Orientation and mobility in ultrathin oligothiophene films: UV-Vis, IR and fluorescence studies

Abstract

Thin films of oligothiophenes αnTs 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 αnTs are adsorbed in the submonolayer region with their long molecular axes parallel to the surface plane. In the monolayer region, the αnTs organize with their long axes perpendicular to the surface plane forming aggregates of the pin cushion type with a mean tilt angle of θ = 15 ± 3° relative to the normal of the macroscopic surface. Terminal alkylation of αnT 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.