Extracting single and two-exciton couplings in photosynthetic complexes by coherent two-dimensional electronic spectra.

Signatures of "strong" J and "weak" K exciton couplings in the nonlinear femtosecond optical response of the FMO photosynthetic complex are identified. The two types of couplings originate from interactions of molecular transition charge dipoles and change of molecular permanent dipoles in their ground and excited states, respectively. We demonstrate that by combining various two-dimensional optical signals it should be possible to invert spectroscopic data to reconstruct the full exciton Hamiltonian (energies and couplings).