=chemistry =suggestion =chemical safety
UV light can cause damage when molecules in cells absorb it by breaking into free radicals. Sunscreens block that UV light, but current sunscreens usually use molecules that absorb it by breaking into free radicals.
Somewhat recently, sunscreens with ZnO and TiO2 have become more popular. These work more by reflecting UV light. Also reflecting visible light means they make you look more white than other sunscreens, but the newer nanoparticle sunscreen types mostly reflect UV light by being smaller than visible light wavelengths. ZnO and especially TiO2 can be photocatalytic, generating oxidizing radicals under UV. To deal with this, most TiO2 particles in sunscreen are treated in such a way that you get a rutile form (somewhat less photocatalytic) at the surface, and some are coated with silicone or something. At least metal oxide particles are less likely to get in cells and less likely to penetrate skin, though I suppose some nanoparticles could end up "eaten" by macrophages. Anyway, metal oxide particle sunscreens do seem better than current other kinds and worth using.
Alizarin and quinizarin are dihydroxy anthraquinones that are easily made by acylation with phthalic anhydride and have been used as dyes. (Generally made using chlorophenol, while I'd want to use catechol or hydroquinone from phenol oxidation with H2O2 and acylate with phosphoric acid. They're components of madder plant red dye that was used historically.) Alizarin is red and quinizarin is red/brown, but a reddish brown color might not be so bad for a sunscreen.
Those are well-known examples, but other hydroxylated anthraquinones, ones with more than 2 OH groups, are probably better choices for sunscreen. Like alizarin and quinizarin, they should tend to be red to brown.
If using hydroxylated anthraquinones as sunscreen you should probably add a sulfonate group after acylation or something so they'd have some negative charge before getting inside cells. Molecules with negative charge are much less likely to get inside cells.
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