Trends in the nematic–isotropic liquid transition temperatures for the homologous series of 4-n-alkoxy- and 4-n-alkyl-4′-cyanobiphenyls

Abstract

HUG 7RXThe lower homologues of the series of 4-n-alkoxy-and 4-n-alkyl-4'-cyanobiphenyls have been prepared and their liquid crystal transition temperatures determined. The trends in the nematic-isotropic liquid (N-I) transition temperatures along the series are compared with observed trends for other homologous series. These results reveal deficiencies in the theoretical treatments That have been proposed to explain trends in N-l transition temperatures in such series. However, the results may be explained by the existence, in the case of the higher homologues, of certain alkyl chain conformations which are especially favourable to the stability of the nematic phase.WE report the nematic-amorphous isotropic liquid (N-I) transition temperatures of the lower members of the homologous series of 4-n-alkyl-and 4-n-alkoxy-4'cyanobiphenyls. These compounds give unexpectedly high values for the N-I transition temperatures compared with those for the higher homo1ogues.l Before discussing the new results, we shall present views on the observed variation in N-I transition temperatures on ascending other homologous series of mesogens.Studies of many homologous series of mesogens have shown that the temperatures for mesophase-mesophase and mesophase-amorphous isotropic liquid transitions show regular trends as the series are ascended.,~~ For series of nematogens, a regular alternation of N-I transition temperatures occurs and when these are plotted against the number of carbons in the terminal alkyl chain, the points lie on two curves. When the alkyl group is directly attached to a ring, the upper curve is for the odd members and the lower curve for the even members. 1471ien the terminal group is an n-alkoxy-group, the reverse situation occurs because the oxygen is geometrically equivalent to a CH, group. A common behaviour for the two separate curves for a series is that both either rise or fall initially and then level out, the alternation becoming less pronounced as the series is ascended. Alternatively, the upper curve may sometimes fall and level out, while the lower curve rises slightly or stays almost level as the series is ascended.At one time, most series gave the types of curve which fall for both odd and even members, the initial slopes being greatest for series involving high N-I transition temperatures. Later,4 series were found for which both curves rise as the chain is lengthened. These series involve rather low N-I transition temperatures, and the quest €or mesogens of low m.p. has increased the number of examples of series giving this type of N-I c ~r v e . ~. 6Attempts have been made to explain the shapes of N-I curves in terms of the effects of increasing chain length on the anisotropy of molecular p~larisability.~ Assuming a rigid, extended zig-zag chain, the axial polarisability is increased about twice as much as the polarisability at

Liquid Crystal Polymers. V. Thermotropic Polyesters with Either Dyad or Triad Aromatic Ester Mesogenic Units and Flexible Polymethylene Spacers in the Main Chain

“…The phenomenon of the increase in T1 at n values of 9 and I 0 after the decrease up to an n value of 8 as n increased has been attributed in the case of low molecular weight compounds to a conformational change from the fully extended trans conformation of the polymethylene spacer to that having a central gauche unit in the longer spacers which, through slightly energentrically less favorable, would stabilize the mesophase order. 16 An alternate possibility for the unexpected trends in T m and T; for polymers with n = 8 and higher may be the formation of a smectic phase because of the presence of the long polymethylene spacers, but the existence of such a phase has not yet been observed in this work by other characterization methods.…”

“…For low molecular weight, liquid crystal organic compounds, it is well known that structural variations in the mesogenic units can give rise to substantial changes in the thermal behavior of the mesophases, 16 although the theoretical interpretation of such a dependence remains to be established.…”

“…The test for stir-opalescence of the polymer melts was performed by placing the polymer between two cover glasses and sliding the upper one over the 16 lower when the polymer was observed to be in a molten state on a Fisher-Johns melting point apparatus. The thermal behavior of the polymers were studied under a nitrogen atmosphere on a PerkinElmer DSC-IB at the heating and cooling rates of 20°C min -1 .…”

Nickel- and Palladium-Catalyzed Cross-Coupling Reactions at the Bridgehead of Bicyclo[1.1.1]pentane Derivatives - A Convenient Access to Liquid Crystalline Compounds Containing Bicyclo[1.1.1]pentane Moieties

“…[33] 7g, [34] 7i, [35] 4-bromobutyl-and propylbenzenes, [36] 4-bromo-4Ј-ethylbiphenyl (22f), [37] (4-bromophenyl)trimethylsilane, [38] 22g, [39] lithium 4,4Ј-di-tert-butylbiphenylide, [16a] and PdCl 2 (dppf) [22] were prepared according to published procedures. Vinyl iodide was obtained in 50% yield by treating vinylmagnesium bromide with I 2 in THF solution.…”

Synthesis and Properties of Thermotropic Liquid Crystal Polymers with Main Chain Mesogenic Units

“…However, another possible explanation for the abrupt change from nematic to smectic order along the series may be that a conformational change of the polyalkylene spacer, from the fully extended trans conformation to the one with a central gauche unit, can occur for either the odd numbered or longer spacers. 15 Such a conformational change would represent a higher energy state but it would stabilize the mesophase order.…”