Photoyellowing of milled Wood Lignin and Peroxide-Bleached Milled Wood Lignin in Solid 2-Hydroxypropylcellulose Films After Sodium Borohydride Reduction and Catalytic Hydrogenation in Solution: a Fluorescence Spectroscopic

Abstract

Fluorescence spectra and emission quantum yields of 2-hydroxypropyl-cellulose films, incorporating milled wood lignin that had been treated in solution by HgOg/NaOH and/or NaBfy and/or H₂ (Pd/C), were measured before and after irradiation by UV light (λ> 300 nm). Bleaching, reduction (NaBH₄), and hydrogenation (H₂, Pd/C) increase the quantum yield of fluorescence and emission in the blue region (400 nm). The destruction of carbonyl chromophores (α-carbonyl, coniferaldehyde, and quinones), which quench the fluorescence of biphenyl groups, the main structures emitting in this part of the spectrum, appeared to be mainly responsible for this increase. Irradiation restores i) emission in the long wavelength part of the spectra (maximum emission: 500 nm, maximum excitation: 400 nm), and ii) quenching of the fluorescence in the blue part (400 nm) of the spectra, even for hydrogenated films. These results are interpretated in relation to the formation, under irradiation, of coniferaldehyde groups and also the generation of complex structures formed by photooxidation of phenolic biphenyls.