Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives
作者:C. Scott Hartley、Nadia Kapernaum、Jeffrey C. Roberts、Frank Giesselmann、Robert P. Lemieux
DOI:10.1039/b515313a
日期:——
The atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4′-heptyl[1,1′-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76), and electroclinic coefficients ec were measured as a function of the dopant mole fraction x1 in the chiral SmA* phase at T
−
TC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ec values reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1 in 2-PhP is amplified by achiral 2-phenylpyrimidine additives (5 mol%) that are longer than 2-PhP; in the best case, ec is amplified by a factor of 3.2 with 5-(tetradecyloxy)-2-(4-(tetradecyloxy)phenyl)pyrimidine (3g), which is almost twice as long as 2-PhP. However, no amplification is observed in a 4 mol% mixture of (R)-1 in NCB76 using the same series of additives. A correlation between ec values and the temperature range of the SmA* phase suggests that the amplification of ec with increasing length of the additive 3 in the (R)-1/2-PhP mixture is due primarily to a decrease in the tilt susceptibility coefficient α as the second-order SmA*–SmC* phase transition moves away from the tricritical point. Measurements of smectic layer spacing as a function of T
−
TC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with T
−
TC for the pure host 2-PhP fits to a square-root law, which indicates that the second-order SmA–C transition is nearly tricritical. On the other hand, the corresponding variation in dA/dC with T
−
TC for a 5 mol% mixture of 3g in 2-PhP fits to a linear relation, which indicates that the second-order SmA–C transition approaches typical mean-field behavior.
将手性化合物(R)-2,2',6,6'-四甲基-3,3'-二硝基-4,4'-双[(4-壬氧基苯甲酰氧基)联苯]((R)-1)掺杂到非手性液晶主体2-(4-丁氧基苯基)-5-辛氧基嘧啶(2-PhP)和4-(4'-庚基[1,1'-联苯]-4-基)-1-己基环己烷氰基(NCB76)中,并在T-TC=+5K的条件下,测得手性SmA*相中掺杂物的摩尔分数x1与电场驱动斜率系数ec的关系。外推得到的ec值为3.07和2.28 deg µm V−1,与一些报道的最高纯手性SmA*材料的ec值相当。在2-PhP中掺杂4摩尔%的(R)-1,再掺杂5摩尔%的非手性2-苯基嘧啶添加剂(比2-PhP长),可以使电场驱动斜率系数放大;在最优情况下,使用长度几乎是2-PhP两倍的5-(十四烷氧基)-2-(4-(十四烷氧基)苯基)嘧啶(3g),ec放大了3.2倍。然而,在NCB76中掺杂4摩尔%的(R)-1,再使用同样系列的添加剂,并没有观察到放大效果。电场驱动斜率系数与SmA*相温度范围的相关性表明,在(R) -1/2-PhP混合物中,随着添加剂3的长度增加,ec的放大主要是由于倾斜敏感系数α的降低,使得SmA*-SmC*二阶相变远离三临界点。小角度X射线散射法测量层间距随T-TC的变化,结果与此解释一致。结果显示,纯主体2-PhP的层间距减少量dA/dC随T-TC的变化符合平方根规律,表明二阶SmA-C转变近似于三临界点。另一方面,2-PhP掺杂5摩尔%的3g,层间距减少量dA/dc随T-TC的变化符合线性关系,表明二阶SmA-C转变接近典型的平均场行为。