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 (C(t)). This form can be transformed by light excitation into the cis-chalcone (C(c)) isomer, which is in equilibrium with the hemiacetal form (B(2)). This mixture is relatively inert due to the existence of a kinetic barrier that slows down the back thermal isomerization of C(c) to the stable C(t) form. Such a back reaction to C(t) 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 C(c)^- and C(t)^- of the cis and trans chalcone. C(t)^- is a stable, not photosensitive and luminescent species, whereas C(c)^- is not stable, being converted into C(t)^- 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-unlockerase cycles for an optical molecular-level memory with multiple readout capacity.