UV Light Sets Crystals in Motion

Tiny crystals of a cobalt coordination compound leap, twirl and burst apart like popcorn when exposed to UV light.

A team led by researchers at the National University of Singapore suggests the microscopic phenomenon could find macroscopic applications turning sunlight into mechanical energy.

The researchers call the crystal motion a photosalient effect. It involves crystals of the compound [Co(NH₃)₅(NO₂)]Cl(NO₃) that undergo sudden jumps over distances 10² to 10⁵ times their own size, they said. The phenomenon demonstrates the conversion and transfer of light energy into mechanical motion through a sudden expansion of volume to relieve stress, they said.

The diagram shows the popping nature of crystals under UV light.

This is a rare phenomenon, the researchers said, and the first reported evidence of such events driven by a photochemical reaction in solids. The researchers made the discovery while studying metal complex polymerization in its solid state.

“In our work, we observed that the conversion of energy in the crystals may be able to mimic the motility of biological systems,” said Jagadese J. Vittal, a chemistry professor at NUS.

Working with researchers from New York University, the NUS team used an optical microscope in conjunction with a high-speed camera to capture the crystals’ rapid motion.

Through a variety of analytical methods, the researchers found that the popping and disintegration of the crystals happened when the strain was generated during the photochemical reaction; this led to the formation of metal coordination polymers.

The research shows UV light sources such as the sun could be used to trigger chemical reactions that drive practical mechanical motions. In addition, there exists the potential to better harness solar energy, as well as alternative energy conversion.

“Knowledge and application of such behavior is very important toward addressing the global energy crisis,” he said.

The research was published in Angewandte Chemie International Edition (doi: 10.1002/anie.201303757).