A molecular material that inverts like an umbrella blown inside-out is the surprise discovery of a study into host–guest chemistry.

Molecular ‘host’ materials, which can incorporate small guest molecules have applications ranging from separation of chemical mixtures to gas storage or sensing.

Now, a team led by Kanazawa University researchers has examined the chemistry of a vanadium oxide host material, in which 12 units of VO₅ arrange themselves in a bowl-shaped structure. The bowl’s cavity is the perfect size for a carbon dioxide molecule.

Using X-ray analysis, the team imaged the structure with the guest molecule in place. They then heated the material in a vacuum to remove the guest and imaged the structure again. The researchers discovered that stripped of its guest, one of the square pyramid-shaped VO₅ units had flipped inside out to fill the bowl’s void.