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

• Published in: New journal of chemistry 2017, Vol.41 (14), p.6385-6394
• Main Authors: Ogata, Shuhei, Shimizu, Tomohito, Ishibashi, Takashi, Ishiyone, Yushi, Hanami, Mitsuhiro, Ito, Minami, Ishii, Ayumi, Kawaguchi, Shogo, Sugimoto, Kunihisa, Hasegawa, Miki
• Format: Article
• Language: English
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/C7NJ01444A

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 .