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Download fileWater-Soluble and Highly Luminescent Europium(III) Complexes with Favorable Photostability and Sensitive pH Response Behavior
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posted on 26.09.2016, 16:48 authored by Chen Wei, Huibo Wei, Weibo Yan, Zifeng Zhao, Zelun Cai, Boxun Sun, Zhaosha Meng, Zhiwei Liu, Zuqiang Bian, Chunhui HuangTwo highly luminescent and water-soluble Eu(III) complexes, Eu1 and Eu2, based on novel carboxyl-functionalized
1,5-naphthyridine derivatives 8-hydroxy-1,5-naphthyridine-2-carboxylic
acid (H2L1) 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.