A supramolecular ferroelectric realized by collective proton transfer
A supramolecular ferroelectric realized by collective proton transfer
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DOI：
10.1002/anie.200700407
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Tokura, Yoshinori
中科院分区：
文献类型：
--
作者：
Horiuchi, Sachio;Kumai, Reiji;Tokura, Yoshinori
氢键将两个或多个电负性原子结合成X±H···Y，在晶体工程、超分子化学和分子识别中被用来构建分子组件。[1]通过氢键进行质子转移的动力学引起了广泛的兴趣，因为质子转移在许多化学反应和生物功能中起着至关重要的作用，也因为它在电子和光学器件中潜在的广泛应用。[2]在有机分子固体中，质子转移通常伴随着π-电子构型的互变异构，这可以与新的功能联系在一起；据报道，酮和烯醇形式的分子之间的转换会产生可逆的颜色变化[3]或偶极反转。[4]重要的是，即使在分子间空间，如果每个分子的质子总数受到限制，就像在冰的情况下典型的那样，这样的质子运动也可以关联起来。方酸(H_2C_4O_4)[5]提供了被称为“冰规则”的约束的一个很好的例子：在短O···O键(2.55)上的协同质子转移可以反转氢键分子的极性。因此，在有序-无序相变附近出现了高介电常数。在这个例子中，极性分子的三维排列通常是反平行的(反铁电)，抵消了净电极化。如果把可移动的质子排列到一个极点阵中，它们就会产生自发极化，它们的运动可以通过外加电场来控制，就像铁电体一样。在这里，我们报道了第一个表现出铁电性的有机系统，它由分子间的协同质子转移引起。KH_2PO_4(KDP)是一种典型的无机铁电体，它通过PO4~+离子之间很短的O···O键(2.50)，通过一个点到点的质子转移来逆转电场的极化。[6]KDP的分子类似物仍然没有开发出来，除了有机-无机杂化的盐[dabcoH]+[ReO4]±(dabco=diazabbiclo[2.2])。[7]为了设计真正的有机铁电体，我们利用了一种强氢键的酸碱体系，因为一些这样的化合物已知具有新的热诱导质子迁移。[8]具有相似质子亲和力的酸和碱的组合保证氢键的质子可以很容易地附着在这两个分子上。[9]作为酸的2，5-二羟基-对苯二酚和作为碱的吡啶衍生物的晶体(方案1)最近被测试以满足这一要求。例如，已经证明了氯苯酸(H_2ca；方案1，X=Cl)和溴苯酸(X=Br)与吩嗪(Phz)生成铁电加合物。然而，在这些加合物中，没有明显的位置到位置的质子转移，相反，铁电性被归类为位移，正如BaTiO_3或PbTiO_3家族所发现的那样。另一方面，在具有较强碱的质子转移的一价盐H_2ca中实现了具有质子有序的反铁电相变，5，5‘-二甲基-2，2’-联吡啶(55DMBP；方案1)。[11]这里提出的新的铁电是由H_2ca的碘类似物，碘苯酸(H_2ia；方案1，X=I)和55DMBP衍生而来。该化合物也是一种质子转移的1：1盐，由[H-]表示。
Hydrogen bonds, which bind two or more electronegative atoms as XÀH··· Y, have been utilized to construct molecular assemblies in crystal engineering, supramolecular chemistry, and molecular recognition.[1] The dynamics of proton transfer through hydrogen bonds has attracted extensive interest, because of the crucial role played by proton transfer in many chemical reactions and biological functions, and also because of its potentially diverse applications in electronic and optical devices.[2] In organic molecular solids, proton transfer is often accompanied by a tautomerism of the π-electron configuration, which can be associated with novel functionalities; transformations between the keto and enol forms of molecules have been reported to produce a reversible color change [3] or a dipole inversion.[4] Importantly, such a motion of protons can be correlated even through intermolecular space, if each molecule is restricted in its total number of protons, as is typically found in the case of ice. Squaric acid (H2C4O4)[5] provides a good example of this constraint called the “ice rule”: cooperative proton transfer over the short O··· O bonds (2.55) can reverse the polarity of the hydrogen-bonded molecular sheets. As a consequence, a high dielectric permittivity emerges near the order–disorder phase transition. As in this example, the three-dimensional arrangement of polar molecules is usually antiparallel (antiferroelectric), canceling out the net electric polarization. If the mobile protons were ordered into a polar lattice, they would generate a spontaneous polarization, and their motion could be controlled with an external electric field, as a ferroelectric. Herein, we report the first organic system that exhibits ferroelectricity induced by a cooperative intermolecular proton transfer. KH2PO4 (KDP) is a representative inorganic ferroelectric, in which the electric polarization is reversed by an electric field through a site-to-site proton transfer over very short O··· O bonds (2.50) between the PO4 2À ions.[6] Molecular analogues of KDP are still undeveloped, except for an organic–inorganic hybrid, the salt [dabcoH]+[ReO4] À (dabco= diazabicyclo [2.2. 2] octane).[7] To design genuinely organic ferroelectrics, we have exploited a strongly hydrogen-bonded acid–base system, because some such compounds are known to exhibit novel thermally induced proton migration.[8] The combination of acids and bases with similar proton affinities guarantees that the protons of the hydrogen bonds can easily attach to both molecules.[9] Cocrystals of 2, 5-dihydroxy-p-benzoquinones as the acid and pyridine derivatives as the base (Scheme 1) have recently been tested to fulfill this requirement. For example, it was demonstrated that chloranilic acid (H2ca; Scheme 1, X= Cl) and bromanilic acid (X= Br) produce ferroelectric adducts with phenazine (Phz).[10] In these adducts, however, there is no apparent site-to-site proton transfer and the ferroelectricity is, rather, classified as displacive, as is found for the BaTiO3 or PbTiO3 family. On the other hand, an antiferroelectric phase transition with proton ordering was realized in a proton-transferred monovalent salt of H2ca with a stronger base, 5, 5’-dimethyl-2, 2’-bipyridine (55DMBP; Scheme 1).[11] The new ferroelectric presented herein is derived from the iodine analogue of H2ca, iodanilic acid (H2ia; Scheme 1, X= I), and 55DMBP. This compound is also a proton-transferred 1: 1 salt, as formulated by [H-