A new colorimetric receptor for selective detection of maleate vs. fumarate and ratiometric detection of F− ions

Abstract

Two new receptors, R1 and R2, were designed and synthesized for the isomeric discrimination of maleate ions over fumarate ions. Receptor R1 showed high selectivity with a significant color change from colorless to orange red with Δλ_max of 104 nm. This was owing to the hydrogen bonded electrostatic interactions of maleate ions with receptor R1. The formation of hydrogen bond between receptor R1 and maleate ions was confirmed by ¹H NMR titrations, where the signal at δ 12.25 corresponding to the benzohydrazide –NH proton showed a downfield shift upon binding with the anion. Receptor R3 did not show any binding with maleate ions or fumarate ions due to steric hindrance. Receptors R1 and R2 detected F⁻ ions with color changes from colorless to orange and colorless to pale yellow, respectively. Upon incremental addition of F⁻ ions, the color was further transformed from orange to blood red in receptor R1 and pale yellow to dark yellow in receptor R2. This result was due to the formation of initial hydrogen bonds followed by the deprotonation of receptors at higher concentrations of F⁻ ions.