Diffuse-reflectance laser flash photolysis studies of the photochemistry of bleached thermomechanical pulp

Abstract

Diffuse-reflectance laser flash photolysis was used to study the photochemistry of bleached, black spruce mechanical pulp. Following laser excitation of bleached pulp samples at 354 nm, a transient absorption spectrum was observed with λ max = 450 nm. Oxygen and the phenolic hydroxyl groups in lignin reduced the detected amount of transient by static quenching, and the intensity of the transient spectrum also decreased in pulps reduced with sodium borohydride. The transient was assigned as the electronic triplet state of the aryl ketone group in lignin. Computer simulations indicated that the concentration of the ketone triplet states varied exponentially with depth into the sample, and kinetic analysis was carried out on this basis. The decay of the transient was remarkably long, and was not described by either simple first-order or higher-order kinetics. We interpret this as a manifestation of the heteogeneity of lignin; the aromatic ketone group is distributed over a variety of structurally or chemically distinct sites in the macromolecule.