Interactions of NO{sub 2}{sup {minus}} and SO{sub 3}{sup 2{minus}} with organic triplets. Charge transfer versus energy transfer: The role of reorganization energy in triplet-anion interactions and spectroscopic methods for its evaluation
- Hebrew Univ., Jerusalem (Israel)
- Brandeis Univ., Waltham, MA (United States)
Charge-transfer (CT) and energy-transfer (NT) interactions of simple anions with organic triplets are reviewed and discussed in connection with new quenching rate constant (K{sub q}) and radical yield measurements for SO{sub 3}{sup 2{minus}} and No{sub 2}{sup {minus}}. In the latter case both processes may occur at high organic triplet energies. Reorganization energies for one-electron oxidations are obtained for several anions, using data on charge-transfer-to-solvent (CTTS) spectra and photoelectron emission thresholds, which like the kinetic parameters of Marcus-Hush theory, also reflect Franck-Condon strains. The results, combined with thermodynamic free energies, give vertical redox potentials which correlate better than do equilibrium potentials with quenching rates. The theoretical basis for correlation between k{sub q} and Hv{sub CTTS} is discussed in the framework of Marcus rate theory. Assigning the total reorganization energy in the CT quenching reaction to the small anion component of the D-A pair gives reasonable agreement with data on quenching of dye triplets but too slow rates for aryl carbonyl triplets where exciplex formation may possibly occur. The optical reorganization energy for NO{sub 2}{sup {minus}} leads to values of the thermal self-exchange rate agreeing with those computed from the Marcus-Hush cross-relations, which also neglect bonding effects. The mechanism of NO{sub 2}{sup {minus}} interaction with triplets is discussed in detail, including indirect kinetic evidence for quenching of a short-lived exciplex by NO{sub 2}{sup {minus}} without radical formation. The possibility of reduction by triplet NO{sub 2}{sup {minus}} formed by initial NT from the organic triplet is also considered. Finally, a scheme is presented involving an equilibrium between CT and NT states and relating the free energy difference between these states to radical yields. 54 refs., 8 figs., 3 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 457763
- Journal Information:
- Journal of Physical Chemistry, Vol. 96, Issue 13; Other Information: PBD: 25 Jun 1992
- Country of Publication:
- United States
- Language:
- English
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