Sensing Zn2+ in Aqueous Solution with a Fluorescent Scorpiand Macrocyclic Ligand Decorated with an Anthracene Bearing Tail

Abstract

Synthesis of the new scorpiand ligand L composed of a [9]aneN₃ macrocyclic ring bearing a CH₂CH₂NHCH₂-anthracene tail is reported. L forms both cation (Zn²⁺) and anion (phosphate, benzoate) complexes. In addition, the zinc complexes of L bind these anions. The equilibrium constants for ligand protonation and complex formation were determined in 0.1 M NaCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric (pH-metric) titrations. pH Controlled coordination/detachment of the ligand tail to Zn²⁺ switch on and off the fluorescence emission from the anthracene fluorophore. Accordingly, L is able to sense Zn²⁺ in the pH range 6-10 down to nM concentrations of the metal ion. L can efficiently sense Zn²⁺ even in the presence of large excess of coordinating anions, such as cyanide, sulphide, phosphate and benzoate, despite their ability to bind the metal ion.

Keywords: anion binding; azamacrocycles; chemosensor; fluorescence; scorpiand; supramolecular interactions; zinc.