Room-temperature Liquid Crystal Research Articles

Synthesis and Mesomorphism of Rigid Bridge Triphenylene Discotic Liquid Crystal Dimers

Pd-Cu catalyzed homocoupling reaction has been used for the synthesis of symmetrical triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)5(O2CC6H4C≡C-)]2 (n = 4, 5, 6, 7, 8, 9), which connected two triphenylene discogens via a long rigid spacer. The purity and structure of monomers and dimers have been characterized with 1H NMR, IR and high resolution Mass. Their fine thermal stability has been confirmed by the results of thermogravimetry analysis (TGA). Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) results have shown that the monomers and dimers exhibit columnar mesophases (Col). The dimers display wide mesophase ranges and are room temperature liquid crystals, and their clearing points increased with the lengthening of peripheral alkyl chain.

Aggregate Structure and Free Energy Changes in Chromonic Liquid Crystals

Past and recent x-ray and absorption data on chromonic liquid crystals are analyzed in a consistent manner to reveal both the cross-sectional areas and stacking free energy changes for a number of systems. Two tentative conclusions result from this study. First, the locations of the charged groups on the dye molecule determine the number of molecules in the cross-section of the aggregate. Second, among systems with the same aggregate structure, the higher the stacking free energy change as measured in dilute solutions, the lower the concentration at which a room temperature liquid crystal phase forms at higher concentrations.

Light-Induced Deformation of Photoresponsive Liquid Crystals on a Water Surface

Photodeformation: Azobenzene derivatives showing a room-temperature liquid crystal (LC) phase exhibit photoinduced deformation on a water surface. While a droplet of a LC sample floating on the surface expands upon UV irradiation, a LC sample containing a solvent is condensed towards the center of the illuminated regions (see figure).

Room-temperature discotic liquid crystals based on oligothiophenes—attached and fused triazatruxenes

A new class of discotic liquid crystals based on oligothiophenes - attached and fused triazatruxenes, were synthesized in high yields. The synthesis was carried out first by microwave-assisted six-fold Stille cross-coupling reactions to yield the hexakis-oligothiophenes - substituted triazatruxenes TAT-T1 and TAT-T2 and followed by successful oxidative cyclodehydrogenation of TAT-T1 to afford the planar TAT-T1-C with an extended core. Their optical properties, electronic properties, thermal behavior and self-assembly were investigated by various techniques. UV-vis absorption and fluorescence spectroscopic measurements demonstrate that TAT-T1-C is a largely delocalized and rigid polycyclic aromatic π-system. All of these disk-like molecules, in particular TAT-T1-C, show a strong π–π interaction in the solid state and exhibit columnar liquid crystalline properties at room temperature with a wide-range mesophase. Elongation of the peripheral oligothiophene arms or expansion of the rigid core size upon cyclization of the neighbouring thiophene moieties gives rise to dramatic increases of both the isotropic temperatures and the mesophase width.

Room-temperature electron-deficient discotic liquid crystals: facile synthesis and mesophase characterization

Two novel series of rufigallol-based room-temperature discotic liquid crystals have been designed and synthesized using microwave dielectric heating within a few minutes in good yield. The chemical structures of all the compounds have been characterized using spectral techniques and elemental analysis. The mesophase behaviors of all the compounds have been investigated by polarizing optical microscopy and differential scanning calorimetry. Columnar hexagonal structure of the mesophase of these compounds has been established with the help of X-ray diffraction studies.

Imidazolium Ionic Liquid Crystals with Pendant Mesogenic Groups

Ionic liquid crystals were obtained by coupling one or two mesogenic units (cholesterol or cyanobiphenyl) to an imidazolium cation. Anions are bromide, bis(trifluoromethylsulfonyl)imide, and tetrakis(2-thenoyltrifluoroacetonato)europate(III). The mesomorphism of the compounds depends on the type and number of mesogenic units and on the type of anion. In general, the most stable mesophases are observed for the bis(trifluoromethylsulfonyl)imide salts. Most of the compounds containing cholesterol moieties show enantiotropic SmA* phases over a broad temperature range, and some of them are room temperature liquid crystals. Modeling of the small-angle X-ray scattering patterns revealed the molecular arrangement in these mesophases. On the contrary, most of the compounds containing cyanobiphenyl groups exhibit monotropic lamellar or nematic mesophases, depending on the number of mesogenic units. The imidazolium salts containing the tetrakis(2-thenoyltrifluoroacetonato)europate(III) anion show an intense red photoluminescence.

