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

Proton and CuII, ZnII, CdII and PbII 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 ZnII over CdII and PbII, due to the better accommodation of the smaller ZnII ion within the macrocyclic cavity. The fluorescence emission study points out that ZnII binding at neutral pH is accompanied by a marked increase of the acridine emission. CdII binding gives rise to a much less intense increase of the emission, whereas CuII and PbII complexation leads to fluorescence quenching. Both thermodynamic and sensing selectivity are lost in the case of the larger macrocycle L2.