Luminescent Lanthanide Complexes of a Bis-bipyridine-phosphine-oxide Ligand as Tools for Anion Detection

Abstract

The Gd³⁺, Tb³⁺, and Eu³⁺ complexes of a bis-bipyridine-phenylphosphine oxide ligand PhP(O)(bipy)₂1 (bipy for 6-methylene-6‘-methyl-2,2‘-bipyridine) have been synthesized. In acetonitrile solutions at room temperature, the Tb³⁺ and Eu³⁺ complexes show a metal-centered luminescence, indicative of an efficient energy transfer from the two bipy subunits to the Ln center. The photophysical properties drastically depend on the nature of the anions present in solution. In particular, addition of 2 equiv of nitrate anions to a solution containing the [Ln·1](OTf^-)₃ leads to an 11-fold increase of the luminescence intensity for the Eu³⁺ and a 7-fold increase for the Tb³⁺ complexes. Similar effects are provided with Cl^-, F^-, and CH₃COO^- anions. UV−vis titration experiments were used to determine association constants for binding of, respectively, one, two, and three anions. Stepwise anion addition has also been investigated on the molecular level using quantum mechanical (QM) calculations for the Eu complexes. These calculations reproduce the experimental findings, especially if solvent molecule addition is taken into account. The X-ray crystal structure of the nitrate salt of the Tb complex, as well as QM calculation of a similar Eu complex, demonstrates the coordination of three nitrate anions in a bidentate mode and the step-by-step relegation of the bipy subunits in the second coordination sphere. These features give valuable insights into the mechanism of the overall light amplification process.