Studies on some smectogenic compounds with large bilayer spacings

Abstract

In mesomorphic compounds with strongly polar end groups, near neighbor molecules have antiparallel correlations. This results in a bilayer structure which in turn leads to several interesting phenomena like reentrance and smectic polymorphism. After a brief review of the development of this subject, we report some unusual properties in several compounds with relatively large bilayer spacings (≥ 1.6 times the molecular length). For example, 4-n-decylphenyl-3'-methyl-4'-(4"-nitrobenzoyloxy)benzoate (10 PMNBB) shows a remarkably large expansion (~20%) of the bilayer spacing on cooling in the S_A phase, the rate of expansion increasing with decrease of temperature. Further, the dielectric anisotropy reverses sign, becoming negative at lower temperatures. When the nitro group is replaced by a cyano group (10 PMCBB), (a) the rate of expansion of the bilayer spacing decreases with decrease of temperature, and (b) an S_A - S_A transition takes place with a jump of ~0.4 Å in the layer spacing at T_AN - T ≈ 15°. We present results on several homologues of both the compounds as also on compounds with a lateral methoxy substituent.