CHROMOPHORE PRODUCTION IN LIGNIN MODEL COMPOUNDS - ANAEROBIC REACTIONS

The anaerobic photochemistry of a number of plausible lignin model compounds (i.e. I: 3,4-Dimethoxy-alpha-(2-methoxyphenoxy)-beta-hydroxypropiophenone; II: 1-(3,4-Dimethoxyphenyl)-2-(2-methoxyphenoxy)-propan-1,3-diol; Pol A: Poly(4-methoxyacrylophenone); Pol B: Poly(3,4-dimethxyacrylophenone); 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 (lambda greater than or equal to 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 < lambda < 350 nm region on account of the greater extent of red-shifting of the longest-wave pi-pi(*) 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 C-13 NMR confirm the presence of o-quinones in all the models.