Coherent Picosecond Exciton Dynamics in a Photosynthetic Reaction Center

Photosynthetic reaction centers convert sunlight into a transmembrane electrochemical potential difference, providing chemical energy to almost all life on earth. Light energy is efficiently transferred through chromophore cofactors to the sites, where charge separation occurs. We applied two-dimensional electronic spectroscopy to assess the role of coherences in the photoresponse of the bacterial reaction center of Rhodobacter sphaeroides. By controlling the polarization of the laser beams, we were able to assign unambiguously the oscillatory dynamics to electronic (intermolecular) coherences. The data show that these coherences are sustained for more than 1 ps, indicating that the protein coherently retains some excitation energy on this time scale. Our finding provides a mechanism for effective delocalization of the excitations on the picosecond time scale by electronic coherence, setting the stage for efficient charge separation.