Experimental and theoretical studies on structure, bonding and luminescence properties of Eu(III) and Tb(III) complexes of a new macrocyclic based 8HQ ligand

Abstract

8-Hydroxyquinolate complexes of lanthanide have a variety of applications in electroluminescent devices due to their unique luminescence properties. But, the simple 8HQ ligand has a tendency to form polynuclear complexes. However, linking several 8HQ units to a single moiety facilitates the formation of mononuclear complexes due to flexibility, preorganization, π-stacking and entropy effect. Mononuclear complexes of europium and terbium ions with a new chelator, [1,4,7-tris-{(5-methylene-8-hydroxyquinoline)-1,4,7-triazacyclononane}], embedded with these prerequisites are synthesized and characterized. Each Ln(III) is nine-coordinate and bonded to three sets of O,N-donors of 8HQ units and three water resulting in a pseudo-C3 symmetric tricapped trigonal prismatic geometry. In silico studies suggest that the metal ion can easily be encapsulated in the central cavity of the ligand without changing its basic structure. Interesting, diverse behavior was observed for photoluminescence spectral studies in solid and solution state. At room temperature, an antenna effect was observed for the complexes only in the powder form. The experimental photoluminescence studies have been explained with the help of TDDFT. The nature of bonding between the lanthanide and the ligand, interpreted by NBO and ETS-NOCV, suggest that the Ln-L bonds are ∼25% covalent. The Slater Condon parameters, spin-orbital coupling constants, and the nephelauxetic parameters were calculated from LF-DFT.