PH-CONTROLLED PHOTOCHROMISM OF HYDROXYFLAVYLIUM IONS

The structural transformations and photochromic properties of the 7-hy droxyflavylium ion have been investigated by means of the pH jump tech nique and continuous and pulsed light excitation. The primary photopro duct of UV irradiation of the colorless trans-chalcone form, which is the predominant species at pH 4, is its colorless cis isomer, which ra pidly disappears on a time scale of seconds through two competitive pr ocesses: i) back-reaction to yield the trans-chalcone form, and ii) fo rmation of the colored flavylium ion and its conjugated quinoidal base . Over minutes or hours (depending on pH), the system reverts quantita tively to its original state. The rate constants and equilibrium const ants of the various processes have been obtained and compared with tho se previously reported for the 4'-hydroxyflavylium and 4',7-dihydroxyf lavylium ions. This comparison demonstrates the substituent effect on the rate and equilibrium constants; the effect on the rate constant of the cis-->trans thermal isomerization reaction is particularly strong . For the 7-hydroxyflavylium and 4',7-dihydroxyflavylium ions the pH o f the solution plays the role of a tap for the color intensity generat ed by light excitation. This also means that this system can be viewed as a light-switchable pH indicator.