  Metal-organic hybrid structures have been demonstrated a versatile platform
to study primary aspects of light-matter interaction by means of emerging
states comprising excitonic and plasmonic properties. Here we are studying the
wave-vector dependent photo-excitations in gold layers covered by molecular
films of zinc-phthalocyanine and its fluorinated derivatives (FnZnPc, with n =
0,4,8,16). These layered metal-organic samples show up to four anti-crossings
in their dispersions correlating in energy with the respective degree of ZnPc
fluorination. By means of complementary structural and theoretical data, we
attribute the observed anti-crossings to three main scenarios of surface
plasmon coupling: i) to aggregated $\alpha $-phase regions within the FnZnPc
layers at 1.75 eV and 1.85 eV , ii) to a coexisting F16ZnPc $\beta $-polymorph
at 1.51 eV, and iii) to monomers, preferentially located at the metal
interface, at 2.15 eV. Whereas energy and splitting of the monomer
anti-crossings depend on strength and average tilting of the molecular dipole
moments, the aggregate related anti-crossings show a distinct variation with
degree of fluorination. These observations can be consistently explained by a
change in FnZnPc dipole density induced by an increased lattice spacing due to
the larger molecular van der Waals radii upon fluorination. The reported
results prove Au/FnZnPc bilayers a model system to demonstrate the high
sensitivity of exciton-plasmon coupling on the molecular alignment at
microscopic length scales.
