Nonlinear Exciton Annihilation and Local Heating Effects in Photosynthetic Antenna Systems

Abstract— Generation of the nonequilibrium distribution of excited vibrational modes stimulated by electronic energy relaxation in pigment‐protein complexes of the light‐harvesting antenna of some photosynthetic systems is discussed in this paper. It is shown that the simplest approach to this problem can be achieved by introducing a local temperature, which is a good parameter for describing the nonequilibrium distribution of the local vibrational modes of the pigment molecules and its nearest protein surroundings. Then the transient absorption kinetics is determined by the kinetics of the excitation relaxation as I well as the heating/cooling of the local vibrational modes. Experimentally, this process can be investigated in the i singlet‐singlet annihilation conditions that create the i greatest amount of local heating. The systems under in‐: vestigation are trimers of bacteriochlorophyll a contain‐ i ing pigment‐protein complexes from the green sulfur i bacterium Chlorobium tepid urn (so‐called FMO complexes) and aggregates of the light‐harvesting complexes of photosystem II (LHC2) from higher plants containing chlorophyll alb. It was shown that at 77 K the heat redistribution kinetics in LHC2 is on the order of 3040 ps and in FMO it is approximately equal to 26 ps. The local heating effect at room temperature is less pronounced; however, by using longer pulses and at higher excitation energies (on the order of a magnitude higher), an additional kinetics of hundreds of ps, also related to the heating/cooling process, was observed.