The document explains the concepts of chromophores and auxochromes, highlighting that chromophores are covalently bonded groups with characteristic absorption in the UV-visible region, while auxochromes do not absorb light but can enhance absorption when attached to chromophores. It provides examples of both, detailing how auxochromes can shift absorption wavelengths and intensify colors in organic compounds. The text also illustrates these concepts with specific examples such as nitrobenzene and aniline, showing how their respective functional groups influence color.

1. Concept of Chromophore
&
Auxochrome
• Dr. B. Sateesh Kumar
M.Sc., CSIR-JRF, Ph.D
Assistant Professor in
Chemistry

2. @DrBSK

3. @DrBSK
Heme colour-Red
due to
conjugated C=C
chromophore gps

4. ?
See this color due
to
Tomato
Conjugated Chromophore
gps

5. See this color due
to
?
@DrBSK
Answer

6. @DrBSK

7. Chromophore
• Isolated covalent bonded groups that shows a
characteristic absorption in UV-Visible region, those
are called Chromophore.
In the case of multiple bonded compounds, the π-electrons are responsible for
absorption and electronic excitation.
Chromophore must contains
-N=N-
Ethylenic, acetylenic, carbonyls, acids, esters, nitrile group etc.

8. Auxochromes
• The groups does not show any characteristic
absorption in UV-Visible region, those groups are
called Auxochromes.
• These are electron-donating groups and are known
as auxochromes.
• Examples of auxochrome groups are: -NH2, -OR, -
NHR, -SH, -OH ..Etc.
auxochrome (from Ancient
Greek αὐξάνω (auxanō) 'increase',
and χρῶμα (chrōma) 'colour')

9. Auxochrome
• 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.
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10. • An auxochrome can be defined as any group which does not itself act
as a chromophore but whose presence in a compound brings about a
shift of the absorption band towards the red of the spectrum (i.e.
longer wavelength)
Benzene shows an absorption at 255nm whereas aniline
absorbs at 280 nm. Thus amino group is an auxochrome

11. • There are mainly two types of auxochromes:
• Acidic: −COOH, −OH, −SO3H
• Basic: −NH2, −NHR, −NR2
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12. Effects on chromophore
• It increases the color of any organic compound
• Nitrobenzene is pale yellow colorbecause of the presence of a nitro
group (−NO2) which acts as a chromophore.
• But p-hydroxynitrobenzene exhibits a deep yellow color, in which the −OH group
acts as an auxochrome. Here the auxochrome (−OH) is conjugated with the
chromophore −NO2.
• Similar behavior is seen in azobenzene which has a red color, but p-
hydroxyazobenzene is dark red in color.
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