Photoyellowing behavior of non-phenolic lignin models representative of end groups and β-ether structures

Lignin model compounds representative of etherified end groups [(E)-3-(3,4-dimethoxyphenyl)-2-propenl-ol (1), (E)-3-(3,4-dimethoxyphenyl)propenal (2)] and beta-ethers, [1-(3,4-dimethoxyphenyl)-3-hydroxy-1-(2-methoxyphenoxy)-1-propanone (3), erythro-1-(3,4-dimethoxyplienyl)-2-(2-methoxyphenoxy)-1,3-propanediol (4)] have been applied to filter paper and subjected to accelerated photoaging with UV/VIS-light. The changes in optical properties were monitored. In case of all the models, the brightness decreased and the b*-value increased as a result of irradiation, which shows that they are sensitive to light simulating sunlight behind window-glass. In the case of the end group models, the photoproducts identified were veratraldehyde and veratric acid together with the Z-isomers of the starting materials. In the case of the cinnamyl alcohol 1, photo-oxidation to the cinnamaldehyde 2 was observed. The irradiation of the alpha-carbonylic beta-ether 3 resulted in the homolytic cleavage of the Cbeta-O bond and a recombination that led to a beta-5 dimer [1-(3,4-dimethoxyphenyt)-3-hydroxy-2-(2-hydroxy-3-methoxyphenyl)- 1-propanone] and a beta-1 dimer [1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(4-hydroxy-3-methoxyphenyl)- 1-propanone]. The monomeric products veratraldehyde, vanillin, acetoveratrone, and 1-(3,4-dimethoxyphenyl)-3-hydroxy-1-propanone were also formed. The only photoproduct conclusively identified in experiments with the arylglycerol-beta-aryl ether 4 was veratraldehyde and most of the starting material was unaffected by the irradiation. The addition of a photosensitizer (acetoveratrone) to this type of model prior to irradiation resulted in some increase in discoloration.