Two-Dimensional Electronic Spectroscopy of Light-Harvesting Complex II at Ambient Temperature: A Joint Experimental and Theoretical Study.

We have performed broad-band two-dimensional (2D) electronic spectroscopy of light-harvesting complex II (LHCII) at ambient temperature. We found that electronic dephasing occurs within ∼60 fs and inhomogeneous broadening is approximately 120 cm(-1). A three-dimensional global fit analysis allows us to identify several time scales in the dynamics of the 2D spectra ranging from 100 fs to ∼10 ps and to uncover the energy-transfer pathways in LHCII. In particular, the energy transfer between the chlorophyll b and chlorophyll a pools occurs within ∼1.1 ps. Retrieved 2D decay-associated spectra also uncover the spectral positions of corresponding diagonal peaks in the 2D spectra. Residuals in the decay traces exhibit periodic modulations with different oscillation periods. However, only one of them can be associated with the excitonic cross-peaks in the 2D spectrum, while the remaining ones are presumably of vibrational origin. For the interpretation of the spectroscopic data, we have applied a refined exciton model for LHCII. It reproduces the linear absorption, circular dichroism, and 2D spectra at different waiting times. Several components of the energy transport are revealed from theoretical simulations that agree well with the experimental observations.