Energy Transfer Pathways in Light-Harvesting Complexes of Purple Bacteria as Revealed by Global Kinetic Analysis of Two-Dimensional Transient Spectra

Excited state dynamics in LH2 complexes of two purple bacterial species were studied by broad-band two-dimensional electronic spectroscopy. The optical response was measured in the 500-600 nm spectral region on the 0-400 fs time scale. Global target analysis of two-dimensional (2D) transient spectra revealed the main energy transfer pathways between carotenoid S-2, 1B(u)(-) and S-1 states and bacteriochlorophyll Q(x) state. Global analysis ascertained the evolutionary and vibration-associated spectra, which also indicated the presence of a higher-lying vibrational level in the carotenoid S-1 state. The estimation of the spectral overlap between the 1B(u)(-) state and the Q(x) state indicated a significant contribution of the 1B(u)(-) state to the overall S-2-to-Q(x) excitation energy transfer.