Selective binding and fluorescence sensing of ZnII with acridine-based macrocycles

Proton and Cu II , Zn II , Cd II and Pb II binding by ligands L1 and L2, containing a triethylentetraamine and a tetraethylentetraamine aliphatic chain, respectively, linking the 2,7 positions of an acridine moiety, has been analysed by means of potentiometric, UV–Vis and fluorescence emission measurements in aqueous solution. Considering proton binding, L1 and L2 bind up to four and six acidic protons in the pH range 2–11. These protonation steps preferentially occur on the aliphatic amine groups; protonation of acridine takes place only at strongly acidic pH values. In metal complexation, L1 displays a marked selectivity for Zn II over Cd II and Pb II , due to the better accommodation of the smaller Zn II ion within the macrocyclic cavity. The fluorescence emission study points out that Zn II binding at neutral pH is accompanied by a marked increase of the acridine emission. Cd II binding gives rise to a much less intense increase of the emission, whereas Cu II and Pb II complexation leads to fluorescence quenching. Both thermodynamic and sensing selectivity are lost in the case of the larger macrocycle L2.