Organic–inorganic hybrid liquid crystals derived from octameric silsesquioxanes. Effect of the peripheral groups in mesogens on the formation of liquid crystals
作者:Xiaobai Wang、Ching Mui Cho、Wan Yong Say、Angeline Yan Xuan Tan、Chaobin He、Hardy Sze On Chan、Jianwei Xu
DOI:10.1039/c0jm03406a
日期:——
A series of octameric silsesquioxane-based organicâinorganic hybrid compounds 7aâh was synthesised by reacting octakis(dimethylsiloxy)silsesquioxane with 4â²-(undec-10-enyloxy)biphenyl-4-yl mono-, di-, or trialkoxybenzoateviaplatinum-catalysed hydrosilylation reaction. The chemical structures of 7aâh were characterised by 1H, 13C, and 29Si nuclear magnetic resonance spectroscopy, mass spectroscopy and elemental analysis. Liquid crystallinity of 7aâh was investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). Temperature-dependent X-ray diffraction was used to verify liquid crystal phases, revealing that 7aâd assembled to smectic C and smectic A phases. The effects of the number and length of peripheral flexible alkoxy groups in 7aâh on the formation of liquid crystals were studied using DSC and POM, which revealed that 7aâd bore one alkoxy terminal group in each mesogen exhibited a mesophase, 7eâf and 7gâh bore two and three flexible alkoxy terminal groups in each mesogen, respectively, and did not show mesomorphic behaviour. The concept of âflexible group densityâ (FGD), which was defined as the number of the CH2 groups per unit volume in the periphery of the POSS molecule, was introduced to interpret the formation of liquid crystals. The results revealed that the POSS molecules with di- and tri-substituted mesogens exhibited much larger FGD than those with mono-substituted mesogens, hence the absence of mesophases for 7eâf and 7gâh. This study provides a useful archetype to predict the formation of liquid crystals when designing and synthesising liquid crystalline materials, particularly star-like or dendrimer-like molecular materials.
通过八甲基环四硅氧烷与4′-(十一碳-10-烯氧基)联苯-4-基单、二或三烷氧基苯甲酸酯在铂催化硅氢加成反应中合成了系列八聚体硅倍半氧烷基有机-无机杂化化合物7a~7h。通过核磁共振光谱、质谱和元素分析对7a~7h的化学结构进行了表征。利用差示扫描量热法(DSC)和偏光显微镜(POM)研究了7a~7h的液晶性。采用温度依赖的广角X射线衍射法确认液晶相,结果表明7a~7d形成近晶C相和近晶A相。通过DSC和POM研究了7a~7h分子周围末端柔性烷氧基的数量和长度对液晶形成的影响,结果表明7a~7d每个介晶基元中有一个末端烷氧基,具有液晶相。7e~7f有二个、7g~7h有三个柔性烷氧基团。在每个介晶基元中没有呈现液晶行为。提出了“柔性基团密度”(FGD)这一概念,定义为POSS分子周围每单位体积中的CH2基数量,来解释液晶的形成。结果表明,双、三取代的介晶基元的FGD远大于单取代介晶基元的FGD,因此,7e~7f和7g~7h未形成液晶相。该研究表明,FGD可作为设计和合成的液晶材料,特别是星形或树枝状分子材料形成液晶性的预测参数。