pH switchable and fluorescent ratiometric squarylium indocyanine dyes as extremely alkaline solution sensors.

pH indicators based on optical analysis play an important role in monitoring pH changes. Squarylium indocyanine (SICy) dyes have been developed to be used as extremely alkaline solution sensors. The absorbance and fluorescence intensities of the SICy dyes were quenched at a pH value of 13, and then recovered by adding acid to adjust the pH to 7. This reversible switching process could be repeated several times without significant loss of fluorescence. NMR and mass spectrometry analyses demonstrate nucleophilic addition of hydroxide ions to the N heterocycle. This study opens a new avenue to the design of fluorescent ratiometrics in extremely alkaline systems.