Showing papers in "Liquid Crystals in 2009"

Liquid crystal oligomers: going beyond dimers

Abstract: This review focuses on structure-property relationships in liquid crystal oligomers, which consist of molecules containing two or more mesogenic units linked via flexible spacers essentially in a linear fashion and so does not consider, for example, liquid crystal dendrimers and tetrapodes. Previous reviews have tended to focus mainly on liquid crystal dimers in which just two mesogenic units are interconnected by a single spacer. By contrast, this review is largely devoted to higher oligomers such as liquid crystal trimers and tetramers containing three or four mesogenic units connected by two or three spacers, respectively.

Landau–de Gennes modelling of nematic liquid crystal colloids

Abstract: Phenomenological Landau–de Gennes modelling based on the free energy of nematic liquid crystal colloids is reviewed. Nematic phase, gradient of order, and surface anchoring contributions to the total free energy are used. The numerical finite difference relaxation technique is explained as an efficient tool for the minimisation of the free energy. Effects of the mesh and mesh allocation are discussed. Various conceptually different colloidal structures are calculated to show the universality of the model. Single particles, dipolar–quadrupolar dimers, entangled dimers, dimers bound by escaped hyperbolic rings, and hierarchically patterned Saturn-ring colloidal superstructures are presented.

Dual frequency liquid crystals: a review

Abstract: We review the recent progress in developing high performance dual frequency liquid crystal (DFLC) materials. A new figure of merit is derived for evaluating the DFLC performance. The dielectric relaxation and electro-optical properties of several high performance compounds, which possess a positive dielectric anisotropy at low frequencies and a relaxation frequency in the kilohertz region, are presented. Potential applications of these compounds are discussed. Compounds with a negative dielectric anisotropy were also compared and discussed. Two experimental DFLC mixtures were demonstrated with one intended for room temperature operation and another for elevated temperature operation.

Molecular complexity and the control of self-organising processes

Abstract: In this article we investigate the complexity of the molecular architectures of liquid crystals based on rod-like mesogens. Starting from simple monomeric systems founded on fluoroterphenyls, we first examine the effects of aromatic core structure on mesophase formation from the viewpoint of allowable polar interactions, and then we model these interactions as a function of terminal aliphatic chain length. By incorporating a functional group at the end of one, or both, of the aliphatic chains we study the effects caused by intermolecular interfacial interactions in lamellar phases, and in particular the formation of synclinic or anticlinic modifications. We then develop these ideas with respect to dimers, trimers, tetramers, etc. We show, for dendritic systems, that at a certain level of molecular complexity the local mesogenic interactions become irrelevant, and it is gross molecular shape that determines mesophase stability. The outcome of these studies is to link the complexity of the molecular interac...

Why do non-symmetric dimers intercalate? The synthesis and characterisation of the α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)alkanes

Abstract: Ten new non-symmetric liquid crystal dimers belonging to the family of compounds α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)-alkanes have been synthesised and their transitional properties characterised. The dimers contain either a hexamethylene or octamethylene spacer, while the terminal substituents on the 4-benzylideneaniline fragment are H, CH3, F, Cl and Br. The unsubstituted dimers are not liquid crystalline, while the remaining compounds exhibit enantiotropic nematic behaviour. The trends in the clearing temperatures, according to the chemical nature of the terminal substituent, are largely consistent with those established for conventional low molar mass liquid crystals. Three of the dimers also exhibit an intercalated smectic A phase, specifically the two bromo-substituted dimers and the chloro-substituted dimer containing a hexamethylene spacer. The driving force for the formation of this phase is considered to be, at least in part, the specific aniso...

Nematicons: self-localised beams in nematic liquid crystals

Abstract: We review the impressive progress on light self-localisation into spatial optical solitons in nematic liquid crystals. We summarise the basic physics and models, outlining the main properties of nematicons and providing an overview of the most significant experimental achievements in undoped, planar, twisted and chiral nematic liquid crystals.

