  Recent experiments have shown that highly efficient energy transfer can take
place in organic nanocrystals at extremely low acceptor densities. This
striking phenomenon has been ascribed to the formation of exciton polaritons
thanks to the photon confinement provided by the crystal itself. We propose an
alternative theoretical model that accurately reproduces fluorescence lifetime
and spectrum measurements in these systems without such an assumption. Our
approach treats molecule-photon interactions in the weak-coupling regime, and
describes the donor and acceptor population dynamics by means of rate equations
with parameters extracted from electromagnetic simulations. The physical
insight and predictive value of our model also enables us to propose
nanocrystal configurations in which acceptor emission dominates the
fluorescence spectrum at densities orders of magnitude lower than the
experimental ones.
