Ultrafast Exciton Motion in Photosynthetic Antenna Systems:  The FMO-Complex

Excitation energy transfer in the Fenna−Matthews−Olson (FMO) photosynthetic antenna complex of chlorobium tepidum is investigated theoretically. On the basis of a dissipative multiexciton theory, the numerical simulations reproduce the cw-absorption and, in using the same parameters, the differential absorption of ultrafast pump−probe experiments. Exciton relaxation processes are included via a coupling to the vibrations of the protein matrix. In order take into account a delocalized protein−pigment interaction a correlation radius of the protein vibrations is introduced. The model allows for the study of the temperature dependence of optical spectra and enables one to utilize exciton relaxation data as a probe for a global-shape estimation of the spectral density of low-frequency protein vibrations. In fitting the cw-absorption measured at 5 and 107 K, the strength of the exciton-vibrational coupling, the related correlation radius and spectral density, and the inhomogeneous broadening of the exciton levels are determined anew. The obtained parameters are used to reproduce 150 fs pump−probe spectra as well as transient anisotropy pumped and probed at different wavelengths and different temperatures.