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J. Phys. Chem. B 2010, 114, 14550–14558
Photoreactions and Molecular Dynamics of Radical Pairs in a Reversed Micelle Studied by Time-Resolved Measurements of EPR and Magnetic Field Effect† Tomoaki Miura,‡ Atsushi Kageyama,§ Sakiko Torii,| and Hisao Murai*,| Graduate School of Science and Engineering and Department of Chemistry, Shizuoka UniVersity, 836 Oya, Shizuoka 422-8529, Japan, and Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai, Japan ReceiVed: March 14, 2010; ReVised Manuscript ReceiVed: July 6, 2010
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 ) [H2O]/[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 timeresolved 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. Introduction Electron transfer reactions in the vicinity of inhomogeneous water-lipid interfaces are of much interest in terms of mechanisms of redox reactions in biological environments.1-3 For example, the respiratory chain and the photosynthetic electron transport system involve the electron transfer reaction through biomembranes as key reaction processes. Furthermore, it has been suggested that water molecules strongly bound to protein pockets play an important role for the internal electron transfer reaction of proteins and those water molecules exhibit characteristic properties different from that of bulk water.4,5 Reversed micelles (RMs) are one of the best model systems for the research of such a kind of electron transfer reactions in surface bound water because one can easily tune the size of the inner lipid water pool by changing the water/surfactant concentration ratios (W). Some previous studies have indicated the existence of water molecules strongly bound to the headgroup of surfactant molecules probably by hydrogen bonding.6-10 Bulk-like water can be observed if the water pool is large enough (W > 10), whereas RMs are filled solely by bound water at smaller W. It is expected that the dynamics of solute molecules confined in the bound water is very different from that in bulk water. Such peculiar molecular dynamics is possibly responsible for the characteristic electron transfer at water-lipid interfaces. From these aspects, it is important to precisely observe both †
Part of the “Michael R. Wasielewski Festschrift”. * To whom correspondence should be addressed. E-mail: shmurai@ ipc.shizuoka.ac.jp. Phone/Fax: +81-54-238-4753. ‡ Graduate School of Science and Engineering, Shizuoka University. § Tohoku University. | Department of Chemistry, Shizuoka University.
chemical reactions and the molecular dynamics of probe molecules in RMs. Spin chemical techniques as time-resolved EPR (tr-EPR) and the magnetic field effect (MFE) can be powerful tools for studying both photoinduced electron transfer reactions and the molecular dynamics of generated radical ion species. In many cases, tr-EPR spectra show characteristic polarization patterns known as chemically induced dynamics electron polarization (CIDEP).11 Spin correlated radical pairs (SCRPs)12-16 and the radical pair mechanism (RPM)11,17-19 are especially of much importance in terms of electron transfer because they give us information about singlet-triplet energy gaps (ES - ET ) 2J) of radical ion pairs, which are strongly correlated with electronic coupling interactions between donors and acceptors.15,16,19,20 CIDEPs, however, are sometimes problematic in the case of inhomogeneous reaction media because they could pick up very minor reactions if the observed species has a huge polarization. This problem can be solved to some extent by the aid of conventional photospectroscopy as the transient absorption. There have been a few reports on tr-EPR studies of photoreactions in RMs.21-23 In most studies, water-soluble quinones such as anthraquinone disulfonate (AQDS) are used as an electron/hydrogen acceptor and sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (aerosol OT or AOT) as a surfactant. Akiyama and Tero-Kubota22 have reported SCRP signals due to reencounters of freely diffusing radicals in the water pool in the system of AQDS with hydrogen donors. White et al.23 have reported photoreactions of AQDS with the AOT RM itself and shown various species as a result of electron/hydrogen transfer from AOT. However, tr-EPR studies that focus on the difference between bound water and bulk-like water as reaction environ-
10.1021/jp102300z 2010 American Chemical Society Published on Web 08/10/2010
Photoreactions and Spin Dynamics in a Reversed Micelle ments have never been reported. AQDS does not dissolve in nonpolar solvents, which might make it difficult to study its reactions in small RMs. Furthermore, tr-EPR studies of electron attachment to AOT have never been reported. MFE on the radical pair reactions is attributed to a coherent interconversion between the singlet and triplet manifolds of radical pairs and subsequent spin selective recombination reactions.24-26 MFE is quite sensitive to confinement of the radical pairs because enhancement of the reaction yield under an applied field is totally dependent on radical pair lifetimes.24,27,28 Another important point here is that the spin-state mixing process can be perturbed by various relaxation processes, which are induced by dynamic fluctuations of electron spin interactions.29-31 Recent studies by the novel nanosecond-pulsed magnetic field effect30-32 have shown that MFEs in relatively low magnetic fields (
