Photophysics and photochemistry of a lignin model molecule containing α-carbonyl guaiacyl and 4-hydroxy-3-methoxybenzyl alcohol moieties

Abstract

A lignin model molecule (DCOOH), incorporating an α-carbonyl guaiacyl part and a 4-hydroxy-3-methoxybenzyl alcohol moiety linked by an n-hexane chain, has been synthesized. The electronic absorption spectra in the ground and excited states (flash photolysis) and the luminescence properties in dilute solutions (alcohol and alkane) and in hydroxypropylcellulose (HPC) film were studied at room temperature and at 77 K in comparison with models representing the two ends. They show that in non-polar and very good hydrogen donor solvents there is interaction between the excited carbonyl and the phenolic group, the chain probably being in a folded conformation. In alcoholic solvents and in HPC films, an intermolecular interaction between the carbonyl chromophore and the hydroxy group of the solvent was observed, the hexamethylene chain being in an extended conformation. UV irradiation of the dimer and the carbonyl monomer, incorporated in an HPC film, show similar reactivity which appears to involve an intermolecular hydrogen abstraction by the excited-state carbonyl group from the surrounding medium. The reactivity of the carbonyl derivatives in isopropyl alcohol and tetrahydrofuran (THF) was found to be similar to that observed for the films. In THF, two isomeric photoadducts, between the carbonyl monomer (MCOET) and tetrahydrofuran were isolated and characterized. In alkane solvents, UV irradiation of the dieter leads to the appearance of a new band in the long wavelength range characteristic of the production and oxidation of phenoxyl radicals, which are oxidized to coloured structures such as quinones. The main conclusion of the study is that for the first time, it is shown that in the solid state the carbohydrate matrix does not tend to promote the oxidation process between the carbonyl and the phenol. This result is important for the understanding of the mechanism of photoyellowing of lignin-rich pulps