Electronic Excitation Transfer from Carotenoid to Bacteriochlorophyll in the Purple Bacterium <italic>Rhodopseudomonas acidophila</italic>

Ultrafast fluorescence upconversion has been used to probe electronic excitation transfer within the B800B820 light-harvesting antenna of Rhodopseudomonas acidophila strain 7050. Emission from the carotenoid S2 band decays in 54 ( 8 fs, and the bacteriochlorophyll B820 Qy band rises in approximately 110 fs. The B820 Qy rise time is wavelength-dependent. Energy-transfer rates between the carotenoid and several neighboring bacteriochlorophyll are calculated. Coupling strengths are estimated through transition dipoletransition 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 S2 to Qx transfer time between 65 and 130 fs. The S1 to Qy 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.