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-d iphenyl-1,3-oxazole in aqueous solutions of sulfuric acid (pH 7 to H-0 -10) were studied. The spectral parameters are essentially acidity de pendent, which is due to the acid-base equilibria of these heterocycle s both in the ground and in the first excited singlet states. The diff erence between these two compounds is governed by their dissimilar sol vation. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole in the S-0 state (pK(BH+) = -2.49) and 2,5-diphenyl-1,3-oxazole in the S-0 a nd S-1 states (pK(BH+) = -1.93, pK(BH+)() = 1.95) were experimentally obtained. The enthalpies of formation, electron charge density, and g eometry of the bases and corresponding conjugate acids in the S-0 and S-1 states were calculated by the MNDO method.