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

Article abstract of DOI:10.1021/j100192a018

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 SO3^2- 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 the 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 equilbirium potentials with quenching rates.The theoretical basis for correlation between k_q and HνCTTS 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 rate 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.