The Photochemistry of Sunscreens.

To understand how sunscreens work, one must understand how molecules interact with light energy.  Absorption of light by a molecule is associated with the part of its structure called a chromophore, which may or may not encompass the whole molecule.  For organic molecules the chromophores that are responsible for the absorption of ultraviolet light are generally associated with delocalised p-electrons in conjugated systems.

In general, when a molecule absorbs a photon whose energy is high enough, an electron is promoted from a lower energy level to a higher one.  The molecule is said to go from its ground state to an excited state.   The most common excited state for organic molecules is the first excited singlet state, in which the promoted electron is still spin-paired with the one left in the lower energy level.

Once in the excited state the molecule has several different pathways that it can take:
The molecule can emit a photon and return to its ground state.

The molecule can return to its ground state by emitting the energy thermally through a series of vibrational transitions.

The molecule can undergo some type of reaction from the excited state, which is generally termed photochemistry.

Finally, the molecule can convert to a lower energy state.

The favored path will be the one which is most rapid.

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