Optimizing sensitization processes in dinuclear luminescent lanthanide oligomers: selection of rigid aromatic spacers.

Abstract : This work illustrates a simple approach for optimizing the lanthanide luminescence in molecular dinuclear lanthanide complexes and identifies a particular multidentate europium complex as the best candidate for further incorporation into polymeric materials. The central phenyl ring in the bis-tridentate model ligands L3-L5, which are substituted with neutral (X = H, L3), electron-withdrawing (X = F, L4), or electron-donating (X = OCH3, L5) groups, separates the 2,6-bis(benzimidazol-2-yl)pyridine binding units of linear oligomeric multi-tridentate ligand strands that are designed for the complexation of luminescent trivalent lanthanides, Ln(III). Reactions of L3-L5 with [Ln(hfac)3(diglyme)] (hfac- is the hexafluoroacetylacetonate anion) produce saturated single-stranded dumbbell-shaped complexes [Ln2(Lk)(hfac)6] (k = 3-5), in which the lanthanide ions of the two nine-coordinate neutral [N3Ln(hfac)3] units are separated by 12-14 Å. The thermodynamic affinities of [Ln(hfac)3] for the tridentate binding sites in L3-L5 are average (6.6 ≤ log(β(2,1)(Y,Lk)) ≤ 8.4) but still result in 15-30% dissociation at millimolar concentrations in acetonitrile. In addition to the empirical solubility trend found in organic solvents (L4 > L3 > L5), which suggests that the 1,4-difluorophenyl spacer in L4 is preferable, we have developed a novel tool for deciphering the photophysical sensitization processes operating in [Eu2(Lk)(hfac)6]. A simple interpretation of the complete set of rate constants characterizing the energy migration mechanisms provides straightforward objective criteria for the selection of [Eu2(L4)(hfac)6] as the most promising building block.
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Submitted on : Wednesday, July 25, 2012 - 11:54:09 AM
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Jean-François Lemonnier, Laure Guénée, César Beuchat, Tomasz A Wesolowski, Prasun Mukherjee, et al.. Optimizing sensitization processes in dinuclear luminescent lanthanide oligomers: selection of rigid aromatic spacers.. Journal of the American Chemical Society, American Chemical Society, 2011, 133 (40), pp.16219-34. ⟨10.1021/ja206806t⟩. ⟨hal-00720638⟩



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