Exciton dynamics in photosynthetic molecular aggregates

J.A. Nöthling, T.P.J. Krüger, T. Mančal
University of Pretoria, South Africa

During the initial steps of photosynthesis, light energy is absorbed and transported on a molecular level inside plant cells. Pigment molecules, which are responsible for the absorption of light, are held in close proximity to one another by a protein scaffold and exciton states are consequently often formed. In plants, energy is transported amongst exciton states to a reaction centre from where the energy is converted into chemical energy. We discuss the mechanism through which energy is transferred amongst exciton states, highlighting the role of the phonon bath in facilitating this transfer. We also explain Redfield theory, which is used for calculating exciton dynamics theoretically, and its limitations. The computational complexity of Redfield simulations increases dramatically as the number of excitons increases. We explore the possibility of simplifying computation of Redfield dynamics by projecting the multi-excitonic equations of motion onto the space of single-exciton dynamics. We discuss calculated Redfield dynamics in the Fenna-Matthews-Olson complex of green sulphur bacteria. In particular, we discuss the importance of two of this complex’s pigment molecules (between which long-lasting coherence is predicted) for light harvesting.