Triplet states with unusual spin polarization resulting from radical ion pair recombination at short distances
Covalently bound electron donor-acceptor, [D-A], molecules that undergo photoinduced charge separation from a photoexcited singlet state to form radical ion pairs that subsequently undergo back electron transfer to yield a triplet state are very rare. Apart from their importance in the general context of electro-transfer reactions, these molecules provide insights into the primary photochemistry of photosynthetic reaction centers. Photoexcitation of [D-A] produces {sup 1}*[D-A], which undergoes rapid charge separation to yield {sup 1}[D{sup {sm{underscore}bullet}{plus}}-A{sup {sm{underscore}bullet}{minus}}]. At [D{sup {sm{underscore}bullet}{plus}}-A{sup {sm{underscore}bullet}(minus)}] distances {ge}20 A, the exchange integral, J, between the two radicals is generally {le}10{sup {minus}4} cm{sup {minus}1}. This weak interaction allows radical pair intersystem crossing (RP-ISC) within {sup 1}[D{sup {sm{underscore}bullet}{plus}}-A{sup {sm{underscore}bullet}(minus)}] to occur, which leads to the formation of a spin-correlated radical pair {sup 3}[D{sup {sm{underscore}bullet}{plus}}-A{sup {sm{underscore}bullet}{minus}}]. Radical ion pair recombination within {sub 3}[D{sup {sm{underscore}bullet}{plus}}-A{sup {sm{underscore}bullet}{minus}}]. In all of these reactions the spins remain correlated, which results in non-Boltzmann populations of the spin states within each intermediate. The authors report here the first example of a covalently bound [D-A] molecule in which a single electron-transfer step produces a [D{sup {sm{underscore}bullet}{plus}}-A{sup {sm{underscore}bullet}{minus}}] pair at a short 11 {angstrom} distance with a large J value that leads to efficient {sup 3}*[D-A] formation.
- Research Organization:
- Argonne National Lab., IL (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-31109-ENG-38
- OSTI ID:
- 20000136
- Journal Information:
- Journal of the American Chemical Society, Vol. 121, Issue 33; Other Information: PBD: 25 Aug 1999; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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