Chromophores: Why Dyes and Pigments Have Color

Conjugated system

Conjugated double bonds 12

Molecule appearance

Yellow-orangeappears because it absorbs blue light

HOMO-LUMO gap

– eV

Absorbed λ

– nm

Absorption spectrum

Longer conjugation → smaller HOMO-LUMO gap → absorption peak shifts to longer (redder) wavelengths — the bathochromic shift. The visible color you see is the complement of the light that gets absorbed.

Example type

Dye: the chromophore dissolves and forms a chemical bond with the fiber — it soaks in and stays colored after rinsing.

Dye — bonded to fiber

Carbon backbone (sp²) Conjugated π bond Incoming white light Absorbed wavelength

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