Jirakorn Thisayukta

Study on Helical Structure of the B4 Phase Formed from Achiral Banana-Shaped Molecule

The helical structure of the B4 phase in the banana-shaped molecules, the 1,3-benzene bis[4-(4-n-alkoxyphenyliminomethyl) benzoate] (P-n-O-PIMB) series, has been studied by means of optical microscopy, X-ray diffraction and circular dichroism for homogeneously and homeotropically aligned cells. The homogeneously aligned cells showed a transparent blue color and a circular dichroism at about 400 nm, indicating the existence of a helical structure. Detailed texture observation also reveals the existence of two helical domains with opposite helical senses. The two helical domains are formed with equal probability, and with the helical axes parallel to the layers similar to the twist grain boundary (TGB) phase. This helical structure is also supported by X-ray diffraction. The balance between the two domain areas is broken by doping P-n-O-PIMB with its chiral analogue. The texture of the homeotropically aligned cells suggests two different helical axes, both of which are parallel to the layer but with one parallel and the other perpendicular to the bent-core plane.

A racemic layer structure in a chiral bent-core ferroelectric liquid crystal

A fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmCAPF*. The layer chirality of this structure alternates from layer to layer despite their being composed of chiral mesogens. Observations of the optical second harmonic generation signal from well-aligned domains confirm that the ground state of this phase is bistable ferroelectric. In addition to the ground state two types of metastable domains are also observed.

Effect of chemical structure on the liquid crystallinity of banana-shaped molecules

Banana-shaped molecules having two side wings attached to a bent core may exhibit liquid crystallinity. The most studied material is 8-OPIMB that comprises 1,3-dihydroxybenzene as a central core, a Schiff s base moieties as the wing groups and octyloxy tail groups. To clarify the effect of chemical structure on the liquid crystallinity of such a molecule, we have prepared several banana-shaped molecules, with side wings and central cores different from those of 8-OPIMB and examined their liquid crystallinity, which is sensitive to change in chemical structure. Especially, changing the position of the carbonyl group of the ester function linking the central core to the wing and the position of the nitrogen atom in the Schiff s base moiety caused a loss of liquid crystallinity. On the other hand, smectic liquid crystallinity was maintained for five new types of banana-shaped molecule with different central cores. Although all these smectic phases have liquid-like association of the molecules within the smectic layers, they showed unconventional smectic textures through the separation of spiral, fractal and germ textures from the isotropic melt. Moreover, a frustrated smectic phase and chiral smectic phases were found. Several possible smectic structures for those phases will be discussed.

Fast Switching of Frequency Modulation Twisted Nematic Liquid Crystal Display Fabricated by Doping Nanoparticles and Its Mechanism

The electrooptic characteristics of a twisted nematic (TN) liquid crystal display (LCD) fabricated by doping Ag nanoparticles protected with NLC, 5CB (K-15, Merck) molecules have been investigated, and it is shown that the device exhibits a unique electrooptic response characteristic that is sensitive to the high-frequency components of the operating voltage together with the conventional root-mean-square voltage response. We call this device frequency modulation (FM) TN-LCD. The FM-TN-LCD is switched by switching the frequency of the operating voltage while its amplitude is kept unchanged. As an example, the frequency switched from 20 Hz to 500 Hz or 50 Hz to 2 kHz, and the switching is performed with the time constant of 21 ms for the rising process and 3 ms for the falling process. This device shows a peculiar response to a burst AC square wave, where the decay time is reduced by several times compared to that of an undoped TN-LCD. The mechanism of the FM-TN-LCD is investigated through the study of its dielectric properties.

Dielectric Properties of Frequency Modulation Twisted Nematic LCDs Doped with Silver Nanoparticles

Twisted nematic liquid crystal display (TN-LCD) devices, composed of nematic liquid crystal (NLC), doped with silver (Ag) nanoparticles, which are protected with the same NLC molecules of 5CB (Merck, K-15), show electrooptic characteristics that are featured by a frequency modulation and fast response. This device is called FM-TN-LCD. In this paper, we report that the frequency range in the FM-TN-LCD spreading from 40 Hz to an upper limit 2 kHz correlates with that of the dielectric dispersion of the sample cell, which is expressed analytically by the Maxwell–Wagner theory on the heterogeneous dielectric medium, where the dielectric relaxation frequency, and thus the dielectric relaxation time, are primarily governed by the electrical conductivity of the doped Ag nanoparticles.

