Fast excited state proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) micelles have been investigated using the photoacid 4-methyl-7-hydroxyflavylium (HMF) chloride as probe. The acidbase kinetics of excited HMF are straightforward in water, with biexponential fluorescence decays reflecting ultrafast deprotonation of the excited acid (AH + )* (k d ) 1.5 × 10 11 s -1 or ca. 6 ps) and diffusion-controlled protonation of the excited base A* (k p ) 2.3 × 10 10 L mol -1 s -1 at 20°C). In aqueous micellar SDS solutions, the kinetics are much more complex; triple exponential fluorescence decays are observed at all pH values and temperatures examined. The longest decay time (τ 1 ) 760 ps at 22°C), observed only for (AH + )* and uncoupled from the acid-base equilibrium, is assigned to excitation of HMF in orientations incapable of prompt transfer of the proton to water, i.e., that must rotate to expose the acidic OH group to water (k rot ) 1.2 × 10 9 s -1 or ca. 800 ps at 22°C). The other two decay times, τ 3 and τ 2 , are due to emission from the species involved in the acid-base reaction at the micelle surface. Deprotonation of (AH + )* is slightly slower in SDS micelles (k d ) 3.4 × 10 10 s -1 or ca. 20 ps) than in water. Two processes are operative in the back protonation of A*: (i) pH-independent unimolecular reprotonation in the initially formed geminate compartmentalized pair (A*‚‚‚H 3 O + ) (k r ) 8.8 × 10 9 s -1 ) and (ii) pH-dependent bimolecular protonation of A* via entry of an aqueous phase proton into the micelle (k p ) 1.6 × 10 11 M -1 s -1 ). Dissociation of the geminate pair (k diss ) 1.6 × 10 9 s -1 ) forms A* at the micellar surface. The present study thus provides a rather detailed kinetic picture of the initial steps involved in an ultrafast excited state proton transfer process at the surface of a typical anionic micelle.

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.