Making Carbon Nanorings Emit White Light

By wrapping carbon nanorings around iodine, researchers have developed a material that emits white light in response to electrical stimulation.

AsianScientist (Aug. 7, 2017) – A team led by Nagoya University’s JST-ERATO Itami Molecular Nanocarbon Project and the Institute of Transformative Bio-Molecules (ITbM) has developed a simple and reliable method to synthesize a material that emits white light in response to electricity. The results of this study were reported in the journal Angewandte Chemie International Edition.

Stimuli-responsive materials alter their own properties in response to external factors, such as light, heat, pressure and electricity. This feature can be controlled for a wide range of uses, such as in optical discs, computer memories and displays, as well as artificial muscles and drug delivery systems.

Researchers have been working to develop new stimuli-responsive materials in a predictable fashion. However, it has been extremely difficult to design and control the complex molecular arrangements of the materials.

In this study, the research team led by Dr. Hirotoshi Sakamoto, group leader of the JST-ERATO project at Nagoya University, designed a new material which is a mixture of carbon nanorings and iodine. This material conducts electricity and emits white light when exposed to electricity.

Synthesizing the material was surprisingly simple—the researchers mixed carbon nanorings made from cycloparaphenylene (CPP) with iodine and let it dry. X-ray crystallography confirmed that the iodine molecules lined up inside the hollow core of the aligned nanorings.

The team tried several variations of the mixture, changing the number of carbon nanorings. They eventually found that [10]cycloparaphenylene ([10]CPP) functioned as an optimal ‘responsive porous host’ and when combined with iodine, resulted in the most dynamic iodine atom movement and produced the most sensitive response to external environmental changes. The researchers called the new material [10]CPP-I.

“This ‘responsive porous host’ approach is expected to be applicable to different stimuli, such as photo-irradiation, heat application and pH change, and opens the path for devising a generic strategy for the development of stimuli-responsive materials in a controllable and predictable fashion,” said Dr. Hirotoshi Sakamoto, group leader of the JST-ERATO project.

When a direct current was applied to [10]CPP-I, the bulk resistivity of the sample became approximately 380 times lower, indicating that it conducted electricity. The bulk resistivity in mixtures with 9 or 12 nanorings did not decrease nearly as much. These results showed that pore size in the nanoring assembly controls the response to electrical stimulation.

“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,” said Dr. Noriaki Ozaki, a postdoctoral researcher of the JST-ERATO project. “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.”

Using X-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy, and fluorescence spectroscopy, the team showed that the iodine atoms in the carbon nanorings formed extended polyiodide chains when stimulated by electricity, which gave the material electrical conductivity.

Interestingly, not only did [10]CPP-I conduct electricity, the electric stimuli switched the photoluminescence color of [10]CPP-I from a green-blue color to a white color. White luminescence means that the fluorescence spectrum of [10]CPP-I covers the whole visible light range. Spectral broadening is attributed to the irregular distribution of the electronic structures of CPPs, which is caused by the formation of polyiodide chains.

The white luminescence of [10]CPP-I is a rare example of white illumination material arising from a single molecular assembly. Typically, white light emission is achieved by mixing several components of different colors.

“We were really excited to develop this simple yet powerful method to achieve the synthesis of external-stimuli-response materials,” said Professor Kenichiro Itami, director of the JST-ERATO project who is also the center director of ITbM.

The team’s new approach could help generate a range of reliable stimuli-responsive materials, which can be used in memory devices, artificial muscles and drug delivery systems, among other applications.

The article can be found at: Ozaki et al. (2017) Electrically-Activated Conductivity and White Light Emission of a Hydrocarbon Nanoring-Iodine Assembly.


Source: Nagoya University.
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