Recombination of radical ion pairs of triplet multiplicity

Abstract

Radical ion pairs generated by photo-induced electron transfer or formed upon encounter of independently generated radical ions may undergo back electron pairs of triplet multiplicity. Triplet recombination has been probed by luminescence time-resolved optical spectroscopy (in combination with magnetic field effects), CIDNP spectroscopy, and opto-acoustic calorimetry. The free energy of back electron transfer and the topologies of the potential surfaces of parent molecule, radical ion and the accessible triplet state or biradical play significant roles in this recombination. The free energy governs the rate of back electron transfer and determines whether this process is competitive. The topologies of the intermediates may influence the course of the overall reaction. The topologies of the potential energy surfaces of ground state, radical ion, and triplet related in three different ways. First, aromatic donors/acceptors typically undergo only minor geometry upon oxidation, reduction or excitation to the triplet state. Therefore, rearrangements are unlikely. Alternatively, the connectivities of some radical ions resemble those molecule, whereas the corresponding triplet states (biradicals) have structures of different connectivities. In these cases, back electron transfer will populate a triplet (biradical) with cleaved bonds (“dissociative” electron return) or one with newly formed bonds (“associative” electron return) resulting in structural reorganizations. Third, the radical ions of some strained ring compounds may rearrange to ring-opened structures of lower energy. Here, triplet states or biradicals with geometries related to the rearrangement radical ions may be populated. Finally, triplet radical ion pairs may form CC, CN, or CO bonds between radical anion and cation (charge recombination), generating “bimolecular” states. Following intersystem crossing, the biradicals may fragment to regenerate the reagents, generate adducts by forming a second bond (cyclization) or transfer of an atom or group between the two functions. The different intra-pair reactions of triplet radical are illustrated with appropriate examples.