Chlorophylls and bacteriochlorophylls, together with other chlorin and carotenoid pigments and with quinone molecules, are organized in special pigment-protein complexes (light-harvesting antenna complexes and RC proteins) found in the photosynthetic membrane of cells for efficient photoelectric energy conversion .
Although the difference in the degree of the delocalization between the chlorophyll and bacteriochlorophyll pigments is small (note that the delocalization on the B-ring is disrupted and the ethylene group is substituted by acetyl group on the A-ring in bacteriochlorophyll), this difference has great influence on chemical and biological functions.
Figure 2 shows that the electric energy of the [S.sub.0] [right arrow] [S.sub.1] transition of bacteriochlorophyll corresponds to about 770 nm in organic solvent.
The basic structure of geoporphyrins, especially those with an exocyclic ring in the southern part of the molecule, is strongly suggestive of origin from chlorophylls and bacteriochlorophylls
, which are major photosynthetic pigments of photoautotrophs.
This hypothesis has been recently boosted by experimental results which reveal long-lived quantum coherent excitonic dynamics in the energy transfer among bacteriochlorophylls
in photosynthetic complexes (Engel et al., 2007; Lee et al., 2007).