Ground- and Excited-State Proton Transfers in Reversed Micelles. Polarity Restrictions and Isotope Effects

摘要

Ground- and excited-state proton transfers have been investigated with 8-hydroxy-1,3,6-pyrenetrisulfonate, POH, in sodium bis(2-ethylhexyl) sulfosuccinate, AOT, reversed micelle solubilized water pools in isooctane. Since POH is a much stronger acid in the singlet excited state, (POH)*, than in the ground state, excitation of POH by 1-5-mJ, 8-ns, 353-nm laser pulses, at pH values in between results in the dissociation of POH, governed by K* sub off. PO(-) reprotonation rates k sub on values, have been determined by laser flash photolysis. In reversed micelles K sub on values were found to depend on the water-to-AOT ratios (w values). In the absence of added base, no proton ejection from AOT reversed micelles could be observed at w < 7. The increase in the apparent K sub on values in AOT solubilized w = 7 and 12 water pools with increasing pH has been discussed in terms of altered water activity, ionic strengths, dielectric constant, and pH. Deuterium solvent isotope effects of 1.3 and 2.2 have been determined for k sub on/ k sub on (D2O) in w = 33 and 12 AOT solubilized reversed micelles in isooctane. Combining these values with the pK sub a of POH led to isotope effects of 7.8 and 8.4 on k sub off in the corresponding solutions. An isotope effect of 2.2 has also been determined for the laser pH jump initiated bromocresol green deprotonation rate in AOT solubilized pools. Steady-state and subnanosecond time-resolved fluorescene measurements have been utilized for assessing POH excited-state deprotonation. (Reprints)