Synthesis, characterization and photochromism of 3′-butoxyflavylium derivatives

The compounds 3′-butoxy-7-methoxyflavylium and 3′-butoxy-7-hydroxyflavylium were synthesized and the respective equilibrium and rate constants determined by two complementary techniques, pH jumps and flash photolysis. An experimental strategy based on these two techniques allowed calculation of all the equilibrium and rate constants of the system carried out for the first time in flavylium compounds lacking of the high cis–trans isomerization barrier. Irradiation of the trans-chalcone gives rise to the formation of the cis-chalcone still during the lifetime of the flash, which disappears through two parallel reactions: (i) one leading to the recovery of the trans-chalcone and the other, (ii) forming flavylium cation via hemiketal. This last reaction is globally dependent on pH and at less acidic pH the system reverts back to the trans-chalcone. The highest yield of colour production upon the flash takes place in the pH range 2–3.5.