Femtosecond Energy-Transfer Dynamics between Bacteriochlorophylls in the B800−820 Antenna Complex of the Photosynthetic Purple Bacterium Rhodopseudomonas acidophila (Strain 7750)

Femtosecond one-color pump−probe measurements were performed on the B800−820 light-harvesting antenna complex of Rhodopseudomonas acidophila, strain 7750, at room and low temperature (mainly 77 K). The isotropic and anisotropic decays obtained within the B800 band are clearly wavelength- and temperature-dependent. A fast isotropic decay component at room temperature was found to have a lifetime of 0.30 ps at 784 nm, 0.54 ps at 790 nm, and 0.75 ps at 800 nm. The interband energy-transfer time was found to be ∼0.75 ps at room temperature, which slows down to ∼0.90 ps at 77 K. The time-resolved anisotropy, obtained around 790 and 800 nm, exhibits a single-exponential decay feature with a depolarization time of ∼1.1 ps at room temperature and ∼450 fs at 77 K. Measurements within the B820 band showed a fast isotropic component having a lifetime 60−80 fs. The corresponding time-resolved anisotropy decay occurs in 100−130 fs at room temperature and ∼240 fs at 77 K. In view of the general similarity in the energy-transfer dynamics between the B800−820 complex and other LH2 antenna, we conclude that the markedly blue-shifted B820 band has no substantial effect on the interband transfer rate.