Large Negative Dielectric Anisotropy Research Articles

Benzoxazole-terminated liquid crystals with large birefringence and negative dielectric anisotropy

We synthesized heterocyclic liquid crystals with large birefringence and negative dielectric anisotropy, namely, 5–4-(2-(4-(alkoxy)phenyl)-2,3-difluorophenyl)benzo(d)oxazole derivatives (nPFO). These compounds display enantiotropic nematic mesophases with wide mesophase range of 45.9–84.0 °C (heating process) and 61.3–119.6 °C (cooling process). Compared with reference analogues of traditional benzoxazole compound 6PFB with different orientation of benzoxazole-terminated structure, the compounds nPFO roughly give lower melting points and clearing points because of their increased dihedral angle. Meanwhile, compounds nPFO show obviously large birefringence (Δn, 0.28–0.39 and 0.39–0.48 for experimental measurement and DFT theoretical calculation, respectively) due to the enhanced π-conjugation caused by the ethynyl bridge group in molecules. Benefitting from the larger perpendicular dipole moments and orientation angles, nPFO displays negative dielectric anisotropy (Δɛ) ranging from −3.60 to −4.10. These compounds can endow a commercial negative LC mixture the larger Δn and higher values of figure of merit, suggesting they are suitable dopant to further improve the performance of some commercial LC mixtures.

Electrooptic, pyroelectric and dielectric spectroscopic studies of nematic and twist bend nematic phases of achiral hockey-shaped bent-core liquid crystal

Experimental and theoretical investigations of the twist-bend nematic (NTB) phase in liquid crystals have recently received significant attention for reasons of unusual and interesting characteristics of the NTB. Though NTB was predicted to exist for rigid bent-core LCs by Ivan Dozov in 2001, nevertheless it has unambiguously been proven to exist only in a few bent-core systems so far. Here the characteristics of both the nematic (N) and NTB phases are investigated for a hockey shaped achiral rigid bent-core LC, called BCI, in planar and homeotropic aligned cells, using polarizing optical microscopy, electro-optics, pyroelectricity and wide band dielectric spectroscopy. Spontaneous polarization measured for a bias field of 2.2 V/μm of a chiral domain using the pyroelectric effect is found to be only ∼5 nC/cm2. A measurement of the spontaneous polarization using pyroelectricity in general is unaffected by the sample’s dc conductivity except when its conductance dominates the admittance. As helix of NTB is partially unwound by the field, results lead to the identification of NTB as polar and chiral. The first and the second harmonics of the applied field are observed of higher amplitudes in NTB and N phases, respectively. The hockey-shaped bent-core system exhibits a large negative dielectric anisotropy over a wide range of temperatures than observed for any other compound so far. The large negative dielectric anisotropy is the highly desirable characteristic parameter of the NTB for exploring the field induced phases at relatively lower field strengths. The orientational order parameter characteristically jumps at the N-NTB transition temperature. The complex dielectric permittivity is measured as a function of frequency in the range 1 Hz to 10 MHz, the analysis of results reveales two collective modes in the dielectric spectra. Amplitude of the lower frequency mode is much higher than of higher frequency, former is assigned to the hydrodynamic mode q=qzz^ with z-dependent rotation of the heloconical director n^(r), this in turn involves compression and dilation of the pseudo-layers. The higher frequency mode corresponds to fluctuations of the tilt director and closer to the transition temperature, it exhibits a typical soft mode characteristic feature.

Investigation of a partially fluorinated chiral antiferroelectric liquid crystalline material with large negative dielectric anisotropy

Liquid crystal is a state of matter that has both liquid-like fluidity and crystal-like ordering in varying degrees. In this work, we study 4-{[(2S)-octan-2-yloxy]carbonyl}phenyl4′-{4-[(2,2,3,3,4,4,4-heptafluorobutanoyl)oxy]butoxy}-[1,1′-biphenyl]-4-carboxylate, a partially fluorinated chiral liquid crystalline compound. The mesomorphic studies of the compound highlight the existence of polar smectic phases. The molecular geometry, chemical reactivity, nonlinear optics, and charge dynamics are investigated by density functional theoretical calculations. The structural characterization of the compound is performed through vibrational spectroscopy. Dielectric spectroscopy highlights five collective relaxation processes that perfectly fit the modified Cole-Cole relaxation model. The compound 3F4Bi exhibits a bandgap of 4.05 eV, high birefringence (0.16), optical nonlinearity, and a large negative dielectric anisotropy for the tilted ferroelectric phase. The compound could be used as an optically active component to form ferroelectric multi-component mixtures.

