SCANNING-TUNNELING-MICROSCOPY OF ALKYLATED OLIGOTHIOPHENES AT INTERFACES WITH GRAPHITE

Monolayers of alpha,omega-dihexylsexithiophene (alpha,omega-T6D2) and a non-regioselectively as well as a regioselectively substituted 2,5-didodecylsexithiophene (2,5-T6D2) were physisorbed from solution onto the basal plane of highly oriented pyrolytic graphite (HOPG). Scanning tunnelling microscopy (STM) in situ at the solid/fluid interface revealed that 2,5-T6D2 forms highly ordered monolayers with bath the sexithiophene backbones and the alkyl chains oriented parallel to the graphite surface, which is in contrast to vacuum-deposited films on silicon, for which no order has previously been found. On HOPG a difference in the regularity and the average value of intermolecular spacings within monolayers of non-regioselectively and regioselectively substituted 2,5-T6D2 was observed, which is the first time the influence of regioselectivity in a substitution has been detected by STM. Common to the two- and three-dimensional crystal structures of 2,5-T6D2 is a one-dimensional structural unit. alpha,omega-T6H2 can also form highly ordered monolayers at the interface between an organic solution and the basal plane of graphite with the sexithiophene aromatic backbone and the hexyl side chains oriented parallel to the substrate in a lamella structure. However, in this case it cannot be excluded that other, even more stable adsorbate structures may exist, which could not be imaged by STM.