Effect of chiral dopant on a helical Sm1 phase of banana-shaped N-n-O-PIMB molecules

The chiral helical structures of the Sm1 phase of naphthalene-2,7-diyl bis[4-(4-alkoxyphenyliminomethyl)benzoates] (N-n-O-PIMB) occur with equal probability for right- and left-handed structures. In this report, we investigated the effect of chiral dopants on the formation of these structures. Two chiral homologues, N-4-O-PIMB2* with (S)-(+)-2-methylbutoxy tails and N-8-O-PIMB6* with (S)-(+)-6-methyloctyloxy tails, were synthesized for this purpose. We found that the chiral agent induces an imbalance between the two chiral domains such that the area of one of the two domains increases with chiral dopant content. The position of the chiral centre of the doping agent plays an important role in controlling the helical sense as well as the strength of the effect. A stronger effect was observed for N-4-O-PIMB2* in which the asymmetric carbon is closer to the central core. The helical pitch is not affected by the amount of chiral dopant as observed from the wavelength maximum in the CD curve. Further, it is interesting that N-4-O-PIMB2* induces preferentially left-handed helical structures, while N-8-O-PIMB6* forms right-handed structures. These unusual effects of the chiral dopant are discussed with a comparison with the conventional chiral phases such as the chiral nematic and SmC phases.

High speed parallel synthesis of banana-shaped molecules and phase transition behaviour of 4-bromo-substituted derivatives

Novel derivative series of the well known bent-shaped P-n-O-PIMB liquid crystal mesogens, referred to as ‘4Br-P-n-O-PIMB’, ‘4Cl-P-n-O-PIMB’ and ‘5Cl-P-n-O-PIMB’, having halogen atoms substituted on the phenyl ring in the central core, were synthesized by solution phase parallel synthesis based on a combinatorial approach. The mesomorphic behaviour and physical properties of all the new compounds were studied by means of optical microscopy, differential scanning calorimetry, X-ray and circular dichroism spectroscopy. We found interesting transitional behaviour for the 4Br-P-n-O-PIMB homologous series. The homologues with alkyl tails having carbon numbers of n=3–10, 12, 14 exhibit rather complicated mesomorphic behaviour, which is strongly sensitive to n. The chiral fluid smectic B2 phase with SmCAPA structure and unidentified smectic Bx phase were observed in the homologues with n=9, 10, 12, 14 and n=3–5, respectively. Interestingly, every member exhibits frustrated and/or helical ordered phases at low temperatures, designated as X1, X2, and X3 phases, which result from a spontaneous escape from a macroscopic polarization. The mesomorphic behaviour and mesophase structures differ remarkably from those of the parent P-n-O-PIMB homologues, suggesting that substitution of the halogen atoms at the central core essentially creates a particular interaction between molecules.

Electrooptic and Dielectric Properties in Bent-Shaped Liquid Crystals

Electrooptic and dielectric properties were studied in the antiferroelectric phase, B2, of bent-shaped liquid crystal molecules. We have succeeded in selectively obtaining the homogeneously chiral domain, the racemic domain, and the intermediate domain in which layers with opposite chirality are partially mixed, by properly choosing particular waveforms of the electric field applied. Tristable switching was observed for all domains under a triangular wave field. The switching current profiles under a rectangular wave field indicate that the switching time of homogeneously chiral domains is slower than that of racemic domains. The magnitude of spontaneous polarization depends not on temperature but on the carbon number of the terminal chain of the molecules. Two relaxation modes were detected in dielectric measurements, one of which was attributed to the distortion of the antiferroelectric ordering.

Evaluation of the First-Order Hyperpolarizability Tensor of Bent Molecules by Means of Hyper-Rayleigh Scattering Method

The first-order hyperpolarizabilities βs were determined by hyper-Rayleigh scattering in two homologous series of two-dimensional bent molecules with alkyl and alkoxy end chains. It was found that the β values of alkyl and alkoxy compounds are nearly the same and increase slightly with increasing chain length from ca. 10 ×10-30 esu to ca. 40 ×10-30 esu. Using the β value determined in the present study and the dihedral angle of the bent molecule, the tensor elements of β were also determined. The second-order nonlinear susceptibilities d33 and d31 were estimated and compared with an experimental result obtained by a bulk second-harmonic generation (SHG) measurement by Macdonald et al. [Phys. Rev. Lett. 81 (1998) 4408]. This comparison leads to a conclusion that the high SHG activity in bent molecular liquid crystal systems is attributed to highly ordered molecules possessing high hyperpolarizability in smectic layers.

MOLECULAR CHIRALITY DUE TO TWISTED CONFORMATION IN A BENT-SHAPED LIQUID CRYSTAL STUDIED BY POLARIZED FT-IR SPECTROSCOPY

Abstract Microscopic polarized fourier-transform infrared (FT-IR) measurements were performed in the antiferroelectric B2 phase of a bent-shaped liquid crystal molecule. In homogeneously chiral domains, the absorbance maximum appeared when the transmission axis of a polarizer was parallel to the layer normal for the phenyl ring stretching and C-O-C asymmetric stretching and that for the C = O stretching was almost perpendicular to that for C-O-C in the absence of a field. When a dc field was applied, the maximum for C-O-C rotated by about 35°, which is almost the same as the apparent tilt angle of the molecule. In contrast, the axis of maximum absorbance for C = O was neither parallel nor perpendicular to that of C-O-C under a field. It indicates that the two ester groups are twisted with respect to the central phenyl ring and are not perpendicular to the molecular long axis. Moreover, one of the twist conformations is chosen and the rotation around the molecular long axis is strongly hindered.