Exciton dynamics in photosynthetic complexes: excitation by coherent and incoherent light

In this paper, we consider the dynamics of a molecular system subjected to external pumping by a light source. Within a completely quantum mechanical treatment, we derive a general formula, which enables us to assess the effects of different light properties on the photo-induced dynamics of excitations in a molecular system. We show that, once the properties of light are known in terms of a certain two-point correlation function, the only information needed to reconstruct the system dynamics is the reduced evolution superoperator. The latter quantity is, in principle, accessible through ultrafast nonlinear spectroscopy. Considering a direct excitation of a small molecular antenna by incoherent light, we find that excitation of coherences is possible due to the overlap of homogeneous line shapes associated with different excitonic states. In Markov and secular approximations, the amount of coherence is significant only under fast relaxation, and both the populations and coherences between exciton states become static at long times. We also study the case when the excitation of a photosynthetic complex is mediated by a mesoscopic system. We find that such a case can be treated by the same formalism with a special correlation function characterizing ultrafast fluctuations of the mesoscopic system. We discuss bacterial chlorosome as an example of such a mesoscopic mediator and propose that the properties of energy-transferring chromophore-protein complexes might be specially tuned to the fluctuation properties of their associated antennae.