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 (L-amino) was synthesized by reducing its precursor, L-imino, with sodium borohydride in methanol. The L-amino sample binds to anions due to the more flexible and bent conformation of the capped aminophenolic binding core, compared to the precursor L-imino. The L-amino 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) mu m ((116 +/- 20) ppb) over 15 anions studied, including other phosphates, such as P2O74-, adenosine monophosphate (AMP(2-)), adenosine diphosphate (ADP(2-)), and adenosine triphosphate (ATP(2-)). The L-amino sample shows an increase in the absorbance at lambda = 315 nm in the presence of H2PO4-, CO32-, HCO3-, CH3CO2-, and F-. The H-1 NMR spectroscopic titration of L-amino 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 L-amino sample showed reversible sensing towards H2PO4- and F- in the presence of Mg2+ and Ca2+, respectively. NOESY studies of L-amino, 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 L-amino and H2PO4- or F-. In the SEM micrographs of L-amino, 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.