Photochromic Properties of 3-Methyl-Substituted Flavylium Salts

The photochromic properties of some 3-methyl-substituted synthetic flavylium compounds have been investigated. The main structural feature in these compounds is the existence of a steric effect, caused by the methyl substituent, that forces the phenyl ring to move out of coplanarity with the benzopyrylium moiety. The X-ray structure of 3-methylflavylium tetrafluoroborate shows a torsion angle of 40.4° between the benzopyrylium and the benzene ring. In 3-methyl-substituted synthetic flavylium compounds, the steric effect impedes the formation of trans-chalcone. In the case of the 4′-hydroxy-3-methylflavylium ion, for example, the trans-chalcone form could only be obtained (up to a maximum yield of 60%) by protonation of the trans-chalcone anion, obtained in turn by irradiation of anionic cis-chalcone in basic solution. The multistate/multifunctional properties of the 4′-hydroxy-3-methylflavylium compound are also discussed in detail. For the 4′,7-dihydroxy-3-methylflavylium ion, a detailed investigation of the kinetic and thermodynamic properties of the various forms was performed by means of pH jumps and photochemical experiments. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)