An auxochrome (from

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...

''auxanō'' "increase" and ''chrōma'' "colour") is a group of atoms attached to a

chromophore A chromophore is the part of a molecule responsible for its color. The color that is seen by our eyes is the one not absorbed by the reflecting object within a certain wavelength spectrum of visible light. The chromophore is a region in the mo ...

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 In chemistry, the term chromogen refers to a colourless (or faintly coloured) chemical compound that can be converted by chemical reaction into a compound which can be described as "coloured". There is no universally agreed definition of the term. ...

. Examples include the hydroxyl group (−OH), the

amino group In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

(−NH₂), the

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

group (−CHO), and the

methyl mercaptan Methanethiol (also known as methyl mercaptan) is an organosulfur compound with the chemical formula . It is a colorless gas with a distinctive putrid smell. It is a natural substance found in the blood, brain and feces of animals (including humans ...

group (−SCH₃). An auxochrome is a

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...

of atoms with one or more lone pairs of electrons when attached to a chromophore, alters both the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

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 Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...

Effects on chromophore

It increases the color of any

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

. For example,

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

does not display color as it does not have a chromophore; but nitrobenzene is pale yellow color because of the presence of a

nitro group In organic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups (). The nitro group is one of the most common explosophores (functional group that makes a compound explosive) used globally. The nitr ...

(−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 Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of simi ...

which has a red color, but ''p''-hydroxyazobenzene is dark red in color. The presence of an auxochrome in the

molecule is essential to make a dye. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the color. An auxochrome is known as a compound that produces a

bathochromic shift Bathochromic shift (from Greek βαθύς ''bathys'', "deep"; and χρῶμα ''chrōma'', "color"; hence less common alternate spelling "bathychromic") is a change of spectral band position in the absorption, reflectance, transmittance, or emissi ...

, also known as red shift because it increases the wavelength of absorption, therefore moving closer to

infrared light Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...

. Woodward−Fieser rules estimate the shift in wavelength of maximum absorption for several auxochromes attached to a conjugated system in an organic molecule. An auxochrome helps a dye to bind to the object that is to be colored. Electrolytic dissociation of the auxochrome group helps in binding and it is due to this reason a basic substance takes an acidic dye.

Explanation for the colour modification

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 their

natural frequency Natural frequency, also known as eigenfrequency, is the frequency at which a system tends to oscillate in the absence of any driving force. The motion pattern of a system oscillating at its natural frequency is called the normal mode (if all pa ...

are absorbed readily. This phenomenon, known as

, 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 different molecular orbitals falls within the range of the

visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye will respond to wa ...

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.{{cite journal , title=The Dye Spectrum , url=https://books.google.com/books?id=TaGZ6a_ZXEcC&q=Chromophore&pg=PA52-IA2 , journal=New Scientist , date=May 1989 , volume=122 , issue=1665 , pages=52 , issn=0262-4079 , publisher=Reed Business Information

Classification

There are mainly two types of auxochromes: * Acidic: −COOH, −OH, −SO₃H * Basic: −NH₂, −NHR, −NR₂

References

Chemical compounds Color Chemical reactions