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

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  Comby,S., Imbert,D., Chauvin,A.S., Bunzli,J.C.G. Inorganic   Chemistry 2006 45 (2) 732-743

  Abstract: 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

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