Optical properties of highly oriented quaterthiophene thin films grown by organic molecular-beam deposition

The molecular-beam deposition of quaterthiophene (4T) oligomer on potassium acid phtalate (001) single crystals yields thin films exhibiting a peculiar supramolecular organization induced by the interaction with the substrate. The 4T thin films, which exhibit an extremely large optical anisotropy arising from the alignment of the 4T molecular axis along the b crystallographic axis of the substrate, allow one to carry out polarized transmission measurements over the whole π-π* absorption band. The observation of a strong polarized band, together with very weak unpolarized absorption peaks originating from isolated molecules, is consistent with the formation of a tilted H aggregate in which the molecular transition moments form a 38° angle with the substrate surface. The low luminescence quantum yield further supports the formation of this very ordered structure. Exciton migration and decay paths in the 4T films have been established by studying, with time-resolved photoluminescence spectroscopy, the emission originating from traps and imperfect aggregates.