From vine to wine: photophysics of a pyranoflavylium analog of red wine pyranoanthocyanins

In the ground state, the p-methoxyphenyl-substituted pyranoflavylium cation I, prepared by the reaction of the 5,7-dihydroxy-4-methylflavylium cation with p-methoxybenzaldehyde, is a weak acid (pK(a) = 3.7 +/- 0.1). In its lowest excited singlet state, I is a moderate photoacid (pK(a)* = 0.67) in 30% methanol- water acidified with trifluoroacetic acid (TFA). In comparison to anthocyanins and 7-hydroxyflavylium cations, the photoacidity of I is much less pronounced and the rate of proton loss from the excited acid form of I much slower (by a factor of up to 100). In 50% ethanol: 0.10 mol dm(-3) HClO4, the excited state of the acid form of I undergoes fast (12 ps) initial relaxation (potentially in the direction of an intramolecular charge transfer state), followed by much slower (340 ps) adiabatic deprotonation to form the excited base. The excited base in turn exhibits a moderately fast relaxation (70 ps), consistent with solvent hydrogen-bond reorganization times, followed by slower but efficient decay (1240 ps) back to the ground state. As in uncomplexed anthocyanins and 7-hydroxyflavylium cations, the photophysical behavior of I points to excited state proton transfer as the dominant excited state deactivation pathway of pyranoanthocyanins, consistent with relatively good photostability of natural pyranoanthocyanins.