Exciton delocalization and initial dephasing dynamics of purple bacterial LH2

Ultrafast four-wave mixing spectroscopies are employed to study exciton dyn amics associated with the B800 and B850 transitions of LH2 from Rhodobacter sphaeroides. Bacteriochlorophyll(a), the constituent chromophore of the B8 00 and B850 aggregates, is studied as a monomer in solution for comparison. Frequency-resolved pump-probe spectra measured across the B800 and B850 ba nds establish that at zero delay the transition dipole moment of B850 is su bstantially larger than that of B800, indicating an initial coherence size of similar to 13 chromophores in the (18-member) B850 aggregate. Novel freq uency-resolved stimulated photon echo measurements show that intermolecular interactions in the B850 ring reduce the coupling of this band's electroni c transition to nuclear motion. In contrast, linear electron-nuclear coupli ng is comparable in the bacteriochlorophyll, monomer and B800, where excito n coupling is weak. Photon echo peak shift data are consistent with these o bservations. The initial localization dynamics of the B850 exciton are reso lved 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 similar to 50 fs (for transient grating) due to exciton dephasing resulting in localizat ion. Pump-probe anisotropy measurements reveal substantial transition dipol e moment orientational relaxation with nearly the same time constant. These experimental data will be useful for the development of a rigorous theoret ical picture of ultrafast exciton dynamics in LH2. B850's large transition dipole moment for absorption may play an important role in the biological f unction of LH2, as it would enhance the energy transfer rate between B800 a nd B850.