A 1,3-Capped Calix[4] Conjugate Possessing an Amine Moiety as an Anion Receptor: Reversible Anion Sensing Detected by Spectroscopy and Characterization of the Supramolecular Features by Microscopy.

A phenylenediamine-capped conjugate of calix[4]arene ( Lamino) was synthesized by reducing its precursor, Limino, with sodium borohydride in methanol. The Lamino sample binds to anions due to the more flexible and bent conformation of the capped aminophenolic binding core, compared to the precursor Limino. The Lamino sample showed selectivity towards H2PO4− by exhibiting a ratiometric increase in emission by about 11-fold with a detection limit of (1.2±0.2) μ m ((116±20) ppb) over 15 anions studied, including other phosphates, such as P2O74−, adenosine monophosphate (AMP2−), adenosine diphosphate (ADP2−), and adenosine triphosphate (ATP2−). The Lamino sample shows an increase in the absorbance at λ=315 nm in the presence of H2PO4−, CO32−, HCO3−, CH3CO2−, and F−. The 1H NMR spectroscopic titration of Lamino with H2PO4−, F−, and CH3CO2− showed major changes in the phenylene-capped and salicyl moieties, and thereby, confirming the aminophenolic region as the binding core. However, the binding strength of these anions followed the trend H2PO4−>F−≫CH3CO2−>HSO4−. The heat changes observed by isothermal titration calorimetry support this trend. The Lamino sample showed reversible sensing towards H2PO4− and F− in the presence of Mg2+ and Ca2+, respectively. NOESY studies of Lamino, in comparison with its anionic complexes, revealed that major conformational changes occurred in the capping region to facilitate the binding of anion. ESI-MS and the Job's method revealed 1:1 stoichiometry between Lamino and H2PO4− or F−. In the SEM micrographs of Lamino, the spherical particles are converted into spherical aggregates and further form large agglomerates and even branched sheets in the presence of anions, depending upon their binding strength. [ABSTRACT FROM AUTHOR]