Epitaxial multilayer structures of DMe-PTCDI (2,9-dimethyl-antra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10-tetrone) on Ag(110) were prepared by molecular beam epitaxy. Photoluminescence spectra obtained under ultra high vacuum (UHV) conditions of the multilayers show significant polarisation dependence. By combining this result with a real space model derived from low energy electron diffraction (LEED) measurements the molecular transition dipole moment was determined. During an annealing process the fluorescence of the multilayer spectrum changes dramatically. Possible reasons for this result are discussed and compared with tapping-mode atomic force microscope (AFM) and fluorescence microscopy measurements.