Modeling of energy transfer in photosynthetic light harvesting

In this chapter, we describe how energy transfer in photosynthetic light-harvesting antennae can be modeled using physical models. We perform a quantitative comparison of different energy transfer theories, that is, modified Redfield, standard and generalized Förster theories, as well as the combined Redfield–Förster approach. Physical limitations of these approaches are illustrated and critical values of the key parameters indicating their validity is found. We model at a quantitative level the spectra of and dynamics in three photosynthetic antenna complexes: (1) phycoerythrin 545 from cryptophyte algae (with Redfield theory); (2) the trimeric LHCII complex from higher plants (where the Redfield approach should be combined with the generalized Förster theory), and (3) the B800 antenna from purple bacterial LH2 characterized by a parameter range in between the Redfield and Förster limits. For the B800 antenna, we use the hierarchical equation approach, which is compared with other theories. We conclude that the localized (Förster) limit gives rather realistic population kinetics, while it neglects the coherences (that are sizable for B800). On the other hand, the modified Redfield picture fails to explain the dynamics of almost localized excitations in B800 unless the nonsecular (population-to coherence) transfers are included, which is possible only in the standard Redfield approach.