Ferroelectricity Induced by Ordering of Twisting Motion in a Molecular Rotor

A novel mononuclear metal–organic compound, [Cu­(Hdabco)­(H2O)­Cl3] (1, dabco = 1,4-diazabicyclo[2.2.2]­octane)
in which the CuII cation adopts a slightly distorted bipyramidal
geometry where the three Cl anions constitute the equatorial plane
and the Hdabco cation and H2O molecule occupy the two axial
positions, was synthesized. Its paraelectric-to-ferroelectric phase
transition at 235 K (Tc) and dynamic behaviors
were characterized by single crystal X-ray diffraction analysis, thermal
analysis, dielectric and ferroelectric measurements, second harmonic
generation experiments, and solid-state nuclear magnetic resonance
measurements. Compound 1 behaves as a molecular rotor
above room temperature in which the (Hdabco) part rotates around the
N···N axis as a rotator and the [Cu­(H2O)­Cl3] part acts as a stator. In the temperature range 235–301
K, a twisting motion of the rotator is confirmed. Below the Tc, the motions of the rotor are frozen and the
molecules become ordered, corresponding to a ferroelectric phase.
Origin of the ferroelectricity was ascribed to relative movements
of the anions and cations from the equilibrium position, which is
induced by the order–disorder transformation of the twisting
motion of the molecule between the ferroelectric and paraelectric
phases. Study of the deuterated analogue [Cu­(Ddabco)­(D2O)­Cl3] (2) excludes the possibility of proton
ordering as the origin of the ferroelectricity in 1.