Photoluminescent nickel(II)-metallomesogens derived from salphen ligands: influence of halogens at the spacer on mesomorphism and emission properties

ABSTRACT

Three new series of photoluminescent nickel(II) metallomesogens, [NiL]; H₂L = N,N′-Bis(4-n-alkoxysalicylidene)-4-fluoro/bromo/chloro-1,2-diaminobenzene (n = 12, 14, 16) based on ‘salphen’ ligands have been synthesised and their mesomorphic and photophysical properties explored. The complexes, isolated as orange microcrystalline solids were characterised by elemental analyses, FT-IR, ¹H NMR and UV-visible spectroscopy. Thermal studies show all the compounds to be enantiotropic liquid crystals displaying columnar mesophase over a wide temperature range. Electronegativity and steric requirement of the halogen substituent at the ligand’s spacer remarkably influence the 2-D packing of the columns in the lattice in these complexes controlling the supramolecular mesomorphic order and photoluminescence. The mesophase behaviour of the fluoro-substituted complex is characterised by a transition from a columnar oblique (p1) to columnar rectangular (p2mm) phase, former stable till ambient temperature. The chloro and bromo analogues, on the other hand, displayed exclusively columnar rectangular (p2mm) mesophase with the former transforming into a glassy state and latter into a crystalline phase during cooling to ambient temperature. Molecular model based on interdigitated anti-parallel and back to back arrangements in the different columnar mesophase are proposed on the basis of X-ray diffraction (XRD) studies. The complexes emit in the blue region when excited with near UV wavelength.

Graphical Abstract

KEYWORDS:

• Nickel
• metallomesogen
• enantiotropic
• photoluminescence
• halogen

Acknowledgments

SC acknowledges the Department of Science and Technology (DST), New Delhi, Government of India for an INSPIRE Research Fellowship (Code: IF110692). Authors acknowledge support under DST-FIST program, Ministry of Science and Technology, New Delhi, India and Sophisticated Analytical Instrumentation Facility (SAIF), North Eastern Hill University, Shillong. Thanks are also due to DBT e-Library Consortium (DeLCON) of Bio-Informatics Centre, Assam University, India.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Department of Science and Technology, Ministry of Science and Technology, New Delhi, India [IF110692].