Enantiotropic smectic phase

Subsequently, we focus on LC polythienylenevinylene (PTV) derivatives with CB or PCH groups at the 3-position of the thiophene ring [26-30], in addition to LC-PT. Both types of polymers showed enantiotropic smectic phases and also exhibited fluorescence with greenish yellow and/or green colors. We report linearly dichroic optical properties and anisotropic electrical conductivities of LC-PTs and LC-PTVs after macroscopic alignment under a magnetic field [22-30, 44-46]. 

Mesomorphic dendrimers containing electroactive units have potential for construction of dendrimer based molecular switches. Deschenaux et al. reported [154] the synthesis and liquid-crystalline properties of a novel dendrimer containing six mesomorphic ferrocene units. Apart from exhibiting a broad enantiotropic smectic A phase as determined by polarized optical microscopy, DSC, and XRD studies, thermogravimetry revealed the excellent thermal stability of the macromolecule. 

Apart from the parent compound 1 and its very simple alkyl derivatives, 1,3,4-oxadiazoles are solids. Solid oxadiazoles containing biphenyl or triphenyl substituents exhibit interesting properties upon heating. The symmetric 2,5-bisbiphenyl-4-yl-l,3,4-oxadiazole 38 melts into an isotropic phase showing small monotropic mesophase. By contrast, the asymmetric (hockey stick-shaped) mesogen 2-terphenyl-4-yl-5-phenyl-l,3,4-oxadiazole 39 exhibits a more stable enantiotropic liquid crystalline phase (a smectic phase as well as a nematic phase) <2001PCB8845>. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

Most interesting are the effects of salt complexation on the mesomorphic behavior of liquid crystalline crown ethers and liquid crystalline crown ether polymers. Sodium triflate was added to poly(17) [34] and poly(25) (Scheme 14) [39]. The enantiotropic nematic and smectic phases of poly(17) were changed dramatically [40]. With increasing amounts of salt, the clearing temperatures are shifted to higher values while the melting transition increases only slightly. 

Butyl stearate has an enantiotropic smectic B phase from 14... 27 °C [727],... 

The monomers XXIV-m-n were crystalline solids and only the monomers with the shortest carbon-segments (n=4) showed an enantiotropic smectic C mesophase. Monomers with higher carbon chains did not exhibit a LC phase. 

The smectic state, apart from exhibiting a rich variety of modifications, is also thermodynamically ordered. Thus, there is a well-defined sequence of phases formed on cooling from the isotropic liquid. For instance, the following transition sequence would be observed for a ferrocene liquid crystal exhibiting enantiotropic smectic C, smectic A and nematic phases ... 

Interesting liquid crystal properties resulted from the Schiff-base derivatives 11 [18]. All compounds exhibited mesomorphic behavior (Fig. 9-6). The first members of the series gave rise to enantiotropic nematic phases and the long chain derivatives exhibited enantiotropic smectic A phases. The intermediate chain length derivatives presented monotropic nematic or smectic A phases. 

We designed and studied [27] a family of unsymmetrically l,l -disubstituted ferrocene derivatives obtained by combining the organic units A and B (above), used to prepare the families 13 and 14, respectively within the same molecular framework. Structures 25 (Fig. 9-21) led to remarkable mesomorphic properties. All derivatives exhibited liquid crystal properties. Compound 25 (n = 11) gave rise to an enantiotropic smectic A phase. Complex 25 (n = 12) showed an enantiotropic... 

BS forms an enantiotropic smectic B (Sm-B) phase between 15 and 26 C, in which the individual constituent molecules preferentially adopt fully extended conformations in hexagonally close-packed layers, with their long molecular axes perpendicular to the layer planes. NMR and IR evidence indicates that the constituent molecules rotate rapidly about their long molecular axes in this Sm-B phase, and methylene groups near the middle of the stearyl chains are somewhat more-ordered than those near the ends (35,37-38). This material forms an isotropic liquid phase above 26 C, and a progression of differently structured solid phases below 15°C. 

In our studies, we have used several related materials as reference solvents for comparison purposes. 4 -EthylbicyclohexyI-4-carbonitrile (CCH-2) forms enantiotropic smectic and nematic phases between 28-44°C and 44-48°C, respectively, and is an isotropic liquid above 48°C (40). 

Liquid crystalline polymers containing ferrocene in their side chains have been reported.235-240 Deschenaux used free-radical synthesis to prepare thermotropic liquid crystalline polymethacrylates containing ferrocene (Scheme 2.46).235 Polarized light microscopy showed that monomer 171 and its corresponding polymer 172 exhibited enantiotropic smectic A and C phases. 

The synthesis of a ferrocene-capped dendrimer with a stealth cyclotriphazene core was reported by Sengupta.320 Electrochemical studies showed that this dendrimer contained equivalent iron centers. A liquid crystalline ferrocene-containing six-arm star, 304, has been reported.344 This orange star showed an enantiotropic smectic A phase.349... 

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