Anion receptors with viologen molecular scaffold

We have designed and synthesized new anion receptors 1²⁺ and 2²⁺. These receptors interact with anions through hydrogen bonds and charge transfer complex depending on the basicity of anion. Therefore, anions with weak basicity such as chloride, bromide, and hydrogen sulfate bound to the receptors 1²⁺ and 2²⁺ only through hydrogen bonds while anions with strong basicity such as fluoride, acetate and dihydrogen phosphate bound to the receptors 1²⁺ and 2²⁺ only through charge transfer interactions at UV–vis titration condition (20 μM). However, in more concentrated ¹H NMR titration condition (2 mM), 1²⁺ and 2²⁺ decomposed to form the product one of their amide arm is eliminated. As charge transfer complexes showed colorimetric response, they turned out to be efficient naked eye detector for anions with strong basicity such as fluoride, acetate, and dihydrogen phosphate.