Two-Dimensional Supramolecular Organization in Oligomers of Dialkylterthiophenes-Effect of the Alkyl Substituent Position

Strictly monodispersed fractions of two types of regioisomers of increasing molecular mass, namely, oligomers of 3,3 ''-diocty1-2,2':5',2 ''-terthiophene (3,3 '' DOTT) and 4,4 ''-diocty1-2,2':5',2 ''-terthiophene (4,4 '' DOTT) (n-mers, n = 2-5) were studied by STM with the goal to establish the effect of the type of regioregularity on the 2D supramolecular organization of these compounds on HOPG. Both regioisomers show strikingly different supramolecular organizations. In the 4,4 '' DOTT series, a two-dimensional arrangement is found for short oligomers (2-mers and 3-niers). With increasing oligomer length (4-mers and 5-mers), this two-dimensional structure evolves toward a more one-dimensional one characterized by randomly oriented rows of parallel-aligned molecules. This evolution arises from the interdigitation of the terminal and the side alkyl groups in two perpendicular directions. In the 3,3 '' DOTT series, independent of the oligomer length, a two-dimensional organization is observed. Because of a significantly shorter distance between the two alkyl substituents in the mer, only one type of interdigitation takes place for this regioisomer in the direction perpendicular to the molecule's long axis.