  The interplay between electronic and intramolecular high-frequency
vibrational degrees of freedom is ubiquitous in natural light-harvesting
systems. Recent studies have indicated that an intramolecular vibrational
donor-acceptor frequency difference can enhance energy transport. Here, we
analyze the extent to which different intramolecular donor-acceptor vibrational
frequencies affect excitation energy transport in equilibrium (coherent light
excitation) and the more natural nonequilibrium steady state (incoherent light
excitation) configurations. It is found that if the Huang-Rhys factors remain
constant, the acceptor population increases when the intramolecular vibrational
frequency of the acceptor exceeds that of the donor. The increase in the
acceptor population due to the vibrational frequency difference is higher for
higher values of the Huang-Rhys factors or the vibronic coupling strengths.
However, the nonequilibrium steady state results show that the vibrational
donor-acceptor frequency difference does not significantly enhance energy
transport in the natural scenario of incoherent light excitation and under
biologically relevant parameters. Insight about a potential mechanism to
optimize energy transfer in the NESS based on increasing the harvesting time at
the reaction center is analyzed.
