Tobias Brixner1, Jens Stenger1, Harsha M. Vaswani1, Minhaeng Cho2, Robert E. Blankenship3 & Graham R. Fleming1
1.Department of Chemistry, and the Institute for Quantitative Biomedical Research (QB3), University of California, Berkeley, and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2.Department of Chemistry and Center for Multidimensional Spectroscopy, Division of Chemistry and Molecular Engineering, Korea University, Seoul 136-701, Korea
3.Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
Correspondence to: Minhaeng Cho2Graham R. Fleming1 Correspondence and requests for materials should be addressed to G.R.F. and M.C.
Top of pageTime-resolved optical spectroscopy is widely used to study vibrational and electronic dynamics by monitoring transient changes in excited state populations on a femtosecond timescale1. Yet the fundamental cause of electronic and vibrational dynamics-the coupling between the different energy levels involved?is usually inferred only indirectly. Two-dimensional femtosecond infrared spectroscopy based on the heterodyne detection of three-pulse photon echoes2, 3, 4, 5, 6, 7 has recently allowed the direct mapping of vibrational couplings, yielding transient structural information. Here we extend the approach to the visible range3, 8 and directly measure electronic couplings in a molecular complex, the Fenna-Matthews-Olson photosynthetic light-harvesting protein9, 10. As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings that ensure the efficient transport of energy from light-capturing antenna pigments to the reaction centre11. We monitor this process as a function of time and frequency and show that excitation energy does not simply cascade stepwise down the energy ladder. We find instead distinct energy transport pathways that depend sensitively on the detailed spatial properties of the delocalized excited-state wavefunctions of the whole pigment-protein complex.