Study of radical ions in the condensed phase by fluorescence-detected magnetic resonance

Magnetic resonance studies of transient condensed-phase radical ions have been realized for the first time without the use of a stabilizing matrix. The occurrence, structure, and, reactivity of radical cations and radical anions in the condensed phase using time-resolved fluorescence-detected magnetic resonance (FDMR) is reviewed. FDMR observes the time-resolved EPR spectra of spin-correlated radical-ion pairs on the 10(-8)-10(-6) time scale. FDMR studies have elucidated the identity and fate of primary radical cations in saturated hydrocarbons, alcohols, and other solvent media exposed to ionizing or photoionizing radiation. In photoionization, FDMR allows the determination of the photophysical pathway, i.e., the photon order and the multiplicity of the state which is ionized. The formation of secondary radical cations via electron transfer from solutes to solvent radical cations allows the FDMR/pulse radiolysis method to study a wide range of organic radical cations. Radical-cation reactions that have been studied include unimolecular dissociation, charge transfer, aggregate formation, and ion-molecule reactions. Ion-molecule reactions such as proton transfer are the principal channels besides neutralization for radical-cation decay in the condensed phase.