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

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.