Proc. Natl Acad. Sci. USA 110, 6678–6681 (2013)

Credit: © 2013 NAS

The organization of molecules into periodic patterns is not only intriguing from mathematic and aesthetic perspectives, it also determines the properties of the resulting materials. Among the 11 possible ways to tessellate a surface with symmetric polygonal units, eight are semi-regular Archimedean tilings comprising two or more building units. These patterns can give rise to unusual properties, such as geometrically frustrated magnetism. The best-known example is perhaps the kagome lattice, which consists of triangles and hexagons.

The self-assembly of small molecules at a surface has emerged as a convenient tool for generating interesting surface tilings for study, and although a variety of patterns have been created previously, five-vertex motifs have remained elusive. David Écija and Johannes Barth from the Technical University, Munich, have led a team that have now observed that cerium centres and ditopic linear ligands (L) — polyphenyl segments featuring a binding carbonitrile group at each end — assemble on a Ag(111) surface into five-fold planar {CeL5} coordination complexes that can act as the vertices of a semi-regular tessellation. Although this ligand had previously been observed to form coordination polymers at surfaces with other metal centres, it had not spontaneously arranged in this manner. The unusual five-fold motif observed with cerium is likely to arise from a balance between the surface confinement of the ligands, the coordination at the lanthanide centre and steric hindrance. The patterns were characterized by scanning tunnelling microscopy, and supported by calculations.

The researchers first obtained discrete star-shaped molecules in which each cerium centre coordinates to five different ligands through a carbonitrile group. Adjusting the concentration and the ligand:lanthanide ratio to 5:2 led to each ligand coordinating to two cerium centres. The cerium coordination sphere was flexible enough to accommodate the formation of a continuous 'snub square tiling' (pictured) up to a size of 300 Å × 300 Å. This extended motif is made up of triangles and squares, with the ligands acting as edges and the cerium centres as vertices. The same motif was generated with both a 3- and a 4-phenyl-ring ligand, pointing to the generality of this lanthanide-directed approach for surface tessellation.