4-Carboxy-7-hydroxyflavylium. A Multistate System Involving Twelve Species Reversibly Interconverted by pH and light stimuli

In the frame of the metamorphosis chemistry, where complexity is achieved at the bottom, a multistate system based on 4-carboxy-7-hydroxyflavylium is described. Twelve species reversibly interconverted by pH and light stimuli were identified at the equilibrium or as transients. The carboxylic substituent deprotonates at moderately acidic pH values and as a consequence allows the existence of two photoactive trans-chalcones, whose irradiation leads to different photoproducts. The thermodynamic and kinetic properties of the network of reactions of the multistate system were investigated by means of UV-vis spectroscopy, stopped-flow, continuous irradiation, and flash photolysis. The evolution of the system to the equilibrium when the pH is raised from acidic to higher pH values can take place through two different routes involving the protonated or the deprotonated species at the carboxylic acid substituent. The kinetics and the photochromism of the system were rationalized by means of an energy level diagram.