Jean-Claude Bunzli - Connecting terminal carboxylate groups in nine-coordinate lanthanide podates: Consequences on the thermodynamic, structural, electronic, and photophysical properties

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      Senegas,J.M., Bernardinelli,G., Imbert,D., Bunzli,J.C.G.,   Morgantini,P.Y., Weber,J., Piguet,C. Inorganic Chemistry  2003 42 (15) 4680-4695

      Abstract: The hydrolysis of terminal   (t)butyl-ester groups provides the novel nonadentate podand   tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine}   (L13) which exists as a mixture of slowly interconverting   conformers in solution. At pH = 8.0 in water, its deprotonated   form [L13 - 3H](3-) reacts with Ln(ClO4)(3) to give the poorly   soluble and stable podates [Ln(L13 - 3H)] (log(beta(110)) =   6.7-7.0, Ln = La-Lu). The isolated complexes [Ln(L13 -   3H)](H2O)(7) (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and   their crystal structures show Ln(Ill) to be nine-coordinate in a   pseudotricapped trigonal prismatic site defined by the donor   atoms of the three helically wrapped tridentate binding units of   L13. The Ln-O(carboxamide) bonds are only marginally longer than   the Ln-O(carboxylate) bonds in [Ln(L13 - 3H)], thus producing a   regular triple helix around Ln(Ill) which reverses its screw   direction within the covalent Me-TREN tripod. High-resolution   emission spectroscopy demonstrates that (i) the replacement of   terminal carboxamides with carboxylates induces only minor   electronic changes for the metallic site, (ii) the solid-state   structure is maintained in water, and (iii) the metal in the   podate is efficiently protected from interactions with solvent   molecules. The absolute quantum yields obtained for [Eu(L13 -   3H)] (Phi(Eu)(tot) = 1.8 x 10(-3)) and [Tb(L13 - Eu 3H)]   (Phi(Eu)(tot) = 8.9 X 10(-3)) in water remain modest and strongly   contrast with that obtained for the lanthanicle luminescence step   (Phi(Eu) = 0.28). Detailed photophysical studies assign this   discrepancy to the small energy gap between the ligand-centered   singlet ((1)pipi*) and triplet ((3)pipi*) states which limits the   efficiency of the intersystem crossing process. Theoretical TDDFT   calculations suggest that the connection of a carboxylate group   to the central pyridine ring prevents the sizable stabilization   of the triplet state required for an efficient sensitization   process. The thermodynamic and electronic origins of the   advantages (stability, lanthanide quantum yield) and drawbacks   (solubility, sensitization) brought by the "carboxylate effect"   in lanthanide complexes are evaluated for programming   predetermined properties in functional devices

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