Charge-Transfer Complexes in Kraft Lignin Part 1: Occurrence

Abstract

Charge-transfer complexes were found to occur between kraft lignin and an added model quinone, 3,5-di-tert-butyl-1,2-benzoquinone. The occurrence of charge-transfer interactions was also apparent in an oxidized kraft lignin with an increased quinone content. In these systems, free phenolic groups within the lignin were considered the donor species and ortho-quinones the complementary acceptor moieties. Carbon-14 labeling revealed that the quinone content of the investigated kraft lignin averaged 3%. These quinones were determined to have a molar absorptivity of 528 L/mol-cm. Upon sodium borohydride reduction of this lignin, only one-third of the absorbance decrease could be accounted for by this number of quinones. The remaining two-thirds of the decrease in absorbance was assigned to the disruption of charge-transfer complexes. The quinones, therefore, played a dual role as a chromophore by participating as acceptor species in these complexes.