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 diox o-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 t he 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. Coordin ation requires the energetically expensive deprotonation of the amide nitro gens, so that only Cu2+ and Ni2+ are able to promote it among the series of divalent first-row transition metal cations. Moreover, the complexation re action is pH-dependent and one can distinguish between the two metal cation s on working at the proper pH. The quenching mechanism has been examined by measuring the lifetime of the excited state of the ruthenium luminophore b oth on the metal-free and metal-complexed system and by hash photolysis exp eriments 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)bip y(Cl) luminophore, has been examined as another possible water-soluble ON-O FF fluorescent sensor for the same transition metal cations. Again, only Cu 2+ and Ni2+ are bound with a pH-dependent equilibrium, but incomplete lumin escence quenching was observed, which prevented the determination of the qu enching mechanism.