ABSTRACT
Essentially all living things on earth use the sun as their energy
source which is harvested by photosynthesis. Photosynthetic con-
version of physical energy of sunlight into a chemical form useful
for cellular processes involves many steps [1, 2]. Photosynthesis
starts with the absorption of a photon by a light-harvesting pigment
or chromophore forming an exciton, followed by the transfer of
the exciton to the reaction center, where charge separation is
initiated leading subsequently to chemical storage of the energy.
In this chapter, our focus lies on the transfer dynamics which
surprisingly exhibits a quantum efficiency near unity under low
light conditions. Not only plants but also several bacteria make use
of photosynthesis. In green sulfur bacteria such as Chlorobaculum
tepidum light harvesting is done by an antennae complex, the chlorosome, which is a large conglomerate. The connection to the
reaction center is exclusively through the small Fenna-Matthews-
Olson (FMO) complex, a trimerwhosemonomers each contain seven
chromophores [3-6]. Recently, an eighth pigment was resolved
[7] which is, however, very weakly coupled to the other seven
pigments and thus is of little relevance for our studies. The purpose
of the FMO complex is purely to conduct the excitonic energy
from the antennae to the reaction center. Typical transfer times
through the FMO are on the order of picoseconds, whereas the
decay of an excited chromophore (with subsequent reemission of
a photon) takes on the order of nanoseconds. In addition, typically,
the absorption cross section for photons is small, meaning that
in general only a single exciton is present in an FMO complex.
This allows to restrict later the theoretical consideration to the
single excitation subspace. Furthermore, these timescales reflect the
extraordinary quantum efficiency of the excitonic energy transfer
in photosynthesis. However, it is still unclear why transport is so
much faster than direct decay of excitons. An understanding of this
question, which must lie in the design principle of photosynthetic
complexes, would allow to exploit the near-unity efficiency of energy
transfer within artificial light-harvesting complexes. Organic solar
cells suffer from very high exciton losses during transport and thus
could greatly benefit from biomimetic design principles.
