Synthesis and characterisation of novel isoxazole-based banana liquid crystals from naturally occurring curcumin

ABSTRACT

Isoxazole-based bent-core liquid crystals (LCs) derived from naturally occurring curcumin were synthesised and their LC properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction techniques. Five compounds, including a branched alkyl chain derivative, were prepared and characterised. These derivatives exhibit enantiotropic mesophases. While lower homologues display wide-temperature-range nematic phase, a longer-chain derivative 3d shows smectic C phase in addition to the nematic phase. The bent angle in these compounds is in between calamitic LCs and banana LCs. Therefore, the molecules escape from polar order packing observed in typical bent-core LCs. Increasing the length of alkyl chain reduces both melting and isotropic temperatures in the series. However, the compound with branched alkyl chains exhibits significant reduction in the nematic-isotropic temperature only. Detailed XRD experiments confirm the presence of the N phase in the lower homologues and SmC phase in a higher homologue.

GRAPHICAL ABSTRACT

KEYWORDS:

• isoxazole
• bent-core liquid crystals
• nematic phase
• smectic phase
• curcumin

Acknowledgements

We thank Dr Vijayaragavan for recording NMR spectra; Mr Srinivas HT and Ms Vasuda for their technical support and Mr Hemant Kumar Singh (SRF), Banaras Hindu University, for DFT studies.