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
Yz. Ma et al., 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, JOURNAL OF PHYSICAL CHEMISTRY B, 101(6), 1997, pp. 1087-1095
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.