  One of the promising approaches to revealing the photosynthetic efficiency of
close to one unit is to investigate the quantum regime of excitation energy
transfer (EET). The majority of studies, however, have concluded that different
pigment molecules contribute equally to EET, rather than differently. We
investigate the roles of different site-energies in EET by evaluating the
correlated fluctuations of site-energies in two adjacent pigment molecules
(namely Site 1 and Site 2), and we attempt to demonstrate different site roles
in EET with the j-V characteristics and power via a photosynthetic quantum heat
engine (QHE) model rather than an actual photosynthetic protein. The results
show that fluctuations at Site 1 (the pigment molecule absorbing solar photons)
provide ascending and then descending EET. At Site 2, the EET is reduced
through the use of correlated fluctuation increments (the pigment molecule
acting as the charge-transfer excited state). Furthermore, when investigating
the correlated fluctuations at Site 2, the different gap differences of the
output terminal play a positive role in EET, but a sharply decreasing EET
process is also achieved with less correlated fluctuations at Site 2 compared
to those at Site 1.The findings show that different pigment molecules
contribute differently to EET. The significance of this work is that it not
only clarifies the roles of different pigment molecules in EET, but it also
deepens our understanding of the fundamental physics of EET as it transports
through the molecular chain in photosynthetic light-harvesting complexes.
Furthermore, the results are appropriate to the EET in organic semiconductors,
photovoltaic devices, and quantum networks, when these systems couple to the
environment of photons via the vibrational motion of sites in the molecular
chain.
