Chiral lanthanide complexes

Abstract

The use of chiral lanthanide complexes as probes to investigate interactions with other chiral molecules and macromolecules is of particular interest. A key step in the development of such systems is the preparation of a single, rigid enantiomer of the complex which is conformationally rigid on the NMR timescale or the lifetime of the metal-based emission. Chiral europium and terbium complexes are of particular concern as they may function as emissive probes and are amenable to analysis by circular dichroism and circularly polarised lummescence. Charge neutral and cationic complexes of N-substituted 1,4,7,10- tetraazacyclododecane, functionahsed with three phosphmate and one amide pendent arms, or two, three and four pendent amide arms containing a remote chiral centre, have been prepared. The chirality of the remote stereocentre determmes the helicity of the arrangement of the pendent arms and the conformation of the 12-membered macrocycle.The chiral europium and terbium complexes of the tetraamide complexes exist as single, rigid enantiomers, exhibiting a well-defined metal-based circularly polarised emission. Circular dichroism studies reveal that exciton coupling occurs between adjacent pairs of 1-naphthyl chromophores of tetraamides, whereas the constitutional 2- naphthyl isomers exhibit excimer formation. The behaviour of the di-, tri-, and tetraamide complexes in the presence of added anions has been investigated. The measurement of the luminescent lifetime of the excited state and the circularly polarised emission exhibited by the enantiopure complexes in aqueous solution affords a novel way of signalling the presence of selected oxy-anions.