The Photochemistry of α-Phenoxyacetophenones Investigated by Flash-CIDNP-spectroscopy

Photo- and radiation chemical induced degradation of lignin model compounds

The basic mechanistic aspects of the photo- and radiation chemistry of lignin model compounds (LMCs) are discussed with respect to important processes related to lignin degradation. Several reactions occur after direct irradiation, photosensitized or radiation chemically induced oxidation of LMCs. Direct irradiation studies on LMCs have provided supportive evidence for the involvement of hydrogen abstraction reactions from phenols, beta-cleavage of substituted alpha-aryloxyacetophenones and cleavage of ketyl radicals (formed by photoreduction of aromatic ketones or hydrogen abstraction from arylglycerol beta-aryl ethers) in the photoyellowing of lignin rich pulps. Photosensitized and radiation chemically induced generation of reactive oxygen species and their reaction with LMCs are reviewed. The side-chain reactivity of LMC radical cations, generated by radiation chemical means, is also discussed in relation with the enzymatic degradation of lignin.

Photocatalytic degradation of lignin and lignin models, using titanium dioxide: the role of the hydroxyl radical

The role of hydroxyl radicals on the degradation of lignins during a cellulosic pulp bleaching process including a photocatalytic stage, was assessed using peroxyformic acid lignins EL1 and REL1 and two phenolic niphenyl lignin models 1 and 2. The irradiations were performed in the absence of photocatalyst TiO2 and H2O2 (condition a), in the presence of TiO2 (condition b) and in the presence of H2O2 (condition c). The experiments were conducted in alkaline (pH approximately 11) aqueous ethanol solutions with oxygen bubbling. The relative phenolic content of the irradiated solutions, which is indicative of the involvement of hydroxyl radicals, was determined by ionization absorption spectroscopy. The results obtained show that the catalyzed reaction involves both degradation of the phenolate groups by electron transfer and hydroxylation of the lignin aromatic structure. Benzyl alcohol structural elements in sodium borohydride reduced lignin REL1 and compound 2 were also found as good trapping agents for the hydroxyl radicals. The degradation of EL1 was studied by measuring its fluorescence emission by comparison to the fluorescence of compound 2. The emission spectra indicate that some biphenyl phenolate anions in EL1 are reacting under UV/visible irradiation and some others, probably polyphenolic chromophores emitting less fluorescence, are formed.

Effect of cyclodextrin complexation on the photochemistry of the lignin model α-guaiacoxyacetoveratrone

The photodecomposition of α-guaiacoxyacetoveratrone (GAV) in homogeneous solvents and in aqueous cyclodextrin solutions was studied by following the fluorescence emission of the photoproducts formed. The same qualitative behavior as previously observed for the photodegradation quantum yield measurements was reproduced in the fluorescence studies in dry or wet methanol and acetonitrile. The yield for GAV photodegradation in water is smaller than in the organic solvents. Complexation of GAV to β- and γ-cyclodextrins leads to an increase in the photodecomposition yield, especially during the early part of the photodecomposition kinetics. The transients formed in the photolysis of GAV in water are similar to those observed in acetonitrile, and the lifetime of triplet GAV in water is 130 ns. In addition, solvated electrons were formed when GAV was photolyzed in water. Complexation of GAV to cyclodextrins leads to an increase in the triplet yield compared to the radical yield and a moderate shortening of the triplet lifetime.Key words: α-guaiacoxyacetoveratrone, cyclodextrin, excited singlet states, excited triplet states, radicals.