pH-Controlled photochromism of hydroxyflavylium ions

Fernando Pina, Maria João Melo, A. Jorge Parola, Mauro Maestri, Vincenzo Balzani

Research output: Contribution to journalArticlepeer-review

61 Citations (Scopus)


The structural transformations and photochromic properties of the 7-hydroxyflavylium ion have been investigated by means of the pH jump technique and continuous and pulsed light excitation. The primary photoproduct of UV irradiation of the colorless trans-chalcone form, which is the predominant species at pH 4, is its colorless cis isomer, which rapidly disappears on a time scale of seconds through two competitive processes: i) back-reaction to yield the trans-chalcone form, and ii) formation of the colored flavylium ion and its conjugated quinoidal base. Over minutes or hours (depending on pH), the system reverts quantitatively to its original state. The rate constants and equilibrium constants of the various processes have been obtained and compared with those previously reported for the 4'-hydroxyflavylium and 4',7-dihydroxyflavylium ions. This comparison demonstrates the substituent effect on the rate and equilibrium constants; the effect on the rate constant of the cis→trans thermal isomerization reaction is particularly strong. For the 7-hydroxyflavylium and 4',7-dihydroxyflavylium ions the pH of the solution plays the role of a tap for the color intensity generated by light excitation. This also means that this system can be viewed as a light-switchable pH indicator.

Original languageEnglish
Pages (from-to)2001-2007
Number of pages7
JournalChemistry - A European Journal
Issue number10
Publication statusPublished - 1 Oct 1998


  • Flash photolysis
  • Flavylium salts
  • Kinetics
  • Photochromism
  • UV/Vis spectroscopy


Dive into the research topics of 'pH-Controlled photochromism of hydroxyflavylium ions'. Together they form a unique fingerprint.

Cite this