Excitations and excitons in bacterial light-harvesting complexes.

The arrangement of pigments as recently observed in the structure of the peripheral light-harvesting complex of Rhodopseudomonas acidophila provides a new impulse to the discussion about the nature of the excitation transport and exciton delocalization in these photosynthetic light-harvesting systems. A crucial parameter in this discussion is the strength of the coupling between neighbouring BChl molecules. Their proximity implies that exciton interaction is important in such a structure, and this tends to delocalize the excitation. However, at the same time these structures exhibit strong spectral inhomogeneity, due to which the excitation becomes localized. In this paper we discuss some views on the organization, interactions and energy transfer dynamics in light-harvesting complexes of photosynthetic purple bacteria.