Synthesis and solid-state investigations of oligo-phenylene? : Ethynylene structures with halide end-groups

In the field of material science functionalization of substrate surfaces (e.g. metal, graphite) with organic molecules is of increasing interest. Desirable targets are molecules with functional groups providing for two-dimensional assembly and three-dimensional crystal growth. We have synthesized a series of halogen-end-capped oligo-phenylene-ethynylenes (OPEs) to study the interactions at the solid/liquid interface and in crystal structures. Organohalides can be involved in a wide variety of intermolecular interactions such as C-X···H, C-X···X-C and C-X···pi-orbitals. The range of halogen-based interactions and the diversity of intermolecular forces along different crystal axes makes the investigation of such structures particular interesting and challenging. Here we probe the interplay of halide end-groups and the backbone of an OPE to investigate the intermolecular interactions in both solution depositions (2D) and X-ray crystal structures (3D). The STM images and the crystal structures of each OPE reveal striking packing similarities. For each molecule, a plane in the crystal structure with an arrangement of molecules resembling its two-dimensional packing on a flat surface was found. These results support the hypothesis of sheet-by-sheet crystal growth and suggest that flat surfaces would be ideal interfaces to promote crystal growth for halide-end-capped OPEs.