Majhi Debashis, Dvinskikh Sergey V
Department of Chemistry, KTH Royal Institute of Technology, 10044, Stockholm, Sweden.
School of Chemistry, Tel Aviv University, Ramat Aviv, 6997801, Tel Aviv, Israel.
Sci Rep. 2021 Mar 16;11(1):5985. doi: 10.1038/s41598-021-85021-y.
Ionic liquids crystals belong to a special class of ionic liquids that exhibit thermotropic liquid-crystalline behavior. Recently, dicationic ionic liquid crystals have been reported with a cation containing two single-charged ions covalently linked by a spacer. In ionic liquid crystals, electrostatic and hydrogen bonding interactions in ionic sublayer and van der Waals interaction in hydrophobic domains are the main forces contributing to the mesophase stabilization and determining the molecular orientational order and conformation. How these properties in dicationic materials are compared to those in conventional monocationic analogs? We address this question using a combination of advanced NMR methods and DFT analysis. Dicationic salt 3,3'-(1,6-hexanediyl)bis(1-dodecylimidazolium)dibromide was studied. Local bond order parameters of flexible alkyl side chains, linker chain, and alignment of rigid polar groups were analyzed. The dynamic spacer effectively "decouples" the motion of two ionic moieties. Hence, local order and alignment in dicationic mesophase were similar to those in analogous single-chain monocationic salts. Bond order parameters in the side chains in the dicationic smectic phase were found consistently lower compared to double-chain monocationic analogs, suggesting decreasing contribution of van der Waals forces. Overall dication reorientation in the smectic phase was characterized by low values of orientational order parameter S. With increased interaction energy in the polar domain the layered structure is stabilized despite less ordered dications. The results emphasized the trends in the orientational order in ionic liquid crystals and contributed to a better understanding of interparticle interactions driving smectic assembly in this and analogous ionic mesogens.
离子液体晶体属于一类特殊的离子液体，具有热致液晶行为。最近，已报道了双阳离子离子液体晶体，其阳离子包含通过间隔基共价连接的两个单电荷离子。在离子液体晶体中，离子亚层中的静电和氢键相互作用以及疏水区域中的范德华相互作用是有助于中间相稳定并决定分子取向顺序和构象的主要作用力。与传统的单阳离子类似物相比，双阳离子材料的这些性质如何？我们使用先进的核磁共振方法和密度泛函理论分析相结合的方式来解决这个问题。对双阳离子盐3,3'-(1,6-己二醇)双(1-十二烷基咪唑鎓)二溴化物进行了研究。分析了柔性烷基侧链、连接链的局部键序参数以及刚性极性基团的排列。动态间隔基有效地“解耦”了两个离子部分的运动。因此，双阳离子中间相中的局部有序和排列与类似的单链单阳离子盐中的相似。发现双阳离子近晶相侧链中的键序参数始终低于双链单阳离子类似物，这表明范德华力的贡献在减小。近晶相中双阳离子的整体重新取向以取向序参数S的值较低为特征。尽管双阳离子的有序性较低，但随着极性域中相互作用能的增加，层状结构得以稳定。这些结果强调了离子液体晶体中取向序的趋势，并有助于更好地理解驱动该类及类似离子液晶元近晶组装的粒子间相互作用。
