These molecules are useful in photosynthesis because they help the plant or bacterium to absorb and harvest light energy.

The relationship between the extent of delocalization and wavelength of light absorbed, which we first saw in Chapter 9, applies here. b-Carotene, spirilloxanthin, and isorenieratene have successively larger delocalized systems, 11, 13, and 15 electron pairs, respectively (highlighted in yellow).

Their electronic energy-level spacings therefore become progressively smaller, as diagrammed within the properties table.

The three molecules absorb in the blue-violet, the green, and the purple-red regions of the visible spectrum, respectively, so the molecules are coloured yellow-orange, purple, and green by the unabsorbed wavelengths (click on 'Properties table' button).

b-Carotene occurs in all green plants (usually masked by the green of chlorophyll), and is responsible for the yellow colour of carrots, tomatoes, and autumn leaves.

Spirilloxanthin and isorenieratene give purple and green photosynthetic bacteria their characteristic colours.