Mesophase range

Examples of rod-like molecules with a branched fluorinated chain at one end (compounds 90-93) are collated in Fig. 26 [166]. It is interesting that smectic phases are retained despite the significant size of these chains. This is mainly a result of the intercalation of the aromatic cores and aliphatic spacers of these molecules, which can compensate this steric distortion. These branched chains remove the B and E phases with crystalline layers and replace them by fluid smectic phases, including SmC phases. The comparison in this figure also shows that the bulky and flexible bis(perfluoropropylene oxide) derived chains (compounds 92, 93) can provide LC materials with especially low melting points and broad mesophase ranges due to the higher conformational flexibility of perfluoroethers compared to linear perfluoroalkyl chains [99, 176]. 

Molecules possessing a half disc shape cannot be placed side by side to form a supramolecular disc as the half disc molecules disturb each other, which ultimately leads to smaller mesophase ranges. 

Ionic liquid crystals are liquid crystals containing ionic moieties. Ionic hquid crystals having ammonium and pyridinium salts were reported to show stable thermotropic liquid crystalline behavior [35 0]. Seddon, Bruce, and coworkers showed that some imidazolium and pyridinium salts exhibit low melting points and liquid crystalline behavior [41]. Ionic liquid crystals with lower temperature mesophase ranges are called hquid crystalline ionic liquids. A large variety of thermotropic liquid crystaUine ionic liquids exhibiting anisotropic fluid states were prepared. In particular, the ionic liquid crystals based on imidazolium salts 1 [42-46], 2... 

It is noteworthy that the formation of a mesophase was sustained in the copolymers even incorporating low proportions of mesogenic fluorinated side groups. Moreover, the introduction of repeat co-units with hexyl substituents lowered Tg by internal plasticization and resulted in a somewhat broader mesophase range for the copolymers relative to the fluorinated homopoymer (e.g. T -Tg = 164°C for copolymer). 

Recently Rouike has reported a series of cyclopalladated imine complexes with either acetylacetonate or cyclopentadienyl as the other ligand, as in Figure 8 [29]. Unlike Ghedini s azobenzene complexes these are isomerically pure but still display mesophases at similar temperatures. The square planar 16e acetylacetonate complexes display both nematic and Sa phases with extended mesophase ranges compared to the free ligands. Not surprisingly, the formally trigonal bipyramidal 18e cyclopentadienyl complexes show a nematic phase at much lower temperatures and tend to decompose above c. 180°C. 

Ohta has also studied triphenylene-based porphyrazinato metal(II) complexes (Figure 70), which have an extended core compared to phthalocyanines [133]. It was found that both the nickel(II) and coppeifll) complexes displaced a Coltet.o between — 100°C and the onset of decomposition at 300°C, so that extension of the core size appears to have a beneficial effect with regard to the mesophase range. 

For the non-acetato-bridged dimers, the only mesophase seen was SmC which was typically in the range 100-250 °C (for n = 10) whereas for the acetato-bridged complexes, a nematic phase was seen for 6 < < 8 and for n>7, a SmC phase was also seen (between ca. 100 and 160 °C). Mesophase ranges were very much larger in the planar materials. 

Both hydrogen bonding and halogen bonding can be used to realize new liquid crystals. Bruce and coworker have first reported that complex 22-n formed by a 4-alkoxy stilbazole and iodopentafluorobenzene showed SmA phases with a mesophase range from 81 to 84 °C. Recently, they also reported trimeric new liquid... 

The materials prepared in this way tend to be vray insoluble polymers with high melting points and mesophase ranges, e.g., poIy(p-hydroxybraizoic add) melts at -883 K. This makes them difficult to process, and alternative structures with much lower melting points are more useful. The melting points of the main-chain hquid crystal polymers can be reduced in a nnmber of differrait ways, i.e. ... 

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