Synthesis, characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4′-n-alkoxybenzylideneamino)benzoate

Abstract

A new series of Schiff base calamitic liquid crystal; methyl 4-(4′-n-alkoxybenzylideneamino)benzoate (MABAB), H_2n+1C _n OC₆H₄C(H)=NC₆H₄COOCH₃ (n = 6, 8, 10, 12, 14, 16) has been synthesised and characterised by elemental analyses, Fourier transform infrared spectroscopy (FT-IR), ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy. The mesomorphic properties of these compounds were studied by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). All members of the series exhibit enantiotropic smectic A (SmA) mesophase. Temperature-dependent micro-Raman study of one of the members, MABAB-10 has been employed to identify phase transitions and the molecular rearrangement therein. Analysis of Raman marker bands; C–H in-plane bending, C–C stretching of phenyl rings and –C(H)=N– linking group of core confirms the transitions clearly as observed through DSC and POM. An in situ Raman measurement of C–H in-plane bending mode has also been performed to visualise the molecular changes more clearly. The Raman study gives an evidence of induced co-planarity of rings at Cr→SmA phase transition. The density functional theoretical (DFT) optimisation of monomer, dimer and rotational conformer of MABAB-10 also support the induced co-planarity at Cr→SmA phase transition.

Keywords:

• liquid crystals
• smectic A mesophase
• polarising optical microscopy
• differential scanning calorimetry
• temperature-dependent micro-Raman study

Acknowledgements

RN is grateful to the University Grants Commission, India for providing fellowship under Research Fellowship Scheme for Meritorious Students. HKS (Junior Research Fellow), SKS (Senior Research Fellow) and RKS are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for financial support. BS is grateful to CSIR (Project No. 01(2550)12/EMR-II) for providing financial assistance.

Notes

Rajib Nandi and Hemant Kumar Singh contributed equally to this study.