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Time-Resolved ESR Study of Spin Exchange Processes in the Photoreduction of 9,10-Anthraquinone-1,5-disulfonate

1996; American Chemical Society; Volume: 100; Issue: 5 Linguagem: Inglês

10.1021/jp9510756

1541-5740

D. Beckert, Richard W. Fessenden,

Free Radicals and Antioxidants

The photoreduction of 9,10-anthraquinone-1,5-disulfonate by 2,2,6,6-tetramethylpiperidine in aqueous solution was studied on the nanosecond and microsecond time scales using time-resolved optical and ESR measurements. The primary electron transfer from the amine to the quinone triplet generates solvent-separated radical ion pairs which then diffuse apart. The lifetime of the 9,10-anthraquinone-1,5-disulfonate triplet in aqueous solution was determined to be 135 ns and the electron transfer rate constant from 2,2,6,6-tetramethylpiperidene to be kN = 5.7 × 108 M-1 s-1. The corresponding rate constant for quenching by 2-propanol is lower by 2 orders of magnitude at 1.4 × 106 M-1 s-1. This system allowed both radicals of the pair, the anthraquinone anion and (for the first time) the aminyl radical, to be studied by ESR. The radicals are spin polarized (CIDEP) by both the triplet and geminate radical pair mechanisms. Time-resolved ESR experiments were carried out with the transient nutation and Fourier transform techniques. Because of steric hindrance, the recombination reaction in the quinone−piperidine system is suppressed and, therefore, the spin dynamics are dominated by spin polarization, relaxation, and spin exchange. A quantitative analysis by modified Bloch equations allowed polarization factors (triplet mechanism and radical pair mechanism), spin relaxation times (T1 and T2), and the spin exchange rate constants between the anthraquinone anion and aminyl radicals to be obtained. The spin exchange rate constants between quinone−aminyl radical pairs and between aminyl−aminyl radical pairs are identical and are in the diffusion-controlled limit (kex = 4.5 × 109 M-1 s-1).