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

Abstract

X-ray diffraction analyses reveal the dependence of ‘de Vries-like’ properties on the degree of nanosegregation in carbosilane-terminated smectic liquid crystals.

First Examples of de Vries‐like Smectic A to Smectic C Phase Transitions in Ionic Liquid Crystals

“…For PhPyr10,t he order parameter S 2 in the SmA phase is on the order of 0.35-0.45 and for PhPyr12 it is 0.4-0.5. These values of the orientational order parameter S 2 are in the typical range for de Vries-typeS mA phases [13,14,15] and are substantially smaller than valueso fa bout 0.7, which are usually found in normal, non-ionic thermotropic liquid crystals. [22] The translational order parameter S of the SmA phase is am easure for the quality of the translational periodicity of the smecticl ayers.…”

“…This indicates that their layer shrinkage is far less than that expected for conventionals mecticA to smecticC materials (R value = 1) and is at the borderline for what is observed in nonionic de Vries-type materials (R value = 0.2 to 0.3). [15,17] For these ILCs, low orientational andh igh translational order parameters were found, which are in good agreement with earlier results for non-ionic de Vries materials [13,14,15] as well as with the arguments of Lagerwall et al [12] for the origin of the de Vries behavior.…”

“…This de Vries mechanism was proven experimentally for several non-ionic de Vries-type materials. [13,14,15] Owing to their low ori-entational and high translational order,I LCs should thus be perfectc andidates for finding new de Vries-type materials.…”

Tuning the mesomorphic properties of phenoxy-terminated smectic liquid crystals: the effect of fluoro substitution

“…7,15 The design features of these materials include a trisiloxane or tricarbosilane end-group together with a SmA-promoting element such as a chloroterminated alkyl chain or a 5-phenylpyrimidine core. 17 In the process of developing a diverse library of 'de Vries-like' mesogens, we have sought to expand the scope of nanosegregating elements in a new design that includes phenoxy end-groups, 18 which should segregate from alkyl chains at the smectic layer interface by virtue of their strong non-covalent interactions, and perhaps enable the tuning of mesomorphic properties by appropriate aromatic substitution. 17 In the process of developing a diverse library of 'de Vries-like' mesogens, we have sought to expand the scope of nanosegregating elements in a new design that includes phenoxy end-groups, 18 which should segregate from alkyl chains at the smectic layer interface by virtue of their strong non-covalent interactions, and perhaps enable the tuning of mesomorphic properties by appropriate aromatic substitution.…”

Chiral smectic-Aand smectic-Cphases with de Vries characteristics

“…Some of the materials in Sm-A * called "de Vries smectics" are known to exhibit a large electroclinic effect, a large increase in the birefringence with the field, and a minimal layer shrinkage at the Sm-A * to Sm-C * transition as well as in the Sm-C * phase [4]. In recent years a large number of such mesogens have been synthesized [12,13], with siloxane or perfluorinated segments at the end of side chains (both of which are known to promote increased lamellar order), that exhibit low orientational order due to nanosegregation of the constituents.…”