During a chemical reaction, it is likely that a molecule will undergo some change in shape. While nature harnesses such effects to ‘power’ biological devices such as muscle fibers, there are few artificial analogues. Importantly, the observation of a measurable effect in bulk materials requires a concerted change in a very large number of molecules, which must be arranged in an orderly way — as in crystals — so that individual changes are not cancelled out. However, such ordering, necessitates that the crystal be flexible enough that the bulk material can undergo repeated changes without cracking.

Now, Hideko Koshima and co-workers from Ehime University in Japan1 show that crystals of an azobenzene molecule undergo reversible flexing when irradiated with ultraviolet (UV) light. Azobenzenes contain a nitrogen–nitrogen double bond and a molecular shape that allows two forms (isomers) of the molecule to exist under normal conditions. It is well known that the isomers can be inter-converted (isomerized) by irradiation, but here Koshima and co-workers show that the isomerization of azobenzene can cause repeatable and reversible mechanical motion of the bulk material.

Fig. 1: Repeatable and reversible crystal bending is achieved by irradiation with ultraviolet light (scale bar, 200 µm).

“We have been working in the area of solid-state photochemistry for nearly 20 years,” says Koshima, “but this is the first time we have been able achieve such a bulk mechanical movement.” The researchers demonstrated that 5 µm-thick plate-like crystals of azobenzene bend by 180° when irradiated with UV light (Fig. 1). The reverse isomerization in azobenzenes occurs with the application of heat and, in this case, the ambient room temperature was sufficient to allow the reversible bending to be repeated up to 100 times simply by switching on and off the UV light. The bending occurs because only molecules near the surface of the crystal face being irradiated undergo isomerization, changing the way in which the molecules are packed together.

“We hope to understand in more detail how the mechanical motion is correlated to the change in crystal structure,“ says Koshima. In the future, we hope to discover even more flexible and durable crystals that might be applied in many fields, from medicine to manufacturing.”