Answer to Question #89303 in Organic Chemistry for Harsh

Question

Discuss the effect of substituents on the colour associated with organic dyes.

Answer 1

A chromophore is the part of a moleculeresponsible for its color. The color that is seen by our eyes is the one not absorbedwithin a certain wavelength spectrum of visible light. The chromophore is a region in the molecule where the energy difference between two separate molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state. An auxochrome is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. Examples include the hydroxyl group (−OH), the amino group(−NH₂), the aldehyde group (−CHO), and the methyl mercaptan group (−SCH₃).

An auxochrome is a functional group of atoms with one or more lone pairs of electrons when attached to a chromophore, alters both the wavelength and intensity of absorption. If these groups are in direct conjugation with the pi-system of the chromophore, they may increase the wavelength at which the light is absorbed and as a result intensify the absorption. A feature of these auxochromes is the presence of at least one lone pair of electrons which can be viewed as extending the conjugated system by resonance.

Sources: https://en.m.wikipedia.org/wiki/Auxochrome

https://en.m.wikipedia.org/wiki/Chromophore