Photoreactions and Molecular Dynamics of Radical Pairs in a Reversed Micelle Studied by Time-Resolved Measurements of EPR and Magnetic Field Effect

Abstract

Photoreaction of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) in an aerosol OT (AOT) reversed micelle (RM) is studied by time-resolved EPR (tr-EPR) and the transient absorption detected magnetic field effect (MFE). Tr-EPR and transient absorption spectra indicate electron transfer from a highly excited triplet state of TMPD to the AOT headgroup regardless of W = [H(2)O]/[AOT] values from 0 to 40. Noticeable MFEs on the yield of TMPD cation radical (TMPD(+)) are observed at W > 0 and maximized at W ∼ 10. The dynamics of TMPD(+) in the bound water region of the RM has been precisely analyzed by theoretical analysis of time-resolved magnetically affected reaction yield (MARY) spectra. The simulation of the MARY spectra indicates that two kinds of radical pairs exist, both of which are composed of an AOT alkyl radical and TMPD(+). One system has TMPD(+) strongly bound to the anionic interface, where the radical pair shows very slow relaxation and recombination. Another system has TMPD(+) diffusing in the bound water, which shows a smaller diffusion coefficient than that in bulk water by 1 order of magnitude. In the larger water pool (W > 15), the spin correlated radical pair of the hydrated electron and TMPD(+) generated by photoionization is observed by tr-EPR. The ionization reaction is followed by electron attachment to the AOT headgroup and generation of the sulfite radical. However, these radical pairs are not thought to contribute significantly to the observed MFEs. Spin multiplicities of the precursor state and recombination products have been discussed from the different sign of J values for the radical pairs at larger W.