Ferroelectricity Induced by Ordering of Twisting Motion in a Molecular Rotor

A novel mononuclear metal-organic compound, [Cu(Hdabco)(H(2)O)Cl(3)] (1, dabco = 1,4-diazabicyclo[2.2.2]octane) in which the Cu(II) cation adopts a slightly distorted bipyramidal geometry where the three Cl anions constitute the equatorial plane and the Hdabco cation and H(2)O molecule occupy the two axial positions, was synthesized. Its paraelectric-to-ferroelectric phase transition at 235 K (T(c)) 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(H(2)O)Cl(3)] part acts as a stator. In the temperature range 235-301 K, a twisting motion of the rotator is confirmed. Below the T(c), 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)(D(2)O)Cl(3)] (2) excludes the possibility of proton ordering as the origin of the ferroelectricity in 1.