Photo- and pH-Induced Transformations of Flavylium Cation: “Write–Lock–Read–Unlock-Erase” Cycles

The structural transformations of flavylium ion in aqueous solutions caused by pH jumps and photoexcitation have been investigated. At pH < 1, the stable form is the colored cationic species (AH+). By increasing pH, the concentration of AH+ decreases and, at pH = 5, this form is no longer present. The species obtained immediately after a pH jump undergo transformation processes with pH-dependent rate constants. At pH = 5.2 and 20 °C, the final product is the uncolored trans-chalcone (Ct). This form can be transformed by light excitation into the cis-chalcone (Cc) isomer, which is in equilibrium with the hemiacetal form (B2). This mixture is relatively inert due to the existence of a kinetic barrier that slows down the back thermal isomerization of Cc to the stable Ct form. Such a back reaction to Ct can be totally prevented if the irradiated solution is submitted to a pH jump to pH = 1, which transforms the photoproducts into the stable AH+ species. In basic solution, two more species were detected, namely the anionic forms Cc– and Ct– of the cis and trans chalcone. Ct– is a stable, not photosensitive and luminescent species, whereas Cc– is not stable, being converted into Ct– in the dark. The photochemical and pH-induced transformations of the flavylium cation in the pH range 0–11 can be taken as a basis to design write–lock–read–unlock–erase cycles for an optical molecular-level memory with multiple readout capacity.