Water-soluble lanthanide complexes with a helical ligand modified for strong luminescence in a wide pH region

Abstract

The luminescence and structure of a water-soluble helicate Eu complex with L^COOH, which is a hexadentate ligand with COOH groups, were examined in solutions of various pH. The Eu complex EuL^COOH is more than eight-coordinated around Eu, and one of the two carboxylate groups is not bound, even in the powdered state, based on the FT-IR measurement. The synchrotron X-ray powder diffraction peaks of EuL^COOH broadened because of amorphous-like aggregation. EuL^COOH retains the ff emission of Eu in a wide pH range of 2.6–9.7 and shows spectral reversibility at pH = 1.9–11.9. From Horrocks' equation, the number of coordinated water molecules to Eu of EuL^COOH in the initial condition (3.0 × 10⁻⁵ mol cm⁻³; pH 2.6–9.7) was estimated to be 1, but it increased in both strongly alkaline and acidic solutions. Molecular structural changes in these solutions were assumed from the quantum yields and electrospray ionization-mass spectrometry measurements. For instance, one carboxyl group of EuL^COOH could provide hydrophilicity in the initial condition, and the other would be released from Eu at pH 2.1–2.6. Furthermore, EuL^COOH decomposes below pH 1.9, but recovers with adjustments toward the initial pH. The ff emission of TbL^COOH with a coordinated water molecule also appears in the green wavelength region in the initial condition. The luminescence ability of TbL^COOH in water is higher than that in its powder. The luminescence processes of these complexes are discussed as an energy relaxation through the excited triplet state of L^COOH assigned from the phosphorescence band of GdL^COOH.