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 S₂, 1B_u^– and S₁ states and bacteriochlorophyll Q_<i>x</i> state. Global analysis ascertained the evolutionary and vibration-associated spectra, which also indicated the presence of a higher-lying vibrational level in the carotenoid S₁ state. The estimation of the spectral overlap between the 1B_u^– state and the Q_<i>x</i> state indicated a significant contribution of the 1B_u^– state to the overall S₂-to-Q_<i>x</i> excitation energy transfer