Exploration of Nano-Saturns: A Spectacular Sphere-Ring Supramolecular System.

Saturn-like systems consisting of nanoscale rings and spheres are fascinating motifs in supramolecular chemistry. Several ring molecules are known to include spherical molecules at the center of the cavity via noncovalent attractive interactions. In this minireview, we generalize the molecular design, the structural features, and the supramolecular chemistry of such "nano-Saturns", which consist of monocyclic rings and fullerene spheres (mainly C60), on the basis of previous experimental and theoretical studies. Ring molecules are classified into three types (loop, belt, and disk) according to their shapes and possible interactions. Whereas typical belt-shaped rings tend to form tight complexes due to the wide contact area via π-π interactions, flat disk-shaped rings generally form weak complexes due to the narrow contact area mainly via CH-π interactions. In spite of the small association energies, disk-shaped rings are attractive because such rings can mimic the planet Saturn precisely as exemplified by an anthracene cyclic hexamer-C60 complex.