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A simple ditopic salt receptor able to bind sodium salts selectively is presented. Consisting of a thiourea anion binding domain supported with an anthraquinone sensing unit, these receptors are able to sense anions and ion pairs optically and electrochemically.
We synthesized simple ion pair receptors consisting of a crown ether cation binding site and an anthraquinone-supported thiourea anion binding domain and studied their anion-, cation-, and salt-binding properties using spectroscopic, spectrophotometric, and electrochemical measurements in acetonitrile solution. Apart from carboxylate anions, which cause deprotonation, all the anions tested were found to associate with receptor 1 more strongly in the presence of sodium cations, whereas in the presence of potassium or ammonium cation the anion binding strength was greatly diminished. A homotopic anion receptor 3, lacking a crown ether unit, was unable to bind sodium salt more strongly than tetrabutylammonium salts. Solution and solid-state X-ray measurements revealed that strong sodium coordination with the cation-binding domain is responsible for the salt-binding enhancement. Electrochemical measurements showed that the addition of anions to the receptor 1 pretreated with sodium cations resulted in greater changes in reduction potentials compared to the addition of anions to receptor 1 in the absence of Na+.
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