The structural transformations and the photochromic properties of the 7-hydroxyflavylium ion have been investigated by means of the pH jump technique and continuous and pulsed light excitation. The various forms of flavylium compounds, trans- and cis-chalcone (Ct and Cc), quinoidal base A, and hemiacetal form B, are related by interconversions like communicating vessels containing a fluid. Since the amount of colored species formed upon irradiation depends on the pH of the solution, pH can be viewed as a “tap” that modulates the color intensity generated by light excitation.
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Piña et al. (1998) studied this question.