pH-dependent absorption and fluorescence spectra of hydroxyaryl-squarylium dyes

A range of symmetrical hydroxy-substituted 1,3-diarylsquarylium dyes has been prepared and the light absorption and emission characteristics of the dyes examined. Whereas dyes formed from 3-N,N-dialkylaminophenols show intense fluorescence, analogous compounds derived from polyhydric phenols show little or no fluorescence. Those dyes with multiple hydroxy substituents in the aryl rings are very pH sensitive and readily dissociate in solution to give progressively mono-anionic, di-anionic and tri-anionic species as the pH is increased. In view of these observations, previous literature structures for these species have been reappraised. The pronounced spectral differences between the various prototropic species in solution have potential analytical applications. For example, one derivative can be used to detect water in solvents colorimetrically, and another shows strong fluorescence from its dianionic form, and can be used as a pH fluorescence probe or indicator.