Characterization of Quinoxolinol Salen Ligands as Selective Ligands for Chemosensors for Uranium

Quinoxalinol salen ligands have been characterized as selective ligands for the rapid identification of uranyl. The absorption maximum of ligand 1 presented a hypsochromic (blue) shift with the addition of UO22+ (as the acetate salt), and a bathochromic (red) shift in the presence of Cu2+ or Co2+ acetate salts, resulting in distinct, visible color changes for all three metal ions. The absorption maximum of ligand 2 was not observed to change with the addition of UO22+; however, it does present a bathochromic shift with the addition of Cu2+, and a hypsochromic shift with Co2+ added. Using TDDFT calculations, it was demonstrated that the hypsochromic shift for UO22+ ion complexation with ligand 1 is caused by a ligand-to-metal charge transfer, while the bathochromic shift observed with Cu2+ ion addition was caused by a metal-to-ligand charge transfer. Finally, it was found that the addition of Cu2+ (as metal salts) to either ligand resulted in rapid, complete quenching of the ligand fluorescence.