Chromophore

As a chromophore (Greek " color carrier " ) is referred to that portion of a dye, which provides the basic existence of the color.

The fact that a dye in an observer a color stimulus elicits, ie appear colorful, based on several characteristics: the reflection and scattering of ambient light as well as the selective absorption of a portion of this light is absorbed by the chromophore and this energetically excited in a more or less prolonged state offset.

See also body color, absorption spectrum, optics, photochemistry

Inorganic chromophores

In inorganic dyes the color based, depending on the electronic configuration of the chromophore to charge-transfer transitions, or the excitation of electrons in the inner shell.

The mineralogy differs between idiochromatischen and allochromatischen materials.

• In idiochromatischen substances, the chromophore is directly involved in the construction of the mineral, eg when hydrated blue copper sulphate ( containing anhydrous copper sulfate is white).

• Allochromatische substances, however, contain the chromophore only in traces. Ruby is approximately chemically mainly of aluminum oxide, its color, however, it achieved only by traces of chromium, which are incorporated in the crystal lattice of the alumina.

Inorganic chromophores are commonly found among the transition metals. Examples are vanadium, chromium, manganese, iron, cobalt, nickel and copper. Depending on the oxidation state and complexation of these elements can have very different colors, such as therefore also the "chemical chameleon " called manganese six different ones.

See also ligand field theory

Organic chromophores

Synthetic dyes used in the industry usually have organic chromophores, which consist of systems of conjugated double bonds as in the carotene and / or large molecules, such as the aromatic methyl.

In all of these substances, the selective absorption of light quantum of electronic transitions from the highest occupied molecular orbital (HOMO) of the chromophore provide to its lowest unoccupied molecular orbital (LUMO).

Typical organic chromophores are for example:

• Long chains of conjugated double bonds as in carotene or chlorophyll
• By azo groups (A1 in the adjacent figure) connected aromatics as in the azo dye methyl orange (A2 in the adjacent figure) with the NR2 group ( right) as well as the auxochromer NaO3S group ( left) as antiauxochromer group
• Nitro compounds (C1 in the adjacent figure) as the aromatic nitro dye, picric acid (C2 in the adjacent figure) with the OH group as auxochromer group

In addition to the chromophore organic dyes have usually on further, reinforcing their color groups - are referred to as Auxochromes or auxochrome groups and antiauxochromes or antiauxochromes groups - according to the dye theory Witt. Thus, the absorption spectrum of a chromophore alone must not initially necessary lie in the visible region of the electromagnetic spectrum - the chromophore is, however, by the mesomeric effect of substituents further changed shifts so usually also its absorption spectrum.

For example, benzene due to its absorption in the UV light initially colorless - only by substitution with an ( anti) auxochrome as the nitro group, however, the resulting nitrobenzene is colored.

See also: bathochromic effect, hypsochromic effect

A function of the chromophoric features of the chemical environment

The dependence of the color of chromophores on their chemical environment is illustrated by the reaction of phenolphthalein indicator and the redox transformations of indigo in leucoindigo here. Depending on the particular form of the chromophore, these two dyes are either colored or colorless.

Phenolphthalein

The phenolphthalein can after removal of the two protons open at the peripheral hydroxyl groups in a basic medium, the central lactone ring of the molecule and to go its colorless lactoide form so its red colored quinoid form, in which the quinoid π -electron system between the central carbon atom and terminated oxygen atom with its easily movable electron pairs forming the chromophore of phenolphthalein.

Indigo

In the reduction of indigo to indigo white ( leuco - indigo) the chain of conjugated double bonds between the two oxygen atoms is destroyed at the five- membered rings, which leads to the loss of color of indigo due to the reduction of the π - electron system.