Photochromism of the Synthetic 4',7-Dihydroxyflavylium Chloride

The synthetic compound 4',7-dihydroxyflavylium chloride shows an interesting photochromic effect at moderately acidic pH values. Pale yellow solutions of this compound, equilibrated in the dark at pH 3.4, become bright yellow upon irradiation with 313-nm light. Switching off the light, gives back the initial pale yellow solution. Photochemical methods, 1 H NMR, and molecular orbital calculations were used to elucidate this photochromic behavior. In solutions equilibrated in the dark (pH <5.7), the existence of two main species in equilibrium is demonstrated: the colored flavylium cation and the colorless trans-chalcone