A chromophore is a part of a molecule responsible for its color by absorbing light in the ultraviolet (UV), visible, or infrared (IR) regions of the electromagnetic spectrum. It contains electrons that undergo electronic transitions when exposed to light of specific wavelengths, resulting in absorption.
Characteristics of Chromophores
1. Conjugated Systems:
Chromophores often contain conjugated double bonds or aromatic systems. Conjugation lowers the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), shifting absorption to longer wavelengths.
2. Electronic Transitions:
Typical transitions include π → π or n → π. These transitions are responsible for the absorption of light in the UV-visible range.
3. Energy Gap:
A smaller energy gap corresponds to absorption at longer wavelengths (visible region), giving rise to color.
Types of Chromophores
1. Simple Chromophores:
Contain basic structures with localized π-electrons.
Example: Ethene (π → π transition).
2. Conjugated Chromophores:
Consist of multiple conjugated double bonds or aromatic rings.
Example: Benzene, β-carotene.
3. Metal-Containing Chromophores:
Complexes with metal ions (e.g., porphyrins) that absorb light due to d-d or charge-transfer transitions.
Example: Chlorophyll (green pigment in plants).
Examples of Chromophores
| Chromophore | Structure | Absorption Range | Example |
| Carbonyl group | >C=O | 280–290 nm (n → π) | Acetone |
| Double bonds | –C=C– | 170–190 nm (π → π) | Ethene |
| Conjugated dienes | –C=C–C=C– | 220–250 nm | 1,3-Butadiene |
| Benzene ring | Aromatic system | ~254 nm | Benzene |
| Nitro group | –NO₂ | ~200–400 nm | Nitrophenol |
| Porphyrin ring | Conjugated macrocycle | ~400–700 nm | Hemoglobin, Chlorophyll |
Factors Influencing Chromophore Behavior
1. Auxochromes: Groups like –OH, –NH₂, –Cl, or –NO₂ attached to a chromophore enhance its absorption intensity or shift its wavelength.
Example: Phenol absorbs at longer wavelengths than benzene due to –OH.
2. Solvent Effect: The polarity of the solvent can shift the absorption peak (bathochromic or hypsochromic shift).
3. Conjugation: Increasing conjugation results in lower energy gaps and absorption at longer wavelengths (red shift).
4. Substitution: Electron-donating or withdrawing groups affect absorption:
Electron-donating groups (e.g., –OH, –NH₂) cause a red shift.
Electron-withdrawing groups (e.g., –NO₂, –COOH) may cause a blue shift or enhance absorption intensity.
Applications of Chromophores
1. Colorimetry and Spectrophotometry:
Chromophores are used to determine the concentration of substances by measuring light absorption.
2. Dyes and Pigments:
Natural and synthetic dyes owe their colors to chromophores.
Example: Indigo (blue dye), β-carotene (orange pigment).
3. Biological Molecules:
Chromophores are integral to biological molecules such as:
Hemoglobin (red color from iron porphyrin).
Chlorophyll (green color from magnesium porphyrin).
DNA/RNA Bases (absorb in the UV region).
4. Fluorescence and Photochemistry:
Many chromophores exhibit fluorescence or are involved in photochemical reactions.
5. Drug Development:
Chromophores in drug molecules assist in determining their absorption properties, enhancing bioavailability or targeting specific tissues.
Conclusion
Chromophores are vital to understanding molecular interactions with light. They play a central role in spectroscopic analysis, biological processes, and the design of materials and pharmaceuticals. By studying their absorption properties, chemists and researchers can elucidate molecular structures, reaction mechanisms, and material properties.
