Photochemistry of metal complexes of 3-hydroxyflavone: towards a better understanding of the influence of solar light on the metal–soil organic matter interactions

Abstract

The photo-stability of 3-hydroxyflavone (3HF) and its complexes with Al(III), Zn(II) and Pb(II) in methanol was investigated by a multi-disciplinary strategy, combining UV-Vis, fluorescence, Raman and FT-IR spectroscopies, DFT and time-dependent DFT calculations and a traditional organic photochemistry approach. We found that the presence of metals can slow down (in the case of Pb(II)) or even block (in the case of Al(III)) the photo-degradation of 3HF. The Zn(II) complex shows a reactivity comparable to free 3HF. In all cases (except the Al(III) complex) the photo-product is 3-hydroxy-3-phenyl-1,2-indandione, the same as reported in literature for 3HF photochemistry in methanol. This means that metallic cations do not modify the photochemical mechanism of the reaction. In addition, in the case of Zn(II) and Pb(II) complexes, the photo-produced 3-hydroxy-3-phenyl-1,2-indandione was found to exist in free form, i.e. without metal complexation. This means that similar photo-reactions occurring in soil organic matter under solar irradiation entail desorption of free Zn²⁺ and Pb²⁺ cations (the most toxic form) in the environment. The presence of external parameters—presence of water and oxygen—has also been studied. Interestingly, opposite effects have been found for free 3HF and Zn(II) and Pb(II) complexes. Tentative explanations for these effects are provided, relying on a detailed investigation on the structure and on the photophysical properties of the three complexes.