Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 5

Fig. 3 Examples of typical di-, tri-, and tetra-mesogenic liquid crystals
terior two have aliphatic chains that are tethered, and therefore these two
mesogenic units have different properties from the exterior groups.
In a systematic study led by Nishiyama into dimeric and trimeric li-
quid crystals [34-45] investigations were made firstly on deploying the same
mesogens in dimeric systems but with accompanying variation of the linking
group, and secondly on incorporating non-mesogenic units into supermolec-
ular architectures, but with the result that mesomorphic materials were still
obtained.
The first set of four materials, shown in Fig. 4, all possessed the benzoyloxy-
biphenyl unit commonly deployed to support the formation of antiferro-
electric properties in liquid crystal systems. All of the materials were de-
signed to be chiral via the incorporation of 2-octyl moieties with both RR
and SS variants being available for the resulting bimesogens. With a sim-
ple dimethyleneoxy-linking unit between the mesogens, a phase sequence
of ferroelectric-ferrielectric-antiferroelectric, e.g., 1 , as a function of tem-
perature was found [36, 37]. When the linking group was changed to a di-
thioether [43] unit ferroelectric and ferrielectric properties were suppressed
in favor of antiferroelectric phase formation, see 2 . It is known from other in-
vestigations that the incorporation of sulphur in place of oxygen in aliphatic
chains tends to favor the formation of more ordered phases such as the crystal
B phase over the hexatic smectic B phase [46, 47]. Through the incorpora-
tion of a biphenylyl unit in the bridging chain, thereby creating a trimesogen,
higher ordered antiferroelectric phases, such as the smectic I A phase, are
found, see compound 3 [45]. Lastly for this family of materials, the linking
group was substituted with a silyloxy unit [41]. Silyloxy substitution has the
effect of lowering melting point and, because an odd number of atoms substi-
tuted into an even number of atoms in the methylene chain, antiferroelectric
phases are stabilized. For the last compound, 4 , this is the case with a melting
point of only 34.1 ◦ C and the other supermolecular materials having melting
points above 101.5 ◦ C. As the number of atoms located in the chain between