compounds

When any one of these features is lacking from the molecular structure the color is lost