A new method for the determination of proton-transfer rate constants in the ground state of anthocyanins and related flavylium salts is described. The method is based on the well-known pK a shift of phenols on going from ground to the first excited singlet state coupled to the typically very fast excited state proton transfer and very short lifetimes (picosecond range) occurring in these compounds. Under these conditions, a nanosecond light pulse instantaneously shifts the ground-state equilibrium, after which the ground-state transients can be monitored with nanosecond or microsecond resolution. The method is successfully applied to the determination of the deprotonation rate constants, k d , of two synthetic flavylium salts and a natural anthocyanin (7-hydroxy-4-methylflavylium chloride (HMF), 4′,7-dihydroxyflavylium chloride (DHF), and malvidin-3-glucoside chloride (oenin), respectively) and the protonation rate constants, k p , of their conjugate quinonoidal bases in the ground state. For all three flavylium cations, the protonation of the ground state base form is essentially diffusioncontrolled, and the deprotonation occurs in the submicrosecond range. Our directly determined rate constants are 2 orders of magnitude greater than previous values estimated for oenin by T jump. The flash photolysis approach utilized in the present work seems to be the only technique available for measurement of the kinetics of proton transfer in anthocyanins. In addition, our results show clear laser-induced perturbation of the groundstate protonation of oenin, providing the first direct evidence for excited-state proton transfer as a significant energy dissipation process in natural anthocyanins.

Malvidin-3,5-diglucoside (malvin), cyanidin-3,5-diglucoside (cyanin), and pelargonidin-3,5-diglucoside (pelargonin) are among the most representative anthocyanins because of their abundance in the most common red flowers and fruits. Anthocyanin color is directly affected by the pH-dependent chemistry of the red (acid) form of these compounds, while anthocyanin photostability is a function of the photophysics of the first excited singlet state. In the present work, we employ laser flash photolysis and picosecond time-correlated single-photon counting to determine the dynamics of the proton-transfer reactions of these three anthocyanins in the ground [deprotonation rate constants, k d = 1.3 × 106 s-1 (pelargonin), 1.8 × 106 s-1 (cyanin), and 3.8 × 106 s-1 (malvin)] and first excited singlet state [deprotonation rate constants, k d = 4.3 × 1010 s-1 (pelargonin), 4.0 × 1010 s-1 (cyanin), and 1.6 × 1011 s-1 (malvin)], respectively. The ground- and excited-state proton-transfer rate constants for anthocyanins and for photoacids of the naphthol type are found to correlate with an empirical parameter related to the ionic character of the dissociable OH bond. The present results show that the typically weak fluorescence of the flavylium cation form of anthocyanins is due primarily to competitive ultrafast, adiabatic proton transfer to water. This process is highly efficient as an energy-wasting mechanism, as would be required by an in vivo role such as protection of plant tissues from potentially deleterious excess radiant energy.

The synthesis of a new series of photoluminescent compounds, namely 5,8-diaryl quinoxaline derivatives (aryl = phenyl, 4-fluorophenyl, 4-methoxyphenyl, and 4-cyanophenyl), was achieved by a direct Suzuki cross-coupling reaction with the employment of a NCP-pincer palladacycle. The electrochemical and photophysical properties of these compounds were also investigated. Four new 4,8-diaryl-2,1,3-benzothiadiazoles were also synthesized in order to enable a comparison between the two types of nitrogen-containing π-extended heterocycles. The substitution of a hydrogen atom at the 4-position of the aryl that is groups attached to the quinoxaline or benzothiadiazole base by either elec-

Anthocyanins, the major red, purple, and blue pigments of plants, absorb visible as well as UV radiation and are effective antioxidants and scavengers of active oxygen species. In plant leaves, one of the functional roles proposed for anthocyanins is protection of the photo synthetic apparatus from the effects of excess incident visible or UV-B radiation and photo oxidative stress. In essence, a photoprotective role requires that the excited singlet states of both complexed and uncomplexed anthocyanins deactivate back to the ground state so quickly that intersystem crossing, photoreaction, and diffusion-controlled quenching processes cannot compete. Studies of the photochemical properties of synthetic analogs of anthocyanins and of several naturally occurring anthocyanins show that this is indeed the case, uncomplexed anthocyanins decaying back to the ground state via fast (subnanosecond) excited-state proton transfer (ESPT) and anthocyanin-copigment complexes by fast (subpicosecond) charge-transfer-mediated internal conversion.