Answer
In 1991, Stoddart reported the crystal structure of a catenane, a molecule composed of two or more mechanically interlocked rings. This discovery marked a significant milestone in the field of supramolecular chemistry, as it demonstrated the possibility of constructing complex molecular architectures with controlled topology. The crystal structure revealed the precise arrangement of the interlocked rings, providing insights into the molecular recognition and self-assembly processes involved in catenane formation.