  Under physiological conditions, ballistic long-range transfer of electronic
excitations in molecular aggregates is generally expected to be suppressed by
noise and dissipative processes. Hence, quantum phenomena are not considered to
be relevant for the design of efficient and controllable energy transfer over
significant length and time scales. Contrary to this conventional wisdom, here
we show that the robust quantum properties of small configurations of repeating
clusters of molecules can be used to tune energy transfer mechanism that take
place on much larger scales. With the support of an exactly solvable model, we
demonstrate that coherent exciton delocalization and dark states within unit
cells can be used to harness dissipative phenomena of varying nature
(thermalization, fluorescence, non-radiative decay and weak inter-site
correlations) to support classical propagation over macroscopic distances. In
particular, we argue that coherent delocalization of electronic excitations
over just a few pigments can drastically alter the relevant dissipation
pathways which influence the energy transfer mechanism, and thus serve as a
molecular control tool for large-scale properties of molecular materials.
Building on these principles, we use extensive numerical simulations to
demonstrate that they can explain currently not understood measurements of
micron-scale exciton diffusion in nano-fabricated arrays of bacterial
photosynthetic complexes. Based on these results we provide quantum design
guidelines at the molecular scale to optimize both energy transfer speed and
range over macroscopic distances in artificial light-harvesting architectures.
