Terminal aliphatic chains

For systems that have two terminal aliphatic chains that can be independently varied in length, tilted phases tend to occur when the two lengths are similar to one another. At short chain lengths, usually orthogonal phases such as smectic B, crystal B, and crystal E occur. In some cases the upper temperature transitions to the smectic A and smectic B phases can occur so close together that a transition from the nematic phase or the liquid phase to the smectic B phase appears to occur only through a transient smectic A phase. This transition is sometimes called a liquid or nematic to SmAB transition [10] (Fig. 6). 

As the chain lengths of both peripheral aliphatic groups are increased, tilted smec- 

For systems where there is only one terminal alipahtic chain, typically no tilted smectic phases are observed, and orthogonal phases predominate. The smectic A phase is the commonest modification ob- 

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Fig. 41 Variation of the size of the elementary third and fourth generations dendrimer cylinder with bulky mesogenic groups. L3, Lg and Lg are for the bulky groups containing 3, 6 and 9 terminal aliphatic chains, respectively, h is the height of the cylinder, the diameter, and

TGBG phases with large temperature ranges were also obtained in a remarkable series of materials first prepared by Takatoh [54] and later added to by Lamb [55]. The materials synthesized were derivatives of terphenyl. Using Sharpless chiral epoxidation of hexen-2-ol they made liquid crystals with terminal oxirane moieties which introduced two sequential chiral centers into the terminal aliphatic chain, see general structure 11. 

On account of the square planar coordination around the metal ion, metal complexes with 1,3-(substituted-phenyl)- S-diketonate (52) possess a more disk-like shape than the diketonato complexes described previously, and appeared therefore more promising candidates for the formation of columnar mesophases. Moreover, the versatility in the synthesis of the ligand has allowed the preparation of a wide variety of chemical structures for which the number of terminal aliphatic chains, as well as the position of substitution of these chains on the benzene rings, can easily be controlled. This has led to intensive activity in this area with relevant examples presented below. 

Figure 12 Two dimesogens that have the same length of linking units, but having different terminal aliphatic chains. ...

The liquid crystal properties of the stearoyl, palmitoyl, oleoyl and nervonoyl galactocerebrosides derived from bovine brain are shown in Fig. 17. These four compounds were found to exhibit hexagonal columnar phases over very wide temperature ranges. Interestingly, the stearoyl and oleoyl members of the series have almost identical clearing points demonstrating that unsaturation in at least one of the terminal aliphatic chains does not markedly affect the self-organizing properties of... 

V7 n i where TV,. is the nematic->isotropic transition temperature. The striking feature is the clear alternation (odd-even effect) of the birefringence. The alternation in An is obviously the result of the alternating change in the molecular polarizability anisotropy caused by the alternation in the C-C bond angle in the terminal aliphatic chains. However, the alternation in the orientation order probably also plays a role [27]. 

Figure 10. Effect of branching in the terminal aliphatic chains.

Lateral substituents may be attached to the core system or located in the terminal aliphatic chain. By and large, lateral substituents positioned on the core or in the terminal chain(s) disfavour smectic mesophase formation. Smectic mesophase formation is principally depressed more by the steric bulk of the lateral group than by its polarity. Thus for example, fluoro substituents at the core or in the chain(s) are less effective at depressing smectic mesophase formation than is a methyl substituent [14, 31], nevertheless fluoro substitution still lowers transition temperatures of the phase transition to the smectic state in comparison to the unsubstituted analogue. 

Figure 25. Effect of a methyl branch in the terminal aliphatic chain on mesophase formation.

Thus lateral substitution in both the core region or in the terminal aliphatic chain can be used to control melting points, clearing points, and to some degree mesophase type formed. 

Both microscopic and steric models of the SmC phase postulate a zigzag shape for the terminal aliphatic chains of essentially symmetric compounds for SmC formation [20, 31]. The microscopic model assumes additionally that antiparallel permanent dipoles at an angle to the molecular axis promote SmC behavior due to an induced torque [20]. It has been shown that while such dipole moments are not always absolutely essential for SmC formation, they are to be found in most SmC components and still mostly lead to higher SmC transition temperatures than those of the corresponding compounds without these dipoles [32, 33]. This is not valid for compounds incorporating isolated, nonconjugated dipoles next to aliphatic rings [34]. There are also indications that alternately a cis/trans, linearly-extended conformation of the terminal chains is preferred in the SmC phase [34],... 

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