Rational Tuning of Melting Entropies for Designing Luminescent Lanthanide‐Containing Thermotropic Liquid Crystals at Room Temperature

The connection of twelve peripheral and divergent dodecyloxy chains to a central tridentate aromatic binding unit provides the dodecacatenar ligand L11, for which room-temperature mesomorphism is detected. An enthalpically unbalanced large melting entropy (DeltaSmL11=226 J mol(-1) K(-1)) results from the programmed microsegregation induced in the crystalline phase, a phenomenon which is maintained in the associated lanthanide complexes [Ln(L11)(NO3)3] and [Ln(L11)(CF3CO2)3]2. Low-temperature melting processes (-43<or=Tm<or=-25 degrees C) producing room-temperature hexagonal columnar liquid-crystalline phases thus result for these novel lanthanidomesogens. A combined photophysical (high-resolution emission spectroscopy), thermodynamic (differential scanning calorimetry, DSC) and structural (small-angle X-ray diffraction, SA-XRD) investigation of the melting process shows minor structural changes occurring between the crystal (Cr) and the hexagonal columnar mesophase (Colh) in the complexes, which allows the assignment of the existence of these unusual first-order phase transitions to the negligible mixing entropy produced by the two viscous phases at low temperature. Extension of the concept of chemical tuning of melting entropies for the global design of room-temperature metallomesogens and liquid crystals is discussed.

Trans–cis isomerization of an azoxybenzene liquid crystal

Trans–cis isomerization was investigated in a room temperature liquid crystal mixture of two azoxybenzene compounds. Experiments were performed on isolated molecules in dilute solutions and on the liquid crystal phase composed of the pure compounds. The absorption spectra of the trans and cis isomers were found to be similar to those of azobenzene compounds, as were the birefringence and order parameter of the nematic liquid crystal phase. The photo‐optic properties were also similar in that irradiation by ultraviolet light caused the conversion from trans to cis isomers, while short wavelength visible light incident on these compounds resulted in the conversion from cis to trans isomers. The activation energy for thermal relaxation from the cis to trans isomer in the liquid crystal phase was determined to be (66±7) kJ/mole, which is less than for azobenzene in solution. While a photostationary state in a dilute solution with approximately equal numbers of trans and cis isomers was achieved, the nematic–isotropic transition of the mixture of the pure compounds decreased from 70°C to room temperature with a cis concentration of only about 12%. One unusual finding was that the photostationary concentration of trans and cis isomers due to irradiation with light of a specific visible wavelength depended on the starting concentrations of the two isomers, indicating that there may be a molecular conformation that is not photo‐responsive and relaxes only thermally.

Microwave-assisted synthesis of rufigallol and its novel room-temperature liquid crystalline derivatives

An efficient, simple, rapid and economic microwave-assisted synthesis of rufigallol, a molecule of both biological and physical interest, is reported. The acid-catalyzed self-condensation of gallic acid under microwave irradiation produces rufigallol in high yield within 90 s. Alkylation of rufigallol with branched-chain alkyl halides under microwave irradiation provides novel room-temperature discotic liquid crystals in excellent yield.

Rewinding of Helical Systems by Use of the Cr(III) Complex as a Photoresponsive Chiral Dopant

Vibrational circular dichroism (VCD) spectroscopy has been applied for monitoring in situ the photoinduced rewind of supramolecular helices in a liquid crystal. The monitored system was a room-temperature liquid crystal, ZLI-1132, doped with a novel chiral Cr(III) complex. Under the illumination of UV light (365 nm), the photoracemization of the Cr(III) complex led to the rewind of helices in the chiral nematic phase. In response to this, the VCD spectrum of the system exhibited the transient change, confirming the elongation of helical pitch length. The possibility of using the VCD spectrum as a memory of photoresponsive events is discussed.