Changing liquid crystal elastomer ordering with light – a route to opto-mechanically responsive materials

Abstract: Long, flexible chains with rods and spacers give nematic polymer melts. Crosslinking yields highly extensible nematic elastomers. Molecular fluidity and orientational order are retained. Uniquely, these elastomers change shape drastically when orientational order is lost with temperature since chains lose their elongation. Reversible changes of 400% are easy to achieve. de Gennes envisaged this coupling between macroscopic strain and nematic (and other) liquid crystalline order. Dye molecular rods are excited by photons into bent conformations. Bent rods depress orientational order and large thermal shape changes will be duplicated by light. Equally, rods may rotate away from the optical electric vector to avoid bending, thus also leading to mechanical strain. Mechanical recovery follows decay back to the molecular ground state. We sketch how these effects arise, explain how even polydomain networks deform under polarised and unpolarised light, and present new results on how nuclear magnetic resonance can...

Playing with discs

Abstract: The hierarchical self-assembly of disc-shaped molecules leads to the formation of discotic liquid crystals. These intriguing materials are of fundamental importance not only as models for the study of energy and charge migration in self-organised systems, but also as functional materials for various device applications. This has fostered numerous developments in this field. In this article, we have summarised the author's and the collaborators' research work on discotic liquid crystals.

Liquids with conics

Abstract: A red thread marks the history of liquid crystals from the start; it is the concept of focal conic domains in smectic phases. Discovered by G. Friedel and F. Grandjean early in the beginning of the 20th century, focal conic domains still attract the attention of researchers in the field. The presence of focal conic domains is a touchstone of layered media. They rightly excited the interest of P.-G. de Gennes in a seminal paper about the Apollonius tiling of toric focal conic domains. In this paper we shall review the development of this notion over the last one hundred years, firstly emphasising its importance in the appearance and broadening of the concept of order and of its singularities in liquid crystals and beyond in condensed matter physics, through the renewed notions of dislocations, disclinations, grain boundaries, kinks and focal conic domain imperfections, then describing the empirical evidence of focal conic domains in various liquid crystalline materials, and finally how they have inspired r...

Self-assembling discotic liquid crystal porphyrin into more controllable ordered nanostructure mediated by fluorophobic effect

Abstract: The novel nanoscale discotic liquid crystal porphyrin with partial chain perfluorination, which has the same basic structure as the best photoreceptor in nature (chlorophyll), shows an exceptionally enhanced tendency to self-assemble into ordered nanostructure. Defect-free homeotropically aligned fluorinated porphyrin thin films were, for the first time, fabricated and characterised. The ability to self-assemble large π-conjugated discotic molecules into highly ordered nanostructure via partial chain perfluorination provides new insight for the bottom-up nanofabrication of molecular devices. The controllable ordered porphyrin nanostructure with directed molecular arrangement holds great promise for use in high-performance electronic devices.

Biaxial nematics: symmetries, order domains and field-induced phase transitions

Abstract: We studied the symmetry and spatial uniformity of the orientational order of the biaxial nematic phase in the light of recent experimental observations of phase biaxiality in thermotropic bent-core and calamitic-tetramer nematics. Evidence is presented supporting monoclinic symmetry, instead of the usually assumed orthorhombic symmetry. The use of deuterium nuclear magnetic resonance to differentiate between the possible symmetries is described. The spatial aspects of biaxial order are presented in the context of the cluster model, wherein macroscopic biaxiality can result from the field-induced alignment of biaxial and possibly polar domains. The implications of different symmetries on the alignment of biaxial nematics and on the measurements of biaxial order are discussed in conjunction with the microdomain structure of the biaxial phase.

Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

Abstract: High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are...

Light-responsive wires from side-on liquid crystalline azo polymers

Abstract: The pioneering work of Professor de Gennes in the field of liquid crystalline polymers and elastomers is outlined with an emphasis on artificial muscles. To illustrate the presentation, results on ...

Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties

Abstract: Two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles were synthesised and characterised. Their molecular structures differed wherein the latter comprised a lateral hydroxyl group, unlike the former. Spectroscopic techniques such as FT-IR, 1H & 13C NMR and mass spectrometry together with elemental analysis were employed to elucidate the molecular structures. The transition temperatures and their mesophases were determined by differential scanning calorimetry, optical polarising microscopy and X-ray diffraction techniques. Members with decanoyloxy till hexadecanoloxy chain in the series without the lateral hydroxyl group each exhibited a smectic A phase, while those in the series with the lateral hydroxyl group were non-mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure–mesomorphic properties relationship.

