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 (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.

Bauert et al. have now used tunnelling electron microscopy to observe the packing arrangement of corannulene — a convex aromatic molecule also known as a buckybowl — on metal surfaces (J. Am. Chem. Soc. 131, 3460–3461; 2009). They also studied its chiral pentachloro and pentamethyl derivatives.

All three molecules close-pack, with their centroids on a hexagonal lattice. The corannulenes form a perfectly ordered monolayer by tilting to one side, but this is not possible in the presence of bulky substituents. The pentachloro derivatives form an array of 'striped lattices' (pictured), with pentagonal molecules arranged in antiparallel rows, but the order only extends for a short distance. The pentamethylated molecules form a more disordered 'rotator phase' in which the orientation of the molecules, their position with respect to the substrate and their chirality all vary, though their centroids still form a hexagonal lattice.