Ferroelectric Characterization of a Room Temperature Liquid Crystal: 2Cl.BAAP.180-BBP by Low Frequency Dielectric and Optical Methods

The isotropic to nematic(IN), nematic to cholesteric(NCh), cholesteric to smectic-A (ChA) and smectic-A to smectic-C* (AC*) phase transition temperatures exhibited by a room temperature ferroelectric liquid crystal RTFLC, viz., 2Cl.BAAP.180-BBP are determined by the observed anomaly in differential permittivity, d l '/dT. The low frequency dielectric dispersion along with the activation energy are detailed in the N, Ch, SmA and SmC* phases. The Soft mode and Goldstone mode relaxation in the vicinity of AC* phase transition are discussed. The trend of distribution parameter h-values for the off-centred Debye relaxation is discussed. Temperature variation of dilectric strength for the soft mode relaxation is estimated through ferrolectric Curie-Weiss law. Observed temperature variation of Spontaneous Polarization P s (T), Tilt angle θ(T), Response Time s (T), and Effective Torsional viscosity m (T) is found to agree with the Mean field expectation. Relative strength of P s among the reported values is discussed as a function of extended conjugation about the chiral centre.

Phase Characterization and Properties of Completely Saturated Quaternary Phosphonium Salts. Ordered, Room-Temperature Ionic Liquids

The properties of four completely saturated and one partially saturated salts consisting of methyl-tri-n-decylphosphonium cations and either chloride, chloride monohydrate, bromide, bromide monohydrate, or nitrate anions are reported. Their neat phase behavior has been examined by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The monohydrates form smectic phases that persist in >90 °C ranges and extend from below room temperature; they are ordered, room-temperature ionic liquid crystals. The nitrate salt becomes liquid-crystalline above room temperature and the other two salts remain as soft solids to their isotropization temperatures. The locations of two solvatochromic dyes, Nile Red and 1,1-dicyano-2-[6-(dimethylamino)naphthylen-2-yl]propene, within the ionic assemblies have been approximated from UV/vis absorption and fluorescence spectra, and the temperature (phase) dependence of the conductivities and dielectric constants of some of the salts have been meas...

Heterocyclic 1,3,4-oxadiazole as columnar core

The synthesis, characterization and mesomorphic properties of a new type of liquid crystalline compound, the 2,5-bis(3,4,5-trialkoxyphenyl)-1,3,4-oxadiazoles, 3a-3h, are reported. These heterocyclic compounds are derived from unsaturated 1,3,4-oxadiazole as the core group, and obtained by the condensation reaction of 3,4,5-trialkoxybenzoic acid N-(3,4,5-trialkoxybenzoyl)-hydrazides and phosphorus oxychloride in toluene under reflux. All compounds were characterized by 1H and 13C NMR spectroscopy, and elemental analysis. The mesomorphic properties of these and the related compounds 1, 2 were characterized and studied by differential scanning calorimetry and polarizing optical microscopy. The formation of columnar mesophases was found to be dependent on the numbers of alkoxy sidechains. The compounds 3 exhibited hexagonal columnar (Colh) phases, however compounds 1, 2 formed crystalline phases. Compounds 3b-3e with shorter carbon chains were room temperature liquid crystals. Polar induction by nitrogen and/or oxygen atoms on the heterocyclic core ring might be responsible for the formation and better observed mesomorphic properties in this type of compound.

The history of liquid-crystal displays

The modem history of liquid crystals has been dominated by the development of electronic displays. These developments began in 1964, when Heilmeier of RCA Laboratories discovered the guest-host mode and the dynamic-scattering mode. He thought a wall-sized flat-panel color TV was just around the corner From that point on, twisted-nematic (TN) mode, super TN mode, amorphous-Si field-effect transistor and room-temperature liquid crystals were developed. In the beginning, liquid-crystal displays (LCDs) were limited to niche applications such as small-size displays for digital watches, pocket calculators, and small handheld devices. That all changed with the development of the notebook computer industry. In 1988, Washizuka et al. of Sharp Corporation demonstrated an active-matrix full-color full-motion 14-in display using a thin-film-transistor array. The electronics industries now recognized that Heilmeier's 25-year dream of a wall-hanging television had become reality. LCDs could be used to replace existing cathode ray tubes. Through the cooperation and competition of many electronics giants, the LCD industry was firmly established.

