Molecular crystals are not known for their dynamism, according to Rabih Al-Kaysi. Chemists pay them little interest, because they’re not generally thought of as exciting substances. Al-Kaysi, a professor at  King Saud bin Abdulaziz University for Health Sciencesand KAIMRC, has recently published a paper which bucks this trend. The study, carried out with a team from Saudi Arabia, Japan, and the USA, reports  a ‘smart’ molecular crystal that continuously oscillates when illuminated with two wavelengths of light, a discovery the researchers hope will lead to further developments in microrobotics.

Motile materials are not new. Other researchers have previously demonstrated that certain materials can be made to change form in response to external stimuli, such as light or an electrical or magnetic field. However, this capability has mostly been demonstrated in soft materials, such as polymers. 

Examples of hard molecular crystals that move are few, and their ability to move is limited. Light causes a change in the geometry of individual molecules in these crystals.  However, these hard photomechanical crystals tend to only do one thing when activated, says Al-Kaysi. Whether it’s a jumping motion, a twisting one, or bending, “you shine a light on them, they do that action, it’s gone,” he says. 

In contrast, the crystalline microwires developed by Al-Kaysi and his team oscillate continuously. The microwires are composed of the compound (Z)-DVAM. When exposed to a continuous source of UV and visible light, they oscillate, moving forward at a speed of 7 micrometers per second (0.007 millimeters per second). This mimics the behaviour of flagella, the whip-like apparatus that some biological cells use to move.

This discovery is the fruit of methodical effort to find a crystal structure with such properties, says Al-Kaysi, as theory predicted that one should exist. “With a lot of experiments, eventually you find that ‘sweet spot,’” he says, where the “thickness, length, and the photochemistry [of the crystal microwires] all work together and give you this autonomous motion”. 

The (Z)-DVAM flagella not only exhibit superior properties compared with other molecular machines but are also significantly smaller than polymer ones, expanding their potential uses. With the discovery of these crystalline microwires, researchers now have a light-activated actuator that needs no wires. Al-Kaysi describes this as “a quantum leap forward”.

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