Emissive ‘zinc(II)-salphen’ core: building block for columnar liquid crystals

Abstract

A series of half-discoid [N₂O₂]-donor tetradentate alkoxy substituted salicylaldimine ligands, [N,N′-di- (4-n-alkoxysalicylidene)-4-Cl-l,2-diamino-benzene] (L; n = 12, 14, 16 and 18) have been prepared. The reaction of the ‘salphen’-type Schiff base ligand with Zn(OAc)₂.4H₂O afforded a series of mononuclear zinc(II) complexes. The ligands and the corresponding zinc(II) complexes were characterised by elemental analysis, Fourier transform infrared, proton nuclear magnetic resonance and ultraviolet-visible spectroscopy. Although the ligands are non-mesogenic, columnar mesomorphism is induced upon complexation with the metal. The p2gg symmetry of the rectangular columnar phase is confirmed by variable temperature powder X-ray diffraction study. Two ‘half-disc’-shaped molecules with four alkoxy legs are presumed to self-assemble via a dimeric interaction filling the space. In contrast to the ligands, the zinc(II) complexes all exhibited moderately intense green emission at room temperature both in solution and in the solid state. Density functional theory calculation, carried out using a DMol3 program, revealed a distorted square planar geometry for the complexes. The mesomorphic and photoluminescence property of the zinc complexes are collated as a function of spacer substituent, as well as alkoxy carbon chain length.

Keywords:

• zinc
• metallomesogen
• columnar mesomorphism
• fluorescence
• density functional theory

Acknowledgements

The authors thank INSPIRE, Department of Science and Technology (Ministry of Science & Technology), New Delhi, for financial support and Prof R.C. Deka (Tezpur University, India) for assistance with the theoretical calculations. Sophisticated Analytical Instrumentation Facility, North Eastern Hill University, Shillong, and CDRI, Lucknow, are acknowledged for some spectral results.