Aspects of lanthanide complexes for selectivity, intensity and sharpness in luminescence bands from twenty-four praseodymium, europium and gadolinium complexes with differently distorted-hexadentate ligands

Abstract

We structurally and spectroscopically investigated a series of praseodymium (Pr) complexes with eight ligands that form helicate molecular structures. The mother ligand skeleton (L) has two bipyridine moieties bridged with ethylenediamine. The bridged skeleton of PrL was changed to diamines 1-methyl-ethylenediamine, trimethylenediamine and 2,2′-dimethyl-trimethylenediamine, and the corresponding ligands were designated as L^me, L^pr and L^dmpr, for each Pr in these complexes upon UV-excitation. The luminescence quantum yields of PrL and PrL^pr in the visible and near infrared (NIR) regions indicate that PrL is excited by both the electronic state of the ligand and the ff absorption band, whereas PrL^pr is excited through the ligand. The addition of a methyl group to PrL and PrL^pr has a different effect on the Pr emission intensity with the intensity of PrL^me decreasing more than that of PrL and PrL^dmpr and increasing more than that of PrL^pr. Thus, the coordination of Pr and the increased rigidity of the ligand upon methylation enhance luminescence. The azomethine moieties on L^me, L^pr and L^dmpr were reduced and formed the corresponding PrLH, PrL^meH, PrL^prH and PrL^dmprH complexes. The luminescence of the non-methylated series is due to transitions related to the ¹D₂ level and thus the methylated series luminesces due to high energy levels such as ³P_J arising from the shortened π electronic systems. We also discuss the strong red emission of a series of Eu complexes with eight ligands from the viewpoint of their molecular structures and luminescence efficiencies and evaluate the Judd–Ofelt parameters from the luminescence spectra of Eu complexes.