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. (C) 2011 Elsevier B.V. All rights reserved.