  Elucidating quantum coherence effects and geometrical factors for efficient
energy transfer in photosynthesis has the potential to uncover non-classical
design principles for advanced organic materials. We study energy transfer in a
linear light-harvesting model to reveal that dimerized geometries with strong
electronic coherences within donor and acceptor pairs exhibit significantly
improved efficiency, which is in marked contrast to predictions of the
classical F\"orster theory. We reveal that energy tuning due to coherent
delocalization of photoexcitations is mainly responsible for the efficiency
optimization. This coherence-assisted energy-tuning mechanism also explains the
energetics and chlorophyll arrangements in the widely-studied
Fenna-Matthews-Olson complex. We argue that a clustered network with rapid
energy relaxation among donors and resonant energy transfer from donor to
acceptor states provides a basic formula for constructing efficient
light-harvesting systems, and the general principles revealed here can be
generalized to larger systems and benefit future innovation of efficient
molecular light-harvesting materials.
