Why is beta-carotene orange?
Beta-carotene has the sort of delocalisation that we’ve just been looking at, but on a much greater scale with 11 carbon-carbon double bonds conjugated together. The diagram shows the structure of beta-carotene with the alternating double and single bonds shown in red. The more delocalisation there is, the smaller the gap between the highest energy pi bonding orbital and the lowest energy pi anti-bonding orbital. To promote an electron therefore takes less energy in beta-carotene than in the cases we’ve looked at so far – because the gap between the levels is less. Remember that less energy means a lower frequency of light gets absorbed – and that’s equivalent to a longer wavelength. Beta-carotene absorbs throughout the ultra-violet region into the violet – but particularly strongly in the visible region between about 400 and 500 nm with a peak about 470 nm.
