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

The segmental ligand 2-(6-carboxypyridin-2-yl)-1,1'-dimethyl-2'-(5-methylpyridin-2-yl)-5, benzimidazole) (L-9 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(L-9-H),(3)](2+). Electrospray mass spectrometry and proton NMR spectroscopy show that [LnZn(L-9-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(L-9-H),(3)](2+) in acetonitrile or in water produce strong red luminescence. The Eu (D-5(0),) lifetime and quantum yield indicate that Eu-III is efficiently protected from external interactions for complex concentration in the range 10(-4)-10(-8) M 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(L-9-H),(3)](2+) is maintained at low concentration which strongly contrasts with the lipophilic analogous non-covalent lanthanide podates [EuZn(L-i),(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(-5) M. Detailed photophysical studies have established that [EuZn(L-9-H),(3)](2+) works as an efficient UV-->VIS light-converting device in the solid state and in water.