The chiral ferroelectric smectic C (SmC*) phase, characterized by a helical superstructure, has been well exploited in developing high‐resolution microdisplays that have been effectively employed in the fabrication of a wide varieties of portable devices. Although, an overwhelming number of optically active (chiral) liquid crystals (LCs) exhibiting a SmC* phase have been designed and synthesized, the search for new systems continues so as to realize mesogens capable of meeting technical necessities and specifications for their end‐use. In continuation of our research work in this direction, herein we report the design, synthesis, and thermal behavior of twenty new optically active, three‐ring calamitic LCs belonging to four series. The first two series comprise five pairs of enantiomeric Schiff bases whereas the other two series are composed of five pairs of enantiomeric salicylaldimines. In each pair of optical isomers, the configuration of a chiral center in one stereoisomer is opposite to that of the analogous center in the other isomer as they are derived from (3 S)‐3,7‐dimethyloctyloxy and (3 R)‐3,7‐dimethyloctyloxy tails. To probe the structure–property correlations in each series, the length of the n‐alkoxy tail situated at the other end of the mesogens has been varied from n‐octyloxy to n‐dodecyloxy. The measurement of optical activity of these chiral mesogens was carried out by recording their specific rotations. As expected, enantiomers rotate plane polarized light in the opposite direction but by the same magnitude. The thermal behavior of the compounds was established by using a combination of optical polarizing microscopy, differential scanning calorimetry, and powder X‐ray diffraction. These complementary techniques demonstrate the existence of the expected, thermodynamically stable, chiral smectic C (SmC*) LC phase besides blue phase I/II (BPI or BPII) and chiral nematic (N*) phase. However, as noted in our previous analogous study, the vast majority of the Schiff bases show an additional metastable, unfamiliar smectic (SmX) phase just below the SmC* phase. Notably, the SmC* phase persists over the temperature range ≈80–115 °C. Two mesogens chosen each from Schiff bases and salicylaldimines were investigated for their electrical switching behavior. The study reveals the ferroelectric switching characteristics of the SmC* phase featuring the spontaneous polarization (P_S) in the range 69–96 nC cm⁻². The helical twist sense of the SmC* phase as well as the N* phase formed by a pair of enantiomeric Schiff bases and salicylaldimines has been established with the help of circular dichroism (CD) spectroscopic technique. As expected, the SmC* and the N* phase of a pair of enantiomers showed mirror image CD signals. Most importantly, the reversal of helical handedness from left to right and vice versa has been evidenced during the N* to SmC* phase transition, implying that the screw sense of the helical array of the N* phase and the SmC* phase of an enantiomer is opposite.

中文翻译：

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铁电液晶：光学活性，三环席夫碱和水杨醛亚胺的合成和热行为

具有螺旋上层结构特征的手性铁电层列C（SmC *）相已被广泛用于开发高分辨率微显示器，该显示器已被有效地用于制造各种便携式设备。尽管已经设计和合成了绝大多数具有SmC *相的旋光（手性）液晶（LC），但对新系统的搜索仍在继续，以实现能够满足最终用途技术要求和规格的液晶元。在此方向上我们的研究工作继续进行，在此我们报告了属于四个系列的二十种新的光学活性三环钙钛矿型液相色谱的设计，合成和热行为。前两个系列包括五对对映体席夫碱，而其他两个系列由五对对映体水杨醛亚胺组成。在每对光学异构体中，一个立体异构体中的手性中心构型与另一种异构体中的类似中心构型相反，因为它们衍生自（3 S）-3,7-二甲基辛氧基尾巴和（3  R）-3,7-二甲基辛氧基尾巴。为了探查每个系列中的结构与性质的相关性，位于介晶另一端的正烷氧基尾巴的长度已从正辛氧基变为正构烷氧基。十二烷氧基 这些手性液晶元的旋光性的测量是通过记录它们的比旋光度来进行的。如预期的那样，对映异构体使平面偏振光沿相反的方向旋转，但幅度相同。通过使用光学偏振显微镜，差示扫描量热法和粉末X射线衍射的组合来确定化合物的热行为。这些互补技术证明，除了蓝相I / II（BPI或BPII）和手性向列相（N *）相之外，还存在预期的热力学稳定的手性近晶C（SmC *）LC相。但是，正如我们之前的类似研究所指出的那样，绝大多数席夫碱都在SmC *相的下方显示了一个额外的亚稳态，不熟悉的近晶（SmX）相。尤其，SmC *相在≈80–115°C的温度范围内持续存在。研究了分别从席夫碱和水杨醛亚胺中选择的两种介晶的电开关行为。该研究揭示了具有自发极化的SmC *相的铁电开关特性（P _S）在69–96 nC cm ^-2的范围内。借助于圆二色性（CD）光谱技术，已经确定了SmC *相以及由一对对映体席夫碱和水杨醛亚胺形成的N *相的螺旋扭曲感。正如预期的那样，一对对映异构体的SmC *和N *相显示出镜像CD信号。最重要的是，在从N *到SmC *相变的过程中，已经证明了螺旋惯性从左向右反转，反之亦然，这表明对映异构体N *相和SmC *相的螺旋排列的螺旋感是相反的。