Trapped free radicals and electrons in organic glasses

Free radicals, hydrogen atoms, and electrons produced in rigid organic glasses at sufficiently low temperatures have lifetimes of minutes to years. They can be studied by their electron spin resonance spectra and, in the case of electrons, by their optical spectra, recombination luminescence, and electrical conductivity. The decay kinetics of these reaction intermediates serve to distinguish those trapped as geminate pairs or in spurs of high concentration from those formed with random distributions. Electron spin resonance studies of relaxation times and of the spectra of radical pairs provide further evidence on geometrical distributions. The decay rates of radicals combining with reactive geminate partners are dependent on the size and shape of the radical, the temperature, and the nature of the matrix. Decay is much slower in deuterated matrices than in protiated matrices. The factors that control the physical trapping of electrons in organic glasses are under intensive investigation. There is evidence that many electrons trapped relatively weakly during irradiations at 4 degrees K deepen their traps by orientation of dipoles when the matrix is warmed; that most electrons are trapped in the field of the geminate positive ion; that in some matrices the traps have a bound excited state to which the electron can be promoted without detrapping; and that trapped electrons can tunnel to solute molecules with a higher electron affinity than the trap depth.