On-Surface Polymerization: From Polyarylenes to Graphene Nanoribbons and Two-Dimensional Networks

On-surface polymerization is a novel technique for the fabrication of one- and two-dimensional molecular networks confined on a surface and is a rapidly developing field in surface science. The molecular building blocks exhibit pre-defined connection sites at which, after thermal activation and diffusion on the surface, the molecules are linked in a clean environment. Depending on the position and number of these connection sites, activated molecules polymerize to yield chains or two-dimensional networks. The chemical composition of the resulting polymer is precisely defined by the precursor molecules. We review current developments in the field of on-surface polymerization and present different examples, including the fabrication of graphene nanoribbons. We introduce reductive Ullmann-type coupling as well as Scholl-type cyclodehydrogenation for fabrication of graphene nanoribbons of increasing width. The surface plays a crucial role during the activation and polymerization processes because it serves as a catalyst, promotes molecular diffusion, and has a huge influence on the final molecular architecture. One-dimensional polymers can act as molecular wires and their conductance has been studied at the level of individual chains. In addition, we discuss two-dimensional networks and describe recent progress in attempts to improve their quality using sequential activation or defect-healing.