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

We structually 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 designed Lme, Lpr and Ldmpr. 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 different effect on the Pr emission intensity with the intensity of PrLme decreasing more than that of PrL and PrLdmpr increasing more than that of PrLpr. Thus, the coordination of Pr and the increased rigidity of the ligand pon 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 transition 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.