Ultrafast pump-probe spectroscopy of bacteriochlorophyll c antennae in bacteriochlorophyll a-containing chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus

Time-dependent isotropic and anisotropic absorption difference profiles have been obtained with 2 ps resolution for the (BChl) bacteriochlorophyll c antenna in BChl a-containing chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus. The isotropic absorption difference spectra are bipolar; they are dominated by excited state absorption at wavelengths < 725 nm, and by ground state photobleaching at wavelengths > 735 nm. The entire isotropic spectrum exhibits a dynamic blue shift over approx. 4 nm with a time constant of approx. 7 ps. All of these phenomena are rationalized in terms of a linear exciton model that resembles the theory of J-aggregates. In this theory, the occurrence of the dynamic blue shift corresponds to relaxation between Q_y excitation components; it cannot readily be explained using a model that assumes that the excitations are localized on single chromophores within 1 ps. The anisotropic decay times, resolved here for the first time with our 2 ps fwhm instrument function, are approx. 7 ± 1 ps at 720 nm and 4 ± 1 ps at 740 nm. The residual anisotropy r(∞) is 0.32 ± 0.02 at 740 nm, which is in good agreement with fluorescence and linear dichroism measurements. These anisotropy functions initialize to 0.4 within error, indicating that subpicosecond depolarization is not prevalent.