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 (H2xa) 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] (H2ca = chloranilic acid, H2ba = bromanilic acid) were found to host room-temperature ferroelectricity
with a spontaneous polarization of 3–4 μC/cm2 along the hydrogen-bonded chains. Compared with the Curie points
of other supramolecular 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 (α) form, dppz and H2ba gave two different crystal forms with a 2:3 ratio: the
brown β form of [H1.5-dppz]2[Hba]3 and the brownish-red γ form of [H-dppz]2[Hba]2[H2ba]. Mixed solutions of dppz with
the less acidic fluoranilic acid (H2fa) exhibit valence
instability; the H2fa 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]­[H2fa] cocrystal
and ionic [H-dppz+]­[Hfa–]·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]­[H2fa] crystal.