Photoinduced electron transfer in non-aqueous microemulsions

Abstract

Photoinduced electron transfer from zinc tetraphenyl porphyrin (ZnTPP) incorporated in n-heptane/AOT/ethylene glycol microemulsions was followed by laser flash photolysis and fluorescence quenching. Using two acceptors, duroquinone (DQ) and methyl viologen (MV 2+) which are located on opposite sides of the interfacial region, the apolar and polar pseudophases respectively, it was possible to monitor kinetic and spectroscopically the respective radical ions formed. The determination of local quencher concentrations enabled the evaluation of electron transfer quenching rate constants in each pseudophase. The values obtained showed that when both the fluorophore and the quencher are either in the oil pseudophase or at the interface the processes are diffusion-controlled limited. The magnitude of the rate constants ranges from 10 8 to 10 10 mol −1 dm 3 s −1. By contrast, the forward electron transfer occurring in the polar pool is reaction controlled ( k q T=2.1×10 6 mol −1 dm 3 s −1) whereas the back recombination of radical ions in the pool is also diffusion controlled ( k 2=4.1×10 8 mol −1 dm 3 s −1). The triplet state kinetics is well supported by steady-state and transient fluorescence quenching studies from which effective reactional distances (9–12 Å) and diffusion coefficients (0.5–1.3)×10 −9 m −2 s −1, could be evaluated at both the oil and interface pseudophases. The larger effective reaction distances coupled with lower diffusion coefficients estimated at the interfacial region connected to the polar non-aqueous solvent shows that factors such as the distance, mutual orientation and microviscosity are the controlling physical parameters. On the other hand, beyond the energetics, the efficiency of the whole electron transfer in the inner polar non-aqueous nanophase depends on the solvation of radical ions formed.