A squaraine-based dipicolylamine derivative acting as a turn-on mercury(ii) fluorescent probe in water

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 the mercury(ii) complex having 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.