  The early steps of photosynthesis involve the photo-excitation of reaction
centres (RCs) and light-harvesting (LH) units. Here, we show that the
--historically overlooked-- excitonic delocalisation across RC and LH pigments
results in a redistribution of dipole strengths that benefits the absorption
cross section of the optical bands associated with the RC of several species.
While we prove that this redistribution is robust to the microscopic details of
the dephasing between these units in the purple bacterium Rhodospirillum
rubrum, we are able to show that the redistribution witnesses a more fragile,
but persistent, coherent population dynamics which directs excitations from the
LH towards the RC units under incoherent illumination and physiological
conditions. Stochastic optimisation allows us to delineate clear guidelines and
develop simple analytic expressions, in order to achieve directed coherent
population dynamics in artificial nano-structures.
