Design and synthesis of malonohydrazide based colorimetric receptors for discrimination of maleate over fumarate and detection of F, AcO and AsO ions.

In this study, we have designed and synthesized two new organic receptors R1 and R2 based on malonohydrazide for the recognition of biologically important anions. The receptor R1 capable of colorimetric discrimination of maleate over fumarate ion, exhibit significant color change from pale yellow to wine red due to intermolecular hydrogen bond between the R1 and maleate ion, supported by HNMR titration, where the peak at δ12.0 ppm attributed to the NH proton experiences a downfield shift upon binding with maleate ion. Receptor R1, equipped with two electron-withdrawing NO moieties as the chromogenic signaling unit enhance the hydrogen bonding tendency and acidity, and thus when comparing with receptor R2, R1 displayed substantial higher redshift (∆λ of 148 nm, 143 nm, and 140 nm towards F, AcO, and maleate anion in the DMSO. In addition, the synthesized receptors R1 and R2 are able to detect F, AcO, and AsO ions as their sodium salts in an aqueous solution with visual color change. Receptor R1 exhibit electrochemical response towards F and AcO ions. The receptors R1 and R2 are successfully applied for quantitative detection of F ion in the toothpaste solution in an aqueous medium. Additionally, R1 and R2 exhibit fluorescence enhancement towards F and AcO ions in the DMSO. As well, R1 and R2 demonstrate to be potentially useful colorimetric chemosensor for sensing maleate ion using the test strip. The theoretical calculation based on TD-DFT corroborates well with the experimental results of the receptors R1 and R2 with fluoride, acetate and maleate.