A [Ru^II(bipy)₃]-[1,9-diamino-3,7-diazanonane-4,6-dione] two-component system, as an efficient ON–OFF luminescent chemosensor for Ni²⁺ and Cu²⁺ in water, based on an ET (energy transfer) mechanism

Abstract

A dioxo-tetramine ligand (1,9-diamino-3,7-diazanonane-4,6-dione ≡ dioxo-2,3,2-tet) has been appended to a Ru^II(bipy)₃ unit. This new system, 2, is water-soluble and capable of sensing Cu²⁺ and Ni²⁺ cations thanks to the strong quenching of the Ru(bipy)₃ fluorescence, which takes place when a metal cation is coordinated by the dioxo-2,3,2-tet binding unit. Coordination requires the energetically expensive deprotonation of the amide nitrogens, so that only Cu²⁺ and Ni²⁺ are able to promote it among the series of divalent first-row transition metal cations. Moreover, the complexation reaction is pH-dependent and one can distinguish between the two metal cations on working at the proper pH. The quenching mechanism has been examined by measuring the lifetime of the excited state of the ruthenium luminophore both on the metal-free and metal-complexed system and by flash photolysis experiments carried out on the complexed systems. The results clearly indicate that an energy transfer mechanism holds both for the Cu²⁺ and Ni²⁺ complex. The characterization of 2 as a water soluble ON–OFF sensor for copper and nickel has also been checked for its lowest detection limit, finding that these two metals can be detected down to a 10^–7 M concentration. Moreover, also system 3, containing a dioxo-2,3,2-tet ligand and the Re^I(CO)₃bipy(Cl) luminophore, has been examined as another possible water-soluble ON–OFF fluorescent sensor for the same transition metal cations. Again, only Cu²⁺ and Ni²⁺ are bound with a pH-dependent equilibrium, but incomplete luminescence quenching was observed, which prevented the determination of the quenching mechanism.

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