Molecular switches regulate gas adsorption on porous polymer

Simple Science Summary

Molecular machines are promising tools. However, they are tiny – a fraction of a fraction of a millimetre. To perform work in the visible world, you need a large number of them to work in unison. Chemists from the University of Groningen have created a molecular network that contains molecular switches that can be flipped using ultraviolet light. The scientists created small spheres of this material. The spheres can take up gases, such as nitrogen or carbon dioxide, but as the switch is flipped, their capacity to store gas goes down. Such a system could be used to pick up gases and release them again, for example to retrieve carbon dioxide form industrial chimneys.

Chemists from the University of Groningen (The Netherlands), together with colleagues from the University of Milan (Italy), have created a soft porous aromatic framework containing light-sensitive molecular switches. The adsorption of nitrogen and carbon dioxide gases reduces dramatically when the switches are flipped by light. Such a material might be used to adsorb gases from factories and could be regenerated easily without excessive energy input. The results were published in Nature Chemistry on 26 June.

Molecular motors and switches are abundant in nature and perform important tasks. The creation of artificial molecular machines is a promising development. In 2016, the Nobel Prize for Chemistry was awarded for their development. It is in the laboratory of one of the three laureates, Ben Feringa, that Wojchiech Danowski works on harnessing the power of molecular machines.

Stable

A major challenge is to organize molecular motors and switches in the solid state. In that state, random thermal movement is limited and mechanical effects can be amplified, connecting the nanoscale motors and switches to the macroscopic world. Last year, Danowski published a paper describing how molecular motors could be made to work in unison inside a rigid metal-organic framework. He now has set his sights on using switches to alter the properties of a soft, polymer-based porous framework.

‘These porous frameworks are easy to make and very stable,’ Danowski explains. ‘And it is possible to incorporate optical switches into the framework.’ The porosity allows the material to adsorb gas, but also makes it easier to flip all switches at once by shining ultraviolet light on it. The material is made in solution, where it polymerizes and forms small spheres that precipitate.

Stiffer

When the pale-yellow powder is irradiated with light, it turns orange upon switching. At the same time, gas adsorption in the powder is strongly reduced. ‘This happens for both nitrogen and carbon dioxide, two gases with very different chemical properties,’ says Danowski. Therefore, the reduced adsorption is most likely caused by a mechanical effect, not by a change in electronic properties. Danowski: ‘We are still investigating how exactly this happens. During adsorption, the spheres swell. It appears that they become a little stiffer after switching, so this perhaps forces the gas out.’

The results are a proof of principle and show that this type of soft porous framework can be used to turn the tiny effects of molecular switches into macroscopic effects. A possible application could be in gas washers that absorb carbon dioxide. ‘This is generally done in special fluids which absorb the gas, and these need heating at high temperatures to regenerate them,’ says Danowski. This costs a lot of energy, while the soft porous frameworks could be generated with light, requiring far less energy.

Chromatography

However, for such an application, it must be shown that the material can work under real industrial conditions. This will probably require years of further research. Danowski: ‘But we have taken the first step. Also, the switch is chiral, and we want to selectively incorporate one enantiomer. Perhaps that could be used in chromatography, to selectively bind specific enantiomers.’

Reference: Fabio Castiglioni, Wojciech Danowski, Jacopo Perego, Franco King-Chi Leung, Piero Sozzani, Silvia Bracco, Sander J. Wezenberg, Angiolina Comotti & Ben L. Feringa: Modulation of porosity in a solid material enabled by bulk photoisomerization of an overcrowded alkene. Nature Chemistry 26 juni 2020.