Exciton Delocalization and Initial Dephasing Dynamics of Purple Bacterial LH2

Ultrafast four-wave mixing spectroscopies are employed to study exciton dynamics associated with the B800 and B850 transitions of LH2 from Rhodobacter sphaeroides. Bacteriochlorophylla, the constituent chromophore of the B800 and B850 aggregates, is studied as a monomer in solution for comparison. Frequency-resolved pump−probe spectra measured across the B800 and B850 bands establish that at zero delay the transition dipole moment of B850 is substantially larger than that of B800, indicating an initial coherence size of ∼13 chromophores in the (18-member) B850 aggregate. Novel frequency-resolved stimulated photon echo measurements show that intermolecular interactions in the B850 ring reduce the coupling of this band's electronic transition to nuclear motion. In contrast, linear electron−nuclear coupling is comparable in the bacteriochlorophylla monomer and B800, where exciton coupling is weak. Photon echo peak shift data are consistent with these observations. The initial localization dynamics of the B850 exciton are resolved with transient grating and pump−probe magic angle measurements. These data show that the enhanced transition dipole moment of B850 at the moment of excitation contracts significantly with a time constant of ∼50 fs (for transient grating) due to exciton dephasing resulting in localization. Pump−probe anisotropy measurements reveal substantial transition dipole moment orientational relaxation with nearly the same time constant. These experimental data will be useful for the development of a rigorous theoretical picture of ultrafast exciton dynamics in LH2. B850's large transition dipole moment for absorption may play an important role in the biological function of LH2, as it would enhance the energy transfer rate between B800 and B850.