Synthesis and Solid State Structures of Functionalized Phenyleneethynylene Trimers in 2D and 3D

Novel soluble end-functionalized phenyleneethynylene oligomers were synthesized via efficient Pd-catalyzed coupling. The structures, both of single crystals and of monolayers at the solid-liquid interface, were resolved respectively by means of X-ray diffraction (XRD) and scanning tunneling microscopy (STM) at submolecular resolution. They revealed either 3D or 2D highly ordered architectures consisting of rodlike pi-conjugated skeletons and flexible hexyl side chains which possess a considerable conformational mobility at room temperature. The self-assembly arises from the cooperative effect of intermolecular and intramolecular interactions among the unsaturated backbone, the hexyl side chains, the hydrophilic end groups, and eventually the substrate as discussed in detail. Finally, the STM investigation made it possible to monitor molecular-scale defects in the polycrystals.