Crystal Stretches And Shrinks Under Light

Scientists in Japan have invented a flexible material that changes its ability to absorb carbon dioxide when exposed to light.

AsianScientist (Aug. 1, 2017) – Researchers at Kyoto University’s Institute for Integrated Cell-Material Sciences (iCeMS) and the University of Tokyo have developed a light-responsive crystalline material that is flexible and amenable to repeated use. The work is published in the journal Nature Communications.

Photochromic molecules change their electronic states or chemical structures when exposed to light. They can play key roles in the development of photoresponsive materials that could be used in delivery systems for controlled drug release, or to develop dynamic scaffolds for tissue engineering, among other applications. But so far, their use with solid materials has proven challenging because the materials are too rigid to allow repeatable and reversible changes.

To create a stretchy, light-responsive material, Professor Susumu Kitagawa of iCeMS, Dr. Hiroshi Sato of the University of Tokyo and their colleagues prepared a flexible porous crystal made of a photoresponsive dithienylethene derivative, zinc ions (Zn2+) and 1,4-benenzenedicarboxylate.

The resulting polymer consisted of two-dimensional sheets connected by pillars of photoresponsive molecules, which created a three-dimensional entangled framework. Due to the flexible nature of the entangled framework, the channels changed shape when exposed to light. The distance between the two layers shrank upon ultraviolet irradiation and expanded when lit by visible light.

The researchers tested the material’s ability to uptake carbon dioxide (CO2). When the material was not irradiated, it adsorbed up to 136 ml of CO2. When exposed to ultraviolet light, the pores shrank, decreasing CO2 adsorption to 108 ml. CO2 adsorption then rose to 129 ml when visible light was shone on the material, decreasing again to 96 ml upon re-exposure to ultraviolet light.

The polymer’s entangled framework enables these reversible and repeatable CO2 adsorption changes; it gives room for the photoresponsive molecules to transform while allowing them to release their strain into the flexible material.

Preliminary tests indicated that the porous crystal could also adsorb other gases, such as nitrogen, at various temperatures, but more detailed investigation is required.

“Our strategy will grant access to a new dimension of porous compounds as platforms for various photochemical conversions and the photomodulation of porous properties,” the researchers said.

The article can be found at: Zheng et al. (2017) Flexible Interlocked Porous Frameworks Allow Quantitative Photoisomerization in a Crystalline Solid.


Source: Kyoto University.
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