Exciton states and relaxation in molecular aggregates: Numerical study of photosynthetic light harvesting

In this article we briefly review the molecular exciton theory and apply it to photosynthetic light harvesting systems. We derive expressions for exciton relaxation rates using perturbation theory in Hibert space representation and show how the Förster equation can be obtained from a general relaxation rate expression in case of well localized excitons. A detailed analysis of the exciton level structure and wavefunctions of the ring structure is presented for diagonal and nondiagonal disorder. We conclude that the latter plays only a minor role in light-harvesting systems. We emphasize the relaxation rate dependency on square of the energy gap, which makes the high frequency modes particularly important for the exciton relaxation. We found a reasonably good agreement between the measured and calculated relaxation rates in photosynthetic purple bacteria.