Direct observation of chiral oligo(p-phenylenevinylene)s with scanning tunneling microscopy

Oligo(p-phenylenevinylene) (OPV) dimers, tetramers and hexamers, have been organized in highly organized 2D crystals on graphite by spontaneous self-assembly. The unit cell parameters of these monolayers could be determined and a profound effect of the length of the pi-conjugated backbone on the adlayer structure was discovered. For all oligomers, the OPV-units were aligned in rows, with the alkyl chains residing in between the rows. Only trans-vinylene bonds were observed. In the hexamer, the organization was strongly determined by the pi-system. Shortening the pi-conjugated moiety led to monolayer structures controlled by the alkyl side chains. The chiral oligomers (hexamer and tetramer) formed chiral monolayers. In the case of the tetramer, the pi-conjugated backbone was oriented clockwise with respect to the normal of the rows while in the case of the hexamer this orientation was counterclockwise.