Photodecomposition of lignin model compounds: Part I

Abstract

The plausible lignin model compounds, 3,4-dimethoxy-α-(2-methoxyphenoxy)-β-hydroxy-propiophenone ( I) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-propandiol-1,3 ( II) were irradiated in the form of thin films with long-wave ultraviolet ( λ ≥ 300 nm) radiation under high vacuum. Methane and ethane were the only detectable low molecular mass products, the quantum yields for their formation being low (around 10 −4 mol einstein −1). The compounds also underwent coloration and a number of other changes, as indicated by spectroscopy, and these were attributed to the formation of quinonoid species, their precursors being phenoxy radicals formed by direct photolysis of the OCH 3 bonds in the benzene rings. In the case of I, the long-wave chromophore is composite, involving both the carbonyl ( n→π ∗ transition) and the phenyl group ( π→π ∗ transition). In II the absorption due to the S 0→ S 1 transition of the benzene ring extends into the long-wave region ( λ > 300 nm). It is suggested that OCH 3 fission and subsequent reactions of the phenoxy radicals contribute to the coloration of lignin, which contains identical structural elements.