New bent-core mesogens with carbon–carbon multiple linkages in the terminal chains
作者:Gerhard Pelzl、Maria G. Tamba、Sonja Findeisen-Tandel、Martin W. Schröder、Ute Baumeister、Siegmar Diele、Wolfgang Weissflog
DOI:10.1039/b803493a
日期:——
New five-ring bent-core mesogens have been synthesized in which terminal alkynyl chains are connected with the terminal rings of the aromatic core by means of oxycarbonyl groups, cinnamic esters groups or oxyacetic ester moieties. The insertion of CâC triple bonds in different positions of the terminal chains can change the mesophase behaviour and can result in an increase or decrease of the clearing temperatures in comparison to corresponding compounds with saturated chains. In addition, lateral substituents are attached to different positions of the central ring. The mesophase behaviour of the new compounds has been studied by polarizing microscopy, differential scanning calorimetry, X-ray experiments and electro-optical measurements. On the base of the experimental findings new relationships between the molecular structure and the mesomorphic properties have been obtained. Different polar phases (SmCP, Col, B7â²) could be proved, in some of these the polar (ferro- or antiferroelectric) switching takes place through a collective rotation of the molecules around their long axes. Interestingly, three compounds with terminal alkynyl chains form a mesophase with all textural features of a B7 phase although these phases possess obviously a simple layer structure. For the first time it could be observed that helical filaments formed on slow cooling the isotropic liquid are stable to low temperatures. Some of these spirals can serve as nuclei in the growing process of ribbon-like and oval domains. Surprisingly, the helical pitch of the spirals can be exactly recovered in the equidistant stripes of these domains. It is also remarkable that all compounds derived from 2-methylisophthalic acid exhibit unusually high clearing temperatures and that the SmC phases of these compounds show polar switching about 50 K below the SmAâSmC transition temperature.
我们合成了新的五环弯曲核中间体,其中的末端炔链通过氧羰基、肉桂酸酯基或氧乙酸酯分子与芳香核的末端环相连。在末端链的不同位置插入 CâC 三键可改变介相行为,与具有饱和链的相应化合物相比,可导致清除温度的升高或降低。此外,横向取代基被连接到中心环的不同位置。我们通过偏振显微镜、差示扫描量热法、X 射线实验和电光测量等方法研究了新化合物的介相行为。在实验结果的基础上,我们获得了分子结构与介观性质之间的新关系。可以证明存在不同的极性相(SmCP、Col、B7â²),其中一些极性相(铁电或反铁电)的切换是通过分子绕其长轴的集体旋转实现的。有趣的是,三种带有末端炔链的化合物形成的介相具有 B7 相的所有纹理特征,尽管这些相明显具有简单的层结构。我们首次观察到,在缓慢冷却各向同性液体时形成的螺旋丝在低温下非常稳定。这些螺旋丝中的一些可以作为带状和椭圆形畴生长过程中的核。令人惊讶的是,螺旋线的螺旋间距可以在这些畴的等距条纹中精确复原。同样值得注意的是,所有由 2-甲基间苯二甲酸衍生的化合物都表现出异常高的清澈温度,而且这些化合物的 SmC 相在 SmAâSmC 转换温度以下约 50 K 处出现极性转换。