An Octadentate Luminescent Eu(III) 1,2-HOPO Chelate with Potent Aqueous Stability

The synthesis, characterization, and photophysical properties of
two novel ligands, 5LINMe-1,2-HOPO (1) and H(2,2)-1,2-HOPO (2),
which utilize the 1,2-HOPO chelate as a sensitizer for Eu(III) are
reported. In addition, the former ligand was structurally characterized as the Eu(III) complex by X-ray crystallography. The [Eu(1)2]-
complex of the tetradentate ligand (1) is stable in aqueous solution,
to a limiting concentration of ca. 7 × 10-9 M, and retains the
superior photophysical performance noted for the 1,2-HOPO
sensitizer. By contrast, the octadentate ligand (2) has vastly
improved stability as the [Eu(2)]- complex upon further dilution,
to a limiting concentration of ca. 5 × 10-17 M, which is beyond
the minimum detectable concentration of most fluorimeters. The
presence of a single coordinated water molecule for the latter
complex reduces the overall metal-centered luminescence.