Charge Neutral Europium(III) Complex as a Highly Efficient Fluorescent Probe for Detection of Inorganic and Organic Anions

The development of efficient fluorescent sensors for detecting anions is of paramount importance due to the critical roles that anions play in biological systems, environmental processes, and industrial applications. Sensitive and selective detection methods are essential for monitoring anions in complex and competitive environments. Herein, we report the synthesis and photochemical properties of a novel fluorescent sensor designed for the detection of inorganic and organic anions in competitive media. The sensor is based on a charge-neutral europium(III) complex, formed through the coordination of Eu³⁺ ions with a β-diketonate ligand, resulting in a 3 : 1 complex featuring three urea functional groups positioned outside the metal coordination sphere. In solution, this complex exhibits intense reddish fluorescence (Φ=5.2 %), which remains stable even at very low concentrations (10^-6 m). Photochemical studies revealed that the fluorescence of the complex is quenched upon binding to anions via unlocking of the photoinduced electron transfer (PeT), enabling its use as a quantitative and qualitative turn-off probe for anion detection. Remarkably, the complex maintains high sensitivity and selectivity towards anions in competitive media, with association constants reaching up to 4.9×10⁵ m^-1. These results highlight the potential of this europium(III) complex as a highly efficient tool for detecting anionic species in challenging environments.