A water-stable and strongly luminescent self-assembled non-covalent lanthanide podate†

Abstract

The segmental ligand 2-(6-carboxypyridin-2-yl)-1,1′-dimethyl-2′-(5-methylpyridin-2-yl)-5,5′-methylenebis(1H-benzimidazole) (L⁹) reacted with an equimolar mixture of Ln^III (Ln = La or Eu) and Zn^II in basic conditions to give selectively the self-assembled dinuclear non-covalent podates [LnZn(L⁹ – H)₃]²⁺. Electrospray mass spectrometry and proton NMR spectroscopy show that [LnZn(L⁹–H)₃]²⁺ adopt the expected head-to-head triple-helical structure with Zn^II pseudo-octahedrally co-ordinated by the bidentate binding units of the three segmental ligands and Ln^III occupying the remaining facial pseudo-tricapped trigonal prismatic site produced by the wrapped unsymmetrical tridentate units. Upon UV irradiation, solutions of [EuZn(L⁹ – H)₃]²⁺ in acetonitrile or in water produce strong red luminescence. The Eu (⁵D₀) lifetime and quantum yield indicate that Eu^III is efficiently protected from external interactions for complex concentration in the range 10^–4–10^–8M and that no solvent molecule enters the first co-ordination sphere. Electrospray mass spectrometry combined with high-resolution emission spectroscopy confirm that the structure of the dinuclear triple-helical complex [EuZn(L⁹ – H)₃]²⁺ is maintained at low concentration which strongly contrasts with the lipophilic analogous non-covalent lanthanide podates [EuZn(Lⁱ)₃]⁵⁺ {i = 7 or 8; 2-[6-(organo)pyridin-2-yl]-1,1′-dimethyl-2′-(5-methylpyridin-2-yl)-5,5′-methylenebis(1H-benzimidazole)} which are decomplexed in acetonitrile for concentrations below 10^–5M. Detailed photophysical studies have established that [EuZn(L⁹ – H)₃]²⁺ works as an efficient UV → VIS light-converting device in the solid state and in water.