Ultrafast energy transfer in FMO trimers from the green bacterium Chlorobium tepidum

Time-resolved absorption difference profiles were obtained for FMO trimers, isolated from the green thermophilic bacterium Chlorobium tepidum, using one- and two-color femtosecond pump-probe techniques. Uphill and downhill energy transfers between inequivalent pigments in this antenna contribute to lifetime components that range from approximately 100 to approximately 900 fs in the isotropic absorption difference signals, depending on the pump and probe wavelengths. Vibrational thermalization of BChl a pigments may also influence the kinetics. The major lifetime components in the anisotropy decays at most wavelengths are 75-135 fs and 1.4-2.0 ps. The slower anisotropy decays probably stem from equilibration among equivalent, lowest-energy pigments belonging to different subunits in the trimer. The initial anisotropy r(0) is appreciably larger than 0.4 at several wavelengths, but r(t) typically decays to a value less than 0.4 within approximately 100 fs.