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

Multistate/Multifunctional Systems. A Thermodynamic, Kinetic, and Photochemical Investigation of the 4‘-Dimethylaminoflavylium Compound

“…Spectrophotometric measurements show that reacidification to pH 1 of a solution equilibrated for 15 min at pH 9.5 leads to fast and complete recovery of A H + , whereas reacidification to pH 1 of a solution equilibrated for 15 days (in the dark) at pH 11 yields only 3% of A H + , again in a fast process. In agreement with the behavior previously observed for other flavylium species, ,,− it is possible to identify the first species, formed within 15 min, with the anionic cis -chalcone form Cc - , and the second species, formed after days, with the anionic trans -chalcone form Ct - (Figure ). These assignments are also supported by 1 H NMR experiments that show (i) a high field shift of the position of the peaks corresponding to protons 3 and 4 during the slow process (conversion of Cc - to Ct - ) and (ii) a scalar coupling constant of 15.6 Hz between protons 3 and 4 for the second species, a value fully consistent with a trans configuration (compare with the value of 12.4 Hz for the cis isomer) …”

“…This means that the rate-determining step must involve the participation of OH - ions and can be identified with the hydration reaction that leads from A H + to B2 and/or B4 , occurring via OH - attack on the 2 and/or 4 positions of the pyrylium moiety. Note that, in other flavylium systems, this hydration reaction can occur at much lower pH values and takes place by water attack of A H + . ,,− …”

Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium

“…The behavior of these molecules has some analogy with the life cycle of insects; a molecule suffers successive changes in its structure and properties along time by the action of external stimuli (metamorphosis). This concept fits very well the description of the chemical reaction networks characteristic of the flavylium compounds, − as illustrated in Scheme for 7-hydroxyflavylium …”

Multistate reaction kinetics of 6-hydroxy-4′-(dimethylamino)flavylium driven by pH. A stopped-flow study

“…Such a band is characteristic of other flavylium compounds and is assigned to AH + . 11 As the pH increases, this band gradually disappears, reflecting the disappearance of AH + . On the other hand, a new transition appears at 410 nm, which is assigned to the formation of chalcone species (both Cc and Ct).…”