Influence of medium acidity upon the luminescence properties of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyl-1,3-oxazole

The luminescence properties of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyl-1,3-oxazole in aqueous solutions of sulfuric acid (pH 7 to H_o − 10) were studied. The spectral parameters are essentially acidity dependent, which is due to the acid-base equilibria of these heterocycles both in the ground and in the first excited singlet states. The difference between these two compounds is governed by their dissimilar solvation. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole in the S₀ state (pK_BH+ = − 2.49) and 2,5-diphenyl-1,3-oxazole in the S₀ and S₁ states (pK_BH+ = −1.93, pK_BH+^* = 1.95) were experimentally obtained. The enthalpies of formation, electron charge density, and geometry of the bases and corresponding conjugate acids in the S₀ and S₁ states were calculated by the MNDO method.