Photochemical study of an o-ethyl dibenzodioxocin molecule as a model for the photodegradation of non-phenolic lignin units of lignocellulosics

The photochemical behavior of compound 1 was studied as representative of a non-phenolic dibenzodioxocin lignin model, because the presence of both photoreactive α-O-4 and β-O-4 linkages in these structures should bring a high photosensivity of the framework. Fluorescence emission of 1 is situated at shorter wavelength (340 nm) than the emission of the corresponding biphenylbiphenol 5 (380 nm). This observation, which has to be correlated with conjugation between the phenyl rings of the biphenyl chromophore, indicates that dibenzodioxocins are not contributing to the long wavelength emission in lignins (>480 nm). Irradiation of 1 in non-degassed benzene solutions, using Pyrex vessel, which eliminates wavelengths below 290 nm, gives colored solutions. The reaction is very efficient (ΦR=0.37) and analysis of the photoproducts indicates that the first step of the reaction is a cleavage of the α-O-4 bond followed either by β-O-4 cleavage or formation of rearranged products. The color is due to condensation of biphenyl-oxidized products (quinone type). A mechanism is proposed based on separation and identification of photoproducts. The reactivity of 1 supports the fact that dibenzodioxocins have to be considered as important chromophores in photodegradation of lignocellulosic materials.