Natural Organic Ultraviolet Absorbers from Lignin

Excessive exposure of skin to UV-A (315–400 nm) and UV-B (280–315 nm) radiation causes premature skin aging, inflammation, and skin cancers. The harmful effects of solar radiation can be minimized using UV-absorbing agents. Most available organic UV-absorbing agents are synthetic compounds. UV absorbers were developed herein from a reproducible biomass resource, lignin. Lignin is a heterogeneous aromatic polymer with a major absorption maximum in the UV-C (100–280 nm) region but lacks UV-A and UV-B absorption. To produce UV absorbers from lignin, conjugated structures connecting the side chain and aromatic ring must be incorporated to shift the maximum absorption of lignin to the UV-A and UV-B regions. Extensive screening of the degradation reactions of lignin showed that the microwave-assisted alkaline copper oxide reactions yielded lignin-derived monomers and seven dimers with high absorbance in the UV-A and UV-B regions, including the novel compounds 1-(4-hydroxy-3,5-dimethoxyphenyl)-3-(4-hydroxy-3-methoxyphenyl)-2-propen-1-one and 1-(4-hydroxy-3,5-dimethoxyphenyl)-5-(4-hydroxy-3-methoxyphenyl)penta-1,4-dien-3-one. The UV-absorbing compounds have a long conjugated structure comprising aromatic rings and side chains with double bonds and carbonyl groups. The production of UV-A and UV-B absorbing lignin-derived compounds with defined chemical structures from a wide variety of lignified plant biomass sources provides natural UV-absorbing materials with prospects for cosmetics and resin additives.