Selective potassium ion recognition by benzo-15-crown-5 fluoroionophore/y-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/gamma -cyclodextrin (gamma -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 gamma -CyD and exhibit the pyrene dimer emission in water. Equilibrium analysis of the C3/gamma -CyD complex reveals that the observed dimer emission arises from a 2:1:1 complex of C3 with K+ and gamma -CyD, In the absence of KC, the fluorescence lifetimes for the dimer species ((C3)(2)CyD 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 KCI, a rising component corresponding to pyrene excimer formation is observed at the dimer emission region. For the C3/gamma -CyD complex, the apparent association constant for K+ of (3.8 +/- 1.3) x 10(9) M-2 is only slightly affected by the presence of Nat. Although the C5/gamma -CyD complex shows high sensitivity for K+, the selectivity for K+ over Na+ is lower than that of the C3/gamma -CyD complex. In contrast, fluoroionophore C1 with the shortest methylene spacer exhibits no response for alkali metal cations in the presence of gamma -CyD, These results demonstrate that the response function of supramolecular Cn/gamma -CyD complexes is strongly affected by the methylene spacer length of Cn, The highest K+ selectivity is obtained for the C3/gamma -CyD complexes in water.