Circularly polarized luminescence in enantiopure europium and terbium complexes with modular, all-oxygen donor ligands.

The modular syntheses of three new octadentate, enantiopure ligands are reported, one with the bidentate chelating unit 2-hydroxyisophthalamide (IAM) and two with bidentate 1-hydroxy-2-pyridinone (1,2-HOPO) units. A new design principle is introduced for the chiral, non-racemic hexamines which constitute the central backbones for the presented class of ligands. The terbium(III) complex of the IAM ligand, as well as the europium(III) complexes of the 1,2-HOPO ligands, are synthesized and characterized by various techniques (NMR, UV, CD, luminescence spectroscopy). All species exhibit excellent stability and moderate to high luminescence efficiency (quantum yields Phi(Eu) = 0.05-0.08 and Phi(Tb) = 0.30-0.57) in aqueous solution at physiological pH. Special focus is put onto the properties of the complexes in regard to circularly polarized luminescence (CPL). The maximum luminescence dissymmetry factors (g(lum)) in aqueous solution are high with |g(lum)|(max) = 0.08-0.40. Together with the very favorable general properties (good stability, high quantum yields, long lifetimes), the presented lanthanide complexes can be considered as good candidates for analytical probes based on CPL in biologically relevant environments.

A promising change in the selection of the circular polarization excitation used in the measurement of Eu(III) circularly polarized luminescence.

A judicious change in the selected transition used for circular polarization excitation will overcome the low oscillator strength limitation of the currently allowed magnetic-dipole (5)D1 <-- (7)F2 (Eu(III)) transition chosen for circularly polarized luminescence (CPL) measurement. The proposed allowed magnetic-dipole (5)D1 <-- (7)F0 (Eu(III)) transition will facilitate the detection of CPL from the Eu(III) systems of interest. CPL on the acetonitrile solution of the chiral tris complex of Eu(III) with (R,R)-N,N'-bis(1-phenylethyl)-2,6-pyridinedicarboxamide ([Eu((R,R)-1)3](3+)), recently suggested as an effective and reliable CPL calibrating agent, confirms the feasibility of the proposed experimental procedure. A comparable CPL activity exhibited by the acetonitrile solution of [Eu((R,R)-1)3](3+) following direct excitation in the spectral range of the (5)D1 <-- (7)F0 transition and upon indirect excitation through the ligand absorption bands (lambda(exc) = 308 nm) was observed. This confirms that the recommended magnetic-dipole allowed absorption transition, (5)D1 <-- (7)F0, is the transition to be considered in the measurement of CPL. This work provides critical direction for the continued instrumental improvements that can be done for developing CPL into a biomolecular structural probe.

A [Cyclentetrakis(methylene)]tetrakis[2-hydroxybenzamide] Ligand That Complexes and Sensitizes Lanthanide(III) Ions.

The synthesis of the cyclen derivative H(4)L(1)·2 HBr containing four 2-hydroxybenzamide groups is described. The spectroscopic properties of the Ln(III) conplexes of L(1) (Ln=Gd, Tb, Yb, and Eu) reveal changes of the UV/VIS-absorption, circular-dichroism-absorption, luminescence, and circularly polarized luminescence spectra. It is shown that at least two metal-complex species are present in solution, whose relative amounts are pH dependent. At pH > 8.0, an intense long-lived emission is observed (for [TbL(1)] and [YbL(1)]), while at pH < 8.0, a weaker, shorter-lived species predominates. Unconventional Ln(III) emitters (Pr, Nd, Sm, Dy, and Tm) were sensitized in basic solution, both in the VIS and in the near-IR, to measure the emission of these ions.