Selective potassium ion recognition by benzo-15-crown-5 fluoroionophore/γ-cyclodextrin complex sensors in water

Three fluoroionophores Cn (n = 1, 3, 5), in which the crown ether unit and pyrenyl moiety are connected by a - (CH₂)(n) - spacer, have been used to construct supramolecular Cn/γ-cyclodextrin (γ-CyD) complexes for alkali metal ion sensing in water. The Cn (n = 3, 5) are found to selectively form 2:1 complexes with K⁺ in the presence of γ-CyD and exhibit the pyrene dimer emission in water. Equilibrium analysis of the C3/γ-CyD complex reveals that the observed dimer emission arises from a 2:1:1 complex of C3 with K⁺ and γ-CyD. In the absence of K⁺, the fluorescence lifetimes for the dimer species ((C3)₂CyD and (C3)₂(CyD)₂) and the monomer species (C3CyD and C3(CyD)₂) are 13-18 and 130-180 ns, respectively. Upon addition of 0.10 M KCl, a rising component corresponding to pyrene excimer formation is observed at the dimer emission region. For the C3/γ-CyD complex, the apparent association constant for K⁺ of (3.8 ± 1.3) x 10⁹ M^-2 is only slightly affected by the presence of Na⁺. Although the C5/γ-CyD complex shows high sensitivity for K⁺, the selectivity for K⁺ over Na⁺ is lower than that of the C3/γ-CyD complex. In contrast, fluoroionophore C1 with the shortest methylene spacer exhibits no response for alkali metal cations in the presence of γ-CyD. These results demonstrate that the response function of supramolecular Cn/γ-CyD complexes is strongly affected by the methylene spacer length of Cn. The highest K⁺ selectivity is obtained for the C3/γ-CyD complexes in water.