Nanostructured ultrathin films of water-soluble sexithiophene bolaform amphiphiles prepared by layer-by-layer self-assembly

Sexithiophene bolaform amphiphiles with pendant cationic end groups (6TN) were synthesized and alternating multilayers of 6TN and poly(styrenesulfonate) (PSS) were successfully prepared by the layer-by-layer self-assembly deposition technique. The aggregation properties and layer ordering were investigated in view of their potential application for organic field effect transistors (FETs). 6TN ultrathin films were prepared from aqueous solution and water/tetrahydrofuran (THF) (70/30 v/v) mixtures. The amphiphiles showed fine aggregates in water, probably due to the intermolecular interactions of the hydrophobic sexithiophene group. Deaggregation was observed when 30 vol % THF was added, which is a better solvent for the hydrophobic portion of the molecule. A stepwise and regular deposition of 6TN/PSS multilayer in both water and water/THF (70/30) was confirmed by UV-visible spectroscopy, ellipsometry, and surface plasmon spectroscopy. A lower energy vibronic structure was observed with the UV-vis spectra on multilayers prepared with 6TN in water and water/THF (70/30). This is indicative of aggregates of 6TN in water directly adsorbed on the substrate. In the case of 6TN in a water/THF (70/30) mixture, well-stacked aggregates were only formed during adsorption to the surface. These observations were also confirmed from the atomic force microscopic (AFM) images, distinguishing the two types of aggregation and adsorption profiles. The difference in layer thickness between the two systems is most likely due to the reduced surface coverage in the 30% THF system. Polarized UV-vis spectra indicated that the rigid sexithiophene group of 6TN is oriented more parallel to the substrate than perpendicular.