Lignin-quinone interactions: Implications for optical properties of lignin

Charge transfer (CT) excited states are well-known and examined for artificial solution and solid-state donor-acceptor systems. Their possible existence in lignin has, however, not been considered before. In the present work we have studied the optical modifications resulting from the co-adsorption of the acceptor p-benzoquinone with selected donor compounds, some of which are related to lignin, onto filter paper. The quinone was also incorporated into the lignin of beech and spruce thermomechanical pulp fibers, and their optical absorption and emission properties were then examined. Evidence for the formation of CT states of donor compounds-quinone or (wood fiber) lignin-quinone is presented. Most notable is the formation of a broad-band visible absorption, at lower energies than what can be explained by quinone absorption alone, and the significant wood fiber emission quenching caused by these CT states. Since quinones commonly occur in various lignin types (e.g., wood pulps), this has the implication that lignin photo-oxidation may proceed via such CT states.