Organic Radical Cations and Neutral Radicals Produced by Radiation in Low-Temperature Matrices

Abstract

Paramagnetic species produced by ionising radiation in organic materials have been studied extensively by EPR for fifty years. Using low temperature matrices made it possible to characterise a wide class of highly reactive radicals. More recently, the focus was shifted to the investigations of ionised molecules (radical cations). Several approaches based on frozen solution technique, trapping in porous media and rigorous matrix isolation method has been developed up to 1990s. This chapter presents a review of recent progress in the field with special attention to the EPR studies of radical cations and radicals generated by high-energy irradiation of moderate-size organic molecules in solid rare gas matrices. The following aspects are discussed: (i) trap-to-trap positive hole transfer between organic solute molecules in low-temperature matrices; (ii) matrix effects on trapping and reactivity of organic radical cations in rigid inert media; (iii) the role of excess energy in the reactions of ionised organic molecules in solids; (iv) the nature of selectivity of the primary bond rupture in organic molecules and macromolecules. The prospects and problems of EPR and combined spectroscopic studies of the radiation-induced species in organic systems are outlined.