Stable 8-hydroxyquinolinate-based podates as efficient sensitizers of lanthanide near-infrared luminescence

New polydentate ligands (e.g., Tsox and TsoxMe) have been synthesized to take advantage of the chelating effect of bidentate 8-hydroxyquinolinate subunits connected to a N,N,N',N'-tetraaminopropyl-1,2-ethylenediamine framework and with the aim of sensitizing the NIR luminescence of Nd-III and Yb-III ions. Ten pK(a)'s have been determined and the interaction between the ligands and Ln(III) ions in dilute aqueous solution has been probed both by potentiometric and spectrophotometric titrations. These studies have been mostly performed with the Eu-III ion, which is in the middle of the lanthanide series, and extended to other ions (La-III, Er-III, Lu-III). Stable complexes with Ln(III) 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(H2L)](3-) exist as the major species at physiological pH regardless of the lanthanide used. Lifetime determinations of the Nd(F-4(3/2)) and Yb(F-2(5/2)) excited levels in both H2O and D2O at buffered pH point to the absence of water molecules bound in the inner coordination sphere of the Ln(III). 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-III- or Yb-III-centered luminescence (Q(Nd)(L) = 0.02% and Q(Yb)(L) = 0.18%) for Tsox 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 TsoxMe chelates (Q(Nd)(L) = 0.04% and Q(Yb)(L) = 0.37%). In fact, TsoxMe yields one of the most luminescent Yb-III compounds known in water, and this ligand appears to be suitable for the development of NIR probes for bioanalyses.