PULSE-RADIOLYSIS AND LASER FLASH-PHOTOLYSIS STUDIES OF THE LIGNIN MODEL ALPHA(P-METHOXYPHENOXY)-P-METHOXYACETOPHENONE AND RELATED-COMPOUNDS

The relativity of reaction intermediates derived from alpha-(p-anisyl) -p-methoxyacetophenone (IIa) and related compounds has been studied in solution and in the solid state, by a combination of laser photolysis techniques, pulse radiolysis, X-ray crystallography, solid-state NMR, emission spectroscopy molecular modeling, and product studies. In sol ution the photolysis of IIa proceeds predominantly through the nonfluo rescent singlet state that fragments to yield p-CH3OC6H4O. (VI) and p- CH3OC6H4COCH2. (VII) radicals. The latter shows an absorption band at 540 nm. Molecular modeling calculations support the presence of this b and and provide a rationalization for this absorption. In the solid st ate IIa is essentially photostable. The long-lived triplet state decay s exclusively by second-order kinetics (triplet-triplet anihilation). In solution the ketyl radical derived from IIa fragments rapidly to yi eld VI and CH3OC6H4C(OH)=CH2 with a lifetime of <20 ns. In the context of the lignin photodegradation processes responsible for the yellowin g of lignin-rich papers, these results imply that the fragmentation of both the triplet state of Ila and ketyl radicals derived from alcohol groups in lignin will be very difficult to inhibit. Prevention of col oration is more likely to succeed if the radicals produced from the fr agmentations mentioned above are targeted for scavenging before they u ndergo the oxidative processes that promote yellowing.