Research Highlight

Subject Category: Organic, carbon-based and soft materials

NPG Asia Materials research highlight; doi:10.1038/asiamat.2008.157
Published online 5 November 2008

Liquid Crystals: Colorful pitch

Phototunable chiral dopants enable the creation of colorful displays from liquid crystals.

Electronic displays made from liquid crystals require the incorporation of filters to create different colors. Some twisted liquid crystals have the ability to reflect certain colors of light depending on the pitch of their helix, and if the pitch could be externally altered, then full-colour displays and recording without filters would be possible. Until now, most systems for this purpose have employed at least two independent dopants—one to twist the liquid crystals into a helix and another to change its pitch in response to a stimulus. Those that have used a single dopant for both purposes have shown poor tunability or used heat to alter the pitch, giving slow and impractical response times for applications.

Manoj Mathews and Nobuyuki Tamaoki from the National Institute of Advanced Industrial Science and Technology in Tsukuba1 report a new chiral molecule that can act to both induce a helix in commercial host liquid crystals and change the helix pitch in response to light. The key components in the dopant design are a photoresponsive double nitrogen bond and a molecular twist; when dispersed in a host, the resulting blends can quickly achieve colors across the visible spectrum.

The liquid crystals used by the researchers were not helically ordered—the chiral dopant induces the helix, and adding more dopant tightens the twist.

When a blue blend was irradiated with ultraviolet light, the nitrogen-nitrogen bond in the dopant changed from trans to cis, which affected the molecules’ ability to twist the helix. The helix unwound during two minutes of irradiation and the blend became red (Fig. 1). Two minutes of visible light irradiation or three hours in the dark reversed the process.

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Fig. 1: Reversible change in reflection color of the doped liquid crystal film with under UV and visible-light irradiation: a, initial state without any irradiation; b, after 120s UV irradiation; c, after 30s visible-light irradiation; d, after 120s visible-light irradiation.

Tamaoki says, “By using light, and not heat, the color was controlled from blue to red via green reversibly for the first time. As a result the response time was shortened from several tens of minutes to two minutes, and can be shortened further by increasing the light power.”

The researchers also believe that full-color displays are an attractive application of these findings because no cell, electrode or color filter is needed, and the images can be changed by light exposure without touching the materials.

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References

  1. Mathews, M. & Tamaoki, N. Planar Chiral Azobenzenophanes as Chiroptic Switches for Photon Mode Reversible Reflection Color Control in Induced Chiral Nematic Liquid Crystals. J. Am. Chem. Soc. 130, 11409–11416 (2008). | Article | PubMed | ISI |

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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