Raman and UV–vis spectroscopy studies on luteolin–Al(III) complexes

Luteolin is one of the most common flavonoids, but its ability to complex metal ions (e.g. aluminum) is still being discussed. This work presents the results of structural investigation of the luteolin–Al(III) complexes in methanol:water solutions and in the solid state. The analysis was carried out using UV–vis and FT-Raman spectroscopy accompanied by Factor Analysis, deconvolution and quantum-chemical calculations. It was found that in acidic solutions two complexes of luteolin–Al(III) of 1:1 and 1:2 ligand:metal ratio are formed, where luteolin is one- and twofold deprotonated, respectively. Additionally, a third complex precipitated from a basic solution of 1:2 stoichiometry with a threefold deprotonated ligand was obtained.