Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)₆^3- in AOT reverse micelles: Separation of redox products by electron-transfer-induced hydrophobicity

The photoinduced electron transfer from eosin, Eo^2- (1), and ethyl eosin, EoEt (2), to Fe(CN)₆^3- is examined in AOT reverse micelles in heptane. For a microheterogeneous system having a water-to-surfactant molar ratio w = 30, the lifetime of the photogenerated redox products in the system that includes EoEt^- is ca. 300-fold longer than in the photosystem that includes Eo^2-: τ = 4.3 μs for Eo^2- and τ = 1400 μs for EoEt^-. Stabilization of the redox products against recombination in the system containing EoEt^- is attributed to the extraction of the hydrophobic oxidized photoproduct ²EoEt^• from the water pool of the reverse micelles to the continuous oil phase. Photoinduced electron transfer from Eo^2- to Fe(CN)₆^3- in the reverse micelles has been quantitatively analyzed by assuming a Poisson distribution of the quencher over the reverse micelles. Kinetic analysis of the transients allowed determination of the quencher distribution, micelle concentration [m] = 1.44 × 10^-4 M, and water-pool diameter 2R = 82 Å. The kinetics of photoinduced electron transfer from EoEt^- to Fe(CN)₆^3- could be analyzed in terms of a similar quencher distribution. Detailed kinetic analysis revealed that, in the Eo2^-/Fe(CN)₆^3- reverse-micellar photosystem, photoinduced electron transfer is followed by a fast intramicellar recombination. In the EoEt^-/Fe(CN)₆^3- photosystem, fast escape of the neutral oxidized species ²EoEt^• from the reverse micelle competes with the intramicellar recombination (escape efficiency: θ_esc = 0.52), leading to separation of the redox products. The separated photoproducts undergo a slow secondary recombination. A kinetic model for the overall photochemical processes is presented, and kinetic equations for the photoinduced electron transfer in the reverse micelles followed by intramicellar recombination and escape are provided.