Exciton dynamics of chlorosomal light harvesting complex probed by two-dimensional electronic spectroscopy

초록

Chlorosome is the most efficient photosynthetic light-harvesting system that is composed of many bateriochlorophylls in aggregates. In this work, we investigate the exciton dynamics of chlorosome from Chl. Phaeobacteroides using two-dimensional (2D) electronic spectroscopy and pump-probe transient anisotropy at cryogenic temperature. The spectral broadening of the major peak in 2D spectra occurs rapidly with ~20 fs time constant. Also, pump-probe transient anisotropy exhibits 25-fs decay component. These dynamics correspond to ultrafast exciton diffusion in a BChl layer of roll in chlorosome, as supported by an atomistic calculation of model chlorosome. The oscillation of 100-150 cm^(-1) frequency was observed at many locations in 2D spectra and also in both rephrasing and nonrephasing 2D spectra, implying that the slow oscillation arises from vibronic coherence, not electronic coherence. Besides the low-frequency oscillation, the oscillations of higher frequencies are also observed, but they have small amplitude and exhibit ultrafast damping, preventing unambiguous assignment to the electronic coherence. Therefore, the quantum coherence does not seem to play a significant role in the energy transfer in chlorosome.