Electronic Excitation Transfer from Carotenoid to Bacteriochlorophyll in the Purple Bacterium Rhodopseudomonas acidophila

Ultrafast fluorescence upconversion has been used to probe electronic excitation transfer within the B800−B820 light-harvesting antenna of Rhodopseudomonas acidophila strain 7050. Emission from the carotenoid S 2 band decays in 54 ± 8 fs, and the bacteriochlorophyll B820 Q y band rises in approximately 110 fs. The B820 Q y rise time is wavelength-dependent. Energy-transfer rates between the carotenoid and several neighboring bacteriochlorophyll are calculated. Coupling strengths are estimated through transition dipole−transition dipole, polarization, and higher-order Coulombic coupling along with a new transition density volume coupling calculation. Data are compared to calculated energy-transfer rates through the use of a four-state model representing direct carotenoid to B820 energy transfer. The carotenoid emission data bound the S 2 to Q x transfer time between 65 and 130 fs. The S 1 to Q y transfer is assumed to be mediated by polarization and Coulombic coupling rather than by exchange; the transfer time is estimated to be in the picosecond regime, consistent with fluorescence quantum yield data.