Liquid crystal mixtures for potential infrared applications

Liquid crystals (LCs) with diazo linkages have high dielectric and optical anisotropy. Two newly synthesized liquid crystalline compounds were dissolved in room temperature LC hosts, E7 and PTTP-24/36, to assess their properties. It was found that these mixtures have higher birefringence, larger dielectric anisotropy, and better elastic properties than the hosts. Satisfactory viscoelastic coefficients were also obtained for these mixtures, showing that they are promising LC materials for applications in the near IR region.

Ionic hexagonal columnar metallomesogens derived from tetrabenzo[b,f,j,n] [1,5,9,13]tetraazacyclohexadecine

A series of transition metal (Ni, Cu, Pd) complexes derived from macrocyclic tetrabenzo[b,f,j,n] [1,5,9,13]tetraazacyclohexadecine (TAAB) was synthesized and their mesomorphic properties studied by differential scanning calorimetry, polarized optical microscopy and X-ray powder diffraction (XRD). These compounds have eight alkoxy side chains attached around the central molecular core and form disc-like molecules. All the derivatives exhibited columnar mesophases over a wide range of temperature. The mesomorphic behaviour was found to be dependent on the incorporated metal and the carbon length of the alkoxy side chains. The clearing temperatures decreased in the order M = Ni > Pd > Cu; this decrease was probably due to the size of the metal ions. Some derivatives with shorter side chains (n = 10, 12) were room temperature liquid crystals. All compounds were found to exhibit hexagonal columnar (Colh) phases which were confirmed by powder XRD.

Preparation of a Surface-Oriented Liquid Crystal. An Experiment for the Undergraduate Organic Chemistry Laboratory

In this laboratory experiment, sophomore-level organic chemistry students use a simple procedure to prepare a room-temperature liquid crystal, MBBA. The preparation can be carried out in about one hour with high yields. The students then prepare ordered samples of the liquid crystal and observe their optical properties. This exercise utilizes a fairly traditional laboratory preparation of a compound but also serves as an introduction to liquid crystals and materials chemistry. It has been quite popular with the students and has minimal equipment needs.

Electrophilic aromatic substitution in triphenylene discotics: Synthesis of alkoxynitrotriphenylenes

Functionalization of triphenylene-based discotic liquid crystals by direct nitration of hexaalkoxytriphenylene has very recently been reported to produce exclusively the mononitrohexaalkoxytriphenylene. W e have found that nitration of such discotics is not restricted to only one ring of triphenylene, but all the three rings can be nitrated successively to produce mononitro-, dinitro- and trinitro-hexaalkoxytriphenylenes. All the mononitro derivatives show a very broad mesophase range. While the dinitrohexabutyloxytriphenylene is non-liquid crystalline, its higher homologues are room temperature liquid crystals. Photomicroscopic pictures of these liquid crystalline materials show classical textures of an ordered hexagonal columnar phase. X-ray diffraction studies of two representative compounds, 4a and 4b, confirmed the above conclusions. None of the trinitro derivatives are liquid crystalline. Nitrotriphenylenes are valuable precursors to several other triphenylene derivatives like amino, acyl, alkylamino, diazo, etc. Mononitration of 2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylenes gives the 1-nitro-2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylenes. These double functionalized triphenylene derivatives are extremely important precursors as the functional group such as nitro, amino, azo, etc. can be utilized to modify the electronic nature of the core and at the same time the hydroxy functional group may be converted to a polymerizable group. Thus, processable oligomers and polymers can be synthesized.

2-10a 自己組織性低分子と室温液晶よりなる異方性ゲルの電場応答性

2-10a 自己組織性低分子と室温液晶よりなる異方性ゲルの電場応答性

Columnar Mesophases from Nondiscoid Metallomesogens: Rhodium and Iridium Dicarbonyl Diketonates

A series of nondiscoid rhodium and iridium dicarbonyl β-diketonate complexes have been shown to organize in columnar liquid crystal phases with hexagonal disordered structures (Colhd). The shape and dipolar attributes of these materials produce highly correlated antiparallel pairwise arrangements. This organization was investigated by X-ray diffraction and miscibility studies. The liquid crystallinity was found to be stabilized by decreasing the number of carbon atoms in the side chains, and single component room-temperature liquid crystals were developed. The nature of the metal is also important, and iridium mesogens exhibited higher clearing points than the analogous rhodium materials.