Excitation energy transfer and coherent oscillations in chlorosome elucidated by two-dimensional elecronic spectroscopy

초록

Energy transfer dynamics in photosynthetic light harvesting complexes have attracted much interest due to their potential applications in solar cells. From recent spectroscopic studies of natural light harvesting complexes, it was proposed that electronic coherence created among a manifold of exciton states might play an important role in highly efficient energy transfer involved in photosynthesis [1], stimulating intense studies in the emerging field of quantum biology. Chlorosome, the most efficient photosynthetic light-harvesting complex found in nature, consists of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. In this work, we elucidate the dynamics of ultrafast downhill energy transfer and the origin of coherent oscillations in chlorosome isolated from Chlorobaculum limnaeum at cryogenic temperature using two-dimensional (2D) electronic spectroscopy [2]. The line shape of a major peak in 2D spectra changes rapidly in 20 - 30 fs, representing ultrafast downhill energy transfer in a coherent domain and/or among coherent domains within a BChl layer in chlorosome. In addition, coherent oscillations superimposed on the ultrafast amplitude decay were observed. From the analysis of the oscillations, we identified the origins of these oscillations. In particular, the most pronounced oscillation of 140 cm-1 frequency arises from vibrational coherence while the oscillation of 620 cm-1 frequency arises from electronic coherence. Although the 620 cm-1 oscillation dephases rapidly on ~60 fs time scale, the electronic coherence may still contribute to the initial step of energy transfer in chlorosome, which occurs on the time scale comparable to the dephasing time.