PHOTOPHYSICS AND PHOTOCHEMISTRY OF A LIGNIN MODEL MOLECULE CONTAININGALPHA-CARBONYL GUAIACYL AND 4-HYDROXY-3-METHOXYBENZYL ALCOHOL MOIETIES

A lignin model molecule (DCOOH), incorporating an a-carbonyl guaiacyl part and a 4-hydroxy-3-methoxybenzyl alcohol moiety linked by an n-hex ane chain, has been synthesized, The electronic absorption spectra in the ground and excited states (hash photolysis) and the luminescence p roperties in dilute solutions (alcohol and alkane) and in hydroxypropy lcellulose (HPC) film were studied at room temperature and at 77 K in comparison with models representing the two ends, They show that in no npolar and very good hydrogen donor solvents there is interaction betw een the excited carbonyl and the phenolic group, the chain probably be ing 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 be ing in an extended conformation. UV irradiation of the dimer and the c arbonyl monomer, incorporated in an HPC him, show similar reactivity w hich appears to involve an intermolecular hydrogen abstraction by the excited-state carbonyl group from the surrounding medium, The reactivi ty of the carbonyl derivatives in isopropyl alcohol and tetrahydrofura n (THF) was found to be similar to that observed for the films. In THF , two isomeric photoadducts between the carbonyl monomer (MCOET) and t etrahydrofuran were isolated and characterized, In alkane solvents, UV irradiation of the dimer 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 a s quinones, The main conclusion of the study is that for the first tim e, it is shown that in the solid state the carbohydrate matrix does no t tend to promote the oxidation process between the carbonyl and the p henol, This result is important for the understanding of the mechanism of photoyellowing of lignin-rich pulps.