Study of 2D supramolecular self-assembled layers of phenyleneethynylene oligomers on HOPG by scanning tunneling microscopy

The type of supramolecular organization of conjugated macromolecules in solid state is an important feature to be considered for the optimization of optoelectronic properties of active layers for solar cells. We report on the scanning tunneling microscopy (STM) and small-wide angle X-ray scattering (SWAXS) study of four linear and rigid π-conjugated p-phenyleneethynylene (PPE) oligomers. The four compounds possess two dodecanoxy side chains in the central PPE moiety, and differ in conjugation length (3 and 5 phenyleneethynylene moieties). Two oligomers are benzyl (Bz3PEBz, Bz5PEBz) and two are carboxylic acid groups (Ac3PEAc, Ac5PEAc) end-capped. We found that in carboxylic acids terminated oligomers, a lineal type arrangement is obtained, likely due to the hydrogen bridge created between carboxylic acids; in contrast, for the benzylbenzoate terminated oligomers a rather step-wise arrangement is obtained, promoted by the π-π interaction between benzyls. In general, all of the molecules form a lamellar like pattern adopting a face-on arrangement, where the conjugated backbones are flat-lying on the HOPG surface, whose distances correspond to the length of the conjugated backbone, while within the lamellas, a distance of 1.52 nm corresponds to the space between conjugated backbones; values that are consistent with those found by WAXS and DFT/6-311G(d,p) theoretical calculations, for oligomers that self-assemble with the interdigitated lateral alkyl chains.