Luminescence of molecules adsorbed on surfaces of wide gap materials

We report on the photoluminescence spectra of thin films of chain-like π-conjugated molecules on well-defined surfaces of wide gap inorganic materials. The aim is to study the energy transfer processes across the organic/substrate interface which limit the luminescence of molecules in close contact to substrate surfaces. We discuss quaterthiophene (4 T) adsorbed on the c(2×2)-ZnSe(0 0 1) surface and tetracene (Tc) on the surface of a thin epitaxial Al2O3 film on Ni3Al(1 1 1). For thin films vapor-deposited at low substrate temperatures, we observe no luminescence signal for both systems, which indicates the presence of fast luminescence quenching processes at the organic/inorganic interface. After annealing, luminescence spectra corresponding to those of the bulk crystals are obtained. This can be explained by the formation of 3D-crystallites, which effectively separate most molecules from the organic/inorganic interface.