Coherent versus incoherent energy transfer and trapping in photosynthetic antenna complexes

In this paper we present a study of a model in which there is energy transfer as well as a special site where an irreversible reaction takes place. This model has an arbitrary ratio of homogeneous broadening versus site interaction energy. This allows us to study the crossover from hopping dynamics to exciton dynamics. We show that for the survival time the hopping (Forster) approximation gives a surprisingly accurate final result even when the energy transfer is excitonic. Fluorescence depolarization however is a sensitive probe for the nature of the energy transfer. We study the number of coherent molecules by considering a generalization of the inverse participation ratio. For LH1, assuming that it is a ring of 16 dimers, we estimate that the excitation is, on the average, delocalized over two dimers. The excitation is localized by phonons.