New fluorescent Schiff bases linked azo chromophore: synthesis, complex formation with Cu(II) and Ni(II) ions, characterization, X-ray, SEM and optical properties

Azo-aldehyde compound L (1) was prepared by coupling 4-ethylbenzenediazonium chloride ions with salicylaldehyde, and the subsequent azo-Schiff bases HL¹ (2) and HL² (3) were obtained by the condensation of compound L (1) with o-chloroaniline and p-chloroaniline, respectively. New azo-Schiff base ligands and their nickel(II) and copper(II) complexes (4–7) were also prepared and characterized by various analytical and spectroscopic methods, such as, elemental analysis and FTIR, GC–MS and NMR spectra. The metal complexes revealed the central metal atom as being coordinated to two phenolate oxygen atoms and two imine nitrogen atoms of two azo-Schiff bases, bearing approximate square-planar geometries. Further, spectral anaylyses data and metal-ion complexations revealed azo-hydrazone tautomerism within the two azo-Schiff bases HL¹ (2) and HL² (3). Azo-hydrazone tautomerism was evident through the formation of novel neutral mononuclear dye-metal complexes [M(L¹)₂] and [M(L²)₂], with M=Cu(II) or Ni(II) and all the ligands being of the azo form. Characteristic evaluations of the dyes and their metal complexes were conducted using X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDX). XRD data revealed that the synthesized compounds were of crystalline structure. SEM studies showed that the surface morphologies of all the samples had different shapes and particle sizes. TGA studies were conducted for the thermal properties of the compounds synthesized. The ligands and the corresponding metal complexes displayed effective emissions upon UV irradiation. Ligands (1–3) showed 32, 38 and 35% quantum yields and 3.02, 3.57 and 3.30 ns excited-state lifetimes, respectively.