Influence of halogen substituent on the mesomorphic properties of five-ring banana-shaped molecules with azobenzene wings

Abstract

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B₆ phases, a B₄-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B₆-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-induced trans–cis-isomerisation on the prepared compounds.

Keywords:

• bent-core liquid crystals
• azobenzene
• nematic phase
• anticlinic SmC phase
• B₆ phases
• B₄-like dark conglomerate phase

Acknowledgement

M. Alaasar is grateful to the Alexander von Humboldt Foundation for the research fellowship.

Notes

1. As an exception, it was reported that increasing the spacer length in dimeric rod-like LC decreases the smectic tendencies of the mesogen and increases the nematic tendencies for a given homologues series.