ORIENTATION AND MOBILITY IN ULTRATHIN OLIGOTHIOPHENE FILMS - UV-VIS, IR AND FLUORESCENCE STUDIES

Thin films of oligothiophenes alphanTs are prepared by vapour deposition on fused silica and crystalline sapphire substrates, and characterized with respect to molecular orientation by polarized fluorescence, UV-Vis and IR absorption spectroscopy. Isolated alphanTs are adsorbed in the submonolayer region with their long molecular axes parallel to the surface plane. In the monolayer region, the alphanTs organize with their long axes perpendicular to the surface plane forming aggregates of the pin cushion type with a mean tilt angle of theta = 15 +/- 3-degrees relative to the normal of the macroscopic surface. Terminal alkylation of alphanT or multilayer coverage results in a loss of orientation with respect to the surface. Heating the substrate during film deposition either reduces or enhances the molecular order depending on temperature and molecular chain length.