Proton Dynamics and Room-Temperature Ferroelectricity in Anilate Salts with a Proton Sponge

Abstract

Ferroelectricity as well as characteristic proton-transfer dynamics are achieved by combining a 2,3,5,6-tetra(2′-pyridyl)pyrazine (TPPZ) molecule with anilic acids (H₂xa). Dielectric measurements revealed phase transitions at T_c = 334 and 172 K for bromanilate (Hba⁻) and chloranilate (Hca⁻) salts, respectively. The room-temperature ferroelectricity of the (H₂-TPPZ)(Hba)₂ crystal is evidenced by the slow polarization reversal with modest pyroelectricity. In accord with the observed large deuteration effect, synchrotron X-ray diffraction studies disclosed proton dynamics in an intramolecular N···H⁺···N bond of the H₂-TPPZ²⁺ dication and in an O−H···O⁻ hydrogen-bonded cyclic dimer of the ortho-quinoid Hxa⁻ anions. The disordered (Hxa⁻)₂ dimer in two-fold orientation manifests its double-proton transfer process above T_c, whereas these protons are ordered in the ferroelectric phase. The H₂-TPPZ²⁺ dication acts as a proton sponge by forming two intramolecular N···H⁺···N hydrogen bridges between the pyridyl units with a very short N···N distance. The dication in the paraelectric state adopts a nonpolar geometry due to the delocalization of the protons over two sites in the respective N···H⁺···N bonds. Below T_c, only one of the two protons gets localized, and the resultant acentric H₂-TPPZ²⁺ ion generates the dipole moment responsible for the ferroelectricity.