Jean-Claude Bunzli - Self-Assembly and Photophysical Properties of Lanthanide Dinuclear Triple-Helical Complexes

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  Piguet,C., Bunzli,J.C.G., Bernardinelli,G., Hopfgartner,G.,   Williams,A.F. Journal of the American Chemical Society  1993 115 (18) 8197-8

  Abstract: The dinucleating ligand   bis[I-methyl-2-(6'-[1''-(3,5-dimethoxybenzyl)benzimidazol-2''-yl]pyrid-2'-yl)be nzimidazol-5-yl]methane   (L) reacts with lanthanide perchlorates to give dinuclear 2:3   complexes [Ln2(L)3]6+ (Ln = La, Eu, Gd, Tb, and Lu). Detailed   ES-MS, H-1-NMR, luminescence, and spectrophotometric measurements   in acetonitrile show that the cations [Ln2(L)3]6+ are produced by   strict self-assembly and adopt a triple-helical structure in   solution (pseudo-D3 symmetry). The crystal structure of   [EU2(L)3](ClO4)6(CH3CN)9 (11, Eu2C195H177N39O36Cl6, a=17.634-(3)   angstrom, b = 21.408(4) angstrom, c = 29.437(7) angstrom, alpha =   82.13(1)degrees, beta = 85.76(1)degrees, gamma = 89.79(1)degrees,   triclinic, P1BAR, Z = 2) shows a dinuclear pseudo-D3   triple-helical cation, [EU2(L)3]6+, where the three   bis(terdentate) ligands L are wrapped around the helical axis   defined by the europium atoms. The Eu(III) of each site is   9-coordinated by six nitrogen atoms of the benzimidazole units   occupying the vertices and three nitrogen atoms of the pyridine   units occupying the capping positions of a slightly distorted,   tricapped trigonal prism. Luminescence studies of the crystalline   complex [Eu2(L)3]-(ClO4)6.nsolv (n = 9, solv = CH3CN, 11; n = 2,   solv = H2O, 6; n = 9, solv = H2O, 7) confirm the pseudo-D3   symmetry of the Eu(III) sites in 11 and show that secondary   interactions with water molecules in 6 and 7 destroy the trigonal   symmetry. An efficient intramolecular energy transfer between the   3pipi* excited state centered on L and the excited levels of   Eu(III) and Tb(III) is observed (antenna effect) together with a   dipole-dipolar Tb-->Eu intramolecular energy transfer in the   heterodinuclear-doped Eu-Tb compound. Stability constants and H-1   NMR in acetonitrile show that the homodinuclear complexes   [Ln2(L)3]6+ are less stable for the heavier lanthanides Tb and   Lu. The origin of this effect is discussed together with the   nonstatistical distribution of the different species observed   when stoichiometric quantities of L (3 equiv) are mixed with   Ln1(ClO4)3 (1 equiv) and Ln2(ClO4)3 (1 equiv) in solution (Ln1   not-equal Ln2; Ln1 = La, Eu, Tb, and Lu; Ln2 = Tb and Lu)

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