Chlorophyll b absorbs most effectively at blue 470 but also peaks at 430 and 640.
Chlorophyl a absorbs reasonably well at blue 450nm but absorbs most with a broader peak at red 680 – 700nm.
Pigments have an alternating arrangement of single and double bonds in the molecule’s carbons; these are conjugated bonds that share electrons. Pigments conjugated bonds share electrons in pi orbitals across the whole structure.
Conjugated Bond length and arrangement determines wavelength to be absorbed.
Chlorophyll’s conjugated bonds are found in the porphyrin ring, made up of a cyclic tertapyrrole that chelates Magnesium and in the side chain, which forms a long hydrocarbon tail to the heme ring. Electrons move readily along a conjugated bond series like they do in copper wire.http://www.webexhibits.org/causesofcolor…
When a photon of just the right amount of energy strikes an electron resonating in the pigment, the electron can absorb the photon and get promoted to a higher quantum level. The photon must have just the exact amount of energy to boost the electron from its current level to its new level or it cannot be absorbed. Each chlorophyll pigment A or B molecule differs in the bonds so absorbs a different wavelength. The only difference between Chlorophyll A and chl B is one methyl group is changed to a formyl group but this changes the wavelength that can be absorbed by the molecule.http://www.centenary.edu/attachments/bio…http://www.bio.umass.edu/biology/conn.ri…
If the question is about why the chlorophylls have a ‘green gap’ in the absorption spectra then the answer is the ‘Purple World’ hypothesis.
Early Earth’s life reflected purple and used the abundant midspectra wavelengths absorbed by rhodopsin leaving the flanking spectral wavelengths as open but marginal niches bacteria using chlorophyll adapted to fill.http://www.livescience.com/environment/0…http://www.msnbc.msn.com/id/18042280/