Jean-Claude Bunzli - Encapsulation of labile trivalent lanthanides into a homobimetallic chromium(III)-containing triple-stranded helicate. Synthesis, characterization, and divergent intramolecular energy transfers

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  Cantuel,M., Gumy,F., Bunzli,J.C.G., Piguet,C. Dalton   Transactions 2006 (22) 2647-2660

  Abstract: The segmental   bidentate-tridentate-bidentate ligand L2 reacts with M(II) (M =   Cr, Zn) and Ln(III) (Ln La, Eu, Gd, Tb, Lu) to give the   heterotrimetallic triple-stranded helicates [MLnM(L2)(3)](7+).   For M = Zn(II), the isolated complexes   [ZnLnZn(L2)(3)](CF(3)SO(3))(7) (Ln = Eu, Tb) display only   lanthanide-centred luminescence arising from the pseudo-tricapped   trigonal prismatic LnN(9) coordination site. For M = Cr(II),   rapid air oxidation provides Cr(III) and leads to the isolation   of inert [CrLnCr(L2)(3)](CF(3)SO(3))(9) (Ln Eu, Tb) complexes, in   which divergent intramolecular Ln -> Cr energy transfers can   be evidenced. Taking [ZnEuZn(L2)(3)](7+) as a luminescent   standard for Eu-centred emission, a quantitative treatment of the   energy migration processes indicates that the rate constant   characterizing the Eu -> Cr energy transfer is more efficient   in the trimetallic system, than in the analogous simple   bimetallic edifice. Particular attention is focused on potential   control of directional energy transfer processes in Cr-Ln pairs

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