Benzothiazolium styryl dyes containing a monoazacrown ether: Protonation and complexation with metal and ammonium cations in solution

Abstract

Solutions of the trans-isomers of a model benzothiazolium styryl dye and two novel derivatives containing monoaza-15-crown-5 and -18-crown-6 ether rings have been studied using UV–VIS absorption spectroscopy. Protonation of these dyes causes the strongest colour changes, arising from hypsochromic shifts of ca. 140 nm in their absorption band maxima. The azacrown ether dyes have been found to complex alkali-metal (Li ⁺ , Na ⁺ , and K ⁺ ), alkaline-earth-metal (Mg ²⁺ , Ca ²⁺ , and Ba ²⁺ ), and ammonium (NH ₄ ⁺ ) cations, but not silver (Ag ⁺ ), in acetonitrile solution, while of these cations only Ag ⁺ forms a complex in aqueous solution. Spectra and stability constants are reported for all of these complexes and indicate that the dyes are effective chromoionophores. The stability of the metal cation complexes in acetonitrile solution has been found to depend on the presence of trace amounts of water; this effect has been quantified and is interpreted in terms of the competitive solvation of these cations by water. The stability constants, spectra, and hypsochromic shifts together indicate that small cations complex with the crown ether ring, but not with the azacrown nitrogen atom.