UV irradiation during fluorescence measurements affects the structures and fluorescence properties of isolated lignins in solution.

There are no reports on the stability of autofluorescence, although lignins and their solutions are known to exhibit autofluorescence. An aim of this study is to investigate spectral change of lignin autofluorescence under prolonged excitation with ultraviolet (UV). Both fluorescence spectra of softwood milled wood lignin (MWL) and kraft lignin (SKL) in propylene glycol significantly changed upon an UV irradiation at 280 nm under O2 and Ar atmospheres even for 10 min. By the further irradiation, the autofluorescence intensities were remarkably changed together with the emission wavelengths. However, no tendency in the autofluorescence change was observed. The change varied between lignin preparations and gas atmosphere. These phenomena should be caused by autoxidation upon UV irradiation, and the reaction resulted in not only photodegradation but also photopolymerization. [ABSTRACT FROM AUTHOR]