Singlet-singlet annihilation and local heating in FMO complexes

Energy transfer in a trimeric Bchl a containing FMO pigment-protein complex from the green sulfur bacterium Chlorobium tepidum has been studied by means of picosecond transient absorption spectroscopy under high-excitation conditions. At room temperature the excited state absorption spectrum of the FMO complex was found to be similar to that of noninteracting Bchl molecules in solution, which suggests that the influence of exciton coupling on the spectroscopic properties of the FMO complex at room temperature is not substantial. Analysis of the excited state relaxation kinetics in singlet-singlet annihilation conditions shows that the energy transfer from the excited monomer to another excited monomer is independent of the oxidation-reduction state of the complex and is slower than the intermonomer excitation migration rate. The difference spectrum at 77 K resembles the absorption spectrum, showing three exciton subbands. In addition to the singlet-singlet annihilation, the 7 ps rate of which is similar to that at room temperature, and to the intrinsic exciton decay, which is also temperature independent, energy redistribution between exciton states with a mean time of 26 ps is evident. This redistribution is explained as being due to local heating/cooling kinetics stimulated by the excitation pulses.