Mesogenic cyanobiphenyl

T. Ikeda, S. Kurihara, D. B. Karanjit, and S. Tazuke, Photochemically induced isothermal phase transition in polymer liquid crystals with mesogenic cyanobiphenyl side chain, Macromolecules 23, 3938-3943 (1990). 

Nierengarten et al. reported liquid crystalline pillar[5]- and pillar[6]ar-enes. They synthesized pillar[5]arene 6.32 and pillar[6]arene 6.33 with 10 and 12 mesogenic cyanobiphenyl units, respectively (Figure 6.20). Both compounds showed smectic A phase (focal-conic fan texture and homotropic areas) over a broader temperature range when compared with the model compound. This can be explained as the macrocyclic effect Inter-molecular 71-71 interactions between neighboring pillar[n]arene cores stabilized the smectic A phase. 

Scheme 3. Lowest energy conformations for the mesogen 5CB (4-n-pentyl-4 -cyanobiphenyl)...

Table 2. Crystal and molecular data of mesogenic 4 -n-alkoxy-4-cyanobiphenyls (CBOn)... 

The crystal structure of the mesogenic compound 4 -(4-hydroxy-1-n-butoxy)-4-cyanobiphenyl (CB0(CH2)40H) was described by Gehring et al. [59]. In... 

There are also some trends when looking at the main chain flexibility. Table II demonstrates that when the main chain flexibility decreases from cyanobiphenyl containing polymethacrylates to polysiloxanes, not only does the Tg drop, but the isotropization temperature increases. However, the trend is the opposite when the mesogen is methoxyphenyl benzoate (18). Therefore, this effect of the main chain flexibility is still ambiguous. 

For device purposes, molecules used should be stable to heat and light. The LC phases have to be stable over a wide temperature range around room-temperature. This can be achieved by using mixtures of mesogens. A mixture of cyanobiphenyls, one with alkyl substituent (CjH,) and the other with alkoxy substituent (CgHi70), has proved to be most useful. 

As the formation of LC phase in comb-like polymers is predetermined by the interaction of mesogenic groups, it would have seemed, that the temperature range of LC state for such systems should not depend on the length of the main chain, i.e. on the degree of polymerization (DP). However, studies 44,49) on the dependence of Tc] on DP carried out for some polyacrylic and polymethacrylic derivatives of cyanobiphenyl, as well as for polyparabiphenylacrylate 51 (Fig. 5), have shown that... 

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

Biphenyl mesogens with R=CN [(4 -cyanobiphenyl-4-yl-)oxy-] or R=OCH3 [(4 -methoxybiphenyl-4-yl)-oxy-] were attached to norborn-2-ene moieties, and separated by different spacer lengths and carboxylic acid ester, ether or imido-groups as anchoring moieties, as shown in Fig. 4. 

Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

Our group investigated the influence of the spacer length of 2,3-disubsti-tuted norbornene derivatives with cyanobiphenyl-mesogenic units [42] and with methoxybiphenyl-mesogenic units [43] on the mesophases. Exo,endo-his-[(4 -cyanobiphenyl-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylates with different alkyl spacer lengths (IV-n, n=2-12) were synthesized in a straightforward manner. 

In this chapter we want to discuss the correlation of the mesophase behavior of a cyanobiphenyl-based SCLCP with its backbone structure. As shown before, the backbone structure, the spacer lengths, and the mesogen density per repeat unit have great influence on the LC mesophase evolved. Ligure 8 shows some examples of backbone structures bearing the cyanobiphenyl-moiety that have been reported in literature. The above-mentioned ROMP-derived polymers poly-(II-n) [39],poly-(IV-n) [42,47],poly-(VI-n) [41],andpoly-(VII-n) [53] will be compared with each other and with acrylate-based [56-59], siloxane-based [60] and vinylcyclopropane-based systems [61]. The detected mesophases and their transition temperatures are summarized in Table 6. 

Fig. 8 Polymer backbone structures of SCLCPs based on cyanobiphenyl mesogens...

Table 6 Comparison of SCLCPs with different main chains based on the cyanobiphenyl-mesogen...

Fig. 33 Illustration of the two extremes, homogeneous and segregated, of the distribution of hydrogen-bonded side chains along the backbone in the case of a substoichiometric composition. System PAA(LC) investigated consists of poly(acrylic acid) and side chains containing cyanobiphenyl mesogens connected with an alkyl chain to imidazole-based hydrogen-bond acceptors (Scheme 10c). Reprinted with permission from [242]. 2006 American Chemical Society...

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