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Chromophore Production in Lignin Model Compounds - Anaerobic Reactions

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The anaerobic photochemistry of a number of plausible lignin model compounds (i.e. I: 3,4-Dimethoxy-α-(2-methoxyphenoxy)-β-hydroxypropiophenone; II: 1-(3,4-Dimethoxyphenyl)-2-(2-methoxyphenoxy)-propan-1,3-diol ; Pol A: Poly(4-methoxyacrylophenone); Pol B: Poly(3,4-dimethoxyacrylophenone) ; St 1: 3,5-Dimethoxy-4-hydroxystilbene; and St 2: 3,5,3',5'-Tetramethoxy-4-hydroxystilbene) was studied, thin films of these materials being exposed to long-wave (λ > 300 nm) radiation under high vacuum conditions (10-6 torr). In all cases, the only low molecular weight products formed were methane and ethane, and quantum yields were estimated for these reactions. All materials underwent colouration (yellow) and a number of changes were also observed in both the absorbing and emitting characteristics. The colouration was attributed to the presence of o-quinones which were formed (by further photolysis) from the phenoxy radicals, which were, in turn, produced by O-CH3 fission, the resulting methyl radicals being the precursors of methane and ethane. The stilbenes were in all cases much more reactive (by a factor of about 100); however, they also absorbed higher intensities of radiation in the 300 < λ < 350 nm region on account of the greater extent of red-shifting of the longest-wave π-π* aromatic transitions. Gel permeation data indicate the formation of products of cyclization and isomerization of stilbenes and the dimerization of phenoxy radicals while new absorbances in the infrared and 13C NMR confirm the presence of o-quinones in all the models.

Affiliations: 1: Chemistry Department, Lakehead University, Thunder Bay, Ontario P7B 5E1


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