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 Lme, Lpr and Ldmpr, for each Pr in these complexes upon UV-excitation. The luminescence quantum yields of PrL and PrLpr 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 PrLpr is excited through the ligand. The addition of a methyl group to PrL and PrLpr has a different effect on the Pr emission intensity with the intensity of PrLme decreasing more than that of PrL and PrLdmpr and increasing more than that of PrLpr. Thus, the coordination of Pr and the increased rigidity of the ligand upon methylation enhance luminescence. The azomethine moieties on Lme, Lpr and Ldmpr were reduced and formed the corresponding PrLH, PrLmeH, PrLprH and PrLdmprH 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 3PJ 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.