Theoretical aspects of the effects of coupled coherent-incoherent dynamics on the primary processes of photosynthesis

Abstract

The general method of the coupled coherent-incoherent mechanism including the effect of quenching has been applied to the study of excitation energy transfer in the model of the photosynthetic unit. Due to the rotational symmetry of the model used, the compact analytic form of the physical observables, e.g. time-dependent fluorescence intensities at the reaction center and antenna molecules, the rate of fluorescence quenching and the quantum yield have been derived. Based on the obtained results, the general mechanism of coupled coherent-incoherent transfer is compared to the extreme regimes of the purely coherent and purely incoherent transport. It is shown that the calculated high value of the quantum yield at infinitely long time, ≈0.93, of the incoherent transfer mechanism is not the factor evidencing the superiority of the incoherence at the energy transfer of the photosynthetic process assuming that the geometry of the photosynthetic unit is of centro-symmetric. In contrast, on the relevant pico-second time scale, this value is only ≈0.45. It is the presence of the coherent hopping coupled to incoherence that makes the energy transfer so extremely effective. Contribution of coherence increases the quantum yield on the relevant time scale up to 0.98. The time constants of the fluorescence, i.e. the rate of fluorescence quenching calculated by the general method of the coupled coherent-incoherent mechanism are in very good agreement with experiment.