Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit.

Govindaswamy Shanker,Carsten Tschierske,Marko Prehm

doi:10.3762/bjoc.8.54

Abstract

Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic phases (NcybA and NcybC), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and cybotactic nematic phases (SmC, NcybC), which are replaced by the nontilted CybA phases and nematic phases composed of SmA-type clusters (NcybA). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving biaxial nematic phases of the orthorhombic type. The orthogonal (NcybA) and skewed (NcybC) cybotactic nematic phases were distinguished by XRD and optical observations.

Highlights

Liquid-crystalline (LC) dimers of low-molar-mass compounds formed by the coupling of two mesogenic segments are of contemporary interest [1,2,3,4]
The dimesogens and trimesogens were obtained by following the synthetic protocols described in Scheme 2 and Scheme 3 by DCC coupling of the ω-mesogen-functionalized alkanoic acids 2 [66] and 4 [66] with 4-[(4-hydroxyphenyl)-1,2,4-oxadiazol-5yl]phenyl 4-hexylbenzoate (3) [67] and 3,5-bis(4-hydroxyphenyl)-1,2,4-oxadiazole (1) [65], respectively
The obtained di- and trimesogens were investigated by polarizing microscopy (PM) (Optiphot 2, Nikon) in conjunction with a heating stage (FP82HT, Mettler) and differential scanning calorimetry (DSC, DSC-7, Perkin-Elmer)

Summary

Introduction

Liquid-crystalline (LC) dimers of low-molar-mass compounds formed by the coupling of two mesogenic segments are of contemporary interest [1,2,3,4]. Dimesogens can be categorized into mesogenic dimers (homodimesogens), composed of identical mesogenic units, and heterodimesogens combining different types of units, such as two distinct rod-like units [4], or combining rod-like with discotic [1,9,10], phasmidic or bentcore units, linked together through flexible chains [1]. These flexible chains, which in most cases represent linear alkyl.

Methods

Results

Conclusion