A Squaraine-based Dipicolylamine Derivative Acting as a Turn-on Mercury(II) Fluorescent Probe in Water

International audience; A symmetrical squaraine-based ligand, sbdpa, bearing two dipicolylamine (dpa) units, was synthesized for the first time, using an environment friendly procedure, such as one-pot and solvent-free reaction. The sbdpa ligand was found to have a small blue-green emission, in aqueous solution, due to a photoinduced electron transfer (PET) mechanism. Metal complexes of sbdpa with a large panel of cations, Na+, K+, Ag+, Mg2+, Ca2+, Ba2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+, were prepared in aqueous buffered solution and screened, in search of a selective response for one of them. A turn-on fluorescence response was then found only for Hg2+, as a consequence of a chelation-enhanced fluorescence (CHEF) effect that disrupts the PET mechanism in sbdpa. Subsequently, the complexation behaviour of Hg2+ with sbdpa was investigated in aqueous solution by potentiometric titrations and other spectroscopic techniques. The acid-base reactions of the ligand were also studied by potentiometry, as well as by 1H-NMR and UV-vis titrations. For comparison purposes, the complexation of two other divalent cations, Cu2+ and Zn2+, was also assessed by potentiometry. Additionally, results are compared with dpa all along the study. Both mono- and dinuclear complexes of sbdpa were found for the three metal cations studied having the mercury(II) complex the largest thermodynamic stability. These findings support a stronger uptake of Hg2+ cation by sbdpa, possibly involving at least one oxygen atom from the squaraine moiety on its coordination sphere.