Amidothiourea based colorimetric receptors for basic anions: evidence of anion induced deprotonation of amide –NH proton and hydroxide induced anion⋯π interaction with the deprotonated receptors

Abstract

Two amidothiourea based receptors (L1 and L2) containing a π-acidic 3,5-dinitrophenyl chromophore have been synthesized in good yields and their anion recognition properties were evaluated both in organic and aqueous organic environment by spectroscopic techniques. Anions such as F⁻, AcO⁻ and H₂PO₄⁻ were examined to be suitable analytes for the receptor molecules, displaying optical signaling from colorless to orange/red, whereas anions of lower basicity such as Cl⁻, Br⁻, I⁻, NO₃⁻ and HSO₄⁻ did not cause any discernable spectral changes. The detailed ¹H NMR titration experiments and single crystal X-ray structural analyses revealed that the receptor–anion(s) interaction encourages deprotonation of the amide –NH proton of the amidothiourea function. Interestingly, the highly basic OH⁻ ion showed stepwise color changes with increasing equivalents, from colorless to red to green. The step wise color changes were found to be the outcome of OH⁻ (1 equiv.) induced mono-deprotonation of the individual receptors (colorless to red), followed by anion⋯π interaction (red to green) with the π-acidic 3,5-dinitrophenyl ring of the receptors beyond one equiv. of OH⁻ addition. This anion⋯π interaction between the OH⁻ ion and the synthesized receptors has also been confirmed by monitoring the OH⁻ induced absorption spectral changes of a control receptor (L_C).