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Auxochrome

All organic compounds are mainly composed of carbon and hydrogen atoms. They have certain functional groups which are responsible for the unique chemical and physical properties of organic compounds. For example, -NH2, -OH, -COOH, -CHO, >C=O are some examples of functional groups. The structure of organic compounds can be determine with the help of different techniques like X-ray diffraction, spectroscopy (NMR, IR), UV-spectrophotometry etc. Spectroscopy involves the interaction of organic compound with electromagnetic radiation. 

The functional groups in the molecule interact with electromagnetic radiations. Molecules absorb certain wavelength of radiations and remaining amount can be detected in digital display to get complete information about the structure of molecule. The visible part of electromagnetic spectrum lies from 400 – 700 nm of wavelength. If any organic molecule absorbs light from visible region, it will be colorful and color of molecule depends on absorption of wavelength of radiations.

 

Chromophore and Auxochrome

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In a molecule, there is a certain part which can absorb and reflect certain color when exposed to light. Such part of molecule is called as chromophore.  Chromophore is an extended delocalized system of electrons in a molecule. When a certain wavelength of light interacts with a molecule, molecule absorbs certain wavelength and reflects remaining light that causes color of molecule. 

The region of chromophore has energy difference between two different molecular orbitals that falls within the range of the visible spectrum. Visible light causes excitation of an electron from its ground into an excited state of chromophore. There are mainly two different types of chromophores; conjugated pi systems and metal complexes. In conjugated pi-bond system chromophores the electrons moves between eight energy levels which are extended pi orbitals due to alternating single and double bonds. Some common examples of such chromophores are retina in the eye, food colorings and dyes for textile industries. Even natural color variation in animals is also due to presence of chromophores. 

Another type of chromophore is metal complex chromophores in which color arise due to splitting of d-orbitals of transition metal. The splitting occurs due to interactions of transition metal with ligand. These chromophores are commonly found in chlorophyll, hemoglobin and malachite. Some of the groups are also impart color when they are attached with a chromophore. But they cannot produce color in the absence of chromophore.  Such groups are called as auxochromes. Auxochrome word arises from two word roots; ‘auxo’ means “to increase” and ‘chrome’ means “color”.  So they can be defined as the group of atoms which can impart a particular color while bonding with a chromophore.  But it fails to produce same color in the absence of chromophore. 

Auxochrome Definition

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The group of atoms which has capacity to alter the color of a chromophore is called as auxochrome. For example azobenzene is one of the most common examples of molecule with chromophore. These dyes contain two groups; the chromophore and the auxochrome. Here chromophore is responsible for color of the dye whereas the auxochrome helps to improve the color of the dye. These two groups are connected by a conjugated system and chromophore is an electron-withdrawing and auxochrome is an electron-donating group.  

Azo dye Auxochrome

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The color of azo dyes is due to chromophore and auxochrome in the molecule. Auxochrome can be acidic groups such as –COOH, -OH, -SO3H or basic groups like -NHR, -NR2, -NH2. We know that light is combination of seven color and an object can absorb certain wavelength of light. The reflected light imparts the complementary light to that object. Wavelength, the corresponding color, and its complementary color are listed below.

 Wavelength Range/nm 
 Colour   Complementary Colour 
 400-435  Violet  Green-yellow
 435-480  Blue  Yellow
 480-490  Blue-blue 
 Orange
 490-500  Blue-green 
 Red
 500-560  Green  Purple
 560-580  Yellow-green 
  Violet
 580-595  Yellow  Blue
 595-605  Orange  Green-blue
 605-750  Red  Blue-green

The conjugated pi-system of azo dyes absorbs specific wavelengths of electro-magnetic radiation.  When absorption falls in visible region, the reflected light is deficient in that particular color and the sample appears colored. So the color of dye depends on the wavelength that absorb by the sample and the radiation absorbed depends on the structure of azo dye. Change in the structure of azo dyes can cause bathochromic shift (towards longer wavelength) or hypsochromic shift (towards shorter wavelength). Presence of auxochrome in structure of azo dye also affects the color of dye as auxochrome supports the color of chromophore. Let’s take a simple example. Benzene is a colorless aromatic compound whereas nitrobenzene is pale yellow liquid. 

Here the color of nitrobenzene is due to presence of nitro group in the molecule. Hence nitro group acts as chromophore. Similarly in para-hydroxynitrobenzene, there are two groups; $-NO_{2}$ and –OH. Here nitro group acts as chromophore and –OH group acts as an auxochrome. The presence of auxochromophre enhances the color of compound and it appears deep yellow color. The conjugation of –OH group with –NO2 group causes the color of compound. Similar behavior can be observed in azobenzene with red color and para-hydroxy azobenzene is dark red in color.

Difference Between Chromophore and Auxochrome

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Both auxochrome and chromophore are responsible for the color of a chemical compound. The conjugation between these two groups occurs with the absorption of light from the visible region and reflected light is complementary color of substance. The major difference between auxochrome and chromophore is that auxochrome cannot impart color in the absence of chromophore. This is because the conjugation between these two groups causes color to a molecule. Auxochrome supports the color of chromophore and enhance the intensity of color.  

Auxochrome Examples

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Azobenzene are aromatic dyes which contain chromophore and auxochrome also. The color of compound is due to absorption of visible light. The groups which absorb the light from visible region are called as chromophores and some of the groups a shift in the absorption and called as auxochrome. So we can say that auxochromes are ‘color helpers’ or ‘color intensifiers’.  Auxochromes can be either positively charged or negatively charged. For example amino group $(-NH_{2}, -NHR, -NR_{2})$ is a positively charged auxochrome whereas carboxyl (-COOH), hydroxyl (-OH) and sulphonic groups are negatively charged auxochromes. 
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