Substitution and solvent effects in the chalcones isomerization barrier of flavylium photochromic systems

Useful application of photochromic compounds as optical memories implies the existence of a large kinetic barrier between the forms interconverted by light. In the case of flavylium salts, the ground state isomerization barrier between the photoisomerizable chalcone isomers is shown to correlate with the electron donating ability of the substituents, measured by their effects in the 1H NMR chemical shifts of the aromatic protons. Substitution with electron donating groups in ring A lowers the barrier while substitution at ring B has the opposite effect. However, in water, the observed increase is higher than expected in the case of compound 4′,9-dihydroxychalcone when compared with the analogous 4′-dimethylamino-9-hydroxychalcone, containing a better electron donating group in the same position. Our interpretation is that the water network is providing an efficient pathway to form tautomers. In acetonitrile, unlike water, the expected order is indeed observed: Ea(4′,9-dihydroxychalcone) = 60 kJ mol−1 < Ea (4′-dimethylamino-9-hydroxychalcone) = 69 kJ mol−1.