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

Three-component systems made of a tetradentate bis-amino bis-quinoline ligand, a transition metal cation (Ni(2+) or Cu(2+)) 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 Cu(2+) containing systems, an "on-off-on" variation of fluorescence intensity vs. pH has been observed, whereas in the presence of Ni(2+) 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 Cu(2+) or Ni(2+) is used as the cation.