Crown ether-lanthanide complexes studied using CPL and TL. I. The solution stoichiometry of europium and terbium nitrate complexes of (2R,3R,11R,12R)-2,3,11,12-tetramethyl-18-crown-6

Abstract

As part of an extensive study of the use of circularly polarized luminescence (CPL) and total luminescence (TL) in probing crown etherlanthanide interactions in nonaqueous solvents, we describe here the results of an anion-exchange experiment designed to elucidate the solution stoichiometry of a chiral crown ether-lanthanide nitrate complex in trifluoroethanol (TFE). The CPL experiment measures the differential emission of left and right circularly polarized light by a chiral luminophore when it is excited with unpolarized light. CPL is sensitive to solutions structure when a (achiral) luminosphore gains chirality through association with a chiral ligand. For lanthanides, CPL is much more easily measured than CD because of the inherent sensitivity of emission measurement and the weakness of f-f transitions in absorption. Although all lanthanides are capable of exhibiting CPL in chiral crystals, only europiym (III) and terbium(III) exhibit significant luminescence in solution, and thus solution CPL studies of lanthanides are normally limited to these ions. This study shows the usefulness of CPL and TL measurements in conjunction with NMR in characterizing subtle aspects of the solution behavior of crown-lanthanide systems that otherwise would be difficult to study.