The Dynamics of Ultrafast Excited State Proton Transfer in Anionic Micelles

Abstract

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 acid−base kinetics of excited HMF are straightforward in water, with biexponential fluorescence decays reflecting ultrafast deprotonation of the excited acid (AH+)* (kd = 1.5 × 1011 s-1 or ca. 6 ps) and diffusion-controlled protonation of the excited base A* (kp = 2.3 × 1010 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 (τ1 = 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 (krot = 1.2 × 109 s-1 or ca. 800 ps at 22 °C). The other two decay times, τ3 and τ2, 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 (kd = 3.4 × 1010 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*···H3O+) (kr = 8.8 × 109 s-1) and (ii) pH-dependent bimolecular protonation of A* via entry of an aqueous phase proton into the micelle (kp = 1.6 × 1011 M-1 s-1). Dissociation of the geminate pair (kdiss = 1.6 × 109 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.