ESR STUDY OF A NITROXIDE RADICAL IN SOL-GEL GLASSES

A spin probe TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy) w as dissolved in a tetraethyl orthosilicate sol-gel reaction system and measured by electron spin resonance spectroscopy at 295 K. The nitrog en hyperfine coupling constant was from 1.64-1.66 mT in the sol-gel so lutions. The values were sensitive to the ethanol-to-water ratio of th e solutions. The hyperfine coupling constant in. the xerogels was 1.70 mT, which was almost the same as that in water, indicating that the p robe molecules were trapped in silica pores with water adsorbed on the silica surfaces. The motion of TEMPOL in the xerogels was considerabl y slower than in the sol-gel solutions. The local viscosity estimated was from 70-90 cP. The ESR spectra of TEMPOL were altered during the s ol-gel process, indicating that adsorbed water on the silica surfaces has an important role for trapping organic molecules in sol-gel glasse s.