New entry for fluorinated carbocycles: Unprecedented 3,6-disubstituted 1,1,2,2-tetrafluorocyclohexane derivatives

The synthetic protocols for symmetrical/unsymmetrical 3,6-disubstituted 1,1,2,2-tetrafluorocyclohexane molecules were successfully established for the first time. Some of thus obtained tetrafluorinated cyclohexanes underwent recrystallization to afford trans-configured products preferentially. One of the unsymmetrical trans-disubstituted tetrafluorocyclohexanes, trans-1-ethyl-2,2,3,3-tetrafluoro-4-[4-(trans-4-n-propylcyclohex-1-yl)phenyl]cyclohexane, was found to possess a low birefringence Δn (0.073) and a large negative dielectric anisotropy Δε (–9.4) in a binary mixture system, which were very prominent as characteristics of LC molecules in VA-mode driving LC displays.

Rational design and synthesis of a novel laterally-tetrafluorinated tricyclic mesogen with large negative dielectric anisotropy.

Guided by theoretical calculations, we predicted that 5,5,6,6-tetrafluoro-1-methoxycyclohex-1-ene, a new structural motif, exhibits a large negative dielectric anisotropy, and then developed a synthetic strategy for the preparation of the novel tetrafluorinated tricyclic mesogen in twelve reaction steps starting from a commercially available fluorinated starting material. During the evaluation of its liquid-crystalline characteristics, the tetrafluorinated mesogen was found to display low birefringence and a large negative dielectric anisotropy, confirming that its molecular structure is a reasonable design for the further development of novel mesogenic molecules with large negative dielectric anisotropies.

Design, synthesis and evaluation of new fluorinated liquid crystals bearing a CF2CF2 fragment with negative dielectric anisotropy.

We designed unique negative dielectric molecules, which were tricyclic compounds with a 5,5,6,6-tetrafluorocyclohexa-1,3-diene moiety in the mesogen, and then for the first time developed an effective synthetic protocol from commercially available 4-bromo-3,3,4,4-tetrafluorobut-1-ene. We also evaluated their physical properties for new mesogenic compounds, such as birefringence, dielectric anisotropy and shear viscosity, which are important characteristics for flat LC display device applications. The molecules clearly showed not only a large negative dielectric anisotropy and birefringence but also high viscosity.

Synthesis and the effect of 2,3-difluoro substitution on the properties of diarylacetylene terminated by an allyloxy group

A series of 2,3-difluoro tolane allyloxy-based liquid crystals (LCs), which are composed of the tolane unit, lateral fluoro substituents, alkoxy and allyloxy terminal groups, were synthesised with 4-bromo-2,3-difluorophenol as the starting material via a five-step reaction. Their structures were confirmed by infrared spectroscopy (IR), mass spectroscopy (MS) and nuclear magnetic resonance (NMR). The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The dielectric measurement was performed using an independent analyser EC-1 and the birefringence was determined using an Abbe refractometer. The results reveal that LCs have large negative dielectric anisotropy and high birefringence, which are potentially important for the fast response displays and liquid crystal photonic devices (LCDs).

Stereoselective synthesis and physicochemical properties of liquid-crystal compounds possessing a trans-2,5-disubstituted tetrahydropyran ring with negative dielectric anisotropy.

