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 B800-850 antenna complex from the purple bacterium Rhodopseudomans acidophila (strain 10050) were studied by one-color transient absorption experiments with a typical pulse length of 50 fs at room temperature and 77 K. The anisotropy kinetics observed within the B800 band are clearly wavelength dependent, indicating that the B800 ↔ B800 energy transfer or excitonic relaxation processes are wavelength dependent. The 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 originate from excitonic relaxation. Energy transfer from the B800 to the B850 occurs in ≃0.8 ps at room temperature and ≃1.30 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 × 10¹⁴ photons pulse^-1 cm^-2, the transient absorption kinetics after ≃3 ps are dominated by a long-lived bleaching. However, in contrast, a slowly recovering excited-state absorption was found to be dominant at lower pump intensities. This intensity dependence is attributed to the variation of the population distribution between the lowest and next higher lying excitonic levels of the B850 ring, a result of exciton annihilation in the lowest-state, following the rapid energy transfer from the B800 to the B850 band and subsequent fast excitonic relaxation within the excitonic manifold of the B850 ring. The time constant for this annihilation process was found to be ≃1 ps. Excitonic calculations indicate that several high-lying excitonic states show good spectral overlap with the B800 band, and thus, they could serve as excellent acceptors for the energy transfer from B800 to B850.