Time-resolved ESR study of spin exchange processes in the photoreduction of 9,10-anthraquinone-1,5-disulfonate
- Univ. of Leipzig (Germany)
- Univ. of Notre Dame, IN (United States)
The photoreduction of 9,10-anthraquinone-1,5-disulfonate by 2,2,6,6-tetramethlypiperdine in aqueous solution was studied on the nanosecond and microsecond time scales using time-resolved optical and ESR measurements. 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-tetramethlypiperidene to be k{sub N}=5.7 x 10{sup 8}M{sup -1} s{sup -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 (T{sub 1} and T{sub 2}), 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 (k{sub ex} = 4.5 x 10{sup 9} M{sup -1} s{sup -1}). 30 refs., 9 figs., 3 tabs.
- DOE Contract Number:
- AC02-76ER00038
- OSTI ID:
- 199376
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 5 Vol. 100; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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