Influence of amidoammonium calix[4] resorcinarenes on methyl orange protolytic equilibrium: supramolecular indicator systems

Here, we report on the study of cationic amidoammonium calix[4]resorcinarenes 1-5 of various lipophilicity capable of binding acid-base indicator methyl orange (MO). We identified the contributions of macrocycle aggregation and conformational mobility in the binding of MO. The effective pK(a) values of bound MO systematically decrease as the size and the packing density of the aggregates increase with an increase in calixresorcinarene lipophilicity. Consideration of a series of macrocycles clearly shows that large aggregates form most stable complexes, binding guests not on individual level but as aggregates. It was found that the most stable MO complex with 5 is formed due to electrostatic binding with ammonium groups of the macrocycle and incapsulation of MO in a hydrophobic layer of the aggregates. We have shown that competitive binding of MO and cationic surfactants by aggregates of 5 is suitable for visual/spectrophotometric detection of colourless anionic substrates.