Jean-Claude Bunzli - High-spin iron(II) as a semitransparent partner for tuning Europium(III) luminescence in heterodimetallic d-f complexes

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  Edder,C., Piguet,C., Bunzli,J.C.G., Hopfgartner,G.   Chemistry-A European Journal 2001  7 (14) 3014-3024

  Abstract: The segmental ligand   2-{6-N,N-diethyrcarbamoyl)pyridin-2-yl}-1,1   dimethyl-5,5'-methylene-2'-(6-methylpyridine-2-yl)bis[1H-benzimidazole]   (L-3) reacts with a stoichiometric mixture of Ln(III) (Ln = La,   Eu, Gd) and M-II (M = Zn, Fe) in acetonitrile to produce   selectively the heterodimetallic triple-stranded helicates   (HHH)-[LnM(L-3)(3)](5+). In these complexes, M-II is   pseudooctahedrally coordinated by the three wrapped bidentate   binding units, thus forming a noncovalent tripod which organizes   the three unsymmetrical tridentate segments to give ninefold   coordination to Ln(III). The introduction of a methyl group at   the 6 position of the terminal pyridine in L-3 sterically reduces   the complexing ability of the bidentate segment for M-II.   Spectroscopic (ESI-MS. UV/Vis/NIR, NMR), magnetic and   electrochemical measurements show that 1) the   head-to-head-to-head triple helical complexes   (HHH)-[LnM(L-3)(3)](5+) are quantitatively formed in solution   only for ligand concentrations larger than 0.01M, 2)Fe-II adopts   a pure high-spin electronic configuration in   (HHH)-[LnFe(L-3)(3)](5+), and 3) the (FeFeIII)-Fe-II/ oxidation   process is prevented by steric constraints. Detailed   photophysical studies of (HHH)-[Eu-Zn(L-3)(3)](5+) confirm that   the pseudotricapped trigonal-prismatic lanthanide coordination   site is not affected by the methyl groups bound to the terminal   pyridine, thus leading to significant Eu-centered emission upon   UV irradiation. In (HHH)-[EuFe(L-3)(3)](5+), a resonant   intramolecular Eu --> Fe-hs(II) energy transfer partially   quenches the Eu-centered luminescence; however, the residual red   emission demonstrates that high-spin iron(Ii) is compatible with   the sensitization of Eu-III heterodimetallic d-f complexes. The   influence of the electronic configuration of Fe-II on the   efficiency of Eu-III--> Fe-II energy-transfer processes is   discussed together with its consequence for the design of   optically active spin-crossover supramolecular devices

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