Three stereoselective syntheses and the physicochemical properties of trans,trans-5-(4-ethoxy-2,3-difluorophenyl)-2-(4-propylcyclohexyl)tetrahydropyran, which is an important liquid-crystal compound with a large negative dielectric anisotropy (Δε=-7.3), are described. The key step in the construction of the trans-2,5-disubstituted tetrahydropyran ring in the first approach involved a benzylic cation mediated intramolecular olefin cyclization of a 2-allyloxy-1-arylethanol derivative. The second method included the Et2 Zn-induced 1,2-aryl shift of a bromohydrin obtained from a hetero-Diels-Alder reaction, followed by stereoselective bromination. The third approach utilized the hetero-Diels-Alder reaction of trans-4-propylcyclohexanecarboxaldehyde and a 2-aryl-3-(trimethylsilyl)oxy-1,3-butadiene, followed by stereoselective protonation. From results obtained by using a quantum chemical calculation method, the reason why the target compound shows a large negative Δε value is discussed.

新型1,2,4-噁二唑类液晶化合物的合成

In order to meet the different demands of the LCD matrix,a serious of 1,2,4-oxadiazoles compounds was synthesized.These compounds were synthesized from aryl iodides through three steps.Their constructions were confirmed by NMR,FT-IR and elemental analysis.The parameters and the performances of these compounds were measured by POM and DSC.Some of these 1,2,4-oxadiazoles compounds not only have small optical anisotropy(Δn = 0.107~0.118)and large negative dielectric anisotropy(Δe =-1.6 ~-4.3),but also have a wide nematic phase temperature range.These compounds can be used in the LCD matrix to improve partial performance of the LCD. The preparation method is of simple operation and low cost,which can also realize industrialization easily.

多氟多氰类负性液晶化合物的合成

In order to meet the different demands of the LCD,two novel polyfluorinated polycyano negative dielectric anisotropy liquid crystalline compounds were synthesized.These compounds were synthesized from phthalonitrile and 1,2-difluorobenzene by several steps.Their constructions were confirmed by NMR and elemental analysis.The parameters and the performances of these products were measured by POM,DSC and rotary viscometer.These polyfluorinated polycyano compounds have small optical anisotropy(Δn=0.096~0.107),large negative dielectric anisotropy(Δe=-15.2~-16.7)and high viscosity(γ1= 234.8~248.6mPa·s).These compounds can be used in the LCDmatrix to improve partial performance of the LCD.The preparation method is of simple operation and low cost,which can also realize industrialization easily.

Synthesis and Properties of Novel Negative Dielectric Liquid Crystal Compounds with a Tetrahydropyran Ring

Novel liquid crystal compounds having a tetrahydropyran ring structure have been developed. These compounds show large optical anisotropy, large absolute negative dielectric anisotropy and higher solubility with other LC compounds compared with conventional high birefringent negative dielectric compounds. The two types of tetrahydropyran derivatives have a chair form cyclohexane ring structure with oxygen in different positions, but they show almost the same dielectric value, and their temperature dependence shows almost the same tendency. In order to clarify this the probabilities of the various conformations were determined. Practical mixtures for the VA-LCD application show improved properties suitable for LCD-TV, also three-dimensional.

Field-induced transition from homeotropic to planar geometry in the SmC* phase of an electroclinic liquid crystal

The optical and electrical behavior was investigated of a symmetric liquid crystal (LC) cell: ITO–silane–LC–silane–ITO. The silane layer induces a perfect homeotropic alignment of the molecules of the studied electroclinic liquid crystal (ELC) material, BDH 764E. A field-induced transition from the perfect homeotropic to planar orientation in the chiral smectic C (SmC*) and smectic A (SmA) phases of the ELC was observed. Optical and dielectric studies were performed for both alignment (geometry) modes. The field-induced transition from the homeotropic to planar orientation was studied vis-à-vis the high negative dielectric anisotropy obtained in the studied material. Such an ELC with large negative dielectric anisotropy and perfect homeotropic alignment may have important implications for modern LC display technology.

Physical characterisation of 4′-butyl-4-heptyl- bicyclohexyl-4-carbonitrile

We report measurements of birefringence, bend elastic constant, principal dielectric constants and conductivities as functions of temperature in the nematic phase of 4′-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). The compound exhibits a small birefringence and a large negative dielectric anisotropy. Surprisingly, it shows a negative conductivity anisotropy, which probably owes its origin to the structural details of the molecules. The orientational order parameters (S) are estimated from both birefringence and dielectric anisotropy, and they agree well in the entire nematic range. The experimental results are compared with those on CCH7, which has a similar core structure but with a longitudinal dipole moment, in place of the transverse one of CCN-47.

