Effect of protonation and Zn(II) coordination on the fluorescence emissionof a phenanthroline-containing macrocycle. An unusual case of "nonemissive'' Zn(II) complex

Ligand 2,5,8-triaza[9]-10,23-phenanthrolinophane (L) contains a triamine ch ain connecting the 2,9 positions of a phenanthroline unit. Protonation of L has been studied by means of potentiometric and H-1 and C-13 NMR technique s, allowing the determination of the basicity constants and of the stepwise protonation sites. Protonation strongly affects the fluorescence emission properties of the chemosensor L. The two benzylic amine groups, namely, the two aliphatic amine groups adjacent to phenanthroline, are the most effici ent nitrogens in fluorescence emission quenching. In the diprotonated recep tor [H2L](2+) both of these nitrogens are protonated, and therefore this sp ecies is the most emissive. In the [H3L](3+) species the three acidic proto ns, are located on the amine groups of the polyamine chain. This species is still emissive, but less so than [H2L](2+), due to formation of a. hydroge n bond network involving the phenanthroline nitrogens, as shown by the crys tal structure of the [H3L]Br-3. H2O salt. A potentiometric investigation of Zn(II) binding in aqueous solution suggests that some nitrogen donors are not involved, or weakly involved in metal coordination. Actually, the cryst al structure of the [ZnL(H2O)](ClO4)(2) complex shows that both of the benz ylic amine groups are weakly bound to the metal. This Zn(II) complex does n ot show any fluorescence emission. This rather unusual feature can be expla ined considering an electron transfer process involving the benzylic nitrog ens.