In
the first picoseconds of photosynthesis, photoexcitations of chlorophyll
molecules are passed through a network of chlorophyll-binding proteins to a
charge transfer site, initiating the conversion of absorbed energy to chemical
fuels. The remarkably high quantum
efficiency of this energy transfer relies on near-field coupling between
adjacent chlorophyll molecules and their interaction with protein phonon
modes. Using two-dimensional
electronic spectroscopy, we track the time-evolution of energy flow in a
chlorophyll-protein complex, CP29, found in green plants. The results from these nonlinear
four-wave mixing experiments elucidate the role of CP29 as a light harvester
and energy conduit by drawing causal relationships between the spatial and electronic
configurations of its chlorophyll molecules. Through independent control of experimental light pulse
polarizations, we have furthermore developed a technique to determine the
relative angles between the transition dipole moments responsible for energy
transfer. This work not only
yields tools for structural and spectral molecular characterization, but also
deepens our understanding of how photosynthetic systems have evolved to
optimize the conversion of light to biomass.