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.