Multistate/multifunctional molecular-level systems: Light and pH switching between the various forms of a synthetic flavylium salt

Fernando Pina, Ana Roque, Maria João Melo, Mauro Maestri, Livia Belladelli, Vincenzo Balzani

Research output: Contribution to journalArticlepeer-review

122 Citations (Scopus)


The photochromic properties of the 4'-hydroxyflavylium ion (AH+) have been investigated in aqueous solution. This system can be interconverted between as many as ten different forms by light excitation and/or pH changes. In neutral or moderately acidic solution the thermodynamically stable form of this compound is trans-2,4'-dihydroxychalcone (Ct). Light excitation of Ct in acidic or neutral solution gives rise to cis-chalcone Cc (Φ=0.04 at 365 nm), which undergoes slow equilibration with three other forms, namely hemiacetal B, flavylium cation AH+, and quinoidal base A. The relative amounts of the photoproducts depend on pH. At pH < 3 the only product is the colored AH+. In neutral and moderately acidic solutions, the four photoproducts revert back to Ct by a slow thermal reaction (k = 4.0 x 10-5 s-1, half-life 27 hours at 25°C, pH = 3), whose rate can be accelerated by increasing the temperature (k= 1.8 x 10-2 s-1 at 75°C, pH=3), and by exploiting the photochemical conversion of Cc to Ct (Φ > 0.16 at 313 nm). At pH < 1, AH+ is the thermodynamically stable form of the system. A pH jump from 1 to 12 causes the complete conversion of AH+ to the Cc2- dianion. This species is relatively stable (half-life 400 hours at pH 12 and 25°C). Its thermal and photochemical reactions (Φ = 0.17 at 313 nm) lead to Ct2-, which is the thermodynamically stable form of the system in basic solution. Ct2- does not undergo any photochemical reaction. All the observed processes are fully reversible and accompanied by large changes in the absorption and emission spectra. The flavylium salt investigated represents a multistate/multifunctional molecular-level system. It has properties required by optical memory devices with multiple storage in two different memory levels and nondestructive readout capacity through a write - lock - read - unlock - erase cycle. Its light- and/or pH-induced transformations can be taken as a basis for simple logic operations and create an intricate network of chemical processes.

Original languageEnglish
Pages (from-to)1184-1191
Number of pages8
JournalChemistry - A European Journal
Issue number7
Publication statusPublished - 1 Jul 1998


  • Flavylium salts
  • Information storage
  • Logic gates
  • Molecular devices
  • Photochromism


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