Three-component systems for conventional and window-shaped response fluorescent pH indicators

Abstract

Three-component systems made of a tetradentate bis-amino bis-quinoline ligand, a transition metal cation (Ni²⁺ or Cu²⁺) and a fluorescent indicator (Coumarin 343) have been studied in a water–dioxane (1 ∶ 4 v/v) mixture, through potentiometric, pH-spectrophotometric and pH-fluorimetric titrations. For the Cu²⁺ containing systems, an “on–off–on” variation of fluorescence intensity vs. pH has been observed, whereas in the presence of Ni²⁺ a simple “on–off” profile of the fluorescence intensity vs. pH was obtained. These ternary systems thus behave as window-shaped or conventional pH-indicators, depending whether Cu²⁺ or Ni²⁺ is used as the cation.