Liquid study: Reveals tuneable colours

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  • Published: Jul 1, 2013
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Liquid study: Reveals tuneable colours

Light-headed liquid

Full-colour luminescence panels under ultraviolet (365nm) irradiation by adjustment of the nonvolatile blue-emitting anthracene liquid (left: photo) in the material.

Researchers in Japan have used NMR spectroscopy to study liquid materials with excellent light stability based on the skeleton of the organic fluorescent dye anthracene that could be used for full-colour tuneable luminescent systems.

Takashi Nakanishi of the National Institute for Materials Science Organic Materials Group, Polymer Materials Unit, and colleagues Sukumaran Santhosh Babu, Martin Hollamby, Junko Aimi, Hiroaki Ozawa, Akinori Saeki, Shu Seki, Kenji Kobayashi, Keita Hagiwara, Michito Yoshizawa and Helmuth Moehwald (Max Planck Institute of Colloids and Interfaces) - have developed a full-colour tuneable luminescent liquid material, which they say has excellent light stability.

Flexible and mobile

Polymers have come to the forefront of technological developments of full-colour display monitors, screens for mobile phones and other electronic devices. Using organic molecular and polymeric materials side steps many of the issues with conventional semiconductors as well as offering advantages such as lower density, some flexibility and the possibility of printing the devices. However, most light-emitting organic molecular materials developed so far have had problems associated with low inherent luminescent performance as well as low photostability, which means they discolour or decolourize after relatively short periods of time. There is also the problem of aggregation of molecules during the coating process, which is difficult to avoid and means some experimental devices fail early in development.

By contrast the ultimate possibility of ease of manufacture, low power consumption and flexibility continue to drive developments in this area. The search is also on for materials that can be used to make full-colour displays rather than requiring different types of dye in the same device to give different colours.

Anthracene in the scene

Nakanishi and colleagues initially developed an anthracene-based blue-emitting liquid material with an absolute quantum fluorescence efficiency of 55%, which does not undergo aggregation. The material has a melting point of approximately -60 Celsius and so will not freeze under normal working conditions, moreover is thermally stable up to approximately 300 Celsius. The team points out that the material has low viscosity at room temperature. The team refers to anthracene as a general fluorescent dye molecule albeit it with their added highly flexible branched alkyl chains to give it these specific properties. The new material has a photostable lifetime 5 to 10 times longer than commercially available anthracene dyes.

It is the ability to "dope" this liquid with other luminescent dye molecules that makes it of potentially great technological interest. Doping retains the homogeneity of the liquid and so its general physical characteristics but allows different coloured luminescence to be generated assisted by up to 96% fluorescence resonance energy transfer (FRET) of dyes by single blue-light (365 nanometres) excitation. Importantly, the material is also non-volatile and so can be used to coat various substrate surfaces to construct organic multicolour devices, the team add. The fact that it is a liquid means it remains contiguous in its active layer regardless of flexing and bending.

The team used ultraviolet-visible absorption, steady-state fluuorescence, proton and carbon-13 NMR spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) to unambiguously characterise all of the anthracenes investigated.

"Of course we need to fabricate the liquids into flexible device and evaluate their performance," Nakanishi conceded to SpectroscopyNOw. "In this direction, we also need to ask other scientists to develop transparent highly flexible electrode production as well." He adds that, "Another interest is to develop a specific luminescent color from the concept. Especially the colour would be useful for applications in security technology."

Related Links

Nature Commun, 2013, 4, Paper No. 1969: "Nonvolatile liquid anthracenes for facile full-colour luminescence tuning at single light excitation"

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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