Lemonnier, J. F. ; Guenee, L. ; Beuchat, C. ; Wesolowski, T. A. ; Mukherjee, P. ; Waldeck, D. H. ; Gogick, K. A. ; Petoud, S. and Piguet, C.
J. Am. Chem. Soc.133 (40) 16219-16234
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 = OCH(3), 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 [Ln(2)(Lk)(hfac)(6)] (k = 3-5), in which the lanthanide ions of the two nine-coordinate neutral [N(3)Ln(hfac)(3)] units are separated by 12-14 angstrom. The thermodynamic affinities of [Ln(hfac)(3)] for the tridentate binding sites in L3-L5 are average (6.6 <= log(beta(Y,Lk)(2,1)) <=> 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 [Eu(2)(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 [Eu(2)(L4)(hfac)(6)] as the most promising building block.