Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
J Am Chem Soc. 2012 Feb 8;134(5):2634-43. doi: 10.1021/ja209010m. Epub 2012 Jan 25.
Two series of wedge-shaped onium salts, one ammonium and the other phosphonium, having 3,4,5-tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontinuous "gyroid" cubic (Cub(bi)) and hexagonal columnar liquid-crystalline (LC) phases by nanosegregation between ionophilic and ionophobic parts. The alkyl chain lengths on the cationic moieties, anion species, and alkyl chain lengths on the benzyl moieties have crucial effects on their thermotropic phase behavior. For example, triethyl-[3,4,5-tris(dodecyloxy)benzyl]ammonium hexafluorophosphate forms the thermotropic Ia3d Cub(bi) LC phase, whereas an analogous compound with trifluoromethanesulfonate anion shows no LC properties. Synchrotron small-angle diffraction intensities from the Ia3d Cub(bi) LC materials provide electron density maps in the bulk state. The resulting maps show convincingly that the Ia3d Cub(bi) structure is composed of three-dimensionally interconnected ion nanochannel networks surrounded by aliphatic domains. A novel differential mapping technique has been applied successfully. The map of triethyl-[3,4,5-tris(decyloxy)benzyl]ammonium tetrafluoroborate has been subtracted from that of the analogous ammonium salt with hexafluorophosphate anion in the Ia3d Cub(bi) phases. The differential map shows that the counteranions are located in the core of the three-dimensionally interconnected nanochannel networks. Changing from trimethyl- via triethyl- to tripropylammonium cation changes the phase from columnar to Cub(bi) to no mesophase, respectively. This sensitivity to the widened shape for the narrow end of the molecule is explained successfully by the previously proposed semiquantitative geometric model based on the radial distribution of volume in wedge-shaped molecules. The LC onium salts dissolve lithium tetrafluoroborate without losing the Ia3d Cub(bi) LC phase. The Cub(bi) LC materials exhibit efficient ion-transporting behavior as a result of their 3D interconnected ion nanochannel networks. The Ia3d Cub(bi) LC material formed by triethyl-[3,4,5-tris(decyloxy)benzyl]phosphonium tetrafluoroborate shows ionic conductivities higher than the analogous Ia3d Cub(bi) material based on ammonium salts. The present study indicates great potential of Cub(bi) LC nanostructures consisting of ionic molecules for development of transportation nanochannel materials.
两类楔形季铵盐和季鏻盐，一类是铵盐，另一类是鏻盐，均具有 3,4,5-三（烷氧基）苄基部分，通过亲电和亲核部分的纳米分离，表现出热致双连续“准晶”立方（Cub(bi)）和六方柱状液晶（LC）相。阳离子部分的烷基链长度、阴离子种类以及苄基部分的烷基链长度对其热致相行为有至关重要的影响。例如，三乙基-[3,4,5-三（十二烷氧基）苄基]铵六氟磷酸盐形成热致 Ia3d Cub(bi) LC 相，而具有三氟甲烷磺酸盐阴离子的类似化合物则没有 LC 性质。来自 Ia3d Cub(bi) LC 材料的同步小角度衍射强度在体相提供电子密度图。得到的图谱令人信服地表明，Ia3d Cub(bi)结构由三维相互连接的离子纳米通道网络组成，周围是脂肪族区域。已经成功应用了一种新颖的差分映射技术。Ia3d Cub(bi)相中，三乙基-[3,4,5-三（癸烷氧基）苄基]铵四氟硼酸盐的图谱已从具有六氟磷酸盐阴离子的类似铵盐的图谱中减去。差分图谱表明，抗衡阴离子位于三维相互连接的纳米通道网络的核心。从三甲基铵到三乙基铵再到三丙铵阳离子的变化，分别使相从柱状变为 Cub(bi)再变为无中间相。这种对分子窄端形状变宽的敏感性可以通过之前提出的基于楔形分子体积径向分布的半定量几何模型成功解释。LC 季铵盐溶解六氟硼酸锂而不失去 Ia3d Cub(bi) LC 相。由于其 3D 相互连接的离子纳米通道网络，Cub(bi) LC 材料表现出有效的离子传输行为。由三乙基-[3,4,5-三（癸烷氧基）苄基]膦酸四氟硼酸盐形成的 Ia3d Cub(bi) LC 材料表现出高于基于铵盐的类似 Ia3d Cub(bi)材料的离子电导率。本研究表明，由离子分子组成的 Cub(bi) LC 纳米结构在开发运输纳米通道材料方面具有巨大潜力。
