Ultrafast nonadiabatic mechanism of plant sunscreens biflavonoids with two excited-state intramolecular proton transfer structures

At present, the safety and environmental compatibility of commercial sunscreens are becoming unprotected. Most of the commercial sunscreens lack absorption and protection in high-energy UVA area. In this work, it has been proved that biflavonoids have sunscreen properties for the first time, especially, Amentoflavone (AME) covers almost the entire UV region and has high absorption. Different substituents have a great influence on the UV absorption of bisflavonoids. Therefore, we can adjust the substituents to obtain a broad protection region for sunscreens. Depending on the two-proton transfer donor and acceptor sites, it is theoretically proved that there is a reasonable competitive relationship between the two structures, and only one part of the proton transfer can be carried out in the molecule, which proved to be the proton donation of H1–O1–H2. When UV excited, biflavonoids will form intramolecular hydrogen bonds through proton transfer and emit energy in an ultra-fast pathway. Biflavonoid precursors could be used in sunscreen field and then the mechanism was proved, and it is a breakthrough in the research of expanding the field of plant filter. AME performs better than the current mature commercial sunscreens, and this work provided theoretical support to design a new biflavonoids for UV protection in future.