chromophore affects the amount of energy needed to excite the ^-electrons. The longer the conjugated system, the easier it is to excite, so longer wavelengths of light can be absorbed. The result is that phytoene, having three conjugated double bonds is colorless, and phytofluene, having five, is colorless, but fluoresces green under UV light.
Zeta-carotene has seven, absorbs light at ^400 nm and appears yellow, while neurosporene has nine, absorbs light at ~451, and appears orange, and lycopene has eleven conjugated double bonds, absorbs at ^472, and appears red. The polyene chain also allows transfer of singlet or triplet energy.
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