Jean-Claude Bunzli - Effect of a halogenide substituent on the stability and photophysical properties of lanthanide triple-stranded helicates with ditopic ligands derived from bis(benzimidazolyl)pyridine

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      Iglesias,C.P., Elhabiri,M., Hollenstein,M., Bunzli,J.C.G.,   Piguet,C. Journal of the Chemical Society-Dalton   Transactions 2000 (13) 2031-2043

      Abstract:  Bis{1-ethyl-2-[6-(N,N-diethylcarbamoyl)-4-halogenopyridin-2-yl]benzimidazol-5-y l}methane   (halogeno=chloro, L-E; bromo, L-F) have been synthesized as   ditopic receptors for the development of lanthanide-containing   helicates able to couple with biological material and to test the   influence of the halogeno substituent on the wrapping process,   the structure of the resulting dimetallic edifices, and the   photophysical properties of the encapsulated ions. The stability   of the [Eu-2(L)(3)](6+) helicates, as determined by NMR   competitive titrations, decreases by respectively one (L-F) and   three (L-E) orders of magnitude compared to the value found for   the unsubstituted ligand (L-B) although it remains large, log   beta(23)=23.8 (L-F) and 21.8 (L-E) in acetonitrile. The   [Ln(2)(L-E)(3)](6+) helicates are shown to be isostructural in   acetonitrile over the lanthanide series (Pr to Yb) and the   crystal structure of [Tb-2(L-B)(3)](6+) appears to be a good   model for their solution structure, as demonstrated by   paramagnetic NMR measurements (lanthanide induced shift method)   and relaxation time determination. Ligand L-E appears to be a   fair sensitiser of Eu-III, the quantum yield of   [Eu-2(L-E)(3)](6+) being 25% larger than that found for   [Eu-2(L-B)(3)](6+), but the ligand (3)pi pi* state and Tb(D-5(4))   excited level are in resonance, which limits the sensitisation of   Tb-III. High resolution luminescence spectra of   [Eu-2(L-E)(3)](6+), both in solution and in the solid state, are   presented and discussed in terms of site symmetry and vibronic   coupling mechanisms

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