Shielded lateral substituents

A particularly striking example of shielded lateral substituents occms in the naphthoic acids (compounds 84-87). Even large lateral substituents cause an increase in clearing point because of the efficient filling of space which enhances intermolecular attractions. The smectic tendency of the naphthoic acids (85-87) with the lateral substituents is very high. 

Table 35. Shielded lateral substituents (courtesy of Coates [37]).

Lateral substitution has also been used in alicycUc systems where the lateral substituent is located at the axial position of the terminal chain location. Such a position is quite shielded by the ring stmcture and so viseosity is minimised and liquid ciystal phases are... 

The 12-vertex carborane unit provides an effective shielding for lateral substitution of the core with fluorine atoms (Z = F in Figure 14.1a). Typically, a lateral substituent decreases the aspect ratio of the mesogenic molecule, which reduces the stability of the mesophase. It has been demonstrated that the NI in such derivatives approximately linearly depends on the size of the lateral substituent. Results demonstrated that lateral fluorination in series 33,34, and 58 (X=F) decreases the NI as compared to the respective unsubstituted compounds 30,29, and 58 (X=H), and the effect of phase... 

There are compounds in which the lateral substituents are shielded, so that they are less effective in broadening the molecule. Table 35 shows some impressive examples of this kind. There are also compounds with axial CN groups in cyclohexane [162, 163] or dioxane [164] rings, in which partly the shielding effect and partly the increase in density due to the polar group may be responsible for the unexpectedly high clearing temperature. 

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. 

Although early attempts to correlate P shifts with various Taft or Hammett substituent constants met with failure, such empirical correlations were later more successful when structural variation in the class of compounds was limited. Thus the P chemical shifts of substituted phenyl-phosphonic dichloiides correlate reasonably well with either Hammett a or Taft O values (Grabiak etal., 980). Grabiak et al. also observed an upfield shift (—1.0 ppm) for the o-tolylphosphonic dichloride relative to the meta and para isomers. An even larger shielding (— 5.2 ppm) was observed for the... 

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