Popcorn-like explosion of single crystals explained


Chemists have created single crystals of metal coordination complexes that explode violently when exposed to UV light, leaping high into the air. The effect, similar to the bursting of popcorn in a hot pan, results from light-activated polymerisation of molecules within the crystal, followed by an extremely rapid phase transition of the nascent polymer in its attempt to find a lower energy configuration. The phenomenon could be harnessed to create light-driven mechanical actuators, the researchers suggest.

For a number of years, Panče Naumov at New York University Abu Dhabi has been interested in how crystals respond mechanically to heat and, more rarely, light. A team at the National University of Singapore, led by Jagadese Vittal, contacted Naumov when their attempts to make metal-organic frameworks with zinc-pyridyl coordination complexes failed. The crystals popped vigorously under even weak UV light.

The researchers collaborated to delve into the phenomenon. Now, having deployed a range of analytical techniques, including powder x-ray diffraction and high-speed motion analysis, the team believes it has revealed the principal mechanisms at work.

The linear molecules align within the crystal in such a way that C=C double bonds lie adjacent to each other, perfectly set up for a [2+2] cycloaddition reaction. When the sample is irradiated with UV light, the alkenes snap together, creating cyclobutane rings linking neighbouring molecules. ‘There is then a very rapid reconstructive phase transition, around 10,000 times faster than is usually seen,’ says Naumov. ‘The structure tries to rearrange itself to release the strain, causing the crystal to explode.’

Polymerisation reaction in crystal

A light-activated reaction polymerises the molecules in the crystal, then the polymer rapidly rearranges to release the strain © Wiley VCH

The researchers are now experimenting with different molecules to fine tune the effect with a long term goal of investigating possible devices that produce powerful mechanical responses to light.

Christopher Bardeen of the University of California at Riverside, US, who has investigated photomechanical responses of solid state materials, says that the new study is ‘an interesting and important advance.’ He adds, ‘This ability to integrate the absorbed light energy and then release it in a burst of motion, rather than as a gradual expansion, could be useful in applications that require impulsive force.’

Philip Coppens, a crystallographer at University at Buffalo, US, describes the study as ‘a nice illustration of the stress induced in crystals by photochemically induced rearrangements,’ and an experiment that could be a valuable way to disseminate certain key scientific principles to an audience of non-specialists.

References

R Medishetty et al, Angew. Chem., Int. Ed., 2014, DOI: 10.1002/anie.201402040


Related Content

Nanoparticle composites make colourful magnetic crystals

20 March 2014 Research

news image

Gel crystallisation loads gold and iron into normally colourless calcite to give it new properties

Chemistry World podcast - May 2014

8 May 2014 Podcast | Monthly

news image

We speak to the 'sultan of synthesis', Phil Baran, and Peter James explains how labs can save cash on energy bills

Most Read

UC Davis chemist sentenced to four years over explosion

19 November 2014 News and Analysis

news image

Postdoc sentenced over attempt to make explosive device and reckless disposal of hazardous waste

Spanish fly

10 October 2013 Podcast | Compounds

news image

Helen Scales looks at cantharidin, the active ingredient in this famous aphrodisiac

Most Commented

A year of water splitting from one device

21 November 2014 Research

news image

Coating protects electrodes from corrosion to keep photoelectrochemical cell running

Nanostripe controversy in new twist

24 November 2014 Research

news image

Creator of striped nanoparticles insists questions over structures have already been answered and accuses critics of a 'perso...