Picosecond dynamics of the prototropic reactions of 7-hydroxyflavylium photoacids anchored at an anionic micellar surface.
Three water-insoluble, micelle-anchored flavylium salts, 7-hydroxy-3-octyl-flavylium chloride, 4'-hexyl-7-hydroxyflavylium chloride, and 6-hexyl-7-hydroxy-4-methyl-flavylium chloride, have been employed to probe excited-state prototropic reactions in micellar sodium dodecyl sulfate (SDS). In SDS micelles, the fluorescence decays of these three flavylium salts are tetraexponential functions in the pH range from 1.0 to 4.6 at temperatures from 293 to 318 K. The four components of the decays are assigned to four kinetically coupled excited species in the micelle: specifically, promptly deprotonable (AH(+)*) and nonpromptly deprotonable (AH(h)(+)*) orientations of the acid in the micelle, the base-proton geminate pair (A*...H(+)), and the free conjugate base (A*). The initial prompt deprotonation to form the geminate pair occurs at essentially the same rate (k(d) approximately 6-7 x 10(10) s(-1)) for all three photoacids. Recombination of the geminate pair is approximately 3-fold faster than the rate of proton escape from the pair (k(rec) approximately 3 x 10(10) s(-1) and k(diss) approximately 1 x 10(10) s(-1)), corresponding to an intrinsic recombination efficiency of the pair of approximately 75%. Finally, the reprotonation of the short-lived free A* (200-350 ps, depending on the photoacid) has two components, only one of which depends on the proton concentration in the intermicellar aqueous phase. Ultrafast transfer of the proton to water and substantial compartmentalization of the photogenerated proton at the micelle surface on the picosecond time scale strongly suggest preferential transfer of the proton to preformed hydrogen-bonded water bridges between the photoacid and the anionic headgroups. This localizes the proton in the vicinity of the excited base much more efficiently than in bulk water, resulting in the predominance of geminate reprotonation at the micelle surface.
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