Laser flash photolysis study of nicotinic acid in water

The photochemical reactions of the three forms of nicotinic acid (pKa1=2.09 and pKa2=4.85) were studied by laser flash photolysis and product studies. Irradiation at pH=1 yielded a pulse end transient (λmax=310/440 nm) assigned to the protonated triplet. Oxygen quenching experiments showed that this triplet is involved in the formation of 2-hydroxy-nicotinic acid. No transient absorptions were detected at pH=2. Irradiation at pH>2.8 yielded a transient species (λmax=290/375 nm) assigned to the anionic triplet. We demonstrated that this latter yielded bipyridine derivatives by reaction with nicotinic acid. Photoionisation of the anionic singlet excited state was also observed. This reaction is likely to be responsible for the phototransformation occurring in oxygenated basic solutions.