Photochemistry of 3,4′-dimethoxy-7-hydroxyflavylium chloride: Photochromism and excited-state proton transfer

The synthetic compound 3,4′-dimethoxy-7-hydroxyflavylium chloride gives rise, in aqueous solution at moderately acidic pH, to a pH-dependent equilibrium between the flavylium cation, hemiacetal, (Z)-chalcone and a small amount of quinonoidal base. The distribution, as a function of pH, of the molar fractions of the several species present in solution have been calculated on the basis of ¹H NMR and pH jump experiments monitored by stopped-flow and conventional UV-VIS spectrophotometry, and high-performance liquid chromatography (HPLC). The compound shows interesting photochemical properties: (i) at pH 4.0 it presents a photochromic effect that converts (Z)-chalcone into hemiacetal, the reaction being reversible in the dark and (ii) excited-state proton transfer is observed between the flavylium cation and quinonoidal base. An appropriate formalism to quantify the experimental results has been developed. The formalism allows determination of the pH-dependent molar fraction distribution of the several anthocyanin forms present at equilibrium, as well as predicting the distribution of the molar fractions prior to equilibrium.