Two copper (II) mononuclear complexes with the general formula [Cu(HL)X2], which L = N-2- ethanolpicolylamine and X = Cl, Br, were synthesized and characterized by elemental analysis infrared, electronic absorption spectroscopy techniques, and conductivity method. The distorted square pyramidal geometry of [Cu(HL)Cl2] was confirmed by the density functional theory level (DFT) calculation and X-ray crystallography. The complexes are soluble in polar solvents and are chromotropic. It was found both complexes have a similar identity in aqueous solution due to dissociation of halides and hydration. Their chromotropism behavior, including solvato-, thermos- and halochromism, was investigated using visible absorption spectroscopy. The original green complex displayed a reversible color change to blue in alkaline and almost colorless in acidic solutions. The pH effect on the visible absorption spectrum of the complex was studied that functions as pH-induced “off–on-off” switches triggered by the deprotonation of ligand in an alkaline aqueous solution and protonation along with loss of the ligand HL in an acidic medium. The complex also exhibited thermochromism and solvatochromism due to solvation and structural change. Further computational analysis of the electronic structure of the complex in different solvents was executed to gain a deeper insight into the properties associated with molecular orbitals. Its electronic spectrum was analyzed, and bands were assigned through the time-dependent density functional theory level (TDDFT) calculation, together with DFT-based molecular modeling
