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

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 Cc^2- 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 Ct^2-, which is the thermodynamically stable form of the system in basic solution. Ct^2- 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.