Effect of 2,4,6-tri(2-pyridyl)-1,3,5-triazine on visible and NIR luminescence of lanthanide tris(trifluoroacetylacetonates)

Abstract

We here report the first example of a praseodymium complex of the type [Pr(tfaa)3(tptz)] (tfaa = trifluoroacetylacetone and tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine), the photophysical properties of the praseodymium complex (visible and NIR region), as well as of the analogous neodymium (NIR region), europium and terbium complexes (visible region), investigated in the solid state. We showed that the ancillary tptz ligand has detrimental effect on the photophysical properties of the terbium complex, showing a 2.5-fold shorter observed lifetime as compared to the hydrated binary complex [Tb(tfaa)3H2O] (τobs = 220 μs). However, it has a positive effect on the NIR emission of the neodymium complex (τobs = 10.33 μs and ϕLn = 3.2%) and visible emission of the europium complex (τobs = 678.54 μs and ϕLn = 89.45%). The Judd-Ofelt parameters (Ω2 and Ω4), the radiative (Arad) and non-radiative (Anrad) decay rates and the energy transfer (WET) and back energy transfer (WBT) rates were calculated and discussed in detail for the europium complex. The high value of Ω2 (30.88 × 10−20 cm2) indicates a greater degree of covalency in the europium – ligand bonds, on the other hand, the higher value of Ω4 (22.36 × 10−20 cm2) could be due to π–π interactions between the non-coordinating arms of the tptz co-ligand. The impact of each ligand (anionic tfaa and neutral tptz) on the photophysical properties in the case of the europium complex was ascertained theoretically by the chemical partition of the radiative decay rate (Arad) and it confirms that the neutral ancillary tptz ligand is solely contributing approximately one third of the Arad (∼32%) with a value of 402.78 s−1 while the three anionic tfaa ligands are contributing ∼68% of the Arad with a value of 855.13 s−1.