Two Schiff base ligands for distinguishing ZnII/CdII sensing—effect of substituent on fluorescent sensing

Abstract

Two Schiff base ligands (HL¹, HL²) were conveniently synthesised by one-step condensation between pyridine 2-ylmethanamine and 3-ethoxy-2-hydroxybenzenaldhyde (for HL¹) or salicylaldehyde (for HL²) as fluorescent sensors for distinguishing sensing of Zn²⁺ or Cd²⁺. Both of the two fluorescent sensors present very weak emission at 463 nm (for HL¹) or 453 nm (for HL²). For HL¹, upon addition of Zn²⁺, the fluorescence intensity of HL¹ enhanced and gradually red shifted to 493 nm with a green emission while addition of Cd²⁺ only induced enhancement of fluorescent intensity at 463 nm. For HL², only addition of Zn²⁺ induced enhancement of fluorescence intensity, presenting a high Zn²⁺/Cd²⁺ selectivity. A Zn²⁺-induced red shift in fluorescent spectra of HL¹ could be attributed to twisted intramolecular charge transfer (TICT) from the interaction between the Zn²⁺ ion and in situ formed ligand L^1′ with the twisted structure in compound 1, which is absent in compound 2. The Zn²⁺/Cd²⁺ selectivity of fluorescent response for HL² correlates with the Cd–HL² and Zn–HL² coordination bond distances. Obviously, introduction of ethoxyl groups onto the benzene ring as an electron-donating group facilitates the Zn-induced in situ dimerization of HL¹ into new ligand L^1′ with a twisted molecular structure, further resulting in a red shift of the fluorescence spectra.