Novel Smectic Polymorphic Behaviour in Homologous Series of Mesogens

The high alternation of nematic thermal stabilities found in the series of o-phenylalkyl4-(4'-phenylbenzylideneamino) cinnamates as the homologous series was ascended has been further exaggerated by substituting the terminal phenyl ester ring with methyl or chloro substituents in the 3"or 4"-positions. Nematic phases were found only when an even number (n) of methylene units occurred in the alkylene chain. Srnectic thermal stabilities also alternated as each of the four series of esters was ascended. However, the srnectic A thermal stabilities decreased and the smectic B thermal stabilities increased on passing from even to odd values of n. In some cases this caused the thermal stabilities of the smectic A and B phases to coincide for odd values of n. When this occurred, a unique texture was observed microscopically at the I-S transition. Although no SA-SB transition could be detected microscopically or by D. T. A., miscibility studies indicated that a transitional SA phase still existed. This suggests that it is necessary to form a transitional SA phase intermediate between the amorphous liquid and a SB phase, both on heating and cooling. This transition has been called SAB-I. Smectic E phases were also found for many of the sixteen new esters, and the thermal stabilities of SE phases, like those of SB phases, increased for odd values of n. The relative effects of 3"and 4"-methyland chloro-substituents on N, SA, Sn and SR thermal stabilities are discussed. Introduction. Several series of phenyl and w-phemates (I) were studied by Gray and Harrison [ I , 21 and nylalkyl 4-(4'-substituted benzylideneamino) cinnavery large alternations in nematic (N) X//CH=N/-\ CH = CH-CO . O(CH2).-\_/ /r\ \d"\Jthermal stabilities were found as the value of n, the high alternations in N thermal stability, compared, for number of methylene units in the alkylene chain, example, with the alternations in N-I transition tempechanged from even to odd (from 0 t o 4). These very ratures normally found for homologous series of Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975160 Cl-366 D. COATES AND G. W. GRAY n-alkyl esters, sometimes led to alternations in phase type as the series were ascended, i. e., from smectic + nematic to smectic and back to smectic + nematic on passing from one member to another. The high alternation in N thermal stability in these series was attributed to the fact that as the series were ascended, the spatial disposition of a bulky terminal ester phenyl ring was being altered (Fig. 1) relative to the rest of the FIG. 1. -Diagrammatic representation of the structures of substituted and unsubstituted phenyl and benzyl csters of 4-(4'-phenylbenzylideneamino) cinnamic acid. The upper structure applies to higher homologues with even values of n (see structure I in text) and the lower structure to higher homologues with odd values of n. The axis lines represent the major axis of the entire elongated molecule (even values of n) or of the major elongated part of the molecule (odd values of n). molecule, and not merely that of a small terminal methyl group as in the case of an homologous series of n-alkyl compounds, i. e., the molecules alternate between being lath-shaped and L-shaped. Particularly when the substituent X in structure (1) was a phenyl group, smectic polymorphism occurred. The thermal stabilities of both smectic A (S,) and smectic B (S,) phases also alternated, but to a much smaller extent ; the alternations of the transition temperatures SA-N or I and S,-S, were in fact of opposite senses. These smaller alternations were also accounted for in terms of molecular shape. Smectic E (S,) phases were commonly exhibited on cooling the S, phases. In the present work. we have again used a phenyl group as the substituent X, but by substituting the ester ring in the 3"or 4-position (Fig. 1) with either a methyl or a chloro substituent, the degree to which the substituted ring lies off-axis for odd values of n may be enhanced, i. e., the molecules should deviate more from being lath-like in shape. If the explanations of Gray and Harrison are correct, even more dramatic alternations in N thermal stabilities should therefore be observed 131, but the S thermal stabilities should be affected much less. It is noted that a 3"-substituent in the ester ring in fact may adopt two extreme spatial positions when the ring is coplanar with the rest of the molecule, because of possible rotation about the CH,-Ph (or 0-Ph) bond. It was considered that the present studies might give information about the significance of such rotational motions in liquid crystals, and that a comparison of the properties of chloro and methyl substituted isomers might be instructive, as these two substituents are of similar sizes, but quite different dipolarities. Results and discussion. Details relating to the mesomorphic properties of the sixteen new 3"and 4-methyland chloro-phenyl and -w-phenylalkyl 4-(4'-phenylbenzylideneamino) cinnamates (see Fig. 1) are given in Table I and depicted graphically in