The synthesis and characterisation of a homologous series of liquid crystal trimers, the 4,4â²-bis[Ï-(4-cyanobiphenyl-4â²-yloxy)alkoxy]biphenyls, is reported in which the length of the flexible spacers is varied from 3 to 12 methylene units. All the members of the series exhibit enantiotropic nematic behaviour and, in addition, monotropic smectic A behaviour is observed for compounds with spacers containing from 4 to 11 methylene units. The formation of a smectic phase in these compounds is attributed to a specific interaction between the unlike mesogenic groups, namely, the central biphenyl and the terminal cyanobiphenyl units. The nematicâisotropic transition temperatures and the associated entropy changes exhibit a dramatic oddâeven effect as the length and parity of the spacers is varied, in which the even members exhibit the higher values. This behaviour is interpreted in terms of the geometry as well as the flexibility of the spacers and how these control the average molecular shape. A comparison of the magnitudes of these oddâeven effects with those for the analogous dimeric series suggests that in the nematic phase the mesogenic units in the trimers are correlated to the same extent as in the dimers but that their orientational ordering is significantly higher.
报告了同源系列液晶三聚体--4,4â²-双[Ï-(
4-氰基联苯-4â²-氧基)烷氧基]
联苯--的合成和表征,其中柔性间隔物的长度在 3 到 12 个亚甲基单元之间变化。该系列的所有成员都表现出了对映向列行为,此外,在含有 4 至 11 个亚甲基单元的间隔物的化合物中,还观察到了单向 Smectic A 行为。在这些化合物中形成的共晶相归因于不同中间基团(即中央
联苯单元和末端
氰基
联苯单元)之间的特殊相互作用。当间隔物的长度和奇偶性发生变化时,向列各向异性转变温度和相关的熵变化呈现出显著的奇偶效应,其中偶数成员的值较高。我们可以从间隔物的几何形状和柔韧性以及它们如何控制平均分子形状的角度来解释这种行为。将这些奇偶效应的大小与类似二聚物系列的奇偶效应进行比较后发现,在向列相中,三聚物中的介原单元与二聚物中的介原单元具有相同程度的相关性,但它们的取向有序性要高得多。