Posted: October 25, 2006

Successful development of a liquid crystalline organic semiconductor forming helical structures

(Nanowerk News) In the National Institute of Advanced Industrial Science and Technology (AIST), Masahiro Funabashi (researcher), and Nobuyuki Tamaoki (group leader) of the Molecular Smart System Group of the Nano Technology Institute, have succeeded in the development of an organic semiconductor exhibiting optical properties as cholesteric liquid crystals. Since it shows a charge transport characteristic as a semiconductor, and also forms helical structures, its application to circularly polarized luminescence devices and organic semiconductor lasers is expected.
A schematic illustration of the application of a cholesteric semiconductor (Image: AIST)
Recently, organic semiconductors have been considered to be useful to realize inexpensive and flexible opto-electronic devices, and investigations regarding electroluminescence devices and lasers using organic semiconductors have been actively carried out.
The introduction of light-wavelength scaled superstructures is essential to create new optical functions for organic semiconductors, and thus, for their introduction, micro-fabrication using lithography has been applied. However, chiral (optical rotatory) liquid crystalline molecules can spontaneously form superstructures with periodicities comparable to light wavelengths.
Utilizing such periodical structures, circularly polarized luminescence and laser oscillation have been investigated, but the application to electroluminescence devices has been impossible, because liquid crystals are usually insulators.
Circularly polarized electroluminescence devices may be used as the back lights for liquid crystal displays. As polarized light filters are not needed in this case, the energy loss due to the filters can be halved, leading to a longer operating life and an enhancement of the reliability of the displays. Moreover, using them together with circularly polarized light transmitting films, high quality displays may be expected.
As cholesteric liquid crystals have helical structures whose periodicities are comparable with the wavelengths of visible light, they can reflect or confine circularly polarized light with specific wavelengths. Thus, utilizing these properties, circularly polarized luminescence and optically induced laser oscillation have been investigated. Cholesteric liquid crystals are usually insulating materials, and hence to realize electrically driven devices (Fig. 1) the development of conducting cholesteric liquid crystals has been needed.
It has been reported that liquid crystals in which molecules are closely packed, like crystals, can exhibit the electron conduction observed for semiconductors. However, for cholesteric liquid crystals, which form liquid-like structures, ionic conduction has been observed, but the hole or electron conduction usually detected for semiconductors has not been observed until now.
In this work, AIST has succeeded in the synthesis of a cholesteric liquid crystal, phenyl-quarter-thiophene derivative (3-QTP-4Me-Ph05). This liquid crystal exhibits hole conduction as a semiconductor in a cholesteric liquid crystal phase. We also have synthesized its dimer, and found that it has a cholesteric structure at room temperature and can emit circularly polarized light by optical excitation.
The results obtained have been published in the July issue of ChemPhysChem, a physico-chemical specialty journal. Furthermore, they will be presented at The International Liquid Crystal Society meeting held at Keystone, Colorado.
Source: AIST
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