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Illumination & Displays

Electro-optical properties and possible applications of bent-core liquid crystals

Liquid crystals composed of banana-shaped molecules can be used in various display types, enabling submillisecond switching and wide viewing angles without the need for precise and uniform alignment control.
10 March 2011, SPIE Newsroom. DOI: 10.1117/2.1201101.003509

All contemporary LCDs contain rod-shaped molecules that are generally referred to as ‘calamitic’ liquid crystals. However liquid crystals of bent (or banana-shaped) molecules (B-LCs)—see Figure 1(a)—can also be used in displays. They exhibit features that are not present in calamitic liquid crystals. In particular, they can combine inexpensive synthesis with ferro-electric (FE) properties. This leads to bistable fast switching, which can either work without polarizers or offer excellent contrast, with a perfect dark state. The underlying physics is based on the combination of polar smectic-phase layering1 and spontaneous director tilt, which makes layers chiral: see Figure 1(b).2 Layers typically stack so that their polar order alternates (antiferro-electric, AFE) in the absence of an electric field, which can then be switched to an FE state. If the ground states of successive layers have the same chirality, they will switch from a uniform (and transparent) to an opaque state: see Figure 1(c). On the other hand, if the chirality changes sign in the ground state, electric-field-induced switching will occur between opaque and transparent states.3 Although chirality is conserved in the majority of the banana smectics during switching between AFE and FE states, in some materials strong fields cause a gradual change in chirality. We have found that both the opaque racemic (containing equal amounts of left- and right-handed chirality) and transparent chiral AFE (‘OFF’) states in some materials are stable and can be interchanged.4 This implies that they can also be used in optical-storage devices.

Figure 1. (a) Typical banana-shaped molecule. (b) Formation of chiral tilted polar smectic layers with opposite handedness. P: Electric polarization. n: Unit vector along smectic-layer normal. θ: Director tilt angle. (c) Electro-optical switching between transparent and opaque states. (top) A written sign placed directly below a 4μm-thick bent-liquid-crystal (B-LC) sample in (left) the ON and (right) the OFF states. (bottom) Texture of the B-LC cell in ON and OFF states without polarizers.

We demonstrated the first scattering-type display modes almost 10 years ago.3 They are promising for development of privacy windows and e-book displays that do not use energy in storage mode but could also be transferred to fast-switching video modes. The bistability can be used in materials exhibiting a glassy state at room temperature, where one can freeze the display in both the opaque and clear states. Both optical states may have three different electrical states, because polarization can be positive, negative, or zero. Such a device can store orders of magnitude more data than current two-state memory devices.5

Another important electro-optical feature of these structural changes is that anticlinic (racemic FE and chiral AFE) states are characterized by low birefringence (down to zero in certain materials), while synclinic (racemic AFE and chiral FE) states exhibit large birefringence. This enables switching between optically isotropic and birefringent states. A perfect dark state from any viewing angle can be achieved if the opening angle of the banana-shaped molecules is 109.5° with an average tilt angle of the molecular plane with respect to the layers of 45°.6 In reality, the relevant numbers are very close to these ‘ideal’ values.

Apparently similar but governed by a completely different physical mechanism, electro-optical switching has been observed recently in polar smectic A-7 and C-type8materials and in dielectric smectic A9 phases. Vertically aligned macroscopically uniaxial states could be switched to biaxial states by application of an electric field to in-plane electrodes. In this case, the OFF state is dark between crossed polarizers, while the field-induced biaxial state becomes birefringent if the optical axis is perpendicular to the electrodes.

In summary, B-LCs can be used in various displays, which provide submillisecond switching and very wide viewing angle without the need for precise and uniform alignment control. We are currently developing materials that can be used over a wide temperature range (including room temperature). We are also looking for industrial partners for commercialization.

Antal Jákli
Liquid Crystal Institute, Kent State University
Kent, OH

Antal Jákli received his PhD and DSc in 1986 and 2000, respectively, from the Hungarian Academy of Sciences. He is currently a professor and an expert in B-LCs. He has published over 190 papers and was granted 10 patents.

1. T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules, J. Mater. Chem. 6, no. 7, pp. 1231-1233, 1996. doi:10.1039/JM9960601231
2. D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark, E. Körblova, D. M. Walba, Spontaneous formation of macroscopic chiral domains in a fluid smectic phase of achiral molecules, Science 278, pp. 1924-1927, 1997. doi:10.1126/science.278.5345.1924
3. A. Jákli, L.-C. Chien, D. Krüerke, H. Sawade, G. Heppke, Light shutters from antiferroelectric liquid crystals of bent-shaped molecules, Liq. Cryst. 29, pp. 377-381, 2002. doi:10.1080/02678290110113504
4. G. Heppke, A. Jákli, S. Rauch, H. Sawade, Electric-field-induced chiral separation in liquid crystals, Phys. Rev. E 60, pp. 5575-5579, 1999. doi:10.1103/PhysRevE.60.5575
5. S. Rauch, C. Selbmann, P. Bault, H. Sawade, G. Heppke, O. Morales-Saavedra, M. Y. M. Huang, A. Jákli, Glass forming banana-shaped compounds: vitrified liquid crystal states, Phys. Rev. E 69, pp. 021707, 2004. doi:10.1103/PhysRevE.69.021707
6. A. Jákli, K. Fodor-Csorba, Electro-optics of liquid crystals of bent-shaped molecules, Int'l Mtg Inform. Disp. (IMID'03) Dig., pp. 1108-1111, 2003.
7. Y. Shimbo, Y. Takanishi, K. Ishikawa, E. Gorecka, D. Pociecha, J. Mieczkowski, K. Gomola, H. Takezoe, Ideal liquid crystal display mode using achiral banana-shaped liquid crystals, Jpn J. Appl. Phys. 45, no. 10, pp. L282-L284, 2006. doi:10.1143/JJAP.45.L282
8. G. Nair, C. A. Bailey, S. Taushanoff, K. Fodor-Csorba, A. Vajda, Z. Varga, A. Bóta, A. Jákli, Electrically tunable color by using mixtures of bent-core and rod-shaped molecules, Adv. Mater. 20, no. 16, p. 3138-3142, 2008. doi:10.1002/adma.200800067
9. K. Fodor-Csorba, A. Jákli, A. Vajda, E. Gács-Baitz, S. K. Prasad, S. Rao, R. Y. Dong, J. Xu, G. Galli, Intercalated smectic A phases in banana-shaped liquid crystals with carbonate end groups, ChemPhysChem 7, pp. 2184-2188, 2006. doi:10.1002/cphc.200600377