Polyenes are poly-unsaturated organic compounds that contain at least three alternating double and single carbon–carbon bonds. These carbon–carbon double bonds interact in a process known as conjugation resulting in some unusual optical properties. Related to polyenes are dienes, where there are only two alternating double and single bonds.
Some polyenes are brightly colored, an otherwise rare property for a hydrocarbon. Normally alkenes absorb in the ultraviolet region of a spectrum, but the absorption energy state of polyenes with numerous conjugated double bonds can be lowered such that they enter the visible region of the spectrum, resulting in compounds which are coloured (because they contain a chromophore). Thus many natural dyes contain linear polyenes, e.g. beta-carotene, which is responsible for the color of carrots.
Polyenes tend to be more reactive than simpler alkenes. For example, polyene-containing triglycerides are reactive towards atmospheric oxygen. Polyacetylene, which partially oxidized or reduced, exhibits high electrical conductivity. Most conductive polymers are polyenes, and many have conjugated structures.
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Beta-carotene is red-orange pigment abundant in plants and fruits, notably carrots.