Synthesis and photoswitching properties of bent-shaped liquid crystals containing azobenzene monomers

Abstract: Three novel bent-shaped monomers, namely 1,3-phenylene bis-{4-[4-(n-allyloxyalkyloxy)phenylazo]benzoate} 5a–c, containing azobenzene as side arms, resorcinol as central units and terminal double bonds as polymerisable functional groups were synthesised and characterised. The mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements and it was found that all three compounds display SmAintercal mesophases. These bent-shaped molecules exhibit strong photoisomerisation behaviour in solutions in which trans to cis isomerisation takes about 50 seconds whereas the reverse process takes almost 31 hours.

Flexoelectric effect in a bent-core mesogen

Abstract: The flexoelectricity in a bent-core liquid crystal was evaluated quantitatively using two independent electrooptic methods. The absolute value of the bend flexoelectric coefficient e 3 was measured to be about 15.8 pC m−1. Our results revealed that there is little difference between the flexoelectricity of bent-core mesogens and that of conventional calamitic mesogens, i.e. in the same pC m−1 order. The principles of the methods used are reviewed in detail.

Dielectric spectroscopy of unsymmetrical liquid crystal dimers showing wide temperature range TGBA and TGBC* phases

Abstract: The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4‐n‐decyloxy‐4′‐(cholesteryloxycarbonyl‐1‐butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (∼200–300 Hz). Activation energies for DC conductivity ha...

Synthesis and characterization of new azomethine derivatives exhibiting liquid crystalline properties

Abstract: Azomethine liquid crystals (LCs), constituted by symmetric and asymmetric bithiophene or fluorene units as chromophores, were designed and synthesised. Their thermotropic behaviour was investigated using optical microscopy, differential scanning calorimetry, and wide angle X‐ray diffraction. A lowering of transition temperatures was obtained by the introduction of the lateral methyl group as compared with unsubstituted compounds. The observed LC phases possess the classical textures of calamitic LCs. The optical properties of the target compounds were evaluated, both in solution and in solid state. A blue fluorescence was observed for fluorene containing compounds and a green fluorescence was observed for analogous bithiophene containing compounds. The introduction of a methyl lateral group onto the aromatic rings led to a remarkable increase in the solution emission properties.

Numerical results for the blue phases

Abstract: We review recent numerical work investigating the equilibrium phase diagram, and the dynamics of the cholesteric blue phases. In equilibrium numerical results confirm the predictions of the classic analytical theories, and extend them to incorporate different values of the elastic constants, or the effects of an applied electric field. There is a striking increase in the stability of blue phase I in systems where the cholesteric undergoes helical sense inversion, and the anomalous electrostriction observed in this phase is reproduced. Solving the equations of motion allows us to present results for the phase transition kinetics of blue phase I under dielectric or flexoelectric coupling to an applied electric field. We also present simulations of the blue phases in a flow field, showing how the disclination network acts to oppose the flow. The results are based on the Landau–de Gennes expansion of the liquid crystal free energy: that such a simple and elegant theory can predict such complex and subtle phys...

Nanoscale segregation, molecular recognition and fluorophobic effect – from molecular design towards complex mesophase morphologies

Abstract: Mesomorphic structure formation of hydrogen-bonded complexes of amino substituted 1,3,5-triazines with complementary (semiperfluoro)alkoxybenzoic acids is presented. The substitution pattern of both components was modified systematically in order to elucidate the influence of molecular parameters on the mesophase morphologies of the binary mixed systems. The phase sequence of the triazines, and of their associates with the acids, spans the range from smectic layer structures to discontinuous cubic phases composed of closed inverted micelles. Columnar phases with various two-dimensional lattice symmetries and bicontinuous cubic phases were found as intermediates. The mesophase morphologies are discussed in terms of the microsegregation of rigid polar, lipophilic and fluorinated molecular blocks in different sub-spaces along with tailoring the shape of (curved) aggregates by the space requirement of incompatible molecular fragments.

p-Alkoxybiphenyls with guanidinium head groups displaying smectic mesophases

Abstract: Novel calamitic guanidinium salts with p-alkoxybiphenyl or p-alkoxyphenyl core unit (7, 11, 12) have been synthesised, utilising the corresponding nitro compounds as synthetic intermediates. Differential scanning calorimetry (DSC), polarising optical microscopy (POM) and temperature-dependent X-ray diffraction studies reveal smectic A (SmA) mesophases for all series. Whereas the p-alkoxybiphenyl guanidinum chlorides 7 display SmA phases with a minimum chain lengths of C8 and broad mesophase width from 80°C (for C8 derivative 7b) up to 180°C (for C16 derivative 7g), the corresponding p-alkoxyphenyl guanidinium chlorides 11 display much smaller mesophase width from 4°C (for C10 derivative 11c) up to 72°C (for C16 derivative 11e). For the series 12 a large influence of the anion on the clearing point is observed, with clearing points ranging from 209°C for the guanidinium chloride 7f down to 127°C for guanidinium hexafluorophosphate 12d.

