Solution thermodynamics and DFT studies of luminescent Eu(III) and Tb(III) complexes with two new flexible 1-hydroxy-2-pyridinone-based hexadentate chelators

Abstract

The fabrication and chelation of Ln(III) metal ions (Ln = Eu and Tb) with ligands i.e.; N-[3,5-bis[(1-hydroxy-6-oxo-pyridine-2-carbonyl)amino]cyclohexyl]-1-hydroxy-6-oxo-pyridine-2-carboxamide, (TACH-1,2-HOPO) (L₁) and N,N',N''-(((1R,3R)-cyclohexane-1,3,5-triyl)tris(methylene))tris(1-hydroxy-6-oxo-1,6-dihydropyridine-2 carboxamide), (TMACH-1,2-HOPO) (L₂) made up of three 1,2-HOPO units attached with tris-aminocyclohexane and tris-aminomethylcyclohexane framework are reported. The Eu(III) and Tb(III) complexes with hexadentate L₁ and L₂ are evaluated in terms of their thermodynamic stability in aqueous solution by potentiometric and spectrophotometric methods. The UV–visible absorption and luminescence properties are also studied. The overall stability constants, logβ₁₁₀ were found to be 13.98 and 10.75 for Eu(III) and Tb(III) complexes respectively with L₁, whereas for L₂ they were found to be 11.29 and 10.24 for Eu(III) and Tb(III) respectively. Complexes of both the ligands exhibit significantly similar characteristics in terms of stability and photophysical aspects and are found to be nine-coordinated in which the ligand is coordinated to three pairs of O–O donor atoms of three 1,2-HOPO moieties along with three coordinated water molecules forming a distorted tricapped trigonal prismatic geometry. From the computational studies, it was observed that the metal ions are wrapped in the ligand's cavity without affecting their fundamental geometry. The density functional theory (DFT) method has been used to elucidate the coordination behavior and experimental absorption properties of lanthanide complexes. Furthermore, NBO and ETS-NOCV studies were also carried out to investigate the nature of bonding.