4 -methoxyflavylium ion

Several investigations concerning the thermodynamic and kinetic aspects of the thermal reactions of flavylium-type compounds have long been in the literature,133-371 while photochemical and photophysical aspects have been systematically examined more recently.[17-19,38 31 As we shall see below, pH jump, temperature jump, and flash photolysis experiments permit measurement of the rate constants of some of the reactions involved, and steady state titration experiments (using UV/Vis and NMR techniques) allow the measurement of equilibrium constants. In order to illustrate the complex reaction network in which these systems operate, we will now focus on the behavior of the 4 -methoxyflavylium ion (Figure 2 R4 = R7=H, R4- = OCH3).[391... 

As described above, in the pH range 2-8 the colorless trans-4 -methoxychalcone Ct is the thermodynamically stable species and therefore it is the final product of the transformations of the strongly colored 4 -methoxyflavylium ion AH+. Even at pH = 1, at which AH+ is the thermally stable species, Ct can be kinetically stable because of the high energy barrier of its transformation into Cc. Ct, however, can be converted into AH+ photochemically. 391 As might be expected from the thermal behavior of the system, the photoreaction may result either in a transient or in an almost permanent effect, depending on the temperature and the p H of the irradiated solution. 

Fig. 5 Spectral changes caused by continuous irradiation of aqueous solutions ofthe Ct form of 4 -methoxyflavylium ion with 365 nm light (a) pH = 1.0, [Ct] = 2.5 x 10 5 Mjthe curves correspond to the following irradiation...

Fig. 7 Behavior of a 1.0 x 10 5 M aqueous solution of the Ct form of the 4 -methoxyflavylium ion at pH = 3.0 and 60°Con 365 nm photoexcitation (dashed lines) followed by dark periods (full lines)J39 ...

As we have seen above, if the solution is sufficiently acidic (pH < 4), the Cc isomer is rapidly protonated, with conversion to the 4 -methoxyflavylium ion AH+, which is kinetically stable under such pH conditions and exhibits an intense absorption band with a maximum at 434 nm (Figure 24) and an emission band with maximum at 530 nm (Figure 6). At higher pH values, however, protonation does not occur and the Cc photoproduct is back-converted into Ct. As far as Co(CN) is concerned, excitation by 254 or 365 nm light in acidic or neutral aqueous solution causes the dissociation of a CN ligand from the metal coordination sphere (quantum yield = 0.31), with a consequent increase in pH [S71... 

In flavylium compounds that bear OH substituents in their 4 - and/or 7-positions, deprotonation of the OH group can result in other forms being obtained, not seen in the case of the 4 -methoxyflavylium compound discussed above. Figure 8 illustrates this for the 4 -hydroxyflavylium ion.1171 The new species are the quinoidal base A, obtained by simple deprotonation of the AH+ flavylium cation, and the dianionic Cc2- and Ct2- forms, obtained by second deprotonations of Cc and Ct. The roles played by these forms depend on the specific compound and the pH conditions. For... 

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