Exploring the ability of the nalidixate to sensitize visible and near-infrared emitting lanthanide(III) cations.

Recently, a strong interest has been directed towards near-infrared (NIR) emitting lanthanide(III) compounds as they do possess complementary advantages in respect to organic molecules and semi-conductor nanocrystals, especially in the fields of biological analysis and imaging. To benefit from their emission, a key requirement to fulfill is the sensitization of lanthanide(III) cations with an appropriate chromophore. This condition is especially challenging to address for the lanthanide(III) cations emitting in the NIR. The quest for new chromophores well adapted to the NIR-emitting lanthanide(III) ions is an important direction of research in order to broaden the rationalization of the parameters that control the sensitization process. In this work, we have investigated the ability of a readily available chromophoric ligand, the nalidixic acid, to sensitize lanthanide(III) cations with a specific interest for those emitting in the NIR. We have therefore performed an extensive study of the luminescence properties of lanthanide(III) complexes emitting in the visible and in the NIR ranges formed in situ upon mixing the corresponding Ln(III) nitrates (Ln(III)  =  Pr, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb) with nalidixic acid (HNA) in a 1:3 molar ratio in the presence of a base. Luminescence spectra, quantum yields and luminescence lifetimes have been measured and discussed. The red emission of Eu with a quantum yield value of 5.90(3)%, red and NIR of Pr (7(1) · 10-4 and 5.6(1) · 10-4%) and Ho (9.3(2) · 10-4 and 2.8(1) · 10-4%), green of Tb (5.21(5)%), yellow and NIR of Dy (0.51(2) and 0.065(4)%), orange and NIR of Sm (0.147(5) and 0.037(2)%), as well as NIR of Nd (0.0321(2)%) and Yb (0.021(1)%) were observed. These results and analysis show that the nalidixate is a versatile chromophoric ligand that is suitable for the sensitization of nine different lanthanide(III) cations, five of them emitting in the NIR.