Enantiotropic, liquid crystals

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

Figure 3. Schematic free energy diagram for an enantiotropic liquid crystal, where the mesophase is metastable (see text).

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

The liquid crystal properties of the complexes were characterised using polarised optical microscopy and showed a nematic phase for n = 4 and 6 and a SmA phase for n = 6, 8, 10 and 12. The mesophases were monotropic for n = 4 and 6 and enantiotropic for the others the progression from a nematic phase for shorter chain lengths to SmA at longer chain lengths is quite typical for simple, polar mesogens. 

Preliminary investigations of the liquid crystal phase behavior of these gold nanoparticles initially revealed an enantiotropic nematic phase (based on polarized light optical microscopy and thermal analysis) as well as some pattern formation of the gold nanoparticles in TEM experiments [540, 541],... 

A similar approach with 4-nitrophenol also generated mesomorphic systems. This time, the mesophases were slightly more stable and some phases appeared to be enantiotropic (i.e., they became more stable than their crystal phase) (69). However, the beauty of the hydrogen-bonding approach to liquid crystals is that it is not always necessary to synthesize complete moleciiles. Thus, it was realized that if the 4-substituted phenols lowered the anisotropy as indicated in Fig. 53, the anisotropy ought to recover to give much more stable systems if the 3-substituted systems were used (Fig. 54) (69). That this approach was viable was... 

A thermotropic liquid crystal (mcsogen) is a compound that, on heating the crystal or on cooling the isotropic liquid, gives rise to mesomorphism. Liquid crystallinity occurs between the crystal and isotropic liquid states. The intermediate phases, or mesophases, can be either enantiotropic, i.e., thermodynamically stable, or monotropic, i.e., thermodynamically unstable. The solid to mesophase transition is referred to as the melting point, while the mesophase to isotropic liquid transition is referred to as the clearing point. 

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. 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

Ferrocene derivative 16 showed an enantiotropic nematic phase between 204 °C and 235 °C. This result indicated that a biphenyl system, when associated with the ferrocenyl moiety substituted in the 1,3-positions, was also propitious to liquid crystal formation (compare 16 with its l,T-isomeric analogue 7 (n = 6)). 

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... 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

While it was assumed above that only G. is affected by thermal history, in the case of main chain polymeric liquid crystals pronounced time dependent variability in G c has recently also been observed (7,8). It was shown that the lack of equilibrium perfection in the nematic phase can lead to substantial depression of the isotropization temperature T c =T. Thus non-equilibrium mesomorphic states can also, in principle, affect the phase sequence-(enantiotropic, monotropic) in the case of polymeric liquid crystals. 

Equilibrium Relations in the Case of Liquid Crystals.— Whether we are dealing here with substances in two strictly crystalline and enantiotropic forms (which we may call the solid and liquid crystalline forms), possessing a definite transition point, or whether we... 

In order to study the general applicability of the above principle for the transformation of the monotropic liquid crystals to the enantiotropic ones by increasing the molecular weight of the monotropic liquid crystal polymers, the polymer, (3.16), reported by Ober et al. (1983), was restudied (Duan et al. 1987). 

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