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-oxyacetam ide HDAC (R=H) and PDAC (R=propyl) are developed for alkali metal cation se nsing. Extraction of certain alkali metal cations into 1,2-dichloroethane b y each ionophore result in a hypsochromic band shift and a fluorescence int ensity increase in its emission spectrum. This fluorescent response is base d upon the proton-ionizable function of the dansylamide fluorophore. Extrac tion constants (K-ex) and selectivity coefficients (S) for Li+, Na+, and K are obtained. HDAC exhibits strong binding ability for both Na+ and K+, wh ereas PDAC shows a much stronger binding ability for Naf than K+. It is pro posed that PDAC has a preorganized conformation in which the geminal propyl group orients the carboxyl group of the fluoroionophore-containing side ar m over the crown ether cavity favoring Na+ complexation. (C) 2000 Elsevier Science B.V. All rights reserved.