One- and Two-Dimensional EPR Studies on the Radical Pair Generated by the Photoreduction of 9,10-Anthraquinone-1,5-disulfonate in Aerosol OT Reverse Micelles

Abstract

Spin-correlated radical pairs (SCRP) formed by the photoreduction of 9,10-anthraquinone-1, 5-disulfonic acid (1,5-AQDS) disodium salt in aerosol OT (AOT) reverse micelles were studied using time-resolved and pulsed electron paramagnetic resonance (EPR) techniques. Photoirradiation of 1,5-AQDS in AOT micellar solution exhibited a strong polarized EPR spectrum in the presence of water-soluble hydroquinones (H2Q). The polarization patterns varied from E*/A of radicals to the antiphase structure (APS) of SCRP in the early delay period after the laser pulse irradiation. By analysis of the spectra, the radicals were assigned to the corresponding benzoquinone anion radicals. On the basis of our results, the initial process of the 1,5-AQDS triplet quenching may be the electron transfer from H2Q. The consecutive reaction mechanism was proposed for the generation of the SCRP within the water pools. The slower evolution of the SCRP polarization is a result of the sequential radical reactions followed by the spin selective reaction of the radical pair (RP). The peak-to-peak distance of the APS pattern and also the component line width were affected by the molar ratios between water and AOT ([H2O]/[AOT], w). A decreasing w value resulted in an increase in the magnitude of the exchange interaction, |2J|. From the peak-to-peak width of each APS spectral shape in the time-resolved EPR (TR-EPR) spectrum, at w = 20, the value of |2J| was estimated of 0.04 mT. The value was extremely small compared with those of SCRP reported previously and corresponded to 1.9 nm as the average separation of the radicals. Comparatively, the half-life of the APS polarization was longer than the T1 values of the typical organic free radicals. The results suggest that the diffusion of the radicals was strongly restricted within the water pool of AOT micellar solution.