Supramolecular assemblies of tetrahydroxyloligo(phenyleneethynylene) with cross-shaped side chains and its coadsorption with diacids on graphite

An oligo(phenyleneethynylene), with hydroxyl as end-groups of cross-shaped four alkoxy side chains (OH-OPE), was designed and synthesized to investigate the effects of the symmetry of molecular structure as well as the hydrogen bonding and interaction between alkoxy side chains on its supramolecular assemblies by scanning tunneling microscopy (STM). OH-OPEs fabricate by themselves two distinct patterns on highly orientated pyrolytic graphite (HOPG) surface with dissimilar surface coverages, symmetries, and stabilities. The surface coverage of both patterns shows a clear dependence on the concentration of OH-OPE. Contrary to the general concept, in the present case, the D1 pattern with higher packing density is favored at low concentration, which is possibly due to the existence of specific interactions (H-bonds) between the molecules and the difference in the number of H-bonds in these patterns. Coadsorption of diacids of shorter alkyl chains could help to stabilize the low density, porous pattern, whereas diacids with too long alkyl chains destabilize it. Due to the flexibility of the side chains, the coadsorption of shorter diacids does not significantly change the packing of OH-OPE. Our results point to the conclusion that the coadsorption is due to host-guest accommodation rather than hydrogen bond complexation.