Photochemical Behavior of the 4-Methoxyflavylium Ion

AH solutions from pH = 1 to higher pH values by the pH-jump technique or by exciting Ct solutions by a light flash. Such species reach a pseudoequilibrium on a time scale of seconds and then undergo a very slow, thermal transformation to Ct. 39  

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. 

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