High birefringence phenyl tolane positive compounds for dual frequency liquid crystals

Abstract: Dual frequency liquid crystals (DFLCs) provide a possibility to achieve sub-millisecond rise and decay times and are useful for various electro-optical devices. In most of DFLC mixtures, ester compounds are utilised in order to obtain a low crossover frequency. However, the ester linking group does not provide π electron conjugation so that the DFLC mixtures usually exhibit a limited birefringence (Δn < 0.3 at λ = 633 nm). For phase modulation a lower birefringence demands a thicker cell gap which, in turn, increases the response time and operating voltage. This problem becomes more severe as the wavelength increases. In this paper, we introduce a series of isothiocyanato phenyl tolane positive compounds with high birefringence and moderate dielectric relaxation frequency. The dielectric and electro-optical properties of these compounds are investigated. We also formulated a DFLC mixture using these new positive compounds which exhibits Δn = 0.39 at λ = 633 nm and 0.33 at λ = 1550 nm.

Photomechanical properties of azobenzene liquid-crystalline elastomers

Abstract: We prepared homogeneously aligned azobenzene liquid-crystalline elastomer (LCE) films with low T g and explored their photomechanical properties. Upon irradiation with UV light, the films bent toward a light source at room temperature. The mechanical force generated upon exposure to UV light was evaluated by thermomechanical analysis. It was found that the mechanical force generated by photo-irradiation increased with an increase in the cross-linking density. In particular, an LCE film containing 80 mol% azobenzene cross-linker produced a force of over 1 MPa by photo-irradiation, which enabled the film to lift an object 20 times heavier than itself. The degree of contraction by photo-irradiation increased with an increase in temperature and light intensity. Bending could be brought about by more than 5000 times with periodic irradiation. Furthermore, it was found that the LCE films exhibited bending and unbending behaviour by irradiation with sunlight.

Photoinduced phase transitions

Abstract: Employing actinic light to alter/stabilise a particular thermodynamic phase via the photo-isomerisation of the constituent molecules is an interesting tool to investigate soft matter from a new dimension This article focuses on our recent results on several aspects of these non-equilibrium phase transitions, which are isothermal in nature We specifically discuss (i) the influence of different parameters, such as confinement, applied electric field, pressure etc, on the dynamics associated with both the photochemical transition driving the equilibrium nematic to the non-equilibrium isotropic phase and the thermal back relaxation recovering the nematic phase, (ii) unique light-driven disorder–order transition in a reentrant system, (iii) dynamic self-assembly of the smectic A phase, which is stabilised only in the presence of actinic light, (iv) novel temperature-intensity phase diagrams and an example of primary and secondary photo-ferroelectric effects in an antiferroelectric smectic C system These re

Cholesteric helix: topological problem, photonics and electro-optics

Abstract: Using numerical simulations and the results of our experiments we discuss the behaviour of a defect-free cholesteric liquid crystal in rather a strong electric field. The smooth helix unwinding predicted by de Gennes (Solid State Commun. 1968, 6, 163,) occurs under the thermodynamic equilibrium conditions and can easily proceed with the assistance of multiple defects. The defect-free helix in the field strictly perpendicular to the helical axis cannot be unwound for topological reasons. The field deforms the helix without a change of its period and induces higher harmonics of the helical structure. Due to the latter, the shape of the Bragg photonic stop-band changes and a second-order photonic band is observed in the optical transmission. Moreover, novel laser effects are predicted by modelling for the dye-doped, distorted cholesteric structure. The same field-induced anharmonicity results in a strong change of the polarisation state of the beam passing the helical structure along its axis. Due to fast re...

