Induced columnar mesomorphism in non-discoid VO²⁺ salphen complexes: Transition between two rectangular columnar phases

Abstract

A new series of oxovanadium(IV) complexes of the type [VOL], L = N, N′-di-(4-n-alkoxysalicylidene)-4-Me-l,2-diamino-benzene (n = 14,16, 18) have been synthesised and their mesomorphic properties investigated. The compounds were characterised by Fourier transform infrared spectroscopy, ¹H and ¹³C nuclear magnetic resonance spectroscopy, ultraviolet–visible spectroscopy, elemental analyses, solution electrical conductivity measurements and fast atom bombardment mass spectrometry. The phase behaviour of the compounds was examined by differential scanning calorimetry and polarised optical microscopy. The ligands are non-mesogenic, but upon coordination with vanadium(IV), exhibited enantiotropic rectangular columnar mesomorphism. The mesophase symmetry and the molecular organisation in the mesophases were confirmed by variable temperature powder X-ray diffraction study. An interesting phase behaviour exhibiting two types of rectangular columnar phases (Colr₁ to Colr₂) was observed for the vanadyl complexes, the former (Colr₁) being stable down to room temperature. Solution electrical conductivity measurements showed non-electrolytic nature of the complexes. Cyclic voltammetry revealed a single electron quasi-reversible response (VO²⁺/VO³⁺) for the complexes. Density functional theory study carried out using the DMol3 program indicated a distorted square pyramidal geometry for the complexes.

Keywords:

• vanadium
• metallomesogen
• columnar mesophase
• density functional theory

Acknowledgements

The authors thank the University Grants Commission, New Delhi, for financial support and Professor R.C. Deka (Tezpur University, India) for assistance with the theoretical calculations. The Sophisticated Analytical Instrumentation Facility, North Eastern Hill University, Shillong, is acknowledged for some spectral results.