Optimizing Sensitization Processes in Dinuclear Luminescent Lanthanide Oligomers: Selection of Rigid Aromatic Spacers

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 [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)) <= 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 [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.