Correlation cyanobiphenyls

Correlations between molecular structure and the LC state have been fruitfully employed to design 4-alkyl and 4-alkoxy-4 -cyanobiphenyls, which form the basis for electrooptic display devices. The reasoning behind the design of these nematogens is as follows. In a family of nematogens of the general formula... 

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. 

Some examples of typical calamitic mesogens are given in Figure 4. Cyanobiphenyls are included in this list due to their special position in the development of LCDs, but it should be noted that in many ways they are not typical as their liquid-crystal mesophases arise from antiparallel molecular correlations. 

A typical measnrement with the nematic liquid crystal 4 — n — octyl — 4— cyanobiphenyl (8CB) is presented in Fig. 4.3. It is obvious that the thickness of the snrface induced ordered region and the amount of order increase close to the phase transition. This increase is due to the growing nematic correlation... 

The expressions for the flexoelectric coefficients presented in this section are derived using the molecular-field approximation. Therefore, care should be taken in the description of nematic liquid crystals composed of strongly polar molecules. In such liquid crystal materials (for example, cyanobiphenyls) the flexoelectric coefficients may be strongly affected by the short-range dipole-dipole correlations, which are considered in the following section. 

It should be noted that dipole-dipole correlations may contribute to the flexoelectric coefficients only when mesogenic molecules have both a longitudinal and a sufficiently large transverse dipole. This may explain why the correlation contribution seems to be very important for oxycyanobiphenyls (and not for cyanobiphenyls, which do not possess any transverse dipoles). For weakly polar molecules d dx 1 D, and in this case the contribution from the dipole-dipole correlations is two orders of magnitude smaller than for 80CB and can be neglected. 

Let us take first the relatively simple example of 4-pentyloxy-4 -cyanobiphenyl (50CB). It has a strong dipole parallel to the long axis of the molecule, associated with the donor alkoxy group and the acceptor nitrile. Therefore, the individual molecules will be polarized and in the bulk phase they will be polarizable. The material will be prone to nanosegregation as the aromatic units will associate more strongly through n-n interactions than the s-s interactions of the chains. Parallel correlations of the molecules will be preferred, but to minimize the dipolar interactions, the molecules will be required to pack in a head-to-tail... 

Figure 4 Cyanobiphenyl liquid crystals showing the characteristic antiparallel correlations present in the mesophase. Disrnption of these interactions suppresses the liquid crystal phase.

Medium resolution measurements have also demonstrated the onset of stochastic motion of a flexible molecule in the crystal phase below the nematic. In TCDCBPh [di-(4-n-butyloxyphenyl) tranj-cyclohexane-1,4-dicarboxylate] the cyclohexane ring starts large angle internal deformations of the molecule 30 °C below the melting point [79]. Closer to the melting point, a more mobile intermediate crystal phase was discovered [80]. Studies of mixtures have shown that the correlation times of a cyanobiphe-nyl mixed with an azoxyanisole are not additive and this is attributed to the breaking up of the antiparaUel correlations in the cyanobiphenyl [81]. 

De Jeu et al. [70] have discussed the influence of molecular geometry on the elastic constant ratio. From simple geometrical considerations they stated that the splay/ bend ratio should correlate with the molecular length/width ratio for rigid molecules, i.e. Kj, Kii=L W with Kj, >Kn- (Pre-transitional effects of smectic-like ordering are discussed below.) This view has been confirmed by some experimental data [66]. If long, flexible alkyl chains are incorporated in the molecular model structure, the trend is reversed ( r33< Tii). In practice, both cases are observed. For example, the 4 -n-alkyl-4-cyanobiphenyls (nCB) are characterized by the bend/splay... 

Figure 3-18. Nematic/isotropic phase correlated 2D NMR spectra of 4-(n-pentyloxy)-4 -cyanobiphenyl (50CB), together with cross sections. Temperature were jumped from (top) 67.1, (middle) 65.1, and (bottom) 63.1 with microwave pulses of duration 5, 20, and 25 ms, respectively. Reprinted from reference 52, with permission from the Journal of Physical Chemistry...

In Table 7.2 phase transition data are presented for three series of mesogen-spacer combinations attached to different backbones, showing the generality of the Tg and AT correlations for nematic methoxyben-zoate combs, smectic cyanobiphenyl and nematic parallel side chain systems. 

Finally, Haase et al. have demonstrated a correlation between DP and dielectric phenomena. Table 7.5 contains data obtained using cyanobiphenyl acrylic combs for the energy of activation (Fa) of dielectric relaxation at 365 K from a state of alignment parallel with an electric field. An inverse relationship between Fa nd DP is indicated the transition temperatures tend to increase with DP as noted previously. 

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