Jean-Claude Bunzli - Self-assembled dinuclear lanthanide helicates: Substantial luminescence enhancement upon replacing terminal benzimidazole groups by carboxamide binding units

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  Martin,N., Bunzli,J.C.G., Mckee,V., Piguet,C., Hopfgartner,G.   Inorganic Chemistry 1998 37  (3) 577-589

  Abstract: The segmental ligands   bis{1-alkyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl}methane    (alkyl methyl (L-5), ethyl (L-6)) react with lanthanide   perchlorates (Ln La, Eu, Gd, Tb) in acetonitrile to yield the f-f   dinuclear homotopic triple-stranded helicates [Ln(2)(L-i)(3)](6+)   (i = 5, 6) under thermodynamic control. The crystal structure of   [Tb-2(L-6)(3)](ClO4)(3)(MeCN)(2)(THF)(0.5)(EtOH)(0.5) (11a,   C124H145N26O31Cl6Tb2, triclinic,

  , Z = 2) shows the wrapping of the ligands about a pseudo-C-3   axis passing through the metal ions. The Tb ions are 9-coordinate   in facial pseudo-tricapped trigonal prismatic sites and are   separated by 9.06 Angstrom. H-1-NMR and ES-MS data establish that   the triple helical structure is maintained in solution.   Spectrophotometric titrations (Ln = La, Eu) indicate log beta(23)   = 24-25 and the formation of a 2:2 complex [Ln(2)(L-5)(2)](6+)   (log beta(22) = 19-20). Quantum yield determination in   acetonitrile shows that the terminal N,N-diethylcarboxamide   groups in L-5 favor efficient intramolecular L-5 --> Eu(III)   energy transfers leading to strong Eu-centered red luminescence,   50 times as intense as the luminescence observed when the   carboxamide groups are replaced by substituted benzimidazole   units in [Eu-2(L-4)(3)](6+). Resistance toward hydrolysis also   results from the use of carboxamide groups, and no quenching of   luminescence is observed for [Eu-2(L-5)(3)](6+) in moist   acetonitrile up to 2.5 M water. The crucial role played by   carboxamide groups for the control of structural, electronic, and   photophysical,properties is discussed. Replacing perchlorates by   triflates allows the isolation of the dinuclear double-stranded   helicate   [Eu-2(L-6)(2)(CF3SO3)(4)(H2O2)(2)](CF3-SO3)(2)(MeOH)(2)(H2O)(5.5),   whose crystal structure (13a, C85H106Eu2F18N16O30S6, monoclinic,   C2/m, Z = 2) reveals a side-by-side arrangement of the two   strands and 9-coordinate Eu ions linked through hydrogen-bonded   water molecules

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