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

Abstract

Abstract: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 HClO 4 , 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.

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

“…In this context, the photophysics of these synthetic pyranoflavylium cations is quite different in aqueous media from that in acidic acetonitrile. In aqueous media, very fast adiabatic excited state proton transfer (ESPT) efficiently deactivates the singlet excited state of these pyranoflavylium cations , to the detriment of intersystem crossing. Moreover, compared to compounds 1 ‐ 8 , the pyranoanthocyanins derived from malvidin (Scheme ) have multiple additional electron‐donating substituents that enhance the charge‐transfer character of the lowest energy excited state and hence modify its photophysics.…”

“…Substituent (ESPT) efficiently deactivates the singlet excited state of these pyranoflavylium cations (13,18), to the detriment of intersystem crossing. Moreover, compared to compounds 1-8, the pyranoanthocyanins derived from malvidin (Scheme 1) have multiple additional electron-donating substituents that enhance the chargetransfer character of the lowest energy excited state and hence modify its photophysics.…”

Ground‐ and Excited‐State Acidity of Analogs of Red Wine Pyranoanthocyanins,

“…Decay times of the acid (AH + *) and base (A*) forms ( k HA+ and k A , respectively) were determined from monoexponential fits of the transients in the solutions of methanol containing 1% TFA (to suppress ESPT) and in 30% methanol–water mixtures at pH 7–8 where only the base is present, respectively. The excited‐state p K a * values in 30% methanol–water mixtures were determined as a function of pH in the range of pH 1.8–3.0 from the relationship p K a * = log ( k p / k d ), where k p and k d are the rate constants for protonation of the excited base ( k p ) and deprotonation of the excited acid . The values of k d and k p [H+] were determined using the lifetimes and pre‐exponentials of the biexponential fluorescence decays of the excited acid and base forms; the values of k p were then determined from the slope of plots of the first‐order protonation rate constants vs. the proton concentration [H + ] as described previously .…”

“…The excited‐state p K a * values in 30% methanol–water mixtures were determined as a function of pH in the range of pH 1.8–3.0 from the relationship p K a * = log ( k p / k d ), where k p and k d are the rate constants for protonation of the excited base ( k p ) and deprotonation of the excited acid . The values of k d and k p [H+] were determined using the lifetimes and pre‐exponentials of the biexponential fluorescence decays of the excited acid and base forms; the values of k p were then determined from the slope of plots of the first‐order protonation rate constants vs. the proton concentration [H + ] as described previously . Individual and global analyses of the fluorescence decays were performed with the Sand program .…”

“…In a previous work, we reported that the p‐methoxyphenyl‐substituted pyranoflavylium cation ( 1 , Scheme 2) is a weak acid (p K a = 3.7 ± 0.1) in the ground state but somewhat more acidic photoacidic (p K a * = 0.67) in its lowest excited singlet state. Thus, in comparison with anthocyanins and 7‐hydroxyflavylium cations, the photoacidity of 1 is much less pronounced and the rate of proton loss from the excited acid form of 1 much slower by nearly two orders of magnitude.…”