Influence of meta-extended rigid-core, complementary hydrogen bonding and flexible chain on polymorphism in Schiff-base liquid crystals: (4)<sub>MeO</sub>BD(3)<sub>Am</sub>BA:nOBAs
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Schiff-base complementary hydrogen-bonded liquid crystals (HBLC), viz., (4)<sub>MeO</sub>BD(3)<sub>Am</sub>BA:nOBAs with flexible chain length for n = 3,4,5,6,7,8,9,10,11 and 12 are reported. <sup>1</sup>H; <sup>13</sup>C-NMR and Infra-Red spectroscopy used to confirm the formation of HBLCs. LC phases and transition temperatures (T<sub>c</sub>) determined by polarised optical microscopy (POM) and differential scanning calorimetry (DSC). T<sub>c</sub> and enthalpy (∆H) determined by DSC also. Odd-Even effect observed at clearing and melting transitions. Influence of Schiff base and Oxygen as bridging atom promote smectic phase abundance. HBLCs exhibit tetra- or penta-phase variance. Maximum (penta) phase variance is exhibited by n = 8 and 12 with long flexible chain. Prevalent abundance of quasi-two-dimensional (2D) LC phases of SmF and SmI is observed. Nematic phase is quenched. Lower (n = 4) and intermediate (n = 6) members exhibited SmB<sub>cryst</sub> phase. Predominant occurrence of enantiotropic LC phases is noticed. All members exhibited 3D tilted SmG phase. A-C transition exhibited by intermediate homologues (for n = 7, 8, 9 and 10) is found to be either second order or with very small enthalpy. Phase diagram reveals the abundance of multi-critical points with LC phasesinvolving exotic symmetries. Influence of meta-extended rigid core, complementary HB and flexibility is studied for the LC phase abundance with characteristic structural order. POM and DSC results are discussed in the wake of reports in other achiral calamitic LCs.
本文报道了席夫碱互补氢键液晶（hydrogen-bonded liquid crystals，HBLC）系列化合物，即(4)<sub>甲氧基</sub>BD(3)<sub>酰胺</sub>BA:nOBAs，其柔性链长n的取值为3、4、5、6、7、8、9、10、11和12。采用氢谱（<sup>1</sup>H NMR）、碳谱（<sup>13</sup>C NMR）与红外光谱对HBLC的形成进行了表征确认。液晶相态与转变温度（transition temperatures，T<sub>c</sub>）分别通过偏光显微镜（polarised optical microscopy，POM）与差示扫描量热法（differential scanning calorimetry，DSC）进行测定；转变温度与焓变（∆H）同样通过差示扫描量热法测得。在清亮点与熔融转变过程中观察到了奇偶效应。席夫碱结构与作为桥连原子的氧原子，可促进近晶相的占比。该系列HBLC可表现出四相或五相相变行为，其中当柔性链长n为8和12时，可呈现最多的五相相变行为。研究观察到准二维（2D）近晶F相（SmF）与近晶I相（SmI）占据主导地位，向列相被淬灭。低链长同系物（n=4）与中间链长同系物（n=6）呈现出结晶近晶B相（SmB<sub>cryst</sub>）；绝大多数观测到的液晶相均为对映异构相。所有化合物均表现出三维倾斜近晶G相。中间链长同系物（n=7、8、9和10）所表现出的A-C相转变，被证实为二级相变或伴随极小的焓变。相图显示，体系中存在大量多临界点，且这些临界点对应着具有特殊对称性的液晶相。本文研究了间位扩展刚性核、互补氢键与柔性链长对具有特征结构有序性的液晶相占比的影响。最后，本文结合其他非手性棒状液晶的相关研究报道，对偏光显微镜与差示扫描量热法的测试结果进行了讨论。
