Ultrafast dynamics of excitation energy transfer and coherent oscillations in chlorosomes elucidated by 2D electronic spectroscopy

초록

Excitation energy transfer (EET) in photosynthetic light harvesting complexes (LHCs) has attracted much interest because of its remarkably high efficiency and potential applications in solar cells. In particular, recent spectroscopic studies using two-dimensional electronic spectroscopy (2D-ES) proposed that quantum coherence formed among manifold of exciton states mediates highly efficient energy transfer involved in photosynthesis. In this talk, I will discuss the ultrafast EET dynamics and the potential role of quantum coherence in EET in a LHC called chlorosome. Chlorosomes are the most efficient photosynthetic light harvesting complexes found in nature and have a unique architecture with many pigments self-assembled into supramolecular J-aggregates without any support of a protein matrix. The chlorosomal J-aggregates not only exhibit high exciton mobility but also are readily synthesizable in vitro, making them an excellent building block of nanoscale optoelectronic devices. We performed 2D-ES measurements of chlorosomes at 77 K and room temperature (RT) to investigate the dynamics of EET in chlorosomes. First, we found that the line shape of a 2D peak changes rapidly on 20 - 30 fs time scale, which corresponds to the initial step of EET, that is, downhill energy equilibration among manifold of exciton states in chlorosomes. Interestingly, the EET becomes slower significantly at low-energy exciton states at RT, but not at 77 K. Such state-dependent EET rate at the high temperature is ascribed to uphill energy transfer activated by thermal excitation at RT. In addition, we identified coherent oscillations of electronic origin, implying that coherent energy transfer is involved in the initial step of EET in chlorosomes.