ENERGY-TRANSFER AND EXCITON ANNIHILATION IN THE B800-850 ANTENNA COMPLEX OF THE PHOTOSYNTHETIC PURPLE BACTERIUM RHODOPSEUDOMONAS-ACIDOPHILA(STRAIN-10050) - A FEMTOSECOND TRANSIENT ABSORPTION STUDY

Excitation energy transfer and exciton annihilation in the isolated B8 00-850 antenna complex from the purple bacterium Rhodopseudomans acido phila (strain 10050) were studied by one-color transient absorption ex periments with a typical pulse length of 50 fs at room temperature and 77 K. The anisotropy kinetics observed within the B800 band are clear ly wavelength dependent, indicating that the B800 <----> B800 energy t ransfer or excitonic relaxation processes are wavelength dependent. Th e depolarization times found at room temperature were 400 fs at 790 nm , 820 fs at 800 nm, and 360 fs at 810 nm. A faster depolarization time of 240 fs was obtained at 801 nm at 77 K, which is suggested to origi nate from excitonic relaxation. Energy transfer from the B800 to the B 850 occurs in similar to 0.8 ps at room temperature and similar to 1.3 0 ps at 77 K. The kinetics obtained within the B800 band were observed for the first time to exhibit a dramatic dependence on the excitation intensity. When the excitation intensity is higher than 1.09 x 10(14) photons pulse(-1) cm(-2), the transient absorption kinetics after sim ilar to 3 ps are dominated by a long-lived bleaching. However, in cont rast, a slowly recovering excited-state absorption was found to be dom inant at lower pump intensities. This intensity dependence is attribut ed to the variation of the population distribution between the lowest and next higher lying excitonic levels of the B850 ring, a result of e xciton annihilation in the lowest-state, following the rapid energy tr ansfer from the B800 to the B850 band and subsequent fast excitonic re laxation within the excitonic manifold of the B850 ring. The time cons tant for this annihilation process was found to be similar to 1 ps. Ex citonic calculations indicate that several high-lying excitonic states show good spectral overlap with the B800 band, and thus, they could s erve as excellent accepters for the energy transfer from B800 to B850.