Antiparallel correlations

Pig. 53. Diagram to show the proposed antiparallel correlations that stabilize the mesophase in 4-cyanophenol complexes. 

Fig. 2.6.3. Preferred orientation of neighbouring dipoles in the end-on and broadside-on positions. However, because of the anisotropic shape of the molecules, situation I is much more important than II in the nematic structure and there results a net antiparallel correlation between neighbouring dipoles.

The first important question that needs to be considered is whether such an antiparallel correlation leads to the right sign and magnitude of the dielectric anisotropy. Proceeding in the usual manner, the principal dielectric constants turn out to be... 

Mesogens with conventional structures of a core with two terminal chains tend to exhibit smectic phases rather than the nematic phase this is especially true if, as described above, the core unit is narrow, linear, and composed of similar, compatible units. However, the change of core results in more dramatic changes than for the terminal cyano-substituted materials, possibly because of the lack of any molecular pairing (antiparallel correlations) in cyano-substituted systems. 

Here, f, Fx and are local field factors, a is a molecular parameter including the dipole moment, second order and third order hyperpolarizabilities. Under condition F = Fx = the formula does not agree with the experiment for 8CB. Indeed, it follows that rji should increase at the transition from the nematic to smectic A phase according to the increase in (F2) However, in the experiment, Fn markedly decreases. The similar temperature behavior was earlier observed for dielectric permittivity of 8CB. In the latter case the decrease in is due to the antiparallel correlation of molecular dipoles in the smectic A phase, which results in a decrease in the effective dipole moment /x Thus, the decrease... 

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

In these materials, the phase sequence is accounted for by a change in the nature and/or degree of antiparallel correlations. This leads, in effect, to the mesogenic unit changing to a different supramolecular species with a different anisotropy and showing a different mesophase, the effect being driven by temperature. Thus, the supramolecular nature of some systems is dynamic. This dynamic nature is further evidenced by the phenomenon of cybotacticity, in which localized clusters that may be, for example, SmA or SmC in nature, are found in nematic phases. Such behavior is now the subject of intense speculation as proposals... 

Figure 33 Postulated antiparallel correlations in hydrogen-bonded complexes of 4-cyanophenol.

A second effect of the same sample shown in Fig. 3 is also interesting. By cooling the isotropic liquid deviations of the static dielectric constant q from the straight line by approaching the is/N transition were measured. Such behavior was at first detected by Bradshaw and Raynes [24], confirmed by Thoen and Menu [25] and explained as an antiparallel correlation of the strong dipoles in the nematic-like clusters formed in the isotropic phase. For less polar molecules the driving force for the antiparallel orientation can also be produced by a combination of the steric and dipolar forces as shown by Kresse and Kremer [26] and demonstrated in Fig. 3. 

Esters are very common liquid crystal compounds, and lateral fluoro substitution has provided some interesting modifications to melting points, transition temperatures, mesophase morpholo and physical properties. Compounds such as 66 [26] allow for the gen ation of high positive dielectric anisotropy, but the fluoro substituent has caused a large reduction in the nematic phase stability when compared with compound 20, due to the reduction in antiparallel correlations. However, the fluoro substituent in compound 67 [26] is somewhat shielded by the zig-zag nature of the ester linkage hence the nematic-isotropic transition temperature is identical to that of compound 20. The lateral fluoro substituent in compound 68 [51] is not as shielded by the zig-zag structure and the nematic-isotropic transition temperature is much reduced, but not to the same extent as for compound 66. 

---