Pathways of energy flow in LHCII from two-dimensional electronic spectroscopy.

Photosynthetic light-harvesting complexes absorb energy and guide photoexcitations to reaction centers with speed and efficacy that produce near-perfect efficiency. Light harvesting complex II (LHCII) is the most abundant light-harvesting complex and is responsible for absorbing the majority of light energy in plants. We apply two-dimensional electronic spectroscopy to examine energy flow in LHCII. This technique allows for direct mapping of excitation energy pathways as a function of absorption and emission wavelength. The experimental and theoretical results reveal that excitation energy transfers through the complex on three time scales: previously unobserved sub-100 fs relaxation through spatially overlapping states, several hundred femtosecond transfer between nearby chlorophylls, and picosecond energy transfer steps between layers of pigments. All energy is observed to collect into the energetically lowest and most delocalized states, which serve as exit sites. We examine the angular distribution of optimal energy transfer produced by this delocalized electronic structure and discuss how it facilitates the exit step in which the energy moves from LHCII to other complexes toward the reaction center.