E-Joon Choi

Elimination of image flicker in fringe-field switching liquid crystal display driven with low frequency electric field

Recently, a low frequency driving of a fringe-field switching-liquid crystal display (FFS-LCD) draws much attention to minimize the power consumption. In the low frequency driving of FFS-LCD, an image flickering effect occurs when the sign of the electric field is reversed. We suggested a method to eliminate the image flickering effect by doping small amount of bent-core liquid crystal (BLC) molecules. The BLC molecules have an opposite sign of flexoelectric polarization and reduce the flexoelectric polarization of the host liquid crystal. By adding 2.0 wt% of BLC, the total transmittance during a positive and a negative electric field could be balanced and the image flickering effect was not observed by eyes.

Smectic A phase in a new bent-shaped mesogen based on a 2,3-naphthalene central core with an acute-subtended angle

A new bent-shaped molecule which consists of an acute-angled central core and all-ester linking groups has been prepared: 2,3-naphthylene bis[4-(4-n-dodecyloxybenzoyloxy)benzoate]. Its mesomorphic properties have been investigated by DSC, polarizing microscopy, and X-ray diffraction measurements. Moreover, we have carried out detailed electro-optical investigations including birefringence, switching current response, IR dichroism, and second-harmonic generation (SHG) measurements. On the basis of the experimental data a plausible model for the mesophase alignments has been proposed.

Synthesis and properties of new materials with banana-shaped achiral cores and chiral end groups

New chiral bent-core mesogens, derivatives of 1,3-phenylene bis[4-(alkanyloxyphenyliminomethyl)benzoate], were synthesized with variation of a substituent (X=F, Cl); their antiferroelectric properties are described. The mesomorphic and switching properties were characterized by differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X-ray diffractometry in the small and wide angle regions. The presence of chiral tails at the terminals of side wings in the bent-core molecules induced a decrease in transition temperature and formation of the switchable SmC* phase in the melt. In addition, the introduction of a lateral halogen substituent in the Schiffs base moiety prevented the regular stacking of the molecules, resulting in the formation of very complex optical textures. The smectic phase with F-substituted PBFDOB and Cl-substituted PBCDOB showed layer spacings of 39 and 38.5 Å, respectively, corresponding to the end-to-end distance of molecules with a bent conformation. Significantly, the smectic phases of PBFDOB and PBCDOB exhibited a period of 179.5 and 131 Å, respectively, compatible with a helical structure with periodicity about 4.6 and 3.4 times the layer spacings.

Flexoelectric effect of a rod-like nematic liquid crystal doped with highly-kinked bent-core molecules for energy converting components

The flexoelectric effect of a rod-like nematic liquid crystal (pentyl-cyanobiphenyl, 5CB) doped with bent-core liquid crystals with a high kink angle ∼130° was studied. Although the previous theoretical models predicted that such highly-kinked molecules with a longitudinal dipole moment would have no significant influence on the flexoelectric effect, the 5CB mixture doped with 7.0 wt% bent-core liquid crystal with a longitudinal dipole moment showed greater flexoelectric anisotropy 37.2 pC m−1 compared to the pure 5CB 10.3 pC m−1. From the small angle X-ray scattering experiments, the bent-core liquid crystal was found to form local domains surrounded with 5CB molecules and the varied orientation and shape of the assembled complexes were considered to be related to the greater flexoelectric anisotropy.

Synthesis and luminescence properties of dumbbell-shaped molecules combined with rod and disc-like mesogens

A novel series of dumbbell-shaped molecules containing 1,3,4-oxadiazole moieties (OXDn: n = 1, 2, and 3) was designed and synthesized. The molecular structures of these 1,3,4-oxadiazole derivatives were confirmed by FT-IR, 1H-NMR spectroscopy and elemental analyzer. The electron excitation and luminescence properties of these compounds were investigated by UV-vis absorption spectroscopy and photoluminescence spectroscopy both in CHCl3 solution and in solid state. The absorption peaks were found at 336 nm-351 nm, and the photoluminescent peaks at 410 nm-450 nm. The thermal properties were investigated by DSC method. Their melting temperatures were in the range of 81-95°C depending on the structure. Especially, OXD1 could show a discotic columnar mesophase at 75°C monotropically.

Pseudo-rodlike molecules with hockey-stick-shaped mesogen

ABSTRACT Hockey-stick-shaped molecules were newly synthesised to obtain pseudo-rodlike molecules. The designed molecules consist of a polar terminal ring (i.e. 2,3,4-, 2,4,6- or 3,4,5-trifluorophenyl group), a rigid middle block (i.e. four rings with aligned ester linkages) and a flexible terminal chain (i.e. dodecyloxy group). We found that the compounds with 2,3,4- and 3,4,5-trifluorophenyl groups formed a smectic A mesophase with head-to-head bi-layer building blocks, whereas the compound with 2,4,6-trifluorophenyl group formed a nematic mesophase. This might be concerned with the behaviour of pseudo-rodlike molecules GRAPHICAL ABSTRACT

Banana‐shaped molecules with 4,6‐dichlorinated central core: effect of lateral substituents and terminal chains on the formation of antiferroelectric smectic mesophases

Six banana‐shaped compounds with a central core based on a 4,6‐dichloro‐1,3‐phenylene group were synthesized by varying the terminal chains (R = OC10H21 or OC11H21) and the lateral substituents (X = H, F or Cl). Their mesophases were characterized by a combination of differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X‐ray diffractometry. Mesomorphic properties of the banana‐shaped mesogens with an olefinic group (R = OC11H21) as a terminal chain are sensitive to lateral halogen substituents as much as those of the analogues with a saturated group (R = OC10H21). The compounds with X = F showed an antiferroelectric switchable smectic phase, which has been designated a B2 phase. The compounds without a lateral halogen substituent only formed a nematic phase, while the compounds with X = Cl did not exhibit a mesophase in the melt.

Synthesis and mesomorphic properties of banana-shaped achiral molecules with central and lateral halogen substituents

Three banana-shaped achiral compounds, derivatives of 4-chloro-1,3-phenylene bis[4-(4-n-decanyloxyphenyliminomethyl)benzoate], were synthesized with variation of a substituent (X=H, F, or Cl); their antiferroelectric properties are described. The mesomorphic and switching properties were characterized by polarizing optical microscopy, differential scanning calorimetry, triangular wave method, and X-ray diffraction. The introduction of a lateral halogen substituent in the Schiffs base moiety prevents the regular stacking of the molecules, resulting in a decrease of melting temperature and leads to formation of the switchable SmC* phase in the melt. All of the compounds showed antiferroelectric switching, and their values of spontaneous polarization on reversal of an applied electric field were 260, 480, and 300 nC cm−2, respectively, for X=H, F and Cl.

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiffs base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm−2 on reversal of an applied electric field.

Antiferroelectric liquid crystal from a banana-shaped achiral molecule

A new banana-shaped achiral compound, 1,3-phenylene bis[4-(3-fluoro-4-n-octyloxyphenyliminomethyl)benzoate] (PBFOB) was synthesized and its antiferroelectric liquid crystallinity determined. The PBFOB was characterized by differential scanning calorimetry, X-ray diffractometry in the small and wide angle regions, and polarizing optical microscopy; its polarization and antiferroelectric properties were also investigated. The presence of a lateral fluoro-substituent in the banana-shaped achiral molecules containing a Schiffs base mesogen induced a decrease in melting temperature and formation of the switchable smectic B2 phase in the melt. The spontaneous polarization for PBFOB was about 250 nC cm-2 and the polarization of this phase switched on the reversal of an applied electric field.