Monotropic materials

Class y, star PLCs. Four-member stars—that is crosses—were synthesized for the first time by Krone, Reck and Ringsdorf. In distinction to polyesters in Class jS, which are monotropic, polymers in Class y are enantiotropic. Monotropic materials can only go from unstable to stable modifications, while enantiotropic undergo reversible transitions between different stable modifications. Other crosses were... 

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). 

Polymers formed between a and c, d and e all failed to show any liquid-crystalline behaviour. However, for all a examined (m = 2,4,6 and 8), nematic phases were observed with b-4 (all monotropic) - a further monotropic nematic material was the copolymer of a-6 and b-3. Unidentified crystal smectic mesophases were reported for a further three examples. 

Metaboric acid is known in three crystalline modifications, which provide a good example of monotropism (Fig. 24.12) and of the increase in density with change from 3- to 4-coordination of B (Table 24.4). The monoclinic form is readily prepared by the dehydration of H3BO3 in an open vessel at 140°C quenching of the molten material gives a glass which later recrystallizes as the orthorhombic form. If the melt is held at 175°C the most stable (cubic) form is slowly precipitated, while complete dehydration at about 230°C yields B2O3. 

The monomer exhibits complex phase behaviour, in particular, a monotropic liquid crystalline phase, which is only apparent on cooling, such materials can only be properly characterized by using differential scanning calorimetry (DSC) in conjuction with optical microscopy and also by X-ray scattering (see Chapter 1). 

The free ligands (where R = Me or Et and R = hexyloxy or undecyloxy) display monotropic nematic phases, however, upon forming the dinuclear palladium complexes the nematic phases are, perhaps surprisingly, enantiotropic. The melting points are, as expected, raised upon complexation but the increase in the clearing point is somewhat unusual as the structural anisotropy would be expected to be lower in the complexes compared to the free ligands. Indeed, it is somewhat remarkable that these complexes are mesomorphic certainly they represent a structure which would not readily be observed in purely organic materials. 

Simon and co-workers have also studied phthalocyanines with crown-ether moieties attached to the periphery, giving the materials shown in Figure 71 [135]. These compounds were reported as having a monotropic mesophase at 150°C (although melting points were not given, nor was it all clear whether 150°C represented a transition temperature) and X-ray scattering studies, showed the mesophase to be based on a two-dimensional square lattice (lattice parameter... 

In terms of stability, determination of whether the forms are monotropic or enantiotropic is of considerable benefit, as such knowledge will theoretically aid the formulator in determining the likely stability of the material at any stated temperature. In practice, the differentiation between these forms may be a nontrivial task due to the existence of kinetically stable metastable polymorphs. A number of authors (e.g., 4,16-18) have used the guidelines developed by Burger (19), which are summarized in Table 3.1. 

In most cases, the observed mesophases are found on both heating and cooling a material, so sequence 1 in Scheme 2 is fully reversible such mesophases are termed enantiotropic. However, in some cases a particular mesophase may only appear on cooling a material and is therefore metastable (sequence 2, Scheme 2) such phases are termed monotropic. ... 

These results were confirmed by Thompson et al., who investigated the thermal and mesomorphic behavior of Fc materials and those of analogous compounds in which Fc was replaced by a benzene or a cyclohexane ring (see, for example, 51-53).They observed that Fc destabilizes the liquid-crystalline state. Indeed, whereas 51 and 52 show enantiotropic SmC and SmA or SmC and N phases, respectively, 53 displays only a monotropic N phase. This behavior indicates that Fc plays the role of a bulky unit, again as a consequence of its three-dimensional structure. 

Related materials 70 were prepared by Han et al All of the ligands showed monotropic phases, except for 70e, where the phases were enantiotropic. All showed a N phase with a SmC phase seen for 70b and 70e. On complexation, mesomorphism was suppressed in 70d and 70e, while 70a and 70c showed a N phase (Cr 101.5 N 142.4 I and Cr 114.6N 130.2 I) and 70b showed a SmA phase (Cr 118.4 SmA 128.5 I). Clearing points for the mesomorphic complexes were rather similar (between 128 and 142 °C), whereas for the free ligands the difference was greater (between 53 and 185 °C). 

The behavior is typical of relatively simple dipolar materials where a N phase was found at short chain lengths, giving way to a SmA phase in higher homologs. While the iridium complexes could be cycled in and out of the isotropic phase with few problems of decomposition, the rhodium complexes began to decompose rapidly on melting in air. Lateral fluorination of air-iridium stilbazole complexes strongly destabilized the mesophase the 2-fluorinated 4-alkoxystilbazole systems exhibited only monotropic N and SmA phases, whereas the structural isomeric 3-fluori-nated compounds were not mesomorphic. " ... 

For X-Y = 8-hydroxyquinolato ( = 0) and X-Y= 1,10-phenanthroline ( =1), the complexes were non-meso-morphic. However, when X-Y = acac (n = 0), a material with a monotropic SmA phase resulted (Cr 83 (SmA 68) I) and when X-Y = 2,2 -bipyridine (n= [BF4] counterion), a material with an enantiotropic nematic phase was produced (Cr 146 N 158 I) related complexes with PF6 or SbF6 anions were non-mesomorphic. These are further, rare examples of ionic materials showing a thermotropic nematic phase. 

Thus, only the acetylacetonate (acac), 17, and 2,2 -bipyridine, 18, derivatives were mesomorphic, showing a monotropic Sa phase, and an enantiotropic nematic phase respectively, and at much lower temperature than the mesophases of the dinuclear parent complex (Crys 177 SA 219T) [27]. Complex 18 is a further rare example of ionic materials showing a nematic phase. Interestingly, related complexes with PFs or SbFs anions were non-mesom-orphic. 

Deschenaux and co-workers have recently reported the first 1,1, 3-trisubsti-tuted ferrocene-containing thermotropic liquid crystals [123]. The first series of materials (59 x=l), showed monotropic Sa and Sc phases (n=12, 14), and monotropic Sc phases (n=16, 18), all the complexes melting in the isotropic... 

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