PHOTOPHYSICAL PROPERTIES AND PHOTODEGRADATION MECHANISM OF 2-(2'-FURANYL)-1H-BENZIMIDAZOLE (FUBERIDAZOLE)

The photodegradation mechanism of 2-(2'-furanyl)-1H-benzimidazole (Fub eridazole) in aqueous solution was studied by coupling photophysical a nd photochemical data with molecular orbital calculations. Several pho tophysical properties were studied, the excited states were characteri zed and the acid-base equilibrium constants (pK(a1) = 4.8, pK(a2)=11.5 , pK(a1)=6.7 and pK(a2)*=10.8) in both ground and excited states were determined; the photoreaction quantum yields of the acidic and neutra l forms were obtained as a function of molecular oxygen concentration. The first singlet and triplet were identified as pi-pi transitions f or the acidic, neutral and basic forms. The experimental results are i n agreement with a planar geometry for both the ground and first excit ed singlet states. The qualitative results of the photodegradation as a function of oxygen concentration, and the higher reactivity of the a cidic form relative to the neutral form, are discussed on the basis of the formation of a cyclopropenyl ketone intermediate. According to mo lecular orbital calculations, the process of formation of this interme diate is energetically favourable through the triplet of the acidic fo rm, but not through the triplet of the neutral form, explaining the ob served reactivity.