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.

冉酸类（anilic acids，H₂xa）与2,3-二(2-吡啶基)吡嗪（2,3-di(2-pyridinyl)pyrazine，dppz）的共结晶可得到涵盖氢键结构与超分子组装体的多种分子构型。研究发现[H-dppz][Hca]与[H-dppz][Hba]这两种质子转移型1:1盐（其中H₂ca为氯冉酸（chloranilic acid），H₂ba为溴冉酸（bromanilic acid））呈现室温铁电性，沿氢键链方向的自发极化强度可达3–4 μC/cm²。相较于其他超分子铁电体的居里温度，该类盐的居里温度相对较高（分别为402 K与>420 K），这归因于其伸长的氢键可稳定质子有序态，抵御热扰动。除具有铁电性的黑色α晶型外，dppz与H₂ba还可得到两种摩尔比为2:3的不同晶型：分别为组成为[H₁.₅-dppz]₂[Hba]₃的棕褐色β晶型，以及组成为[H-dppz]₂[Hba]₂[H₂ba]的棕红色γ晶型。dppz与酸性较弱的氟冉酸（fluoranilic acid，H₂fa）的混合溶液表现出价态不稳定性：在无水乙醇中，H₂fa分子大多保持中性状态；而在甲醇（methanol，MeOH）溶液中，去质子化的Hfa⁻离子占比显著提升。对上述两种溶液进行结晶，分别得到中性的[dppz][H₂fa]共晶体与离子型的[H-dppz⁺][Hfa⁻]·MeOH溶剂化物盐。适度静水压诱导产生的铁电性验证了这一结论：带有偶极[H-dppz⁺][Hfa⁻]链的离子态，在能量上与[dppz][H₂fa]晶体的非极性中性基态十分接近。