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Published: 12.08.2011

Tricolor Liquid Crystals



Thermal and mechanical stimuli switch the luminescence of a liquid-crystal mixture between three different colors.

Luminescent materials that change their light-emitting properties in response to external stimuli could provide interesting new approaches for novel storage materials, sensors, security materials, and information displays.

Typically, such materials can either be switched from “on” to “off” or between two different colors. It has not previously been possible to switch between three different stable colors with materials containing only a single luminescent substance.

In the journal Angewandte Chemie, Takashi Kato and Yoshimitsu Sagara from the University of Tokyo (Japan) have now introduced a liquid-crystalline material that can be switched back and forth between three different colors by means of thermal and mechanical stimuli.

Tricolor Liquid Crystals

A glowing image: The photoluminescent colors reddish-orange, yellow, and green, are generated from a single liquid-crystalline mixture containing one luminophore (see picture). The colors are easily distinguished by the naked eye and can be reversibly written and erased. Moreover, these luminescent colors can be switched by mechanical and thermal stimuli.

[Credit: Angewandte Chemie]

The new material consists of two dumbbell-shaped organic compounds—one small and one large, both consisting of a branched arrangement of aromatic six-membered rings. The large molecule contains an anthracene component as the luminescent group (luminophore). This mixture of molecules aggregates to form liquid crystals. Molecules in a liquid-crystalline state are partially ordered like in a crystal, but are mobile like in a liquid. Liquid crystals are most commonly found in liquid-crystal displays.

The Japanese researchers prepared thin films of their special liquid crystals. Under UV light, these films glow red-orange. Mechanical shearing, such as rubbing, at 90 C changes the arrangement of the liquid crystals - the rubbed areas now appear green. This new, green phase is stable between room temperature and 146 C. This amazing film can do more: both the red-orange and green phases can be changed to a yellow one by rubbing at room temperature. Heating to 145 C and subsequent cooling to room temperature changes the green and yellow back to red-orange.

If this luminescent mixture can be incorporated into materials such as structural polymers, the thermal and stress histories for the materials are easily detected by bright luminescence color changes. Knowledge of these histories is useful for the maintenance of the materials systems, which include coatings and plastics. In addition, artists may be interested in these materials because of the beauty of the luminescence colors and the ease of writing/erasing and stability of the generated images.





Notes:



 
Further Information and Source:
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Dr. Yoshimitsu Sagara, Prof. Takashi Kato:
Brightly Tricolored Mechanochromic Luminescence from a Single-Luminophore Liquid Crystal: Reversible Writing and Erasing of Images.
In: Angewandte Chemie International Edition; article first published online: 27 July 2011
DOI: 10.1002/anie.201100914
URL: direct link
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Takashi Kato, University of Tokyo, Japan
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Source: Angewandte Chemie International Edition, press release 31/2011
 
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