Photoreduction of quinones. I. One-electron transfer to p-benzoquinone in ethanol studied by ESR during continuous photolysis.

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Electron spin resonance was examined during the continuous photolysis of a flowing solution of p-benzoquinone in ethanol at room temperature. It was found that both semiquinone anions and semiquinone radicals were generated, but their relative yields changed depending on the experimental conditions. The semiquinone anion was generated in a comparatively high yield at low concentrations of p-benzoquinone and at high flowing rates. Its formation was, however, quenched by the addition of acetic acid without transformation to the semiquinone radical. The semiquinone radical was generated efficiently at high p-benzoquinone concentrations and at low flowing rates. Its formation was enhanced by the addition of hydroquinone. These results, as well as the previous results on the effect of the wavelength of light, indicate, in contrast with the currently accepted view, that there is effectively no equilibrium in ethanol between the semiquinone radical and the semiquinone anion under the present experimental conditions, and that the latter is formed by a photoinduced one-electron transfer from solvent to p-benzoquinone. The semiquinone radical seems to arise from the hydrogen abstraction of excited p-benzoquinone from the hydroquinone formed as a product during photolysis.
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