Crystalline Supramolecular Gyroscope with a Water Molecule as an Ultrasmall Polar Rotator Modulated by Charge-Assisted Hydrogen Bonds

A new strategy for the construction of crystalline molecular rotors
is presented. The combination of a conformation-modifiable macrocyclic
host and two cooperative guests affords one supramolecular gyroscope-like
compound, (t-BuNH3)­(18-crown-6)­[ZnCl3(H2O)], in which the coordinated water molecule
functions as an ultrasmall polar rotator, revealed by its significant
dielectric relaxation and the molecular dynamics simulations. In addition,
such a compound can reversibly undergo a polar-to-polar phase transition
triggered by the changed conformation of the 18-crown-6 host, leading
to a switchable on/off rotation of water molecule, well controlled
by strength and direction of charge-assisted hydrogen bonds.