Low temperature enantiotropic nematic phases from V-shaped, shape-persistent molecules

• Main Authors: Matthias Lehmann, Jens Seltmann
• Format: Article
• Language: English
• Published: Beilstein-Institut 2009-12-01
• Series: Beilstein Journal of Organic Chemistry
• Subjects: biaxial nematics; liquid crystals; phase engineering; thiadiazoles; V-shaped mesogens
• Online Access: https://doi.org/10.3762/bjoc.5.73

A series of V-shaped, shape-persistent thiadiazole nematogens, based on an oligo(phenylene ethynylene) scaffold with ester groups connected via alkyloxy spacers, was efficiently prepared by a two-step procedure. Phase engineering results in an optimum of the mesophase range and low melting temperature when the nematogens are desymmetrised with a butoxy and a heptyloxy spacer. The mesophases are enantiotropic and over the whole temperature range nematic. For the optimised mesogen structure, optical investigations by conoscopy monitored a uniaxial nematic phase upon cooling from the isotropic phase to room temperature (ΔT = 150 °C). X-ray studies on magnetic-field-aligned samples of this mesogen family revealed a general pattern, indicating the alignment of two molecular axes along individual directors in the magnetic field. These observations may be rationalised with larger assemblies of V-shaped molecules isotropically distributed around the direction of the magnetic field.