title: Encapsulation of labile trivalent lanthanides into a homobimetallic chromium(III)-containing triple-stranded helicate. Synthesis, characterization, and divergent intramolecular energy transfers.
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](CF3SO3)7 (Ln = Eu, Tb) display only lanthanide-centred luminescence arising from the pseudo-tricapped trigonal prismatic LnN9 coordination site. For M = Cr(II), rapid air oxidation provides Cr(III) and leads to the isolation of inert [CrLnCr(L2)3](CF3SO3)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.
authors.affiliation: Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211, Geneva 4, Switzerland.
authors.forename: Martine
authors.initials: M
authors.lastname: Cantuel
authors.forename: Frédéric
authors.initials: F
authors.lastname: Gumy
authors.forename: Jean-Claude G
authors.initials: JC
authors.lastname: Bünzli
authors.forename: Claude
authors.initials: C
authors.lastname: Piguet