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

초록

Excitation energy transfer in photosynthetic light harvesting complexes has attracted much interest due to their potential applications in solar cells. In particular, from recent spectroscopic studies of natural light harvesting complexes, it was proposed that quantum 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 [2]. 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 [3]. The line shape of a major peak in 2D spectra changes rapidly with 20 - 30 fs time constant. These dynamics correspond to ultrafast downhill energy transfer in a coherent domain and/or among coherent domains within a BChl layer in chlorosome. In addition, we observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm-1 frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm-1 oscillation dephases rapidly, the electronic (or vibronic) coherence may still contribute to the initial step of energy transfer in chlorosome, which is comparably fast.