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

The type of supramolecular organization of conjugated macromolecules in solid state is an important feature to be studied for the optimization of optoelectronic properties of active layers for solar cells devices. We report on the scanning tunneling microscopy (STM) and small-wide angle X-ray scattering (SWAXS) survey of four linear and rigid ?-conjugated p-phenyleneethynylene (PPE) oligomers. The four compounds possess two dodecanoxy side chains in the central PPE moiety, and differ in conjugated length (3 and 5 phenyleneethynylene moieties). Two oligomers are benzyl (Bz3PEBz, Bz5PEBz) and two of them are carboxylic acid groups (Ac3PEAc, Ac5PEAc) end-capped. We found that in carboxylic acids terminated oligomers, a linear type arrangement is obtained, likely due to the hydrogen bridge created between carboxylic acids; in contrast, for the benzylbenzoates terminated oligomers a rather step-wise arrangement is obtained, promoted by the ??? 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, which 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/6-311 theoretical calculations, for oligomers that self-arranged with the interdigitated lateral alkyl chains.