Switching on the Fluorescence Emission of Polypyridine Ligands by Simultaneous Zinc(II) Binding and Protonation

The synthesis and characterization of the two new open-chain ligands 1,15-bis-[6-(2,2'-bipyridyl)]-2,5,8,11,14-pentaaza-octadecane (L1) and 1,15-bis-[2-(1,10-phenanthroline)-9-methyl]-2,5,8,11,14-pentaazaoctadecane (L2), both featuring a tetraethylenpentaamine chain linking via methylene bridges the 6 and 2 positions of two identical 2,2'-bipyridyl (bpy) and 9-methyl-1,10-phenanthroline (9-methyl-phen) moieties respectively, are reported. Their protonation and binding ability for Cu2+, Zn2+, Cd2+ and Pb2+ have been studied by coupling potentiometric titrations with UV-vis absorption and fluorescence emission measurements in water. L1 and L2 afford stable mono- and dinuclear complexes, in which the metal ion is bound by a single bpy or 9-methyl-phen unit and the amine groups on the aliphatic chain. However, L1 displays a greater binding ability for Cu2+ and Zn2+ with respect to L2, the stability constants of the [ML1]2+ complexes being 21.8 (Cu2+) and 19.4 (Zn2+) log units vs 20.34 and 16.8 log. units for the corresponding L2 species. Among all the metal ions tested, only the Zn2+ complex with L2 features an enhanced fluorescence emission at neutral pH, thanks to the simultaneous binding of one Zn2+ ion and H+ ion(s), that inhibits any possible photoinduced electron transfer (PET) process from the amine donors to the excited phen moiety. Binding of a second metal switches off the emission again.