Issue 10, 1996

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

Abstract

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.

Article information

Article type
Paper

J. Chem. Soc., Faraday Trans., 1996,92, 1693-1699

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

F. Pina, L. Benedito, M. J. Melo, A. J. Parola and A. Bernardo, J. Chem. Soc., Faraday Trans., 1996, 92, 1693 DOI: 10.1039/FT9969201693

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