SELF-ASSEMBLY OF HELICAL SUPRAMOLECULAR LANTHANIDE COMPLEXES

The spontaneous strict self-assembly of tailored oligo-multidentate ligands with d-transition metal ions leads to the selective formation of polynuclear double- and triple-helical supramolecular complexes. A significant improvement of the selectivity of the processes can be achieved by using segmental ligands which possess different binding units connected by spacers favouring helical coordination and pure heteropolynuclear helical complexes can be isolated under thermodynamic control. The application of these principles to 4f block ions requires the development of tridentate heterocyclic ligands designed for the synthesis of triple-helical self-assembled complexes containing lanthanides. The ligand 2,6 bis(1'-methyl-1H-benzimidazol-2'-yl)pyridine (L(4)) reacts with Ln(3+) in acetonitrile to give the three successive complexes [Ln(L(4))](3+), [Ln(L(4))(2)](3+) and [Ln(L(4))(3)](3+). The three ligands in [Ln(L(4))(3)](3+) are wrapped around Eu(III) and produce a pseudo-D-3 symmetrical structure considered as a building block for triple-helical luminescent supramolecular structures. The introduction of two tridentate binding units into the oligomultidentate ligand L(6) leads to the formation of dinuclear self-assembled triple-helical complexes [Ln(2)(L(6))(3)](6+) which work as molecular light-converting devices on the nanometric scale. The photophysical properties together with the formation of analogous heterodinuclear and homotrinuclear complexes are discussed.