Plastic columnar mesomorphism in half-disc-shaped oxovanadium(IV) Schiff base complexes

Abstract

A new series of non-disc-like oxovanadium(iv) Schiff base complexes of the type [VO((4-C _n H_2n+1O)₂salcn)], where n = 14, 16 or 18 and salcn is N,N ^′-bis-salicylidene-1,2-cyclohexadiamine, containing 4-substituted alkoxy tails in the side aromatic rings, have been synthesised and their mesogenic properties investigated. The compounds were characterised by FT–IR, ¹H NMR, ¹³C NMR, UV–Vis and FAB mass spectrometry. The mesomorphic behaviour of the compounds was studied using polarised optical microscopy and differential scanning calorimetry. The molecular organisation in the mesophase was determined by X-ray diffraction. It was found that the ligands did not show mesogenic behaviour, but their complexes exhibited a thermally stable enantiotropic highly ordered three-dimensional plastic mesophase with a columnar structure in the extended temperature range 155–166°C. The clearing temperature of the complexes was found to be lower than in the structurally analogous copper complexes. A density functional theory study was carried out using DMol3 at BLYP/DNP level to obtain a stable optimised structure. A square pyramidal geometry for the vanadyl complexes has been proposed.

Keywords:

• Vanadium
• metallomesogen
• Schiff base
• plastic columnar mesophase
• density functional theory

Acknowledgements

The authors wish to thank SAIF, NEHU and CDRI, Lucknow for analytical and spectral data. G.D. is grateful for financial support from UGC, Government of India. The computational resources provided by Dr R.C. Deka, Tezpur University, India, are also acknowledged.