Terminal or end-on substitution

Terminal substitution is another factor having significant impact on the properties of liquid crystals. Taking the following homologues of hydrox-ybenzoate, (3.6) and (3.7), as examples, these two compounds have the 

The comparison of the following two compounds, (3.8) and (3.9), is even more remarkable. In this case, only a small end methoxy substitution changes the nature of the compound from non-liquid crystalline to liquid crystalline. (3.8) melts at 162 °C and no liquid crystal phase will be formed. On the other hand, (3.9) melts at a somewhat lower temperature (157 °C) forming a nematic phase with the clearing point of 165 °C. 

We see in these examples that great effect can be created by substitutions on the two ends of the mesogenic cores. The consequence includes not only the formability of liquid crystal phases and transition temperatures, but also the types of the liquid crystalline phase. It is well known that substitution on the end with longer linear alkyls and alkoxyls favor the formation of smectic phases. One example for this remark has been given in Table 3.3. In the design of liquid crystal polymers, especially in the cases of side-group type and molecular shish-kebabs, the concept of end-substitution has been fully utilized. 

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