pH-dependent absorption and fluorescence spectra of hydroxyaryl-squaryliumdyes

A range of symmetrical hydroxy-substituted 1,3-diarylsquarylium dyes has be en prepared and the light absorption and emission characteristics of the dy es examined. Whereas dyes formed from 3-N,N-dialkylaminophenols show intens e fluorescence, analogous compounds derived from polyhydric phenols show li ttle or no fluorescence. Those dyes with multiple hydroxy substituents in t he aryl rings are very pH sensitive and readily dissociate in solution to g ive progressively mono-anionic, di-anionic and tri-anionic species as the p H is increased. In view of these observations, previous literature structur es for these species have been reappraised. The pronounced spectral differe nces between the various prototropic species in solution have potential ana lytical applications. For example, one derivative can be used to detect wat er in solvents colorimetrically, and another shows strong fluorescence from its dianionic form, and can be used as a pH fluorescence probe or indicato r. (C) 1999 Elsevier Science Ltd. All rights reserved.