Role of Thermal Excitation in Ultrafast Energy Transfer in Chlorosomes Revealed by Two-Dimensional Electronic Spectroscopy

초록

Chlorosomes are self-assembled J-aggregates of bacteriochlorophyll pigments and envisioned as a building block of nanoscale optoelectronic devices because they exhibit high exciton mobility and are readily synthesizable in vitro. In this work, we apply two-dimensional electronic spectroscopy (2D-ES) to investigate ultrafast dynamics of excitation energy transfer (EET) in chlorosomes and their temperature dependence. From the time evolution of measured 2D electronic spectra, we directly map out the distribution of EET rate among manifold of exciton states in chlorosomes in a 2D energy space. In particular, we found that the EET rate varies gradually depending on the energies of energy-donor and energy-acceptor states. In addition, from comparative 2D-ES measurements at 77 K and room temperature, we found that the EET rate exhibits subtle dependence on both exciton energy and temperature, which demonstrates the effect of thermal excitation on the EET rate. This observation suggests that active thermal excitation at room temperature prevents excitation trapping at low-energy states and thus promotes efficient exciton diffusion via phonon-assisted energy hopping in chlorosomes at ambient temperature.