Synthesis, Characterization, Theoretical Study and Investigation of Optical Properties of One New Ligand Based on Azo-azomethine Compounds in the Presence of Some Metal Cations

In this work, 4-bromo-1,2-bis[2-hydroxy-5-(4-nitrophenylazo)benzylideneamino]benzene (H2L) based on azo Schiff-base ligands was prepared from condensation reaction of 1-[3-formyl-4-hydroxophenylazo)-4-nitrobenzene and 4-bromo-1,2-diaminobenzene and characterized with several spectroscopic techniques and elemental analysis. It is well known azophenol groups in different compounds may exist in azophenol and quinone-hydrazone tautomeric forms. There are many factors such as solvent, temperature and substitution that affect equilibrium tautomeric forms. The investigation of UV-Vis spectra of H2L in DMSO and the mixture of DMSO and water showed the solvatochromism that exhibited by azo ligands may be to the effect of proton transfer or dipole moment changes in various solvents. In DMSO, an additional absorption maximum which observed at around 500 nm attributed to the existence of tautomeric form. Also, the increasing of pH and deprotonation of hydroxyl groups affect the keto-enol equilibrium in solution, so that the main absorption bonds and color solution change strongly in intensity. The investigation of UV-vis spectra of H2L in the presence of Ca2+, Cd2+, Cu2+, Co2+, Hg2+, Ag+, Pb2+, Zn2+, Na+ and Ni2+ metal ions revealed the intensity of absorbance bonds changes dramatically with the addition of all cations to the solution of ligand (generally, n→π* transition increases in intensity while π→π* transition decreases). It seems that complexation metal ions for H2L affect keto-enol equilibrium, although it is unknown that why different metal ions have one unequal effect on keto-enol equilibrium. Finally, DFT and TD-DFT calculations was done for better understanding of structure and electronic properties of ligand H2L.