Tilting and tiling

The deposition of corannulenes and two chiral derivatives on metal surfaces shows how the molecules lower their five-fold symmetry

Credit: © 2009 ACS

Although a surface can be tiled in many different ways, none of them exhibit five-fold symmetry. Similarly, molecules with five-fold symmetry (for example, pentagonal or star-shaped) reduce their order of symmetry to form two-dimensional crystals — a phenomenon that can help in understanding molecular recognition processes at surfaces.

In a collaboration between the University of Zurich and the Swiss Federal Laboratories for Materials Testing and Research (Empa), Jay Siegel, Karl-Heinz Ernst and co-workers used tunnelling electron microscopy to observe¹ the packing of corannulene on metal surfaces. Corannulene is a convex aromatic molecule also known as a 'buckybowl', and Ernst and colleagues also studied its chiral pentachloro and pentamethyl derivatives.

All three molecules adopt a close-packing arrangement, positioning their centroids on a hexagonal lattice. The corannulenes organize into a perfectly ordered monolayer by significantly tilting to one side, but this was not possible in the presence of bulky substituents. The pentachloro derivatives form an array of 'striped lattices', with pentagonal molecules arranged in antiparallel rows, but the order only extends for a short distance. The pentamethylated molecules form a less dense, more disordered 'rotator phase' in which the orientation of the molecules, their position with respect to the substrate and their chirality all vary. Even though this rotator phase has such disorder, the centroids of each molecule still form a hexagonal lattice.