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

A dioxo-tetramine ligand (1,9-diamino-3, 7-diazanonane-4, 6-dione drop 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 hash 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)(3)bipy(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.