Sunscreen molecules

A sunscreen spreads a thin film of UV-absorbing molecules over your skin, so the UV is absorbed before it reaches your cells.

The molecules that do this job use the trick that gives carrots and tomatoes their colour: a run of alternating double bonds. The longer the run, the further toward red the light it absorbs (lesson 15 shows how). A sunscreen molecule's run is long enough to reach UVB or UVA, and short enough to stop before the visible colours, so the molecule itself is colourless.

Avobenzone

Avobenzone, C₂₀H₂₂O₃, absorbs UVA. It has two benzene rings joined by a bridge of three carbon atoms, and it takes two forms. In the first, a CH₂ in the middle of the bridge breaks the run in two, and this form absorbs mostly UVC, around 265 nm. Move one hydrogen atom from the CH₂ to an oxygen, and the run reaches from one ring to the other. This second form absorbs UVA, most strongly at 357 nm, and in a sunscreen most avobenzone molecules take it.

From light to heat

When the run absorbs a photon, the photon's energy lifts one of its shared electrons into a higher level. In far less than a billionth of a second, the molecule twists and shakes, the electron drops back, and the energy passes to the molecules around it as a tiny amount of heat. The sunscreen molecule is then ready for the next photon.

Sunlight breaks some avobenzone down, so sunscreens pair it with molecules that help it last, such as octocrylene. Octocrylene absorbs UVB, most strongly around 303 nm. Its C=C double bond sits between two benzene rings on one side and a -⁠C≡N group and a C=O on the other, all in one run.

Mineral sunscreens

Zinc oxide, ZnO, and titanium dioxide, TiO₂, are white powders made of tiny particles. They absorb UV too, and scatter a small part of it back. Large particles also scatter visible light, so a thick layer looks white. Particles a few tens of nanometres across scatter far less visible light and look almost clear on the skin.