An auxochrome is a functional group that does not itself absorb light in the visible spectrum but, when attached to a chromophore, modifies the chromophore’s ability to absorb light. It enhances the absorption intensity and often shifts the wavelength of maximum absorption (λmax)​of the chromophore.
Characteristics of Auxochromes
1. Functional Groups:
Auxochromes typically contain lone pairs of electrons or exhibit polar characteristics. Common examples include –OH, –NH₂, –SH, –Cl, –COOH.
2. Interaction with Chromophores:
Auxochromes interact with the π-electrons or conjugated systems of chromophores through resonance or inductive effects, altering the energy gap.
3. Effect on Absorption:
Bathochromic Shift (Red Shift): Shifts absorption to longer wavelengths, often into the visible region.
Hyperchromic Effect: Increases the intensity of absorption.
Mechanism of Action
Auxochromes modify the electronic structure of the chromophore by:
1. Electron Donation: Lone pairs from the auxochrome enhance electron density in the conjugated system, lowering the energy gap (e.g., –OH, –NH₂).
2. Electron Withdrawal: Auxochromes with electronegative atoms reduce electron density through inductive or resonance effects (e.g., –NO₂, –Cl).
Common Auxochromes and Their Effects
| Auxochrome | Structure | Type | Effect on Chromophore | Example |
| Hydroxyl | –OH | Electron donor | Bathochromic and hyperchromic shift | Phenol vs. Benzene |
| Amino | –NH₂ | Electron donor | Bathochromic and hyperchromic shift | Aniline vs. Benzene |
| Carboxyl | –COOH | Electron donor/withdrawer | Bathochromic or hypsochromic shift | Benzoic acid vs. Benzene |
| Methoxy | –OCH₃ | Electron donor | Bathochromic and hyperchromic shift | Methoxybenzene vs. Benzene |
| Nitro | –NO₂ | Electron withdrawer | Hypsochromic and hyperchromic shift | Nitrobenzene vs. Benzene |
| Halogens | –Cl, –Br, –I | Electron withdrawer | Hypsochromic or slight bathochromic shift | Chlorobenzene vs. Benzene |
Examples of Chromophore-Auxochrome Systems
1. Phenol (Chromophore + Hydroxyl Group):
The hydroxyl group in phenol donates electrons to the benzene ring, shifting its absorption to a longer wavelength compared to benzene.
2. Aniline (Chromophore + Amino Group):
The amino group donates electrons to the conjugated system, increasing absorption intensity and shifting the peak wavelength compared to benzene.
3. β-Carotene (Chromophore with Multiple Auxochromes):
Multiple auxochromes in β-carotene enhance its strong absorption in the visible region, giving it an orange color.
Effects of Auxochromes
1. Bathochromic Shift (Red Shift):
Caused by electron-donating auxochromes.
Example: Phenol absorbs at a longer wavelength than benzene.
2. Hypsochromic Shift (Blue Shift):
Caused by electron-withdrawing auxochromes.
Example: Nitrobenzene shows absorption at shorter wavelengths than benzene.
3. Hyperchromic Effect:
An increase in absorption intensity.
Example: Substituted aromatic rings with auxochromes absorb more strongly than unsubstituted ones.
4. Hypochromic Effect:
A decrease in absorption intensity, less common with auxochromes.
Applications of Auxochromes
1. Dye and Pigment Design:
Auxochromes are used to tune the color and intensity of dyes and pigments.
Example: Indigo dye contains auxochromes that shift absorption to the visible range.
2. Spectroscopy:
Auxochromes are used to improve the sensitivity of spectrophotometric measurements.
3. Biological Molecules:
Auxochromes in biomolecules like proteins and nucleic acids enhance their UV-visible absorption for structural and functional studies.
4. Pharmaceutical Analysis:
Functional groups in drugs serve as auxochromes, aiding in their identification and quantification.
Auxochromes are critical modifiers of chromophore properties, enhancing the utility of spectroscopic techniques and enabling the fine-tuning of molecular absorption characteristics. Their ability to influence electronic transitions underpins their significance in research, materials science, and industrial applications.
