Dopant-Enhanced Vertical Alignment of Negative Liquid Crystals

Wen, Chien‐Hui; Wu, Benjamin M.; Gauza, Sebastian; Nie, Xiangyi; Wu, Shin‐Tson

doi:10.1080/15421400600655949

Cited by 3 publications

(4 citation statements)

References 16 publications

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“…Hence, the ultraviolet stability of BCH5CN molecule is higher, which may be exploited for several optoelectronic applications. The dielectric anisotropy (DA) of biphenylcyclohexane (BCH) is ~0 as the terminal groups are all alkyls [29]. Liquid crystals with large negative dielectric anisotropy can be obtained by incorporating a cyano group as a terminal substituent [30].…”

Section: Uv-visible Absorption Spectrummentioning

confidence: 99%

Substituent and solvent effects on UV‐visible absorption spectra of liquid crystalline disubstituted biphenylcyclohexane derivatives – a computational approach

Praveen¹,

Ojha²

2011

Cryst. Res. Technol.

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Key words liquid crystals, absorption spectra, oscillator strength.A computational approach has been carried out on liquid crystalline disubstituted biphenylcyclohexanes (BCHs) of general formula R-C 6 H 10 -C 6 H 4 -C 6 H 4 -X with R: C 3 H 7 ; X: H (BCH30) and R: C 5 H 11 ; X: CN (BCH5CN) using the CNDO/S + CI and INDO/S + CI methods. These methods have been employed to calculate and analyze the spectral shifts, and absorbance measurements in UV-visible range of the systems. The electronic transitions, absorption wavelength, HOMO (Highest Occupied Molecular Orbital), and LUMO (Lowest Unoccupied Molecular Orbital) energies have been calculated. Further, ultraviolet (UV) stability of the molecules has been discussed in the light of absorption wavelength and electronic transition oscillator strength (f). The effect of different solvent media and substituents on transition energies, oscillator strength, and other absorption parameters has also been reported. The present article provides valuable information regarding enhancing the UV stability of molecules by marinating their conductivity. Further, these absorption spectra will provide valuable data base for future computational studies, and the other experimental investigations.

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Section: Uv-visible Absorption Spectrummentioning

confidence: 99%

Substituent and solvent effects on UV‐visible absorption spectra of liquid crystalline disubstituted biphenylcyclohexane derivatives – a computational approach

Praveen¹,

Ojha²

2011

Cryst. Res. Technol.

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“…The nonpolar BCH molecules can be used as a dopant to liquid crystal mixtures to enhance their vertical alignment. As a result, high-contrast vertical alignment of high birefringence liquid crystals can be obtained [20]. The application of the BCH dopant is very useful for developing highcontrast LCD devices such as large screen liquid crystal televisions.…”

Section: Introductionmentioning

confidence: 99%

“…Another application on mixing of liquid crystals is the method of doping commonly used in the liquid crystal science. For example, Wen et al [20] utilized a nonpolar liquid crystal compound as the dopant to liquid crystal mixtures for enhancing vertical alignment of the host liquid crystal mixtures. High-contrast vertical alignment of high birefringence liquid crystal mixtures were attained by their doping method.…”

Section: Introductionmentioning

confidence: 99%

Size and chain length effects on structural behaviors of biphenylcyclohexane-based liquid crystal nanoclusters by a coarse-grained model

Liao¹,

Chang

et al. 2011

J Mol Model

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Size and chain length effects on structural behaviors of liquid crystal nanoclusters were examined by a coarse-grained model and the configurational-bias Monte Carlo (CBMC) simulation. The nanoclusters investigated in this study are composed of the biphenylcyclohexane-based BCH5H liquid crystal molecule and its derivatives. Results of the study show that the average energy decreases (i.e., more negative) as the cluster size (i.e., the number of molecules) increases. With the increasing cluster size, the equilibrium conformation of the nanocluster changes gradually from a pipe-like structure (for the smaller systems) to a ball-like cluster (for the larger systems). The order parameter of the system reduces with the transition of the equilibrium conformation. Regarding the chain length effect, the pipe-like equilibrium conformation (for the smaller systems) was observed more close to a pipe as the length of the tail alkyl chain of the derivatives extended. However, due to the flexibility of the tail alkyl chain, the pipe conformation of the system deflects slightly about its cyclohexyl group as the tail extends further.

