A dibenzo-16-crown-5 fluoroionophore for selective emission ratio sensing of Na⁺ in basic aqueous dioxane solution

For alkali metal cation sensing in aqueous dioxane solution, a novel dibenzo-16-crown-5‡ (DB16C5) fluoroionophore 1, N-(pyren-1-yl)-sym-(propyl)dibenzo-16-crown-5-oxyacetamide, has been synthesized. Examination of fluorescent behavior for 1 in 4:1 1,4-dioxane-water (v/v) containing tetramethylammonium hydroxide (TMAOH) reveals that proton dissociation of the carboxamido moiety is promoted by Na⁺ binding, which results in an emission ratio response due to internal charge transfer (ICT) from the donor carboxamido anion to the pyrene acceptor. The emission intensity ratio (I₄₅₉/I₃₈₇) increases with enhancement of the Na⁺ concentration. No fluorescent response is induced by the presence of Li⁺, K⁺, or Cs⁺. This high Na⁺ selectivity is attributed to a preorganized structure of DB16C5 lariat ether binding site in which Na⁺ binds tightly to the carbonyl oxygen of the side arm to induce selective proton dissociation. Thus a ratiometric emission response with high Na⁺ selectivity has been obtained for 1 in 4:1 1,4-dioxane-water (v/v) containing TMAOH.