Competition between intra- and intermolecular proton transfer in the excited state of ortho-hydroxy derivatives of 2,5-diphenyl-1,3-oxazole in media of various acidity

The acid-base interactions of 2,5-diphenyl-1,3-oxazole and some model compo unds in aqueous-alcoholic media were studied by electronic absorption and f luorescent spectroscopy. The spectral-luminescent characteristics of the pr ototropic forms were obtained, and the equilibrium constants of the protoly tic processes were estimated. It was found that the presence of intramolecu lr hydrogen bond only slightly affects the basicity of the ortho-hydroxy de rivatives with respect to protonation by the oxazole ring in the ground sta te. The elimination of the dimethylamino group from conjugation with the ma jor chromophoric fragment of 2-(2-hydroxyphenyl)-5- [4-(dimethylamino)pheny l]-1,3-oxazole by protonation of the former in a weakly acidic solution giv es rise to fluorescence of the phototautomeric form, which is incharacteris tic of this compound in a neutral medium.