  In natural and artificial light-harvesting complexes (LHC) the resonant
energy transfer (RET) between chromophores enables an efficient and directional
transport of solar energy between collection and reaction centers. The detailed
mechanisms involved in this energy funneling are intensely debated, essentially
because they rely on a succession of individual RET steps that can hardly be
addressed separately. Here, we developed a scanning tunnelling
microscopy-induced luminescence (STML) approach allowing visualizing,
addressing and manipulating energy funneling within multi-chromophoric
structures with sub-molecular precision. We first rationalize the efficiency of
the RET process at the level of chromophore dimers. We then use highly resolved
fluorescence microscopy (HRFM) maps to follow energy transfer paths along an
artificial trimer of descending excitonic energies which reveals a cascaded RET
from high- to low-energy gap molecules. Mimicking strategies developed by
photosynthetic systems, this experiment demonstrates that intermediate gap
molecules can be used as efficient ancillary units to convey energy between
distant donor and acceptor chromophores. Eventually, we demonstrate that the
RET between donors and acceptors is enhanced by the insertion of passive
molecules acting as non-covalent RET bridges. This mechanism, that occurs in
experiments performed in inhomogeneous media and which plays a decisive role in
fastening RET in photosynthetic systems, is reported at the level of individual
chromophores with atomic-scale resolution. As it relies on organic chromophores
as elementary components, our approach constitutes a powerful model to address
fundamental physical processes at play in natural LHC.
