Photochemistry of 5-Hydroxy-4'-Dimethylaminoflavylium in the presence of SDS micelles. The role of metastable states of flavylium cation-quinoidal base and trans-chalcones

The multistate of species originated by the compound 5-hydroxy-4′-dimethylaminoflavylium was studied in the presence of sodium dodecyl sulfate micelles 0.1 M. A series of pH jumps obtained by addition of base to the equilibrated solutions of the flavylium cation results in a metastable equilibrium between the flavylium cation and the quinoidal base with pK_a = 7.6. In neutral to moderately basic medium, only at 60 °C and after 5 days a new distribution involving the flavylium cation and trans-chalcones is achieved. On the other hand, the anionic trans-chalcone can be formed in few minutes in very basic medium at room temperature and from this, another meta-stable state constituted by protonated, neutral and anionic trans-chalcones, pK_Ct+/Ct = 2.6 and pK_Ct/Ct- = 10.7 is reached. The equilibrium is apparently attained between flavylium cation, neutral and anionic trans-chalcones respectively for pK_AH+/Ct = 4.1 and pK_Ct/Ct- = 10.7. Irradiation of the neutral (stable or metastable) and protonated (meta-stable) trans-chalcones yields to the appearance of the quinoidal base or the flavylium cation in an extended pH region 1 < pH < 10. The peculiar properties of this flavylium based photochromic system are explained by means of an energy level diagram where cis-chalcone and hemiketal lay above the quinoidal base.