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.