GENERATION AND SPECTROSCOPIC AND KINETIC CHARACTERIZATION OF METHOXY-SUBSTITUTED PHENOXYL RADICALS IN SOLUTION AND ON PAPER

A number of methoxy-substituted phenoxyl radicals have been generated and characterized by laser flash photolysis techniques in solution and on paper. The radicals have been produced by three routes in solution : hydrogen abstraction from phenols by tert-butoxyl radical, photolysi s of alpha-aryloxyacetophenones, and direct excitation of phenols. Mos t of the phenoxyl radicals studied have a characteristic absorption ne ar 400 nm; the ortho-substituted radicals have an additional broad abs orption in the visible in non-hydroxylic solvents (e.g., 650 nm for 2- methoxyphenoxyl radical). The relative intensities of the 400 nm and t he long-wavelength bands vary with substitution pattern and solvent, w ith the long wavelength band being much weaker in hydroxylic solvents. Direct excitation of phenols provides an alternate source of phenoxyl radicals in some cases. However, for 4-methoxyphenol there are overla pping absorptions due to triplet phenol and phenol radical cation, bot h of which absorb in the 400-460 nm region. The 4-methoxyphenol radica l cation is formed in a biphotonic process and decays by deprotonation to give the phenoxyl radical. The 4-methoxyphenoxyl radical can also be generated on paper from either the ketone or phenol precursor and d ecays with complex kinetics that depend on the precursor loading and l aser intensity. There is no evidence for the formation of significant amounts of long-lived ketone or phenol triplets on paper. The decay ki netics for the phenoxyl radical are modified by the addition of the ra dical scavenger dithioerythritol. The results indicate both static and dynamic components to the radical scavenging process. The generation of phenoxyl radicals on paper in the presence and absence of scavenger s provides the basis for the development of a new method for screening potential photoyellowing inhibitors and understanding their mode of a ction.