Long-Acting Ultraviolet-Blocking Mechanism of Lignin: Generation and Transformation of Semiquinone Radicals

The natural inspired ultraviolet (UV)-absorbing function of lignin has been widely used in cosmetics and photothermal materials; however, its long-acting UV-blocking mechanism is still unclear. Herein, three organosolv lignins (OLs) were extracted from different botanical resources under mild conditions. These OLs preserved the native structure of lignin, and the best one was chosen to investigate the long-acting UV-blocking mechanism of lignin from the perspective of radicals. Results showed that 136% fresh semiquinone (SQ) radicals were formed when OL was irradiated under UV light for 4 h, and beyond half of these newly generated radicals were readily preserved after withdrawing UV irradiation. The generation and reduction of SQ in OL exhibited good photoresponsivity and cyclic stability, indicating that there was an SQ-based hydroquinone/quinone dynamic transformation. After 1 h of irradiation, the radical and phenolic hydroxyl contents in OL and the sun protection factor of an OL-based sunscreen increased by 110, 10, and 61%, respectively. After 12 h of irradiation, the antioxidant property of OL only weakened a little, while the sunscreen performance was still better than its initial state. The color of OL darkened at the initial time and then turned white gradually. This research paves the way to understanding the long-term UV-blocking mechanism and application of biomass lignin.