Effect of Hard Core Scaffolds - Silsesquioxanes

Within the now conventional group of liquid crystal dendrimers, a group of liquid-crystalline dendrimers based on the hexa- and octa-silsesquioxane 

The structure of the nematic phase is proposed to be one where the central cores are surrounded by the mesogenic groups, with the long axes of the mesogens pointing in roughly the same direction, as shown in Fig. 33. 

The next family of materials, which are to be discussed all possess cyanobiphenyl moieties as the mesogenic units. The systematic studies of the liquid crystal properties are described in terms of spacer length and peripheral number density. The first material, 37, shown in Fig. 35, is an octamer based on the octasilsesquioxane core unit [86]. This material was prepared and purified in such a way that by HPLC and 29Si NMR spectroscopy it was shown to be a single compound without dispersity. The material was found to exhibit smectic polymorphism with SmC and SmA phases being formed. The incorporation of cyanobiphenyl mesogenic moieties means that the material has interesting 

The effect of the length of the spacer chain on the liquid-crystalline properties of the octamers of octasilsesquioxane, of general structure 38, was 

The point of bifurcation, so that 16 mesogenic units can be more easily located around the octasilsesquioxane core, can be achieved by moving the bifurcation point away from the central part of the scaffold as shown for 

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