The photodegradation mechanism of 2-(2′-furanyl)-1H-benzimidazole (Fuberidazole) in aqueous solution was studied by coupling photophysical and photchemical data with orbital calculations. Several photophysical properties were studied, the excited states were characterized and the acid—base equilibrium constants (pKa1=4.8, pKa2= 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 neutral forms were obtained as a function of molecular oxygen concentration.
The first singlet and triplet were identified as π-π* transitions for the acidic, neutral and basic forms. The experimental results are in agreement with a planar geometry for both the ground and first excited singlet states.
The qualitative results of the photodegradation as a function of oxygen concentration, and the higher reactivity of the acidic form relative to the neutral form, are discussed on the basis of the formation of a cyclopropenyl ketone intermediate. According to molecular orbital calculations, the process of formation of this intermediate is energetically favourable through the triplet of the acidic form, but not through the triplet of the neutral form, explaining the observed reactivity.
