CHROMOPHORE PRODUCTION IN LIGNIN MODEL COMPOUNDS - ANAEROBIC REACTIONS

The anaerobic photochemistry of a number of plausible lignin model com pounds (i.e. I: lpha-(2-methoxyphenoxy)-beta-hydroxypropiophenone; II: thoxyphenyl)-2-(2-methoxyphenoxy)-propan-1,3-diol; Pol A: Poly(4-meth oxyacrylophenone); Pol B: Poly(3,4-dimethxyacrylophenone); St 1: 3,5-D imethoxy-4-hydroxystilbene; and St 2: 3,5,3',5'-Tetramethoxy-4-hydroxy stilbene) was studied, thin films of these materials being exposed to long-wave (lambda greater than or equal to 300 nm) radiation under hig h vacuum conditions (10(-6) torr). In all cases, the only low molecula r weight products formed were methane and ethane, and quantum yields w ere estimated for these reactions. All materials underwent colouration (yellow) and a number of changes were also observed in both the absor bing and emitting characteristics. The colouration was attributed to t he presence of o-quinones which were formed (by further photolysis) fr om the phenoxy radicals, which were, in turn, produced by O-CH3 fissio n, the resulting methyl radicals being the precursors of methane and e thane. The stilbenes were in all cases much more reactive (by a factor of about 100); however, they also absorbed higher intensities of radi ation in the 300 < lambda < 350 nm region on account of the greater ex tent of red-shifting of the longest-wave pi-pi() aromatic transitions . Gel permeation data indicate the formation of products of cyclizatio n and isomerization of stilbenes and the dimerization of phenoxy radic als while new absorbances in the infrared and C-13 NMR confirm the pre sence of o-quinones in all the models.