Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties

Abstract

Two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles were synthesised and characterised. Their molecular structures differed wherein the latter comprised a lateral hydroxyl group, unlike the former. Spectroscopic techniques such as FT-IR, ¹H & ¹³C NMR and mass spectrometry together with elemental analysis were employed to elucidate the molecular structures. The transition temperatures and their mesophases were determined by differential scanning calorimetry, optical polarising microscopy and X-ray diffraction techniques. Members with decanoyloxy till hexadecanoloxy chain in the series without the lateral hydroxyl group each exhibited a smectic A phase, while those in the series with the lateral hydroxyl group were non-mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure–mesomorphic properties relationship.

Keywords:

• 2-(4-alkanoyloxybenzylidenamino)benzothiazole
• 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazole
• Schiff base
• smectic A
• structure–property relationship

Acknowledgements

The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the UTAR Research Fund and the Malaysia Toray Science Foundation (UTAR Vote No. 4359/000) for funding this project. T.M. Koh would like to acknowledge UTAR for the award of the research and teaching assistantships. The powder XRD measurements were supported by beamline BL17A (charged by Dr. Jey-Jau Lee) of the National Synchrotron Radiation Research Center, Taiwan.