Abstract: In this work, several kinds of 4-substituted phenyl azo-5-(8-hydroxyquinoline) compounds have been synthesized.The tautomeric equilibrium between the azo type and the hydrazone type of these compounds in different solvents and different acidity has also been studied.The introduction of an electron withdrawing group will facilitate the hydrazone formation.On the contrary, in the case of introduction of.electron-repulsive substitute no effect can be observed.In addition, it was found that under irradiation the main process of the excited state of these compounds is azo-cis/trans isomerization reaction rather than tautomerization.

Key words: tautomeric equilibrium, hydroxyquinoline-azo compound, cis/trans isomeri-zation, non-aqueous acid/base indicator