Electromagnetic radiation from space constantly bombards the earth. Most wavelengths are absorbed by
the atmosphere; however, there are two "windows" of nonabsorption through which significant amounts of
radiation reach the ground. The first transmits ultraviolet and visible light, as well as infrared light or heat;
the second transmits radio waves. As a result, terrestrial organisms have evolved a number of pigments that interact with light in various ways: some capture light energy, some provide protection from light-induced damage, and some serve camouflage or signaling purposes.
Among these compounds are many conjugated polyenes, which play important roles as photoreceptors.
For every chemical compound, there are certain wavelengths of light whose quanta possess exactly the correct amount of energy to raise electrons from their ground state to higher-energy orbitals. For most organic compounds, these wavelengths are in the UV range. However, conjugated double bond systems stabilize the electrons, so that they can be excited by lower-frequency photons with wavelengths in the
visible spectrum. Such a pigment, known as a chromophore, will then transmit the "subtraction color," a color complementary to the one absorbed. For instance, carotene, a hydrocarbon compound with eleven conjugated double bonds, absorbs blue light and transmits orange. The wavelength that is absorbed
generally increases with the number of conjugated bonds; rings and side-chains also affect wavelength.
Wavelength Color Subtraction Color
480 nm blue orange
580 nm yellow violet
680 nm red green
Among the many biological molecules that are affected by light is DNA, the genetic material of living
organisms. DNA absorbs ultraviolet light, and may be damaged by UVC (< 280 nm) and UVB (280-315
nm). UVA (315-400 nm) and visible light can actually repair light-induced damage to DNA by a process called photorepair. For this reason, UVA, which also stimulates tanning, was once considered beneficial.
However, there is now increasing evidence that UVA can damage skin.
Two pigments are identical except for the lengths of their conjugated polyene chains. The first transmits yellow light and the second red.
What can be said about the sizes of the chromophores?