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

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 LnN(9) 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.