Vibronically coherent speed-up of the excitation energy transfer in the Fenna-Matthews-Olson complex

We show that underdamped molecular vibrations fuel the efficient excitation energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions. By employing an environmental fluctuation spectral function derived from experiments, we obtain numerically exact results for the exciton quantum dynamics in the presence of underdamped vibrationally coherent quantum states. Assuming the prominent $180-{\mathrm{cm}}^{-1}$ vibrational mode to be underdamped, additional coherent transport channels for the excitation energy transfer open up and we observe an increase of the transfer speed towards the reaction center by up to $24\%$.

Figure 1

Structural arrangement of the seven main BChl molecules in the FMO complex (Chlorobium tepidum) superposed with a schematic representation of the delocalized excitons (color shading). The two main excitation transfer passageways are indicated by the thin arrows. The harmonic oscillator parabola highlights the underdamped molecular vibrations which speeds up the energy transfer.

Figure 2

Time-dependent increase of the FMO reaction center (RC) population at room temperature for (left) ${\rho }_{11}\left(0\right)=1$ and (right) ${\rho }_{66}\left(0\right)=1$ for the cases when the $180-{\mathrm{cm}}^{-1}$ molecular vibrational mode is underdamped [blue (upper) lines], strongly damped [red (lower) lines], and omitted from the model (black lines on top of the red lines). The dotted lines mark the sink populations when the Huang-Rhys factor $S_0$ is increased by a factor of 2, see text.

Figure 3

Time-dependent populations time of site 1, site 2 (both in the vibrational ground state), and the first vibrationally excited state on site 1. The full lines refer to the full treatment of the $180-{\mathrm{cm}}^{-1}$ mode as part of the system, while the dashed lines mark the case when the 180-${\mathrm{cm}}^{-1}$ mode is omitted. The dash-dotted lines refer to the case of the Huang-Rhys factor $S_0$ increased by a factor of 2. All lines show spline polynomials fitted to the data as guide for the eyes, which, for clarity, are shown by red full circles for the full red line only.