Hydrogen bonded mesogen

However, the hydrogen-bonded mesogens that are of most interest in the context of this article are those elaborated initially by Kato and Frechet in the early 1990s [24-33]. In this approach, a pyridine, which may or may not have liquid crystal properties, was hydrogen bonded with a 4-substituted benzoic acid to form a new species with its own, distinct mesomorphism. For example, complex 9 shows a SmA phase that persists to 238 °C (n = 2, m = 4), while its free component pyridine is nematic to 213 °C the component benzoic acid is also nematic (as the H-bonded dimer) to 147 °C (although note that the notional monomer would not be liquid crystalline). 

Scheme 13 Structure of PS- -PMAA and hydrogen bonding mesogen [171]...

An example of side-chain SLCPs whose mesogen consists of part of the main-chain as well as the side chain (Fig. 3h) has been reported by Kato et al. [107]. In this case the hydrogen bond mesogenic motif utilized was a benzoic acid (as part of the small molecule side chain 42) with a 2,6-diamino-... 

Another representative system is the induction of mesomorphism by the self-assembly of nonmesogenic H-bonding components. Complexation of 4-methoxybenzoic acid (2, n = ) and 4,4 -bipyridine (4) results in the formation of novel hydrogen-bonded mesogen 5 [ 2 (n = l)/4] (Fig. 4). A nematic phase is observed for 5 between 153 and 163 °C [31, 32]. 

The mesomorphic properties of hydrogen-bonded mesogens have been compared [40] with those of covalently bonded compounds 16 and 17 [42] (Fig. 6 and Table 2). The melting and isotropization temperatures of the complexes are lower than those of ester compounds, which leads to wider and lower mesomorphic temperature ranges [40]. Moreover, smectic mesophases are observed for fluorinated H-bonded complex 15. 

The question as to whether hydrogen-bonded complexes are miscible with normal covalently bonded liquid-crystalline compounds arises for their application. Binary phase diagrams of a hydrogen-bonded mesogenic complex with normal liquid crystals have been prepared to examine the miscibility of two different classes of liquid crystals [67], Compound 16 or 4 -heptyl-4-cyanobi-phenyl was mixed with complex 14, These are miscible over the whole range of composition, A smectic C phase is induced for a mixture of 16 and the cyanobiphenyl. 

The formation of covalent cross-linking in liquid-crystalline polymers is useful to lock in the molecular order in solid states, which leads to static functionality [114, 115]. For hydrogen-bonded mesogenic networks, the use of the dynamic nature of H-bonding induces dynamic properties [108, 109]. It is important to choose an appropriate method of cross-linking for the purpose of the use of network materials. 

The formation of hetero-intermolecular hydrogen bonds has also been applied for the preparation of discotic complexes. Equimolar amounts of triazine and carboxylic acid were complexed to form doubly hydrogen bonded mesogen 47 [67], which display disordered hexagonal columnar mesophase. The transitional temperature of the resulting complex is lower than that of the triazine component itself. 

The supramolecular self-assembly between PS-Zi-poly(acrylic acid) (PAA) and imidazole-terminated hydrogen-bonding mesogenic groups was also used to prepare non-azo LCBCs (Chao et al., 2004). Owing to the attached LC properties, the nanostructures in the LCBC films obtained can be oriented by using an alternating current (AC) electric field, in a direction parallel to the electrodes. 

Figure 3.17 Schematic representation of an interfacial portion of an acid-dimer microdomain within which a polymer hydrogen-bonded mesogen is incorporated.

However, the hydrogen-bonded mesogens that are of most interest in the context of this chapter are those elaborated initially by Kato and Frechet in the late 1980s. In this approach, a pyridine, which may or may not have... 

Figure 29 Chiral hydrogen-bonded mesogen, which shows ferroelectric switching in its SmC phase. [5].

Figure 35 Hydrogen-bonded mesogens formed from isonitrile complexes of noble metals.

Figure 40 Hydrogen-bonded mesogens form between biphenyl-carboxylic acid and 4-cyano or 4-nitro-stilbazole.

The transition temperatures of the hydrogen-bonded mesogens 10 and 11, consisting of two rings, are compared with those of the structurally related phenyl benzoates... 

Variable temperature infrared spectra for hydrogen-bonded liquid-crystalline complexes have shown that the structure of the hydrogen-bonded mesogen is stabilized by molecular ordering and packing in the liquid-crystalline state [16,18,27]. The stability of the hydrogen bond suddenly decreases once the complex becomes isotropic disordered [16, 18, 27]. 

Triply hydrogen-bonded mesogenic complexes can be obtained through the molecular recognition of complementary compo-... 

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