Conjugated System - Conjugated Systems in Pigments

Conjugated Systems in Pigments

Conjugated systems have unique properties that give rise to strong colors. Many pigments make use of conjugated electron systems, such as the long conjugated hydrocarbon chain in beta-carotene, resulting in a strong orange color. When an electron in the system absorbs a photon of light of the right wavelength, it can be promoted to a higher energy level. (See particle in a box). Most of these electronic transitions are from one conjugated π-system molecular orbital (MO) with an even kind of symmetry to an other conjugated π-system MO with an odd kind of symmetry (π to π*), but electrons from other states can also be promoted to a π-system MO (n to π*) as often happens in charge-transfer complexes. Often a HOMO to LUMO transition is made by an electron if it is allowed by the selection rules for electromagnetic transitions. Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region and are colorless to the human eye. With every double bond added, the system absorbs photons of longer wavelength (and lower energy), and the compound ranges from yellow to red in color. Compounds that are blue or green typically do not rely on conjugated double bonds alone.

This absorption of light in the ultraviolet to visible spectrum can be quantified using ultraviolet–visible spectroscopy, and forms the basis for the entire field of photochemistry.

Conjugated systems that are widely used for synthetic pigments and dyes are diazo and azo compounds and phthalocyanine compounds.