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Bacterial photosynthesis features robust and adaptable energy-harvesting processes in which light-harvesting proteins play a crucial role. The peripheral light-harvesting complex of the purple bacterium Allochromatium vinosum is particularly distinct, featuring a double peak structure in its B800 absorption band. Two hypotheses-not necessarily mutually exclusive-concerning the origin of this splitting have been proposed; either two distinct B800 bacteriochlorophyll site energies are involved, or an excitonic dimerization of bacteriochlorophylls within the B800 ring takes place. Through the use of two-dimensional electronic spectroscopy, we present unambiguous evidence that excitonic interaction shapes the split band. We further identify and characterize all of the energy transfer pathways within this complex by using a global kinetic fitting procedure. Our approach demonstrates how the combination of two-dimensional spectral resolution and self-consistent fitting allows for extraction of information on light-harvesting processes, which would otherwise be inaccessible due to signal congestion.

Förster theory describes electronic exciton energy migration in molecular assemblies as an incoherent hopping process between donor and acceptor molecules. The rate is expressed in terms of the overlap integral between donor fluorescence and acceptor absorption spectra. Typical time scales for systems like photosynthetic antennae are on the order of a few picoseconds. Prior to transfer it is assumed that the initially excited donor molecule has equilibrated with respect to the local environment. However, upon excitation and during the equilibration phase the state of the system needs to be described by the full density matrix, including coherences between donor and acceptor states. While being intuitively clear, addressing this regime experimentally has been a challenge until the recently reported advances in Fluorescence Detected Two-Dimensional Spectroscopy (FD2DS). Here, we demonstrate using fourth order perturbation theory, the conditions for the presence of donor-acceptor coherence induced cross-peaks at zero waiting time between the first and the second pair of pulses.The approach is illustrated for a heterodimer model which facilitates an analytical solution.