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Inorganic chemistry, anauxochrome (from Ancient Greek αὐξάνω (auxanō) 'increase' and χρῶμα (chrōma) 'colour') is agroup of atoms attached to achromophore which modifies the ability of that chromophore toabsorb light. They themselves fail to produce the colour, but instead intensify the colour of thechromogen when present along with the chromophores in anorganic compound.[1] Examples include thehydroxyl (−OH),amino (−NH2),aldehyde (−CHO), andmethyl mercaptan groups (−SCH3).[2]
An auxochrome is afunctional group of atoms with one or morelone pairs of electrons when attached to a chromophore, alters both thewavelength and intensity ofabsorption. If these groups are in directconjugation with thepi-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 onelone pair of electrons which can be viewed as extending the conjugated system byresonance.
It increases the colour of anyorganic compound. For example,benzene does not display colour as it does not have a chromophore; butnitrobenzene is pale yellow colour because of the presence of anitro group (−NO2) which acts as a chromophore. Butp-hydroxynitrobenzene exhibits a deep yellow colour, in which the−OH group acts as an auxochrome. Here the auxochrome (−OH) is conjugated with the chromophore −NO2. Similar behavior is seen inazobenzene which has a red colour, butp-hydroxyazobenzene is dark red in colour.
The presence of an auxochrome in achromogen is essential to make adye. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the colour.
An auxochrome is known as a functional group that produces abathochromic shift, also known as red shift because it increases the wavelength of absorption, therefore moving closer toinfrared light.Woodward−Fieser rules estimate the shift in wavelength of maximum absorption for several auxochromes attached to aconjugated system in an organic molecule.
An auxochrome helps a dye to bind to the object that is to be coloured.Electrolytic dissociation of the auxochrome group helps in binding and it is due to this reason abasic substance takes anacidic dye.
A molecule exhibits colour because it absorbs colours only of certain frequencies and reflects or transmits others. They are capable of absorbing and emitting light of various frequencies. Light waves with frequency very close to theirnatural frequency are absorbed readily. This phenomenon, known asresonance, means that the molecule can absorb radiation of a particular frequency which is the same as the frequency of electron movement within the molecule. The chromophore is the part of the molecule where the energy difference between two differentmolecular orbitals falls within the range of thevisible spectrum and hence absorbs some particular colours from visible light. Hence the molecule appears coloured. When auxochromes are attached to the molecule, the natural frequency of the chromophore gets changed and thus the colour gets modified. Different auxochromes produce different effects in the chromophore which in turn causes absorption of light from other parts of the spectrum. Normally, auxochromes which intensify the colour are chosen.[3]
There are mainly two types of auxochromes:
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