Selective binding and fluorescence sensing of Zn(<scp>ii</scp>)/Cd(<scp>ii</scp>) using macrocyclic tetra-amines with different fluorophores: insights into the design of selective chemosensors for transition metals

Selective binding and optical sensing of Zn(II) and Cd(II) by L1, HL2, L3, H₂L4 and H₂L5 receptors were analysed in aqueous solutions by coupling potentiometric, UV-vis absorption and fluorescence emission measurements, with the aim to determine the effect of complex stability on selective signalling of metals with similar electronic configurations. All receptors share the same cyclic tetra-amine binding unit attached to a single quinoline (Q) or 8-hydroxyquinoline (8-OHQ) unit (L1 and HL2, respectively), two Q or 8-OHQ moieties (L3 and H₂L4, respectively), and, finally, two Q and two acetate groups (H₂L5). The crystal structures of the Cd(II) and Zn(II) complexes show that L3 and H₂L4 feature a cavity in which the larger Cd(II) complex is better fitted than the Zn(II) complex, leading to the formation of more stable Cd(II) complexes. In turn, Zn(II) forms more stable complexes with L1 and HL2, owing to its high tendency to give 5-coordinated complexes. Considering optical selectivity, Zn(II) gives the most emissive complex with L3, while the corresponding Cd(II) complex is basically quenched. The gathered structure of the Zn(II) complex, in which the two Q units are associated with one another-a structural motif not observed in the [CdL3]²⁺ complex-leads to poor solvation of the Q units, favouring complex emission. Among 8-OHQ-containing receptors, the most emissive complex is formed by Cd(II) with HL2, containing a single 8-OHQ moiety. H₂L4 forms non-emissive complexes: the presence of two coordinating 8-OHQ moieties weakens metal interactions with the tetra-amine unit, favouring PET to the excited fluorophore that quench the emission.