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

Abstract

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.

Figure 1

Ligands based on 2-hydroxyisophthalamides (IAM).

Figure 2

Ligands based on 1-hydroxy-2-pyridinone (1,2-HOPO).

Figure 3

Schematic representation of the design change for the 1^st to the 2^nd generation of enantiopure, octadentate ligands: Chiral information (green spheres) in the polyamine backbone for II (blue part) instead of incorporation into the chelating IAM moiety in I (red part).

Figure 4

Schematic representation of the design principle for the 3^rd generation of chiral, octadentate 1,2-HOPO ligands for europium luminescence: Blocking of the bottom coordination site for water molecules through longer ligand arms (C₃ unit vs. C₂ unit).

Figure 5

Aromatic region of the ¹H NMR spectra (500 MHz) of saturated solutions of [Eu(H3)(H₂O)] (top) and [Eu(H4)(H₂O)] (bottom) in CD₃OD.

Figure 6

Normalized absorption (dotted lines) and steady-state emission (solid lines) of [Tb(H1)] (left, λ_exc = 340 nm) and [Tb(H2)(H₂O)] (right, λ_exc = 340 nm) in 0.1 M aqueous Tris buffer at pH 7.4.

Figure 7

Normalized absorption (dotted lines) and steady-state emission (solid lines) of [Eu(H3)(H₂O)] (left, λ_exc = 328 nm) and [Eu(H4)(H₂O)] (right, λ_exc = 340 nm) in 0.1 M aqueous Tris buffer at pH 7.4.

Figure 8

Normalized absorption (dotted line) and steady-state emission (solid line, λ_exc = 347 nm) of [Eu(H5)(H₂O)] in 0.1 M Tris buffer (pH 7.4)/MeOH (1:9, v/v).

Figure 9

CPL (top) and total luminescence (bottom) spectra of the ⁵D₄→⁷F₅ transition of [Tb(H2)(H₂O)] in MeOH (left: c ≈ 1 mM, excitation at λ = 360 nm) and in 0.1 M Tris buffer at pH 7.4 (right: saturated solution, excitation at λ = 346 nm).

Scheme 1

Synthesis of the enantiopure, tetrapodal hexamine backbone 10. a) Benzene, 56%; b) HBr (48%)/HOAc; c) Dowex 1x8 (HO⁻), H₂O, 54% (from 9).

Scheme 2

Assembly of the new octadentate ligands H₄2, H₄3, and H₄5. a) NEt₃, CH₂Cl₂, 39%; b) BBr₃, CH₂Cl₂, 63%; c) NEt₃, CH₂Cl₂, 58%; d) HCl/HOAc, 74%; e) NEt₃, CH₂Cl₂, 34%; f) HCl/HOAc, 97%.

Scheme 3

Complex synthesis.