Water-Soluble and Highly Luminescent Europium(III) Complexes with Favorable Photostability and Sensitive pH Response Behavior

Two 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.