Stable 8-Hydroxyquinolinate-Based Podates as Efficient Sensitizers of Lanthanide Near-Infrared Luminescence

New polydentate ligands (e.g., <b>Tsox</b> and <b>TsoxMe)</b> have been synthesized to take advantage of the chelating effect of bidentate 8-hydroxyquinolinate subunits connected to a <i>N,N,N</i>‘,<i>N</i>‘-tetraaminopropyl-1,2-ethylenediamine framework and with the aim of sensitizing the NIR luminescence of Nd<sup>III</sup> and Yb<sup>III</sup> ions. Ten p<i>K</i><sub>a</sub>'s have been determined and the interaction between the ligands and Ln<sup>III</sup> ions in dilute aqueous solution has been probed both by potentiometric and spectrophotometric titrations. These studies have been mostly performed with the Eu<sup>III</sup> ion, which is in the middle of the lanthanide series, and extended to other ions (La<sup>III</sup>, Er<sup>III</sup>, Lu<sup>IIII</sup>). Stable complexes with Ln<sup>III</sup> ions are formed (pLn in the range of 14−16), the four chromophoric units being coordinated to the metal center, exploiting the entropic effect generated by the anchor. The monometallic complexes [Ln(H<sub>2</sub>L)]<sup>3-</sup> exist as the major species at physiological pH regardless of the lanthanide used. Lifetime determinations of the Nd(<sup>4</sup>F<sub>3/2</sub>) and Yb(<sup>2</sup>F<sub>5/2</sub>) excited levels in both H<sub>2</sub>O and D<sub>2</sub>O at buffered pH point to the absence of water molecules bound in the inner coordination sphere of the Ln<sup>III</sup>. Photophysical properties of the free ligands and of their lanthanide complexes have been investigated in buffered aqueous solutions both at room temperature and 77 K. The low-energy triplet state makes energy transfers from the ligand to the metal ions possible; this leads to a sizable sensitization of the Nd<sup>III</sup>- or Yb<sup>III</sup>-centered luminescence ( = 0.02% and = 0.18%) for <b>Tsox</b> chelates. Methylation of the amide functions removes the quenching mechanism induced by the proximate N−H vibrations and increases both the lifetimes and quantum yields of the <b>TsoxMe</b> chelates ( = 0.04% and = 0.37%). In fact, <b>TsoxMe</b> yields one of the most luminescent Yb<sup>III</sup> compounds known in water, and this ligand appears to be suitable for the development of NIR probes for bioanalyses.