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.

Abstract

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 H2 PO4 (-) by exhibiting a ratiometric increase in emission by about 11-fold with a detection limit of (1.2±0.2) μm ((116±20) ppb) over 15 anions studied, including other phosphates, such as P2 O7 (4-) , adenosine monophosphate (AMP(2-) ), adenosine diphosphate (ADP(2-) ), and adenosine triphosphate (ATP(2-) ). The Lamino sample shows an increase in the absorbance at λ=315 nm in the presence of H2 PO4 (-) , CO3 (2-) , HCO3 (-) , CH3 CO2 (-) , and F(-) . The (1) H NMR spectroscopic titration of Lamino with H2 PO4 (-) , F(-) , and CH3 CO2 (-) 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 H2 PO4 (-) >F(-) ≫CH3 CO2 (-) >HSO4 (-) . The heat changes observed by isothermal titration calorimetry support this trend. The Lamino sample showed reversible sensing towards H2 PO4 (-) and F(-) in the presence of Mg(2+) and Ca(2+) , 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 H2 PO4 (-) 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.