Spectral Properties, Cation Selectivity, and Dynamic Efficiency of Fluorescent Alkali Ion Indicators in Aqueous Solution Around Neutral pH

The spectral properties of two fluorescent alkali ion indicators, the commercially available cryptand CD222 and a new bipyridyl-type cryptand, Fbpy.bpy.2], bearing the trifluorocoumarino residue are investigated in aqueous solution as a function of pH as well as around neutral pH in the presence of alkali and alkaline earth cations. From the values of the acidity constants it is concluded that bridgehead nitrogen deprotonation occurs at a much lower pH for CD222 (pK _a below 5.5) than for Fbpy.bpy.2]. Spectrofluorometric titrations with salts of NH⁺ ₄, TI⁺, and alkali as well as alkaline earth cations indicate that both indicators are K⁺ selective. Fbpy.bpy.2] shows the higher K⁺/Na⁺ selectivity and larger fluorescence intensity changes but the slower dynamic response. Under suitable conditions, alkali ion binding by CD222 can occur in less than 1 ms.