LONG-WAVE PHOTOCHEMISTRY OF DI-METHOXYLATED COMPOUNDS RELATED TO LIGNIN

Four methoxylated compounds which resemble moieties either found in li gnin or produced in lignin, i.e. poly(3,4-dimethoxyacrylophenone) (Pol y 34), guaiacylacetoveratrone (34Keto), methoxyphenyl)-2-(2-methoxyphe noxy)propan-1,3-diol (34Hydroxy) and 3,3',5,5'-tetramethoxy-4-hydroxys tilbene (ST2), were irradiated in the form of thin films under vacuum, wavelengths being restricted to 300 nm and greater. In all cases new absorptions were formed in the long-wave UV and visible regions and th e samples became coloured (yellow), ST2 being particularly reactive. I n all cases methane and ethane were the only two low molecular weight products, and it was proposed that O-CH3 bond fission was occurring, a ll of the compounds having appreciable long-wave absorptions before ir radiation. C-13 NMR spectra showed that the depletion of the methoxy C -atoms was accompanied by the formation of new carbonyls. Further evid ence for O-CH, fission was obtained from the photolysis of 3-ethoxy-4- hydroxystilbene in which ethane, ethylene and n-butane were formed. Sp ectral observations indicated that the new carbonyls were quinones and o-quinones, these being formed from the phenoxy radicals in turn form ed by O-C fissions, and the coloration was attributed to quinonoid spe cies. The work demonstrates that lignin-type compounds can undergo col oration reactions, which are independent of oxidation, and are totally photo-induced. Plausible reaction mechanisms are proposed. (C) 1996 E lsevier Science Limited.