Mesophase stabilization, terminal groups

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

Structurally similar polymers with azo links, Polymers 9 and 10, formed no meso-phases when the rigid aromatic unit contained terminal carbonyl groups, but these formed nematic mesophases when the ester link was reversed The transition temperatures of the azo and azoxy polymers with the same spacer were quite close to each other. Substitution of methyl groups on the phenylene rings of these polymers reduced melting temperatures without adversely affecting nematic mesophase stability... 

When fluorine groups were introduced in the 2- and 3-positions of the terminal aromatic ring of the biphenyl unit ((48) M = Cu R = OC8Hn w=l) no columnar phase was observed, but a monotropic nematic phase instead, probably due to the reduction of the molecular symmetry When the lateral chain was elongated, the columnar phase disappeared at the expense of an enantiotropic nematic phase ((48) M = Cu R = OC H2 +i, n = 6, 8, 10-12, 14 and m = 2 n=l2, m = 3 and n = 4, 8, m = 4), which became monotropic ((48) M = Cu, R = OC H2 +i, n=l2,m = 4 n = 8, m = 8 and n=12, w = 12,). Furthermore, re-entrant (Section 7.9.3.2.5) nematic phases, between two crystalline phases, were also observed for the compounds in the series m = 2. Both the temperatures of the crystal-to-mesophase (163-219 °C) and mesophase-to-isotropic liquid (175-224 °C) decreased very rapidly with increasing m and n. The stability of the nematic mesophase was also found to depend on the bulkiness of the lateral chain. For instance, using a branched side group such as HMeEt did not suppress the monotropic phase but reduced the transition temperatures considerably. The cross-over between the Coh and the nematic phase would then correspond to a decrease of the lateral interactions between molecules due to the steric hindrance of the chains. 

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