Charge-Transfer Complexes in Kraft Lignin Part 2: Contribution to Color

Abstract

Chelation and removal of transition metals from the kraft lignin employed in this study did not reduce the lignin's visible absorbance. Likewise, hydrogenation of carbon-carbon double bonds in this lignin also had no effect on its visible spectrum. This indicated that extended conjugated systems were not a significant contributor to the color of this lignin. Of the structures investigated, quinones were found to be the major visible-light absorbing chromophores. Since a large component of the quinone's absorption results from their participation in charge-transfer complexes (CTC's),¹ CTC's were a significant contributor to the color of this lignin.