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“…Because of this several different methods for achieving homeotropic alignment with a NDALC have been developed. [2][3][4][5] Lu et al 5 explored in some detail the use of obliquely deposited SiO 2 produced using electron beam evaporation. They found that, depending on the angle of evaporation and the nature of the LC ͑positive or negative ⌬ ͒, alignments from homogeneous through to homeotropic could be obtained.…”

mentioning

confidence: 99%

Voltage dependent director of a homeotropic negative liquid crystal cell

Ruan

Yang

Sambles

2008

Applied Physics Letters

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Thin layers of obliquely ͑60°to normal͒ thermally evaporated SiO x lead to homeotropic alignment of a nematic liquid crystal ͑LC͒ having negative dielectric anisotropy. Under application of an ac voltage the director, as characterized by the fully leaky waveguide technique, is found to realign with a voltage controlled tilt along the evaporation direction. This behavior is in complete contrast with that of a LC having positive dielectric anisotropy and may have important implications for modern LC display technology. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2963977͔Liquid crystals ͑LCs͒ with negative dielectric anisotropy ͑NDALC͒ have recently seen extensive use in flat panel display applications, especially in the vertically aligned ͑VA͒ mode. 1 Because such a structure has good contrast combined with quite fast switching times, this type of display has substantial potential. Clearly to obtain good operation it is important to be able to control the director orientation in such a structure incorporating a NDALC. However, it is well known that these materials can be rather difficult to align homeotropically. Poor alignment of course leads to a low contrast ratio and the advantages of using a NDALC are lost. Because of this several different methods for achieving homeotropic alignment with a NDALC have been developed. [2][3][4][5] Lu et al. 5 explored in some detail the use of obliquely deposited SiO 2 produced using electron beam evaporation. They found that, depending on the angle of evaporation and the nature of the LC ͑positive or negative ⌬ ͒, alignments from homogeneous through to homeotropic could be obtained. Other works, which form the basis of US patents, 6,7 also provide methods for homeotropic alignment of negative nematics.In this letter, we provide a simple and inexpensive method for aligning a NDALC in a uniform homeotropic orientation in a thin ͑Ͻ4 m thick͒ cell and show, using optical waveguide studies, how upon application of an ac voltage across the cell a uniform tilted structure is obtained with the LC director tilted along a single direction which lies in the plane conatining the intitial SiO x evaporation direction and the surface normal.First a wedged cell with a thickness varying from ϳ0 to 3 m has been built using glass plates on the inside of which has been deposited thin SiO x layers by oblique vacuum evaporation ͑60°to normal͒. Such a layer gives homogeneously aligned LC for a positive dielectric anisotropy nematic. The cell was assembled with the evaporation directions antiparallel to each other and then vacuum filled with a NDALC ͑MLC-6884, Merck͒, which has a reasonably high dielectric anisotropy of ⌬ = −5. When this cell was observed between two crossed polarizers, there was a very uniform dark figure at all twist angles. However, if the cell is tilted from being normal to the incident light then at certain angles it appears uniformly bright. This means that the director within the cell is everywhere homeotropically aligned. Thus it appears that the 60°to normal evapo...

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Dopant-Enhanced Vertical Alignment of Negative Liquid Crystals

Cited by 3 publications

References 16 publications

Substituent and solvent effects on UV‐visible absorption spectra of liquid crystalline disubstituted biphenylcyclohexane derivatives – a computational approach

Substituent and solvent effects on UV‐visible absorption spectra of liquid crystalline disubstituted biphenylcyclohexane derivatives – a computational approach

Size and chain length effects on structural behaviors of biphenylcyclohexane-based liquid crystal nanoclusters by a coarse-grained model

Voltage dependent director of a homeotropic negative liquid crystal cell

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