Supramolecular Crown Ether Probe/γ -Cyclodextrin Complex Sensors for Alkali Metal Ion Recognition in Water

Crown ether probes C3-12C4 and C3-18C6, in which the pyrenyl moieties as fluorophore and benzo-12-crown-4 or benzo-18-crown-6 acting as ion recognition sites are connected by a trimethylene spacer, have been synthesized. Their supramolecular function for alkali metal ion sensing in water is compared with that of the previously designed C3-15C5/γ -CyD complex sensor. The C3-12C4, C3-15C5, and C3-18C6 are found to selectively form 2 : 1 complexes with Na+, K+, and Cs+, respectively, in the presence of γ -CyD and to exhibit pyrene dimer emission in water. These results demonstrate that the selectivity of the crown ether probe/γ -CyD complexes can be tuned by simply altering their crown ether ring size. The apparent 2 : 1 binding constants of the probes with alkali metal ions are determined at the optimum γ -CyD concentrations for each probe. For C3-12C4/γ -CyD complex, the accurate binding constant could not be obtained due to the relatively large deviation for the response. However, the fitting curve reveals that the binding constant is about 107 M-2. The 2 : 1 binding constants of the C3-15C5/γ -CyD complex for K+ and C3-18C6/γ -CyD complex for Cs+ are (3.8 ± 1.3)×109 M-2 and (5.8 ± 4.6)×107 M-2, respectively. These values are considerably larger than those of the corresponding benzocrown ethers in organic solvents. In the suparamolecular sensing system, the dimer formation of the probes inside the γ -CyD is selectively promoted by alkali metal ion binding in water. This is a novel sensing mechanism in which the dynamic molecular recognition events are successfully utilized for ion sensing in water.