Design of liquid crystals with ‘de Vries-like’ properties: the effect of carbosilane nanosegregation in 5-phenyl-1,3,4-thiadiazole mesogens

Abstract

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. A defining structural characteristic of most ‘de Vries-like’ materials is a nanosegregating element that strongly promotes the formation of smectic layers. We show herein the effect of varying the length of a nanosegregating carbosilane end-group on ‘de Vries-like’ properties in three series of 5-phenyl-1,3,4-thiadiazole mesogens with chloro-terminated alkoxy chains (QL13-n, QL18-n and QL19-n). Shortening the end-group from a tricarbosilane to a monocarbosilane causes a decrease in ‘de Vries-like’ character, as measured by the reduction factor R, which is consistent with a decrease in nanosegregation and quality of the lamellar ordering. Measurements of the orientational order parameter S₂ by 2D X-ray scattering of smectic monodomains show a concomitant increase in S₂ that is consistent with the hypothesis that ‘de Vries-like’ behavior in these materials arises from a combination of high lamellar order and low orientational order. The requirement of a nanosegregating structural element to achieve ‘de Vries-like’ behavior is confirmed by the characterization of the chloro-terminated mesogen 2-(6-chlorohexyloxy)-5-(4-hexyloxyphenyl)-1,3,4-thiadiazole (QL28-6/6), which undergoes a SmA–SmC transition with a maximum layer contraction of 4.1% and behaves like a conventional smectogen.