The self-assembly of amphiphilic oligothiophenes:: Hydrogen bonding and poly(glutamate) complexation

The self-organization behavior of an amphiphilic sexithiophene bearing amide functionalities is studied and compared to that of a derivative bearing an ester group at the same position. The introduction of hydrogen-bond interactions in assemblies of these pi-conjugated oligomers is found to affect the molecular organization both in protic media and in thin deposits on mica. The amphiphilic 2,2';5',2";5'',2''';5''',2'''';5"'',2'''''- sexithiophene-5,5''''' dicarboxylic acid bis[(4,7,10,13,16-pentaoxaheptadecyl)amide] forms assemblies in n-butanol and water and partially aggregates in toluene. Spectroscopy reveals that the presence of a hydrogen-bonding moiety increases the thermal stability of the assemblies in n-butanol and even more in water solution. On mica surfaces, the formation of rod-like one-dimensional nanostructures is observed after deposition from toluene solutions. In addition, transmission electron microscopy in combination with selected area electron diffraction shows that in water plate-like structures are formed built from parallel oriented stacks, with a pi-pi distance of 3.5 A. Comparison of these data to molecular modeling and quantum chemistry calculations is used to better understand the influence of the amide group on the stacking of these compounds. The introduction of these H-bonding interactions leads to denser and more stable stacks. Furthermore, we show that a derivative of the amide compound, bearing terminal ammonium groups, forms a complex with chiral polyanions in aqueous media such that the sexithiophene segments are stacked in a meta-stable helical fashion with preferred handedness. We observed that poly(glutamate) and DNA generate a chiral sexithiophene assembly. In time the induced chirality disappears, which is explained by the meta-stability of the kinetically formed adduct. This constitutes one step forward towards the controlled formation of functional multi-component systems in aqueous solution.