Synthesis and binding properties of calix[4]biscrown-based fluorescent molecular sensors for caesium or potassium ions

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Abstract
The synthesis of two new fluorescent molecular sensors based on calix[4]biscrowns is reported. A dioxycoumarin fluorophore was incorporated into one of the crowns or into both crowns, leading to Calix-COU1 and Calix-COU2, respectively. The stability constants of the 1 ∶ 1 and 1 ∶ 2 complexes with potassium and caesium ions in ethanol or acetonitrile were measured. An anticooperative effect was observed when binding a second cation to the 1 ∶ 1 complex owing to electrostatic repulsion. The selectivity of Calix-COU1 in ethanol expressed as the ratio of the stability constants of the 1 ∶ 1 complexes was found to be about 4 × 104 for Cs+versus Na+ and greater than 500 for K+versus Na+. The origin of the cation-induced changes in the photophysical properties is discussed.
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