Liquid Crystal Trimers Composed of Banana-Shaped and Rodlike Anisometric Segments: Synthesis and Characterization

Abstract

In this article, we report the first examples of mesomorphic linear trimers consisting of bent-core and rodlike anisotropic segments, primarily conceived to explore the possibility of stabilizing optically biaxial mesophase(s), and we briefly review the work carried out in the area of dimers composed of bent-core mesogen(s). Specifically, four C2 symmetric trimers, which are formed by covalently tethering nematogenic cyanobiphenyls axially at the peripheral ends of a strongly shape-biaxial bent-core mesogen through two identical polymethylene spacers of varying length and parity, have been probed for their phase behavior and compared with those of the closely related oligomesogens. Four alkylene spacers, namely, hexamethylene, heptamethylene, octamethylene, and decamethylene, have been employed with the sole purpose of learning about the correlation between the resulting molecular conformation (shape) and mesomorphism. The optical and calorimetric studies revealed that all of the trimers, contrary to our prediction, display an enantiotropic uniaxial nematic phase over a wide thermal range (>50 °C), whereas the compound possessing a heptamethylene (odd-parity) spacer displays a metastable smectic phase, additionally. The nematic phase seems to have an inclination for the homogeneous alignment. The nematic–isotropic transition temperatures and associated enthalpies display a moderate odd–even effect; the even members show relatively higher values in a manner reminiscent of trimers made of conventional rodlike anisometric segments.