Answer
Cyclobis is the chemical composition of the macrocycle used in Stoddart's rotaxane. Cyclobis is a synthetic macrocycle composed of two bipyridine units linked by a phenylene ring. It is a rigid, disk-shaped molecule with a central cavity that can accommodate a variety of guest molecules. In Stoddart's rotaxane, cyclobis is threaded onto a dumbbell-shaped molecule, forming a mechanically interlocked structure.