INTERACTIONS OF NO2- AND SO32- WITH ORGANIC TRIPLETS - CHARGE-TRANSFER VERSUS ENERGY-TRANSFER - THE ROLE OF REORGANIZATION ENERGY IN TRIPLET ANION INTERACTIONS AND SPECTROSCOPIC METHODS FOR ITS EVALUATION

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(q)) and radical yield measurements for SO32- and NO2-. 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 thc kinetic parameters of Marcus-Hush theory, also reflect Franck-Condon strains. These 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(q) and h-nu(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 NO2- leads to values of the thermal self-exchange mte agreeing with those computed from the Marcus-Hush cross-relations, which also neglect bonding effects. The mechanism of NO2- interaction with triplets is discussed in detail, including indirect kinetic evidence for quenching of a short-lived exciplex by NO2- without radical formation. The possibility of reduction by triplet NO2- 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.