Water-soluble lanthanide complexes with a helical ligand modified for strong luminescence in a wide pH region†
The luminescence and structure of a water-soluble helicate Eu complex with LCOOH, which is a hexadentate ligand with COOH groups, were examined in solutions of various pH. The Eu complex EuLCOOH 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 EuLCOOH broadened because of amorphous-like aggregation. EuLCOOH 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 EuLCOOH 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 EuLCOOH could provide hydrophilicity in the initial condition, and the other would be released from Eu at pH 2.1–2.6. Furthermore, EuLCOOH decomposes below pH 1.9, but recovers with adjustments toward the initial pH. The ff emission of TbLCOOH with a coordinated water molecule also appears in the green wavelength region in the initial condition. The luminescence ability of TbLCOOH 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 LCOOH assigned from the phosphorescence band of GdLCOOH.