Chromophores
Definition:
Parts of a molecule responsible for its color. Chromophores contain conjugated systems (alternating single and double bonds) that allow delocalization of electrons, facilitating electronic transitions that absorb light.
Key Features:
Conjugation: More conjugation lowers the energy required for transitions, often shifting absorption into the visible region.
Functional Groups: Common chromophores include carbonyl (C=O), azo (-N=N-), nitro (-NO₂), and aromatic rings.
Examples:
Beta-Carotene: A long-conjugated system, giving it an orange color.
Hemoglobin: Porphyrin rings act as chromophores, responsible for blood's red color.
Auxochromes
Definition:
Groups attached to chromophores that modify their absorption properties. While not absorbing visible light, auxochromes enhance or shift the absorption of chromophores.
Types:
Electron Donating: Groups like -OH and -NH₂ push electron density into the chromophore, causing bathochromic (red) shifts.
Electron Withdrawing: Groups like -NO₂ pull electron density, causing hypsochromic (blue) shifts.
Functions:
Enhance Intensity: Increase molar absorptivity.
Shift Wavelength: Alter absorption maxima (λₘₐₓ).
Examples:
Hydroxyl Group (-OH): Enhances absorption, often causing a red shift.
Amino Group (-NH₂): Modifies chromophore properties, leading to shifts.