Optical and thermal properties of unsymmetrical liquid crystalline compounds based on isoxazole

Abstract: The thermal and optical properties of two series of unsymmetrical liquid crystalline compounds based on an isoxazole ring are described. Of these materials, 3–7 and 8–12 displayed strong absorption at 285 nm and a weak blue fluorescence in solution at around 377 nm, even with the presence of a high conjugated core. The fluorescence quantum yields observed varied from low to moderate (Φ F = 1–62%), with large Stokes shifts (60–178 nm). All compounds exhibited liquid crystalline behaviour with a wide mesomorphism temperature range and characteristic phases of calamitic compounds, among them smectic A (SmA), smectic C (SmC) and nematic (N) phases. Their phases were characterised using polarising optical microscopy and differential scanning calorimetry. In addition, with X-ray diffraction experiments, layer spacing of 33.3–40.0 A were observed for the smectic phases.

Novel amides and Schiff's bases derived from 1,3,4-oxadiazole derivatives: synthesis and mesomorphic behaviour

Abstract: The synthesis and liquid crystalline properties of novel achiral amides (Ia–g, IIa–g and IVa,b), achiral Schiff's bases (IIIa–g and Va–g), chiral amides (VI, VII) and chiral Schiff's bases (VIII–XI) incorporating a 1,3,4-oxadiazole ring are reported. All amides of the series I and II display an enantiotropic smectic A phase. The amide IVa,b did not show mesomorphic properties. Amides of the series Ia–g and IIa–g contain a flexible n-tetradecylthio chain, the other terminal substituent is an n-alkoxy chain and n-alkyl chain, respectively (n = 4–10) and the 1,3,4-oxadiazole is in the terminal rigid core. Amides Ia–g have broader mesomorphic range and higher thermal stability than the corresponding amides IIa–g. Amides IVa,d contain the 1,3,4-oxadiazole ring in the centre of the rigid core and two flexible alkoxy chains as flexible terminal substituents. Thus, the mesomorphic properties are favoured if 1,3,4-oxadiazole is shifted to a terminal position of the rigid core. Schiff's bases IIIa–g display an enan...

Optically active, mesogenic lanthanide complexes: design, synthesis and characterisation

Abstract: The synthesis, molecular structural characterisation and mesomorphic behaviour of lanthanidomesogens with the formula [LnL(LH)2 ][X]2 are reported. These mesogens were derived from ligands (LH- n ) formed by covalently linking the pro-mesogenic cholesterol segment with the N-(n-decyl)salicylaldimine core through either an even-parity (4-oxybutanoyloxy/6-oxyhexanoyloxy/8-oxyoctanoyloxy) or an odd-parity (5-oxypentano-yloxy) spacer. These ligands were designed based on the recently conceived concept of decoupling the anisometric segment from the metal-coordinating site by a flexible spacer to account for the stabilisation of nematic and/or smectic phases at lower temperatures. The even parity spacer ligands are polymesomorphic whereas the odd parity analogue exhibits only the chiral nematic phase. In contrast, the complexes display solely the smectic A phase indicating that the variation in the nature of lanthanide has no influence on the general phase behaviour of the complexes. The clearing temperatures o...

Asymmetric oxadiazole mesogens as candidates for low-temperature biaxial nematics

Abstract: We report the synthesis and properties of a homologous series of asymmetric, mesogenic derivatives of 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP). Benzyloxy (BnO-), alkoxy benzoate ester derivatives BnO-ODBP-Ph-O-C n , where n = 4, 5, 6, 7, 8, 9, 12, were studied by 2H-NMR, X-ray diffraction and polarising microscopy in order to ascertain if the lower temperature nematic phases exhibited biaxiality. Deuterium nuclear magnetic resonance (NMR) of labelled probes in these asymmetric mesogens does not show evidence of biaxiality. The absence of biaxiality is discussed in terms of the statistical non-linearity of these asymmetric ODBP mesogens.

Synthesis, single crystal structure and liquid crystalline properties of bent-shaped 2,5-diaryl 1,3,4-oxadiazoles

Abstract: Two bent-shaped 1,3,4-oxadiazole-based compounds, namely 2-[4-(2-(4-methylphenyl)-(E)-1-ethenyl)]phenyl-5-(4-pentyloxyphenyl)-1,3,4-oxadiazole 5a and 2-[4-(2-(4-fluorophenyl)-(E)- ethenyl-5-(4-pentyloxyphenyl)-1,3,4-oxadiazole 5b, were synthesised and their liquid crystalline properties were studied in this paper. Compound 5a exhibited an enantiotropic nematic mesophase, while 5b displayed an enantiotropic smectic A phase. No banana-shaped mesophases were found in these mesogens, although they adopt a bent-shaped molecular structure as confirmed by the single X-ray diffraction crystallography.