Photochemistry of aqueous solutions of kynurenic acid and kynurenine yellow

The photophysics and photochemistry of kynurenic acid (KNA) and kynurenine yellow (KNY) in neutral aqueous solutions were investigated using time-resolved optical spectroscopy. Both molecules have similar quinoline-like structures, the only difference being the absence of conjugation in the nitrogen containing cycle in KNY. The main channel of S1 excited state decay in the case of partially-unconjugated KNY is the solvent assisted S1 → S0 radiationless transition via intermolecular hydrogen bonds (ΦIC = 0.96), whereas, in the case of fully-conjugated KNA, it is intersystem crossing to the triplet state (ΦT = 0.82). The major intermediate products of the singlet excited KNY deactivation are the triplet state (ΦT = 0.022) and, most probably, the enol form (Φenol = 0.012), which decay with the formation of 2,3-dihydro-4-hydroxyquinoline and 4-hydroxyquinoline, respectively. The results obtained show that KNA and KNY, which are products of the decomposition of the UV filter kynurenine, are significantly more photoactive and less photostable than the parent molecule.