End-Group-Dominated Molecular Order in Self-Assembled Monolayers

Self-assembled monolayers of 6-(4-phenylazo-phenoxy)-hexane-1-thiol (azobenzene) on gold have been studied with atomic force microscopy in ethanol and with scanning tunneling microscopy at high gap impedance in air. These azobenzene functionalized self-assembled monolayers show two stable rectangular lattices having equal
dimensions of 0.61 and 0.79 nm, and an angle of 89 deg. We conclude that the lattices are end-group dominated because they are incommensurate with Au(111), they extend over terrace edges with no phase shift, and they can also be observed on polycrystalline gold. The unit cells are populated with two molecules that have aromatic interactions and expose the upper phenyl ring edge of the azobenzene moiety to the surface. The anisotropic electronic shape of these phenyl rings allows distinct, isoenergetic geometries created by laterally shifting and tilting pairs of phenyl rings. The upper edge of these phenyl rings appears to be elliptical in STM images.