Evaluation of anion selectivity in protic media by squaramide–Cresol Red ensembles

A new series of positively charged squaramide-ammonium compounds 1–6 bind sulfate and hydrogen phosphate in ethanol–water mixtures by a combination of electrostatic and hydrogen bonding interactions. A sensing ensemble composed of Cresol Red and an squaramide receptor signals the association events of these anions. Competitive experiments in ethanol–water mixtures (9:1) reveal a moderate selectivity for sulfate over hydrogen phosphate and suggest that squaramide 4, featuring an intermediate distance between squaramide subunits, is better suited for complexation of sulfate.