  We have investigated the influence of the substrate on the fluorescence of
adsorbed organic molecules. Monolayer films of
perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI), a supramolecular
network formed from PTCDI and melamine, and
perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) have been
deposited on hexagonal boron nitride (hBN). The principal peaks in the
fluorescence spectra of these films were red-shifted by up to 0.37 eV relative
to published measurements for molecules in helium droplets. Smaller shifts
(~0.03 eV) arising from interactions between neighbouring molecules are
investigated by comparing the fluorescence of distinct arrangements of PTCDI,
which are templated by supramolecular self-assembly and determined with
molecular resolution using atomic force microscopy under ambient conditions. We
compare our experimental results with red-shifts calculated using a combination
of a perturbative model and density functional theory which account for,
respectively, resonant and non-resonant effects of a dielectric hBN substrate.
We show that the substrate gives rise to a red-shift in the fluorescence of an
adsorbed molecule and also screens the interactions between neighbouring
transition dipole moments; both these effects depend on the refractive index of
the substrate.
