Light and pH switching between the various forms of the 4′-methylflavylium cation

Mauro Maestri, Fernando Pina, Ana Roque, Paolo Passaniti

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


In aqueous solution, the 4′-methylflavylium ion (AH + ) can be interconverted into several different forms by light excitation and/or pH changes. All the observed processes are fully reversible and accompanied by strong changes in absorption and emission spectra. At pH < 1, the stable form is the coloured cationic species (AH + ). By increasing pH, AH + is no longer stable and undergoes structural transformations that at pH > 3 lead to the uncoloured trans-chalcone (Ct) as a final product. This transformation occurs through the hemiacetal form (B2) and the cis-chalcone (Cc) isomer. This mixture is relatively inert due to the existence of a kinetic barrier that slows down the thermal isomerisation of Cc to the final Ct form. Ct is photosensitive and can be backconverted by light excitation into Cc which at low pH rapidly gives the AH + species. In basic solution, two more species were detected, the anionic forms (Cc - and Ct - ) of the cis and trans-chalcone. Ct - is a stable, not photosensitive species, whereas Cc - is not stable, being converted into Ct - in the dark. A detailed analysis of the chemical behaviour of 4′-methylflavilium ion from the viewpoint of molecular level device shows that this compound behaves as a multistatelmultifunctional system.

Original languageEnglish
Pages (from-to)21-28
Number of pages8
JournalJournal of Photochemistry and Photobiology A: Chemistry
Issue number1
Publication statusPublished - 24 Oct 2000


  • 4′-Methylflavylium salt
  • Multistate/multifunctional systems
  • Photochromic compounds
  • Thermal isomerisation


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