Fluorescent zinc(II) complex exhibiting “on-off-on” switching toward Cu²⁺ and Ag⁺ ions

Abstract

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, ¹H, ¹³C NMR, ESI-MS, electronic absorption, emission), and electrochemical studies. The structures of H₂L and complexes [{Zn(C₂₃H₁₈N₂O₂)}₂] (1) and [{Cu(C₂₃H₁₈N₂O₂)}₂]·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 ¹H 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.