Copper phthalocyanine

Copper phthalocyanine is one of the most important colorants in the world. It's a large, flat, square molecule: four linked ring units arranged around a copper atom, like petals around the center of a flower. Its deep blue is so intense, and so resistant to light, heat and chemicals, that it turns up in car paint, printing inks and plastics.

The ring system is built from carbon and hydrogen, with nitrogen atoms linking the four units and gripping the copper atom in place. Factories make it by heating phthalic anhydride, a carbon ring compound, with urea, which supplies the nitrogen, and a copper salt, plus a catalyst. Four ring pieces snap together around the copper in a single pot. The crude product is purified and milled into fine crystals.

It's the parent of two blocks above it. Treated so its crystals settle into a particular arrangement, it becomes the blue pigment in display color filters. With most of its hydrogen atoms swapped for chlorine and bromine, it becomes green pigment.

Phthalocyanines were found by accident in the late 1920s, when a blue impurity kept turning up in a Scottish factory making phthalimide. The copper version went on sale in the mid-1930s.