Stable electro-optic response in wide-temperature blue phases realized in chiral asymmetric bent dimers [Invited]
Published in Optical Materials Express, Vol. 4 Issue 4, pp.662-671 (2014)
Spotlight summary: In recent years blue phase liquid crystals (BPLCs), also known as self-assembled soft crystals, have attracted increasing attention worldwide because of their exotic architecture, unique optical properties, and potential applications on next-generation displays as well as new photonic devices. However, the main obstacle to the application of BPLCs is their very narrow temperature range. Significant efforts have gone towards widening this range leading to considerable improvements, such as the stabilization of BPLCs by photopolymerization leading to polymer stabilized BPLCs (PSBPLCs), or designing molecules with biaxial shape. Nevertheless, widening the BP range brings with it some problems: PSBPLCs suffer from a high driving voltage and large hysteresis; since biaxial molecules cannot achieve BP without the assistance of a chiral additive due to their achiral structure, the composition of such BP system is rather complicated.
However, it now seems that the above-mentioned deficiencies might be overcome gradually, owing to the recent breakthrough of Aya et al. They designed and synthesized a new molecule with both a bent-shaped mesogen and a chiral center. Such molecular structure is distinct from those reported previously. The bent-shape provides a preferred structure for obtaining the stable BP, while the large helical twisted power (HTP), which is necessary for forming BP, is supported by the chiral center. Therefore, such molecules possess two prerequisites for BPLCs with wide temperature range. In fact, experimental results show a BP range of 37 K. It is worth pointing out that the reported stable BP contains only a single component and requires no additives, unlike conventional BPLCs, and thus this material is a ‘real’ BPLC. The reported BPLC has an evident Kerr effect, with a large Kerr constant as compared to that of PSBPLCs. As in earlier reports, the response time of such BPLC is on the order of several hundreds of microseconds. However, the most important aspect that needs to be stressed is their very weak hysteresis, only 0.1 V, which can be almost neglected compared with the driving voltage of tens of volts. In contrast, the hysteresis of PSBPLC is normally a few tens or even a hundred times larger. Additionally, such BPLC has a unique electrostriction effect: the size of BP lattice shrinks along the direction of the electric field as this field is applied across the cell, so the reflection band is blue-shifted. That is, these are electric-field-controlled photonic materials. The experimental results presented in this paper indicate that the lattice shrinkage is 7% when applying an electric field of 17 V/μm.
All in all, the BPLC molecule proposed by the authors, which presents both a bent-shaped mesogen and a cholesterol part, provides new insights into the molecular design of stable BPLCs with enhanced properties. Further improvements are necessary for their use in practical applications, e.g., the BP range should be decreased to room temperature, and the dielectric constant should be large enough so that the driving voltage is decreased and the electrostriction is enhanced. These properties are determined by the molecular structure, so the molecular design needs further refinement. We look forward to more breakthroughs to be reported in the near future!
--Zhi-gang Zheng and Dong Shen
ToC Category: Liquid Crystals
|OCIS Codes:||(160.2100) Materials : Electro-optical materials|
|(160.3710) Materials : Liquid crystals|
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