Electrical white light: Raman confirmation

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  • Published: Aug 1, 2017
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Electrical white light: Raman confirmation

The next generation

A new material that emits white light when exposed to electricity has been synthesized by researchers in Japan who used X-ray crystallography for confirmation of their structure. Credit: Wiley-VCH/Angew

A new material that emits white light when exposed to electricity has been synthesized by researchers in Japan who used Raman and X-ray techniques to confirm their structure.

Scientists at Nagoya University have developed a dynamic material that is stimuli-responsive in a predictable manner. The material is based on carbon-containing nanorings and iodine and is electrically conducting. When a voltage is applied it emits white light. The general approach to predictable dynamic materials might be used to make other materials for computer memory devices, artificial muscles, and drug delivery systems. The work represents something of a sea change in designer materials with predictable properties, the team suggests.

Writing in the journal Angewandte Chemist, the Nagoya team describes their simple and reliable method for making such materials. In the current work, the team used [10]cycloparaphenylene ([10]CPP), a cyclic hydrocarbon molecule containing ten para-connected benzene rings; this structure acts as the ideal host for an iodine guest. Its responsiveness hinges on the presence of the iodine guest atoms in the pores formed by the array of cyclic molecules in the crystal, the structure of which was verified by X-ray crystallography experiments. The iodine atoms respond to a voltage and thus change the properties of the crystal as a whole leading it to emit white light, an unusual but keenly sought property of organic, as opposed to inorganic, conductors and semiconductors. Usually, solid state white light generation requires an array of materials and a sophisticated setup. The new material or its chemical cousins and descendents might have potential in next generation illumination systems and displays.

Conductivity

"This responsive porous host approach is expected to be applicable to different stimuli, such as photo-irradiation, heat application and pH change, and open the path for devising a generic strategy for the development of stimuli-responsive materials in a controllable and predictable fashion," explains Hirotoshi Sakamoto. He points out that the synthesis is surprisingly simple involving little more than mixing the rings with iodine in solution and drying. The crystal structures shows the rings are aligned in the crystal each with an iodine guest being hosted. The ten-ring system gave the team the most dynamic iodine atom movement and the most sensitive response to external environmental changes.

"One of the most difficult parts of this research was to investigate how the electric conductivity of [10]CPP-I is turned on by electric stimuli," explains post-doctoral researcher Noriaki Ozaki. When a direct current is applied, the resistivity of the bulk sample drops some 380-fold. This indicates a switch from insulating to conducting. "Although it only took us about three months to synthesize the molecule and discover its electric-stimuli-responsive properties, it took another year to discover the origin of its properties," adds Ozaki. They used X-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy, and fluorescence spectroscopy to figure out the mode of action. It turns out that the iodine atoms in the carbon nanorings form extended polyiodide chains when stimulated by electricity, which endows the bulk material with the requisite electrical conductivity.

White light, no heat

Photoluminescence of [10]CPP-I produces a green-blue hue, but a direct current changes this to the much sought after white wherein fluorescence encompasses the whole of the visible spectrum. The team suspects that this spectral broadening is due to the irregular distribution of the electronic structures of CPPs because of the formation of those polyiodide chains.

"We were really excited to develop this simple yet powerful method to achieve the synthesis of external-stimuli-response materials," adds project director Kenichiro Itami.

Related Links

Angew Chem 2017, online: "Electrically-Activated Conductivity and White Light Emission of a Hydrocarbon Nanoring-Iodine Assembly"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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