Supermolecular Liquid Crystals - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 9

Fig. 8 Trimeric dendron composed of cyanobiphenyl mesogenic moieties

were attached to a pentaerythritol unit leaving one hydroxyl group free for at-
tachment to a scaffold. The trimeric dendron, 14 , so formed was also found
to exhibit liquid crystal phases, as shown in Fig. 8. Remarkably this mate-
rial was found to exhibit smectic A and smectic C phases, but not a nematic
phase. The 4- n -decyloxy-4 -cyanobiphenyl parent system exhibits nematic
and smectic A phases. The suppression of the nematic phase is not surpris-
ing because the ester groups will promote smectic A formation over that of
the nematic phase. However, the formation of a smectic C phase is interest-
ing. Molecular simulations show that, at least in the gas phase at 0 K, the
mesogenic moieties interact strongly together and bunch up to form a cylin-
drical cluster of biphenyl groups, see Fig. 9. Thus the overall structure of the
mesophase is rod-like. The tight packing of the mesogenic units together does
not allow for any overlap of the mesogenic units, thereby preventing the for-
mation of bilayer structures. Alkoxy cyanobiphenyls form bilayer smectic A d
phases with layer spacings 1.4

the molecular length. The close packing of
the mesogens in the dendron ensures that the prevention of interdigitation
between the layers results in the formation of a monolayer smectic A 1 phase,
seeFig.10.Thetightpackingalsohastheeffectofintroducingtheformation
of smectic C phase, which is caused by the limited overlapping of only the
nitrile units.

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Fig. 9 Molecular simulation of a trimeric dendron, 14 , composed of cyanobiphenyl meso-
genic moieties