Elementary excitation in photosynthetic purple bacteria: How big is it?

We have studied excitation energy transfer in the photosynthetic antenna systems LH1 and LH2 of purple bacteria. Femtosecond pump-probe experiments were combined with computer simulations using the recently established structure of these systems to assess the nature of electronic excitation. We have measured the transient absorption kinetics and spectra of the LH1 and LH2 complexes in the temperature range from 4.2 K to 296 K with femtosecond time resolution. The Pauli master equation approach suggests that experimentally measured population and anisotropy kinetics in LH1 and LH2 cannot be described as a simple hopping between bacteriochlorophyll (BChl) a molecules. Exciton calculations where also monomeric doubly excited states are included indicate that the coherence length of the excited state is about 4 BChl a molecules at room temperature and it increases if temperature is lowered. Experiments where segments of LH1 antenna with different sizes were used suggest that the upper limit for the coherence length at 4 K is 12 BChl a molecules.