New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases

Abstract

New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases – the synclinic smectic C (SmC_s) and the anticlinic SmC_a phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro–optical and dielectric spectroscopy measurements. The SmC_s–SmC_a phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small ‘soft’ ferroelectric clusters responsible for a low frequency absorption range.

Keywords:

• hockey-shaped molecule
• X-ray diffraction
• optical birefringence
• dielectric constant
• orientational order parameter

Acknowledgements

We gratefully acknowledge financial support from the Department of Science and Technology, New Delhi (Project no.: SR/S2/CMP-20/2005 and SR/S2/CMP-29/2007) and the University Grants Commission (New Delhi) for the award of a Junior Research Fellowship (AC).