Selective extraction of alkali metal cations with proton-ionizable dibenzo-16-crown-5 fluoroionophores

Two fluorescent ionophores of N-dansyl sym-(R)-dibenzo-16-crown-5-oxyacetamide HDAC (R=H) and PDAC (R=propyl) are developed for alkali metal cation sensing. Extraction of certain alkali metal cations into 1,2-dichloroethane by each ionophore result in a hypsochromic band shift and a fluorescence intensity increase in its emission spectrum. This fluorescent response is based upon the proton-ionizable function of the dansylamide fluorophore. Extraction constants (K-ex) and selectivity coefficients (S) for Li+, Na+, and K+ are obtained. HDAC exhibits strong binding ability for both Na+ and K+, whereas PDAC shows a much stronger binding ability for Naf than K+. It is proposed that PDAC has a preorganized conformation in which the geminal propyl group orients the carboxyl group of the fluoroionophore-containing side arm over the crown ether cavity favoring Na+ complexation. (C) 2000 Elsevier Science B.V. All rights reserved.