Mesomorphic, dielectric, and optical properties of fluorosubstituted biphenyls, terphenyls, and quaterphenyls

AbstractCompounds with moderate and large negative dielectric anisotropy (Δɛ) are very attractive liquid crystal (LC) for vertical alignment mode (VA). Materials with such properties can be achieved by lateral substitution of a polar group into a mesogenic molecule. We synthesized some new LC materials with a negative value of Δɛ, a moderately high birefringence (Δn), and a low viscosity. The mesomorphic and physical behaviour of the novel biphenyls, terphenyls and quaterphenyls fluorosubstituted in the rigid core and also with fluorinated alkyl and alkoxy chains are investigated. The prepared series of four LC compounds are promising for new LC mixtures for various applications. Examples of nematic mixtures with Δɛ∼−3.25 will be presented.

High birefringence and large negative dielectric anisotropy phenyl‐tolane liquid crystals

Two phenyl‐tolane compounds with two pairs of (2,3) lateral difluoro substitutions were synthesized and their properties evaluated. These compounds exhibit a high birefringence (Δn ∼ 0.35) and a large negative dielectric anisotropy (Δε∼ −8). These two important features make the phenyl‐tolane compounds useful as a dopant in a negative Δε liquid‐crystal mixture to enhance the performance of the host mixture, or as a negative component in a dual‐frequency liquid‐crystal mixture to improve the dielectric anisotropy and birefringence and lower the crossover frequency.

Experimental Investigation on Pretilt Angle in Binary Liquid Crystal Composed of Highly Polar Molecules

We report the measurement of dielectric constants, birefringence, and pretilt angle in binary mixtures of nematic liquid crystals (CP7B and CCN-47). CP7B has a strong longitudinal dipole moment and exhibits a large positive dielectric anisotropy (Δε=ε|| -ε⊥>0) and CCN-47 has a strong transverse dipole moment and exhibits a large negative dielectric anisotropy. In the mixtures, ε|| increases with increasing CP7B concentration, whereas ε⊥ remains almost constant. We show that the pretilt angle increases with Δε and birefringence (Δn=ne-no) much rapidly than reported earlier. It is suggested that this effect may arise due the presence of strong permanent dipole moments in the molecules.

12.2: Large Negative Dielectric Anisotropy and High‐Birefringence Liquid Crystals

AbstractNew laterally‐fluorinated biphenyl tolane liquid crystals with a large negative dielectric anisotropy (Δn∼−8) and high birefringence (Δn∼0.34) are developed. By doping 15 wt% of such a compound to MLC‐6608 (a negative Δɛ LC mixture), the overall figure‐of‐merit is doubled.

Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Deltan) and the dielectric constant (epsilon( perpendicular)) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T(PN)) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T(PN), indicating the presence of polarised domains. Our measurements show that DeltaT(PN)(= T(PN)(E)-T(NI)(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E(2). It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.

Improved liquid crystals for vertical alignment applications

Liquid crystals with a large negative dielectric anisotropy are difficult to align vertically on silicon oxide surfaces and sometimes even on polyimides. An interesting method has been found to generate high quality vertical alignment by doping the host liquid crystal with materials that have a cyanogroup along their long molecular axis. The effect is explained by considering both long range and short range interactions between liquid crystal and the alignment layer. Using this method we propose a way to design new liquid crystal mixtures with large negative dielectric anisotropy that have improved aligning ability for applications requiring vertical alignment. The method also allows for increased birefringence.

39.3: Distinguished Student Paper: Improved Vertical Alignment of Liquid Crystals with a Large Negative Dielectric Anisotropy

Liquid crystals with a large negative dielectric anisotropy and large birefringence are desirable for vertical alignment applications. But they are difficult to align vertically on SiOx surfaces. An interesting method has been found to generate high quality vertical alignment by doping the host liquid crystal with materials that have a cyano group along their long molecular axis. Using this method we propose a way to design new liquid crystal mixtures with large dielectric anisotropy and birefringence that have improved aligning ability for applications requiring vertical alignment.