CHROMOPHORES AND AUXOCHROMES PDF

The different colours of white light Everyone is familiar with rainbows- see the top picture for a well known example! These combined make up white light. If a light source is deficient in any colour band, the light appears to be coloured in the complementary colour. The table below shows wavelength, the corresponding colour, and its complementary colour 2.

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A chromophore is the part of a molecule responsible for its color. The chromophore is a region in the molecule where the energy difference between two separate molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state. In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.

Just like how two adjacent p-orbitals in a molecule will form a pi-bond , three or more adjacent p-orbitals in a molecule can form a conjugated pi-system. In a conjugated pi-system, electrons are able to capture certain photons as the electrons resonate along a certain distance of p-orbitals - similar to how a radio antenna detects photons along its length.

Typically, the more conjugated longer the pi-system is, the longer the wavelength of photon can be captured. In other words, with every added adjacent double bond we see in a molecule diagram, we can predict the system will be progressively more likely to appear yellow to our eyes as it is less likely to absorb yellow light and more likely to absorb red light.

In the conjugated chromophores, the electrons jump between energy levels that are extended pi orbitals , created by a series of alternating single and double bonds , often in aromatic systems. Various factors in a chromophore's structure go into determining at what wavelength region in a spectrum the chromophore will absorb. Lengthening or extending a conjugated system with more unsaturated multiple bonds in a molecule will tend to shift absorption to longer wavelengths.

Woodward—Fieser rules can be used to approximate ultraviolet -visible maximum absorption wavelength in organic compounds with conjugated pi-bond systems. Some of these are metal complex chromophores, which contain a metal in a coordination complex with ligands.

Examples are chlorophyll , which is used by plants for photosynthesis and hemoglobin , the oxygen transporter in the blood of vertebrate animals. In these two examples, a metal is complexed at the center of a tetrapyrrole macrocycle ring: the metal being iron in the heme group iron in a porphyrin ring of hemoglobin, or magnesium complexed in a chlorin -type ring in the case of chlorophyll.

The highly conjugated pi-bonding system of the macrocycle ring absorbs visible light. The nature of the central metal can also influence the absorption spectrum of the metal-macrocycle complex or properties such as excited state lifetime.

Examples of such compounds include bilirubin and urobilin , which exhibit a yellow color. An auxochrome is a functional group of atoms attached to the chromophore which modifies the ability of the chromophore to absorb light, altering the wavelength or intensity of the absorption. Halochromism occurs when a substance changes color as the pH changes.

This is a property of pH indicators , whose molecular structure changes upon certain changes in the surrounding pH.

This change in structure affects a chromophore in the pH indicator molecule. For example, phenolphthalein is a pH indicator whose structure changes as pH changes as shown in the following table:. Because of their limited extent, the aromatic rings only absorb light in the ultraviolet region, and so the compound appears colorless in the pH range.

However, as the pH increases beyond 8. This makes the three rings conjugate together to form an extended chromophore absorbing longer wavelength visible light to show a fuchsia color. From Wikipedia, the free encyclopedia. Electronic Structure and Chemical Bonding". Purdue University. In Dolphin, D. The porphyrins. Academic Press, New York.

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Chromophore

They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. 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. 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. It increases the color of any organic compound.

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Difference between auxochrome and chromophore

Chromophore , a group of atoms and electrons forming part of an organic molecule that causes it to be coloured. Correlations between the structural features of chemical compounds and their colours have been sought since about , when it was noted that quinones and aromatic azo and nitro compounds often are highly coloured and that the colours are diminished or destroyed when the compounds are hydrogenated. The ability of a compound to take up hydrogen, called unsaturation, is caused by the presence of electrons that are not strongly fixed in covalent bonds between particular pairs of atoms but occupy larger regions of space molecular orbitals that may be associated with several atoms. These electrons can absorb energy from light over a certain range of wavelengths in the visible region; transmission or reflection of the remainder of the light gives rise to the observed colour of the compound. Deep coloration results if several chromophores are closely joined in the same molecule or if another group, called an auxochrome, is present. Info Print Cite. Submit Feedback.

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Auxochrome

Auxochrome is a group of atoms which will impart a particular color when attached to a chromophore but when present alone, will fail to produce that color. Chromophore is that part of the molecule which when exposed to visible light will absorb and reflect a certain color. Auxochrome is a group of atoms which is functional and has the capability to alter the capacity of the chromophore to reflect colors. Azobenzene is an example of a dye which contains a chromophore.

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