Photochromism of naphthoflavylium. On the role of 4-OH hemiketal in flavylium network

Flavylium compounds suffer in aqueous solution the nucleophilic addition of water at moderately acidic pH values (hydration reaction). The hydration is possible in two positions, namely at position 2, forming hemiketal B2, and at position 4, forming hemiketal B4. B2 can subsequently evolve to give the cis- and trans-chalcone species. At the present work the network of chemical reactions involving the naphthoflavylium compound in aqueous solution was studied by means pH jumps, stopped flow, continuous irradiation and flash photolysis. The equilibrium and rate constants of the system were calculated through a mathematical model. The species B4 has a kinetic effect similar to the one observed for the quinoidal base (for flavylium dyes bearing acidic groups), i.e. B4 is a kinetic product retarding the rate of equilibration. Flash photolysis experiments in comparison with reverse pH jumps results show that the appearance of the flavylium ion is faster in the photochemical-induced process than in the thermal one, suggesting an additional photochemical pathway (besides photoisomerization) after the excitation of the trans-chalcone.