1834

Photochemistry

Theodor Grotthuss (1785–1822), John Draper (1811–1882), Hermann Trommsdorff (1811–1884), Giacomo Ciamician (1857–1922)

Some chemical reactions begin as soon as the reactants are brought together, but many more need a push from an external energy source, with heat probably as the most common. But light can also drive chemistry: the fading of colors from exposure to sunlight was possibly the first photochemical reaction noticed by humanity.

Two photochemistry pioneers—German chemist Theodor Grotthuss in 1817 and English chemist John Draper in 1842—independently discovered that light must be absorbed by a chemical substance for a photochemical reaction to take place. Italian chemist Giacomo Ciamician, who in 1900 began the first systematic study of organic compounds exposed to light, helped to provide the groundwork for our understanding of photochemistry. But in 1834 twenty-three-year-old German pharmacist Hermann Trommsdorff had already noted that sunlight caused pure crystals of a plant-based product called santonin to turn yellow and burst apart.

We now know that different molecules can absorb different wavelengths of light, depending on their structures (which is what arranges their electron clouds, which really interact with the light). Absorption in the visible wavelengths gives materials different colors. The more energetic forms of light (shorter wavelengths up into the ultraviolet) can cause some types of bonds to break and others to become more reactive. A wide array of rearrangements and ring formations can take place, often in ways that no other sort of chemistry could easily produce. The bursting santonin crystals, whose full mechanism wasn’t worked out until 2007, are a good example. The sizes and shapes of the molecules change so drastically that the crystal can’t possibly hold together.

Photochemistry isn’t just a lab curiosity, though. Molecular shape changes like these are the chemical signals that make our retinas sensitive to light, and vitamin D is produced by a sunlight-driven photochemical reaction in our skin. Where there’s energy to be found, living systems will find a way to use it.

Photochemistry in a dentist’s office: the ultraviolet light starts a polymerization reaction, which hardens the sealant for filling a tooth cavity.