Calamitic Liquid Crystals Research Articles

Availability of Liquid Crystalline Materials for Organic Polycrystalline Semiconductor Thin Films

ABSTRACTWe have investigated the charge carrier transport properties of polycrystalline films fabricated with discotic and calamitic liquid crystals. The polycrystalline films of these liquid crystals exhibiting particular textures under a polarized microscope, i.e., hexagonal and co-circular patterns for discotic and calamitic liquid crystals, respectively, which indicate that the molecular alignment in the liquid crystalline phase is well preserved exhibit clear hole transport along with molecular stacks even in a 15 μm thick film, judging from transient photocurrents by time of flight method. The hole mobilities are over 0.1 cm2/Vs at room temperature. These results indicate formation of fewer grain boundaries across conduction channels. On the other hand, the carrier transport properties in the polycrystalline films of non-liquid crystalline molecules with the same molecular shape exhibit no particular patterns as described and photocurrents are annihilated in the bulk as a function of time, indicating many deep trap sites in grain boundaries. The present results indicate that the liquid crystalline molecule is a good candidate for preparing quality organic polycrystalline semiconductor thin films in opto-electronic applications.

Sulphur as a Structural Element in Calamitic Liquid Crystals: Terminal, Linking and Lateral Substitutions

ABSTRACT The effect on the physicochemical properties of calamitic liquid crystals of introducing sulphur atoms into their terminal, linking, and lateral groups is discussed and rationalized in terms of existing theories; a comparison is made with the corresponding oxygen-containing and other reference liquid crystals.

Orientational Dynamics in the Isotropic Phase of a Calamitic Liquid-Crystal Model

We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density.

Addressing non‐idealities in NMR experiments on rotating liquid crystals

Unequivocal evidence for a biaxial nematic phase in low‐molar‐mass calamitic thermotropic liquid crystals has been challenging to generate. Recently we provided NMR evidence that nonlinear calamitic mesogens based on the oxadiazole heterocycle exhibit a biaxial nematic phase (Phys. Rev. Lett., 2004, 92, 145505). Herein we probe the robustness of that claim by exploring potential variations of the director distribution from the ideal one that would apply to nematics rotated about an axis perpendicular to the spectrometer magnetic field. Moreover, we demonstrate that our methodology, when applied to the high temperature uniaxial nematic TBBA, yields a biaxial order parameter η = 0.0, thereby confirming the validity of our procedures. Our findings suggest that the original claim of biaxial order (η∼0.1) as reflected by probe molecule studies of oxadiazole mesogens is still valid.

High mobility ambipolar charge transport in a cross-linked reactive mesogen at room temperature

By photochemically cross linking the molecules of a reactive end group functionalized calamitic liquid crystal (bis(heptylphenyl)-bithiophene-oxetane) in the smectic mesophase, an insoluble rigid molecular network is formed which remains structurally unchanged on cooling to room temperature. The charge transport is investigated by the time-of-flight method and the resulting material shows long-range (∼10μm) ambipolar transport with nondispersive field independent hole mobility, μhole=0.016cm2∕Vs and dispersive field dependent electron mobility, typically μelectron=0.028cm2∕Vs (both at 4×104Vcm−1 and at 15 °C). These desirable charge transport properties make this system a promising candidate for organic electronic applications, such as charge transport layers in organic light-emitting diodes and field effect transistors.

Publisher’s Note: “Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation” [J. Chem. Phys. 122, 184513 (2005)

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Ketonization in Calamitic Liquid Crystals

ABSTRACT The effect of introducing the keto groups into their molecular structure on the physicochemical properties of calamitic liquid crystals is discussed and rationalized in terms of existent theories.

Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation

We report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e., smectic A, nematic and isotropic. These quantities have then been used to compute diffusion coefficients. In addition, we have calculated viscosities in the nematic and isotropic phases. Despite its crude nature, the model is capable of providing a faithful reproduction of many features of the transport behavior observed in real liquid-crystalline materials. The simulation results have been compared with the predictions of the modified affine transformation theory, finding only a fair agreement.

A novel calamitic mesophase semiconductor with the fastest mobility of charged carriers: 1,4-di(5′-octyl-2′-thienyl)benzene

The carrier mobility of highly ordered lamellar mesophases was evaluated by a Time-of-Flight (TOF) method for mesogenic 1,4-di(5'-octyl-2'-thienyl)benzene (8-TPT-8) to reveal the fastest drift mobility (0.1 cm(2) V(-1) s(-1) for the lowest temperature mesophase) in calamitic liquid crystals reported to date.

