Tuning the Photochromic Properties of a Flavylium Compound by pH

We studied the intricate pH- and light-dependent network of the thermal and photochemical reactions of the 4′-(dimethyl-amino)-7-hydroxyflavylium ion by continuous irradiation, flash photolysis; ¹ H NMR spectroscopy, and stopped flow analysis. The distribution of the various forms at the pH-dependent thermodynamic equilibrium and the rates of several interconversion processes were measured. The most interesting feature of this multistate/multifunctional system is related to the possibility of pH-tuning the colour change caused by photoirradiation, because the cis-chalcone isomer, obtained from the photoisomerization of the trans-chalcone form, is spontaneously transformed into coloured species (flavylium cation and quinoidal base) at low values of pH, but reverts back to the uncoloured trans-chalcone at high values of pH because of the low energy barrier between the cis and trans chalcone isomers.