Photo-degradation of trans-caffeic acid in aqueous solution and influence of complexation by metal ions

•The photo-degradation of metal complexes of caffeic acid is compared to the photo-degradation of free caffeic acid.•The kinetics and mechanism of trans-caffeic acid photo-degradation are investigated.•The first route is a photo-isomerization which then undergoes a cyclization to form the esculetin.•Two other routes lead to protocatechuic acid and vinylcatechol formation.•Metal cations such as Al(III), Pb(II) and Cu(II) can influence the rate and mechanism of caffeic acid photo-degradation.

The photo-degradation of metal complexes of caffeic acid was compared to the photo-degradation of free caffeic acid by using UV–vis spectroscopy and HPLC–ESI-mass spectrometry. This article reports first the determination of the products that are formed from the photo-degradation of trans-caffeic acid in aqueous solution and the investigation of the mechanism by a kinetic approach. The good fit between the model and the experimental concentration profiles confirms the photo-isomerization route of the molecule to cis-caffeic acid which then undergoes a cyclization to form the esculetin photo-product. In addition, it reveals, for the first time, another route of major importance leading to the product vinylcatechol. The presence of oxygen leads to an increase of the photo-isomerization rate. Then we report that metallic cations such as Al(III), Pb(II) and Cu(II) can influence the rate and mechanism of caffeic acid photo-degradation. Al(III) ions slow down the photo-degradation whereas Pb(II) and Cu(II) ions have a promoter effect on the production of esculetin. In all cases, the photo-isomerization is reduced by the presence of metal ions and the formation of vinylcatechol does not occur.