Liquid crystals are materials in which molecules assemble into ordered arrays under certain conditions in the liquid state. The ability to organize matter in this anisotropic fashion can lead to the emergence of bulk properties that result from the specific arrangement of the molecules. In particular, electroactive liquid crystals can be used to make electronic devices that change color, emit light or move in response to an applied voltage.

Electrochromic materials are particularly useful for display applications because their color can be changed electrically. To date, however, liquid-crystal-based electrochromism has only been observed either in solution or from thin-films treated with a separate electrolyte solution. Now, a team led by Masahiro Funahashi and Takashi Kato from the University of Tokyo1 have shown that electrochromism can be produced in the bulk liquid-crystalline state without the need for an electrolyte.

The team synthesized liquid crystal molecules containing both an ionic imidazolium group and a pi-conjugated system comprising an oligothiophene chain. In the liquid-crystalline phase, these molecules assembled into a nanostructured array in which the ionic groups lined up to form an ion-conducting layer and the oligothiophene chains lined up to produce a charge-conducting layer.

Because such an arrangement improves the mobility of both ions and electronic charges, when a voltage is applied to the system an electrical double layer is formed at the surface of the electrode and no additional electrolyte is required. In contrast, although polymer-based systems containing both ionic groups and pi-conjugated systems have been studied previously, the absence of any significant nanoscale order resulted in inefficient movement of ions and charges, requiring the use of an electrolyte solution.

Fig. 1: Photographs of a liquid crystal cell heated at 120 °C at (a) 5 V and (b) --1 V. At the higher voltage, the ?-conjugated groups are oxidised and form radical cations, which are blue. This process is reversed at the lower voltage and the pale yellow colour is restored.

When an electrical bias is applied to the liquid crystal, ions move towards the electrodes and positive charges (holes) are injected from the electrodes and move into the bulk material. In this process, the pi-conjugated oligothiophene chains of the liquid crystal molecules are oxidized to form radical cations which results in a change in color (Fig. 1) from yellow to blue.

“This work will be key for the realization of new electrochemical devices that are associated with ion and electronic charge transport,” say Funahashi and Kato, “without the need for electrolyte solutions.”