Fractal growth of a conventional calamitic liquid crystal

We report observation of liquid crystal phase ordering via fractal growth aggregates for a calamitic, non-bent-core mesogen. Fractal growth of a conventional smectic- C (Sm-C) phase from the isotropic melt after a temperature quench was experimentally investigated with respect to time, cell gap, quench depth, and quench rate. The determined fractal dimensions relating to the area as well as the perimeter of the growing aggregates suggest a phase formation process via a percolation mechanism. Computer simulations of the phase ordering process give further evidence for percolation growth, qualitatively reproducing the observed textures and quantitatively leading to the same fractal dimensions. We propose a general model of fractal smectic liquid crystal growth, which accounts for all of the different systems observed so far, bent-core or "banana" phases as well as the observation of fractal phase ordering of a conventional Sm-C phase. The model is based on the "breaking" of the commonly observed growth anisotropy by strong in-layer molecular interactions. These are provided by hydrogen bonding in the Sm-C case discussed here and by steric interactions in the case of the bent-core phases discussed in previous publications.

Correlation of the molecular structure of discotic nematic liquid crystals with their orientational order and specific features of the nematic-isotropic phase transition

The orientational order parameter S of molecules in high-temperature discotic nematic liquid-crys- tal phases of triphenylene derivatives is investigated as a function of the length of side flexible molecular chains at different temperatures. It is established that the orientational order parameters S in the range of the transition from the nematic phase to the isotropic liquid phase (the N D - I transition) are smaller than those predicted from the molecular-statistical theory and computer simulation. It is shown that the N D - I transition is close to both the isolated Landau point and the tricritical point (regardless of the chemical structure of the molecules and the anisotropy of dispersion intermolecular interactions). Consistent explanations are offered for a number of experimental findings, such as the anomalously small changes in the enthalpy and entropy upon the N D - I tran- sition (as compared to those revealed upon the N - I transition in calamitic nematic liquid crystals), the anoma- lously strong response of the isotropic phase of discotic nematic liquid crystals to external fields (thermody- namically conjugate to the order parameter S ) and the long relaxation times of this response, and the formation of cybotactic discotic molecular clusters in the isotropic phase in the vicinity of the N D - I transition. © 2004 MAIK "Nauka/Interperiodica".

Evidence for different polymorphisms with and without an external electric field in a series of bent-shaped molecules.

Five fluid tilted mesophases are observed in a series of achiral banana-shaped compounds. The terminal chain length is the pertinent molecular parameter which induces the polymorphism change. All the phases, refer to tilted lamellar structure without in-plane order in the layers. The observation of monolayer, bilayer, ribbon phase, and undulated structures recalls the richness of the polymorphism of the frustrated polar calamitic liquid crystals. Among the mesophases, we highlight two: a Sm-C G2 phase corresponding to a bilayer structure made of Sm-CG layers, and its two-dimensional variant, Sm- Ctilde; G2. The five mesophases observed at zero field are switchable under electric field. At least three ferroelectric phases are induced by an applied field. There is no direct correspondence between the zero field phases and the phases observed under electric field. These observations show that different polymorphisms exist in the series with and without an applied field. A unique ( E,T ) phase diagram is presented, corresponding to the superposition of the ( E,T ) diagrams obtained for each homolog of the series, where the influence of the chain length is equivalent to a shift on the temperature axis.

Calamitic Liquid Crystals as Nanoscale Analogues of a Block Copolymer

The micro-segregation picture of a block copolymer was applied to a low molecular weight liquid crystal (LC) to clarify the similarities between them with respect to structured mesophase formation. Calamitic LC molecules were regarded as block compounds composed of a rigid anisometric core terminated by flexible chains. Then, the incompatibility parameter χcore-chain and volume fraction fcore were considered analogous to χAB and fA in A−B diblock copolymers. Fedors's method, which was originally used for polymers, was utilized to estimate χcore-chain and fcore. We found good correspondence in the χ versus f phase diagram between block copolymers and block LCs.

Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

AbstractOptical birefringence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10‐3 was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Orientational Ordering and Polarizability of Molecules in the Columnar Phase of Discotic Liquid Crystal

The structural and molecular-optical properties of the columnar discotic liquid crystal 2,3,6,7,10,11-hexaheptyloxytriphenylene were studied by using of refractometric and infrared (IR) measurements. Temperature dependence of the orientational order parameter S with allowance for the anisotropy of the local field of a light wave and mixing of molecular excitations was evaluated for C–C aromatic stretching band. The local field parameters have been evaluated within approximation of the effective spheroidal Lorentz cavity model. The anisotropy γa and the mean value $\\overline \\gamma$ of the molecular polarizability of triphenylene were investigated for several wavelengths. The experimentally observed changes of molecular-optical properties in columnar phase seem to be affected by the induction mechanism of the variety of the molecular polarizability with increasing of S, proposed previously for calamitic liquid crystals. Furthermore, these properties are not related to the effects of aliphatic tails disordering.

Dynamics and Phase Transitions in Discotic and Calamitic Liquid Crystal Side-chain Polymers

We have investigated the effect of structure modification of various discotic liquid crystal side chain polymers on their phase transitions and the resulting dynamics. This has resulted in a better understanding of the structure of the various nematic phases that can be formed, in particular giving rise to the identification of supramolecular thermotropic nematic phases where the fundamental units comprise assemblies of molecules. In addition we have examined the dynamics of field-induced order parameters in the isotropic phase of calamitic side chain polymers using dielectric relaxation spectroscopy and dynamic Kerr effect measurements.

Esters derived from 7-decanoyloxychromone-3-carboxylic acid: synthesis and mesomorphic properties

The synthesis and liquid crystalline properties of a new series of calamitic liquid crystals containing 4-H-benzopyran-4-one (chromone) within the central core is reported. The first homologue in the series display SmA phase (8a), the homologues (8b–e) exhibit SmA and SmC mesophases and the homologues (8f–h) exhibit only a SmC mesophase.

Shear alignment behavior of nematic solutions induced by ultralong side-group liquid crystal polymers.

Addition of a low concentration of a very long (430 kg/mol) side group liquid crystal polymer is shown to produce dramatic changes in the flow characteristics of a calamitic nematic liquid crystal. This polymer causes a typical flow-aligning nematic liquid crystal to align near the velocity gradient direction rather than near the velocity direction, corresponding to having a tumbling parameter lambda<-1, for concentrations greater than 7.5% polymer. Such flow-aligning behavior has not been reported previously in a calamitic nematic. The large molecular weight of the present polymer relative to those examined in the prior literature is responsible for these new phenomena.

Liquid crystalline macrocycles containing phenylpyrimidine unitsElectronic supplementary information (ESI) available: the procedures and analytical data for all compounds and a table with selected bond distances and bond angles of the crystal structure of compound C are available as supplementary data. See http://www.rsc.org/suppdata/jm/b2/b210271d/

Several different macrocyclic liquid crystals consisting of two calamitic 2-phenylpyrimidine or 5-phenylpyrimidine units connected at both ends by polyether or alkyl chains have been synthesised by macrocyclisation reactions under high dilution conditions. The nature of the rigid core has a strong impact on the liquid crystalline phases formed. All 2-phenylpyrimidine paracyclophanes show nematic phases, whereas for the series of the 5-phenylpyrimidine derivatives smectic A-phases are exclusively formed. This behaviour is related to conventional calamitic phenylpyrimidine liquid crystals, but the mesophases are strongly stabilised in the macrocycles. An exchange of polyether chains by alkyl chains leads to significant mesophase stabilisation, whereas increasing the spacer length reduces mesophase stability. Pre-organization of the calamitic cores and micro-segregation of chemically distinct molecular parts are discussed as reasons for the observed effects of molecular structure on the mesophase behaviour of these compounds.

Influence of the structure of the mesogenic core on the thermotropic properties of ω-unsaturated fluorinated liquid crystals

Three series of calamitic liquid crystals have been prepared, consisting of a mesogenic core attached to which is a perfluorinated chain via a thioester linkage, and a hydrocarbon chain containing a terminal double bond. The rigid core is either a monophenyl, biphenyl or phenyl benzoate group. The mesomorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of the mesogenic core and of the hydrocarbon chain length on mesomorphic behaviour was studied. Increasing the length of the alkyl chain strongly reduces the mesomorphic behaviour while increasing the number of aromatic rings in the core increases the transition temperatures, with the widest LC range observed for derivatives with the phenyl benzoate core. The introduction of a single ring as the mesogenic core is considered of great interest in the development of low cost liquid crystal materials.