In the vast majority of cases, supramolecular structures made of benzene rings are planar, because arranging benzene rings in three dimensions results in increased strain and therefore unstable structures. There are, however, notable examples of all-unsaturated carbon compounds, such as carbon nanotubes, helicenes and polyphenylenes, that show specific properties due to their not being flat. Adding to this set of compounds, Dumslaff et al. now report a novel interlocked structure made of only sp2-hybridized carbons that can form the basic unit for larger nanostructures.
The researchers synthesized a molecule composed on two cyclohexa-m-phenylene macrocycles. These macrocycles are interlocked through a covalent bond between two opposite phenylene rings of each macrocycle. As a result, these four phenylene rings form a rigid backbone from which the two macrocycles extend in an orthogonal fashion — the final structure resembles a paddlewheel.
These molecules arrange in an extremely stable amorphous solid. In fact, the glass transition temperature of an analogue of these interlocked structures containing tert-butyl substituents is as high as 152 °C. The stability of these three-dimensional structures is likely due to tight packing of the phenylene rings.