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What are Chromophores & Auxochromes? - Definitions & Types

Instructor: Sarah Pierce

Sarah has taught high school chemistry and biology, as well as college level chemistry(general, organic, analytical, biochemistry), and has a doctorate in chemistry.

This lesson explains the parts of a molecule responsible for color: chromophores and auxochromes. Chromophores and auxochromes are defined and examples of both are given. Different types of auxochromes (hydroxyl, amino, ether groups, etc.) are identified.

The Color of Royalty

Have you ever wondered why purple is a royal color? The dye used to color cloth purple was very rare, so only very rich or people who were royalty could afford it. As a matter of fact, the Roman emperor Nero decreed that he was the only person who could wear purple.

The Shroud of Charlemagne, made for Constantinople, was died purple using Tyrian purple
shroud

Why was this so expensive and rare? Before 1856, when William Perkin discovered a synthetic purple dye, purple dye was obtained from plants or animals. It takes between 9,000 and 10,000 Murex snails to obtain 1 gram of the dye! This purple is called Tyrian or royal purple.

Have you ever wondered what causes the dye molecules to be so colorful? Part of the dye molecule's structure is responsible for the color. Let's find out a little more about color and molecular structure!

The molecular structure of Tyrian purple is responsible for the color we see
the structural formula of Tyrian purple, or 6,6?-dibromoindigo

The Color We See

Let's review color really quickly. Remember the color wheel or color spectrum: Red, Orange, Yellow, Green, Blue, Indigo, and Violet or ROYGBIV? This is called the visible spectrum of light. White light is a mixture of all these colors. When an object absorbs some of the light, we see the remaining colors. Some colors are absorbed more strongly than others, and if a molecule strongly absorbs light of one wavelength, we see the opposite color of light reflected. We can use a color wheel to help us determine which color we will see.

We see the opposite color of the light that is absorbed. For example, if red light is absorbed, we see green
colorwheel

The molecular structure of the molecule can influence the wavelength of light that is absorbed, and as a result, influence the color we see. There are two broad groups of molecular components that are responsible for color in molecules: chromophores and auxochromes.

Chromophores

A chromophore is the section of a molecule that causes us to see color. The chromophore portion of the molecule will have alternating double bonds, or conjugated double bonds. For example, beta-carotene, the molecule responsible for the color in carrots, has many double bonds. It absorbs light at approximately 455 nm (blue), so it appears orange.

The conjugated double bonds of beta-carotene that are responsible for its color are shown in red
the structural formula of beta-carotene, or 1,3,3-Trimethyl-2-[3,7,12,16-tetramethyl-18-(2,6,6-trimethylcyclohex-1-en-1-yl)octadeca-1,3,5,7,9,11,13,15,17-nonaen-1-yl]cyclohex-1-ene, which has eleven double bonds

As the number of double bonds increases, the wavelength of light absorbed by the molecule also increases. For example, lycopene, the molecule responsible for the color in tomatoes, has more double bonds than beta-carotene and it absorbs light at 474 nm (blue-green), so it appears red.

Lycopene has more conjugated bonds than beta carotene, so it absorbs light at a larger wavelength (474 nm)
the structural formula of lycopene, or (6E,8E,10E,12E,14E,16E,18E,20E,22E,24E,26E)- 2,6,10,14,19,23,27,31-Octamethyldotriaconta-2,6,8,10,12,14,16,18,20,22,24,26,30-tridecaene, which has thirteen double bonds

Look at the structure of Tyrian purple that we talked about at the beginning of the lesson and see if you can find the alternating double bonds!


The conjugated double bonds are in a red box. Notice that they can be between two carbons or a carbon and an oxygen.
structural formula of Tyrian purple, which has nine double bonds


Take a moment to count the conjugated double bonds and compare Tyrian purple to beta-carotene and lycopene. Based solely on the number of double bonds, what color would you expect Tyrian purple to reflect?

It wouldn't be purple -- maybe more like yellow! So something else must affect the color. As it turns out, color shifts can also be caused by auxochromes.

Auxochromes

When specific molecular groups are attached to a chromophore, they can enhance the color we see and change the absorbance of the molecule. There are many different types of auxochromes, including, but not limited to:

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