Angew. Chem. Int. Ed. 50, 11729–11730 (2011)

Credit: © 2011 WILEY

An object that cannot be superimposed on its mirror image is said to be chiral. This condition is satisfied for organic molecules in which one of the carbon atoms is bonded to four different groups. This 'point chirality' is typically a static feature: the arrangement of the four substituents around a central carbon atom is fixed when the molecules are made, and does not change unless bonds are broken and reformed.

Now, Nobuyuki Tamaoki and P. K. Hashim from Hokkaido University have designed and made an achiral molecule in which they can induce point chirality by exposing it to ultraviolet light. The compound contains a carbon atom bonded to a methyl group, a phenyl group and two azobenzene groups. Each azobenzene substituent can exist in either a cis (Z) or trans (E) configuration and can be switched reversibly between each form using light or heat. Both azobenzene groups adopt the same E configuration in the ground state and, because two of the four substituents are identical, the compound is achiral. When exposed to ultraviolet light, some of the azobenzene groups isomerize from E to Z and a mixture of compounds is formed, including the EE, EZ and ZZ isomers.

Both the EE and ZZ isomers are achiral, but the EZ compound is chiral. The point chirality comes from the two azobenzene groups adopting different configurations (hence four different groups around a central carbon atom) and both the EZ and ZE enantiomers are formed.