Jean-Claude Bunzli - Influence of anionic functions on the coordination and photophysical properties of lanthanide(III) complexes with tridentate bipyridines

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  Comby,S., Imbert,D., Chauvin,A.S., Bunzli,J.C.G.,   Charbonniere,L.J., Ziessel,R.F. Inorganic Chemistry  2004 43 (23) 7369-7379

  Abstract: A series of four ligands based on a   5'-methyl-2,2'-bipyridyl framework substituted in the 6 position   by a carboxylic acid, a phosphonic acid, a monoethyl ester   phosphonic acid, or a diethyl ester phosphonic acid are   described. The pK(a) values of all ligands and their assignments   are determined by a combination of UV-vis absorption spectroscopy   and H-1 and P-31 NMR spectroscopy. The ability of the tridentate   ligands to form complexes with trivalent lanthanide cations (Ln =   La, Nd, Eu, and Lu) in buffered water solutions (Tris-HCI, pH =   7.4) is studied by UV-vis absorption spectroscopy and H-1 NMR.   While the two ester ligands display a weak coordination ability   toward lanthanide cations, the acid ligands form stable complexes   with 1:1, 1:2, and 1:3 Ln/L ratios. A weak selectivity is   observed for the middle of the lanthanide series, and the   complexes of the phosphonic acid derivative are up to 2 orders of   magnitude more stable than those of the carboxylic acid ligand.   Photophysical properties of the free phosphonic and carboxylic   acid ligands and of their complexes with La, Eu, Gd, Tb, and Lu   are investigated in buffered aqueous solutions both at room   temperature and 77 K. An efficient ligand-to-metal energy   transfer is observed for both the Eu and Tb complexes. Despite a   relatively large energy gap between the ligand-centered   (3)pipi(*) and the Eu(D-5(0)) or Tb(D-5(4)) emitting states, the   metal-centered luminescence is well sensitized with quantum   yields reaching up to 45.5 and 42.2% for the Tb 1:3 complexes   with carboxylic and phosphonic acid ligands, respectively

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