What is the Difference Between Auxochrome and Chromophore

The main difference between auxochrome and chromophore is that auxochrome is a gathering of atoms that change the design of a chromophore, whereas a chromophore is a part of a molecule that absorbs visible light to reflect a certain color.

Auxochromes and chromophores are two parts of a molecule responsible for the color of the molecule. Generally, the auxochrome increases the color of the chromophore.

Key Areas Covered

1. What is Auxochrome
   • Definition, Characteristics, Importance
2. What is Chromophore
   • Definition, Characteristics, Importance
3. Similarities Between Auxochrome and Chromophore
   • Outline of Common Features
4. Difference Between Auxochrome and Chromophore
   • Comparison with Key Differences

Key Terms

Auxochrome, Chromophore

What is Auxochrome

Auxochrome is a group of atoms attached to the chromophore of a particular molecule. However, it modifies the ability of the chromophore to absorb light. Auxochrome is a Greek word that means “to increase the color”.  auxochromes themselves fail to produce the color, but their presence along with chromophores in an organic compound intensifies the color of the chromogen. Chromogens are colorless organic compounds that can be converted into colored organic compounds by a chemical reaction.

Figure 1: Auxochromes

Furthermore, some examples of auxochromes include the hydroxyl group (−OH), the amino group (−NH2), the aldehyde group (−CHO), and the methyl mercaptan group (−SCH3). Therefore, auxochromes are functional groups with one or more lone pairs of electrons. For instance, benzene 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 (−NO2), which acts as a chromophore. Similarly, p-hydroxynitrobenzene exhibits a deep yellow color, in which the −OH group acts as an auxochrome. In this, the auxochrome (−OH) is conjugated with the chromophore −NO2. Also, similar behavior is seen in azobenzene, which has a red color, but p-hydroxyazobenzene is dark red in color.

What is a Chromophore

The chromophore is an atom or group whose presence is responsible for the color of a compound. Molecules absorb some wavelengths of the visible spectrum and reflect other wavelengths. What we can see as the color is the wavelength that is reflected by the molecule. In a chromophore, the energy difference between two separate molecular orbitals falls within the visible spectrum range. Then, the visible spectrum hit the chromophore, exiting electrons from the ground state to the excited state. Therefore, when hit by light, a chromophore changes the conformation.

Figure 2: Beta Carotene with conjugation

Moreover, some examples of conjugated chromophores include retinal (used in the eye to detect light), various food colorings, fabric dyes (azo compounds), pH indicators, lycopene, β-carotene, and anthocyanins. They are for electron cloud-like systems in which electron jump between energy levels. Some other examples of metal chromophores include chlorophyll and hemoglobin in which the metal is in the coordination complex with ligands.

Similarities Between Auxochrome and Chromophore

• Auxochrome and chromophore are two parts or functional groups of a molecule.
• Moreover, they are responsible for the color development of the molecule.

Difference Between Auxochrome and Chromophore

Definition

Auxochrome refers to a group of atoms attached to a chromophore that modifies the ability of that chromophore to absorb light. Meanwhile, the chromophore refers to an atom or group whose presence is responsible for the color of a compound.

Type

Usually, auxochrome is a functional group of a molecule while chromophore is a part of a molecule.

Function

Auxochrome increases the color of a molecule while chromophore gives color to a molecule.

Conclusion

In brief, an auxochrome is a group of functional groups of a molecule that increases the color of the chromophore. Meanwhile, the chromophore is a part of the molecule that gives color to the molecule. Generally, light hits the chromophore and the chromophore undergoes excitation. When the chromophore comes to the ground state, it reflects a color. Therefore, the main difference between auxochrome and chromophore is their function.

References:

1. Wikimedia Foundation. (2021, December 23). Auxochrome. Wikipedia. Retrieved September 1, 2022.
2. Wikimedia Foundation. (2022, May 21). Chromophore. Wikipedia. Retrieved September 1, 2022. 

Image Courtesy:

1. “Auxochromes001” By Alasjourn – Own Work (CC BY-SA 3.0) via Commons Wikimedia
2. “Beta-Carotene conjugation” By Rubber Duck – Own Work (Public Domain) via Commons Wikimedia