Rational Design and Synthesis of Lignin-Derived Smart Sunscreens

Lignin exhibits a long-active UV-blocking property due to its macromolecular aromatic structure and unique semiquione-quione-hydroquione transition. However, its poor absorbance in UVA region (320–400 nm) and irregular aggregation are still the obstacles. Herein, active spirapyrane (SP-Br) is synthesized and covalently introduced into alkali lignin (AL) to construct photo-stimulated UV-absorbing enhancement sunscreen actives (AL-SP_n). The introduction of SP significantly improved the absorbance of AL in UVA region and its aggregation is effectively eased. When the optimal AL-SP₃ solution is exposed to UV irradiation, the color turned purple, and the absorbance in UVA region further improved as the unconjugated SP transformed into conjugated merocyanine (MC) structure. When the light is removed, the solution recovered light yellow. The antioxidant property of AL endow the reversible SP-MC transformation with good cyclic stability. As-prepared AL-SP₃ based sunscreen exhibit obvious photo-stimulated enhancement effect. The sun protection factor (SPF) improved from 23 to 89 after 4 h irradiation, and can maintain the high performance for another 8 h. In addition, it exhibit low permeation and good biocompatibily in vitro and vivo, demonstrating its good potential in practical use.