含薄荷基的手性液晶单体的合成、结构与性能研究

合成了五种新型含薄荷基的手性单体(M1～M5), 它们的结构、纯度及旋光性质通过了1H NMR, FT-IR、元素分析仪及旋光仪等手段的表征, 采用DSC, POM, UV/Vis/NIR等研究了单体的介晶性能、相行为及选择反射性能. 结果表明: 单体的比旋光度值随苯环数目的增加而降低, 通过在薄荷基与液晶核之间引入柔性间隔基元, 实现了含薄荷基单体具有液晶性能的目的. 除M1外, 其余四种单体均呈现手性近晶C (SC*)相和胆甾(Ch)相, 此外M5还出现了蓝相织构. M2～M4只在SC*相区能观察到选择反射现象, 而M5在SC*相区和Ch相区均出现明显的选择反射现象, 且随温度的升高, SC*相区的反射波长发生“红移”, 而Ch相区的反射波长则发生“蓝移”. 随着液晶核刚性的增加, 对应单体的熔点和清亮点增大, 液晶相范围变宽. 液晶核中的酯基桥键与组合方式也对单体的熔点和清亮点具有一定的影响.

Synthesis, Structures and Properties of Chiral Liquid Crystalline Monomers Containing Menthyl Group

One new chiral non-mesogenic monomer (M1) and four new chiral mesogenic monomers (M2～M5) containing menthyl groups were prepared. The chemical structures and purities of the target monomers were characterized with 1H NMR, FT-IR and elemental analysis, and specific optical rotations were evaluated with a polarimeter. Their structure-property relationships were discussed. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry and polarizing optical microscopy. The selective reflection wavelength of light was studied with UV/visible/NIR. The chiral monomers with more aryl segments showed lower specific optical rotation values. The monomers M2～M5 formed the mesophases when a flexible spacer was inserted between the rigid mesogenic core and the terminal menthyl groups by reducing the steric effect. M1 did not show any texture, and M2～M5 showed enantiotropic broken fan-shaped chiral smectic C (SC*) phase, and oily streak texture and focal conic texture of cholesteric phase. Moreover, M5 also exhibited platelet texture of a cubic blue phase on cooling cycles. With increasing temperature, the selective reflection of light shifted to the long wavelength region in the SC* phase range, and the short wavelength region in the cholesteric phase range, respectively. The melting and clearing temperatures increased, and the mesophase temperature range widened with increasing rigidity of the mesogenic core. Moreover, the ester linkage bond and aryl arrangement in the mesogenic core also affected the mesophase behavior.