Chemistry of 7,8-dihydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium. A photochromic system switching from yellow to green

The bio-inspired compound 7,8-dihydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium (styrylflavylium) was synthesized through the insertion of a double bond between rings B and C of the flavylium cation. The pH dependent network of chemical reactions of the new compound is similar to the one of anthocyanins and related compounds. The styrylflavylium is the dominant species at very low pH values. At moderately acidic pH values the respective quinoidal base is formed by deprotonation of the styrylflavylium in competition with the hydration reaction to give an analogous of the anthocyanins hemiketal. The cis-chalcone is formed through the hemiketal (ring C opening) and the trans-chalcone by the cis–trans isomerization of the former. All the rate and equilibrium constants of these reactions have been calculated, permitting to completely characterize the styrylflavylium network. While in water no photochromism could be observed in the presence of CTAB micelles, formation of the flavylium cation upon irradiation of the trans-chalcone takes place. Complexation of the quinoidal base with Al3+ was clearly identified.