Water-Soluble and Highly Luminescent Europium(III) Complexes with

Sep 26, 2016 - ... Luminescent Signaling in the Physiological pH Range J. Am. Chem. ...... Structural Elucidation by Sparkle/AM1 Calculations, and Dop...
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Water-Soluble and Highly Luminescent Europium(III) Complexes with Favorable Photostability and Sensitive pH Response Behavior Chen Wei, Huibo Wei,* Weibo Yan, Zifeng Zhao, Zelun Cai, Boxun Sun, Zhaosha Meng, Zhiwei Liu, Zuqiang Bian,* and Chunhui Huang Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: Two highly luminescent and water-soluble Eu(III) complexes, Eu1 and Eu2, based on novel carboxyl-functionalized 1,5naphthyridine derivatives 8-hydroxy-1,5-naphthyridine-2-carboxylic acid (H2 L1) and 7-cyano-8-hydroxy-1,5-naphthyridine-2-carboxylic acid (H2L2), respectively, are designed and synthesized. The crystal structure of Eu2 indicates that the central Eu(III) ion is nine-coordinated by three tridentate ligands (O^N^O). Both Eu1 and Eu2 show strong luminescence in aqueous solution with quantum yields (lifetimes) of 28% (1.1 ms) and 14% (0.76 ms), respectively. The chelates display unique UV-light stability in solution and remain highly emissive after 100 min of strong UV irradiation (∼300 W·m−2 at 345 nm). Moreover, they exhibit reversible luminescence intensity changes with varied pH values, and the response mechanism is investigated. “Turn-on” of the Eu(III) emission upon increasing pH is realized by ligand structure change from keto to enol anion form, resulting in red-shifted absorption band and suppressed quenching from solvents and N−H vibration upon deprotonating. The results show that these novel Eu(III) complexes are quite intriguing for potential application as bioimaging agents and pH probes.

phenanthridine derivatives,16 pyridyl,13,17 dipyridyl,18 and terpyridyl7b derivatives as well as azathiaxanthone19 chromophore. However, most of these complexes suffer from relatively low quantum yield in aqueous solutions (