High-Temperature and Pressure-Induced Ferroelectricity in Hydrogen-Bonded Supramolecular Crystals of Anilic Acids and 2,3-Di(2-pyridinyl)pyrazine

Cocrystallization of anilic acids (H(2)xa) and 2,3-di(2-pyridinyl)pyrazine (dppz) affords a variety of molecular geometries, including hydrogen-bonding and supramolecular structures. Proton-transferred 1:1 salts of [H-dppz][Hca] and [H-dppz][Hba] (H(2)ca = chloranilic acid, H(2)ba = bromanilic acid)were found to host room temperature ferroelectricity with a spontaneous polarization of 3-4 mu C/cm(2) along the hydrogen bonded chains. Compared with the Curie points of other supramoleailar ferroelectrics, those of the salts are relatively high (402 K and >420 K, respectively) because of the elongated hydrogen bonds, which stabilize the proton ordered state against thermal agitation In addition to the ferroelectric black (alpha) form, dppz and H(2)ba gave two different crystal forms with a 2:3 ratio: the brown beta form of [H-1.5-dppz](2)[Hba](3) and the brownish red gamma form of [H-dppz](2)[Hba](2)[H(2)ba]. Mixed solutions of dppz with the less acidic fluoranilic acid (H(2)fa) exhibit valence instability; the H(2)fa molecules remain mostly neutral in absolute ethanol, whereas methanol (MeOH) solution apparently increases the deprotonated Hfa(-) content Crystallizations of these solutions gave a neutral [dppz][H(2)fa] cocrystal and ionic [H-dppz(+)][Hfa(-)]center dot MeOH salt, respectively. The ferroelectricity induced by a modest hydrostatic pressure corroborates the conclusion that the ionic state with a dipolar [H-dppz(+)][Hfa-] chain is energetically close to the nonpolar neutral ground state of the [dppz][H(2)fa] crystal.