TIME-RESOLVED ELECTRON-PARAMAGNETIC RESONANCE SPECTROSCOPY OF ORGANIC FREE-RADICALS ANCHORED TO SIO2 SURFACES

The first time-resolved electron paramagnetic resonance spectra of organic free radicals anchored to polycrystalline and amorphous SiO2 surfaces via siloxane bonds are reported. Spectra of the dimethylpentyl and tert-butyl radicals were obtained 0.4-mu-s after a slurry of modified silica gel in benzene was flash photolyzed in the cavity of an EPR spectrometer. The spectra are strongly spin-polarized with the triplet spin polarization mechanism (TM) dominating over the radical pair mechanism (RPM). This is in contrast to spectra taken in free solution, where there is an equal amount of TM versus RPM. The TM dominates because of the longer spin relaxation time of the parent molecular ketone triplet state when anchored to the surface. Motional averaging is less efficient for radicals anchored five bonds away from the surface versus seven bonds, resulting in a broadening of the spectral lines in the former case. The spectrum of the dimethylpentyl radical disappeared almost completely when it was anchored to amorphous rather than polycrystalline SiO2, while the tert-butyl radical in solution remained intense.