A selective fluorescent probe for gadolinium^III in water based on a Pd^II-preorganized chromone-receptor

The synthesis, solution studies, photochemical properties and the X-ray structure of a chromone based fluorescent Pd^II complex are reported. The ligand contains two chromone units linked as side arms to an ethylenediamine moiety; in the Pd^II complex the metal ion preorganizes the two hydroxychromone units forming a rigid structure with a negatively charged pocket formed by four oxygen atoms that is able to interact with hard metal cations, such as ions, giving rise to stable bimetallic complexes. Upon interaction with La^III and Gd^III, in particular, the emission intensity at 423 nm increases by a factor of 2 and 8, respectively, while the other rare earth ions quench the fluorescence. Spectrofluorimetric studies on real matrices showed the possibility to use this system as a selective fluorescence probe to detect and trace the presence of Gadolinium in environmental water acting as an OFF-ON chemosensor, with a LOD of 0.4 ppm and a LOQ of 1.2 ppm.