Fluorescent Zinc(II) Complex Exhibiting "On-Off-On" Switching Toward Cu2+ and Ag+ Ions

Binuclear zinc(II) and copper(II) complexes based on a new Schiff base ligand N,N'-bis(2-hydroxybenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H2L) have been synthesized. The ligand H2L and complexes under investigation have been characterized by elemental analyses, spectral (FT-IR H-1, C-13 NMR, ESI-MS, electronic absorption, emission), and electrochemical studies. The structures of H2L and complexes [{Zn(C23H18N2O2)}(2)] (1) and [{Cu(C23H18N2O2)}(2)]center dot H2O (2) have been determined crystallographically. Selective "On-Off-On" switching behavior of the fluorescent complex 1 has been studied. The fluorescence intensity of 1 quenches (turns-off) upon addition of Cu2+, while enhances (turns-on) in the presence of Ag+ ions. The mechanisms of "On-Off-On" signaling have been supported by H-1 NMR, ESI-MS, electronic absorption, and emission spectral studies. Job's plot analysis supported 1:1 and 1:2 stoichiometries for Cu2+ and Ag+ ions, respectively. Association and quenching constants have been estimated by the Benesi-Hildebrand method and Stern-Volmer plot. Moreover, 1 mimics a molecular keypad lock that follows correct chemical input order to give maximum output signal.