An electrochromic nanostructured liquid crystal consisting of π-conjugated and ionic moieties

Article abstract of DOI:10.1021/ja805339q

A new electrochromic molecule comprised of π-conjugated and ionic moieties has been designed and synthesized. It forms a nanosegregated smectic phase in which ion-conductive layers of imidazolium salts are located between hole transport layers of phenylterthiophene moieties. Electrochromism is observed in the bulk liquid crystal state of this compound without an electrolyte solution. In this nanosegregated smectic phase, an electrical double layer is formed rapidly at the electrode. Consequently holes are injected from the electrode, resulting in oxidation of the π-conjugated moieties. Copyright

Full text of DOI:10.1021/ja805339q

Published on Web 09/11/2008
An Electrochromic Nanostructured Liquid Crystal Consisting of π-Conjugated
and Ionic Moieties
Sanami Yazaki, Masahiro Funahashi,* and Takashi Kato*
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Received July 10, 2008; E-mail: funahashi@chembio.t.u-tokyo.ac.jp; kato@chiral.t.u-tokyo.ac.jp
Electroactive nanostructured liquid crystals1 have attracted
attention because self-organized assemblies of π-conjugated mol-
ecules2 can induce additional enhanced or anisotropic properties
such as electric functions.3,4 For example, redox-active liquid
crystals5 are expected to be applied to electrochromic materials,6
actuators,7 and light-emitting electrochemical cells.8 However, the
electrochromism of the electroactive liquid crystal materials has
been observed only in solutions or in thin films dipped in electrolyte
solutions.5b,d,e The reversible electrochromism in the bulk of the
liquid-crystalline state should enable realization of single-layer
electrochromic devices without electrolyte layers.
Figure 1. Schematic image of a nanostructured liquid crystal consisting
of ionic and π-conjugated moieties. Red cylinders are neutral phenylter-
thiophene moieties. Dark green cylinders are the oxidized ones. Dark blue
For efficient electrochromism, fast formation of an electrical
double layer at an electrode surface is essential. Fast electronic
charge transfer from the electrode is also required. π-Conjugated
polymers bearing ionic moieties have been examined.9 However
the mobilities of the ionic species are very low in the polymers,
and electrical double layers are not formed under the application
of the electric field. Therefore electrolyte solutions are essential
for the electrochromic devices using π-conjugated polymers.9
spheres and light blue plates are triflate anions and imidazolium moieties,
respectively.
Our interest is to use nanostructures of liquid crystals that
combine an ion-conductive group and a π-conjugated moiety
transporting electronic charges for the construction of redox-active
anisotropic materials.
Figure 2. Molecular structures of compound consisting of ionic and
π-conjugated moieties 1 and reference compound without an ionic moiety
2.
Herein, we report on the electrochromism in the bulk liquid-
crystalline state without an electrolyte solution. We synthesized
liquid crystal 1 consisting of an imidazolium group as an ion-
conductive part and phenylterthiophene moiety associated with the
electronic charge transport.
spacing of the smectic A phase at 120 °C is 47 Å. Reference
compound 2 shows a monotropic smectic A phase between 86 and
82 °C.
Ionic liquid crystals such as imidazolium-based liquid crystals
exhibit efficient ion transport in smectic layers or in inner
columns.4a-f For π-conjugated liquid crystals, enhanced electronic
charge carrier transport has been observed in the smectic phases
of oligothiophene derivatives3f-l and the columnar phases of disk-
like molecules.3a-e If we use ionic and π-conjugated liquid crystals
to obtain electrochromic liquid crystals, ion-conductive and hole-
conductive channels should be constructed separately because of
the microscopic phase separation1e,4a of these moieties. As shown
in Figure 1, the ions would be accumulated on the surface of the
electrode forming an electrical double layer when an electric field
is applied to the liquid crystal. Subsequently, electronic charges
should be injected from the electrode into the π-conjugated systems
of the liquid-crystalline molecules and transported between them,
leading to electrochromism6 or light emission.8
We have found that compound 1 exhibits electrochromism in
the bulk liquid-crystalline state without a liquid electrolyte layer,
unlike conventional redox-active conjugated polymers6 and liquid
crystals.5 Liquid crystal 1 was capillary-filled into a cell consisting
of two ITO-coated glass plates. The observation of micrographic
texture reveals homogeneous alignment of the liquid-crystalline
molecules. When the voltage increases to 5 V, the color of the
liquid crystal layer changes from pale yellow to dark blue (Figure
3a). The color is restored to the original state when the voltage is
decreased to -1 V (Figure 3b). For a dichloromethane solution of
compound 1, a reversible one-electron oxidation is observed in
cyclic voltammetry. This oxidation accompanies a color change
due to the formation of the radical cation of the π-conjugated
moiety. This observation for the solution state suggests that, for
the bulk liquid-crystalline state, the color change should also be
attributed to the formation of radical cations of 1. Such a color
change does not occur in the bulk liquid-crystalline states of
phenylterthiophene derivative 2 which has no ionic moiety.
The transmittance of the liquid crystal in the cell changes under
the bias application, as shown in Figure 4. A He-Ne laser (λ )
As shown in Figure 2, liquid-crystalline molecule 1 has an
imidazolium triflate moiety and a 3-methyl-5-phenyl-2,2′:5′,2′′-
terthiophene group. For a reference, phenylterthiophene derivative
2 with no ionic moiety was synthesized. Compound 1 exhibits a
smectic A phase between 150 and 47 °C on cooling. The layer
9
13206 J. AM. CHEM. SOC. 2008, 130, 13206–13207
10.1021/ja805339q CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
chemical measurements of compound 1. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 3. Photographs of the liquid crystal cell heated at 120 °C. After
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Figure 4. Response of the transmittance on the application of the DC
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632.8 nm) is illuminated on the liquid crystal cell, and the
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Acknowledgment. This study was partially supported by Cre-
ative Scientific Research of “Invention of Conjugated Electronic
Structures and Novel Functions” (No. 16GS0209) and the Global
COE Program for Chemistry Innovation from the Ministry of
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Supporting Information Available: Synthesis, polarized optical
microscopic images, DSC charts, X-ray diffraction patterns, and electro-
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