Revealing the antipolar order in the antiferroelectric SmZA phase by means of circular alignment.

Many ferroelectric nematic liquid crystals, like one of the archetype materials, DIO, do not have a direct paraelectric N to ferroelectric NF phase transition, but exhibit yet another phase between N and NF. This phase has recently been proposed to be antiferroelectric, with a layered structure of alternating polarization normal to the average director and is sometimes referred to as Smectic ZA (SmZA). We have examined the SmZA phase in circularly rubbed (CR) cells, known to discriminate between the polar NF and the non-polar N phase from the configuration of disclination lines formed. We find that the ground state of SmZA has the same disclination configuration as the non-polar N phase, demonstrating that the SmZA phase is also non-polar, i.e., it has no net ferroelectric polarization. At the same time, the SmZA texture generally has a grainy appearance, which we suggest is partly a result of the frustration related to layered order combined with the imposed twist in CR cells. We discuss possible orientations of the smectic layers, depending on the alignment conditions. While a horizontal SmZA layer structure is always compatible with surface-induced twist, a vertical layer structure would tend to break up in a twisted bookshelf structure to match non-parallel alignment directions at the two surfaces.

Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts.

Ionic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF6 based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.

Time Dependence of Gel Formation in Lyotropic Nematic Liquid Crystals: From Hours to Weeks.

The combination of lyotropic liquid crystals (LLCs) and low-molecular-weight gelators (LMWGs) for the formation of lyotropic liquid crystal gels (LLC gels) leads to a versatile and complex material combining properties of both parent systems. We gelled the calamitic nematic NC phases of a binary and ternary system using the LMWG 3,5-bis-(5-hexylcarbamoyl-pentoxy)-benzoic acid hexyl ester (BHPB-6). This binary system consists of the surfactant N,N-dimethyl-N-ethyl-1-hexadecylammonium bromide (CDEAB) and water, whereas the ternary system consists of the surfactant N,N,N-trimethyl-N-tetradecylammonium bromide (C14TAB), the cosurfactant n-decanol, and water. Though containing similar surfactants, the gelled NC phases of the binary and ternary systems show differences in their visual and gel properties. The gelled NC phase of the binary system remains clear for several days after preparation, whereas the gelled NC phase of the ternary system turns turbid within 24 h. We investigated the time evolution of the gel strength with oscillation rheology measurements (a) within the first 24 h and (b) up to two weeks after gel formation. The shape of the fibers was investigated over different time scales with freeze fracture electron microscopy (FFEM). We demonstrate that despite their similarities, the two LLC gels also have distinct differences.

New examples of ferroelectric nematic materials showing evidence for the antiferroelectric smectic-Z phase.

We present a new ferroelectric nematic material, 4-((4'-((trans)-5-ethyloxan-2-yl)-2',3,5,6'-tetrafluoro-[1,1'-biphenyl]-4-yl)difluoromethoxy)-2,6-difluorobenzonitrile (AUUQU-2-N) and its higher homologues, the molecular structures of which include fluorinated building blocks, an oxane ring, and a terminal cyano group, all contributing to a large molecular dipole moment of about 12.5 D. We observed that AUUQU-2-N has three distinct liquid crystal phases, two of which were found to be polar phases with a spontaneous electric polarization Ps of up to 6 µC cm-2. The highest temperature phase is a common enantiotropic nematic (N) exhibiting only field-induced polarization. The lowest-temperature, monotropic phase proved to be a new example of the ferroelectric nematic phase (NF), evidenced by a single-peak polarization reversal current response, a giant imaginary dielectric permittivity on the order of 103, and the absence of any smectic layer X-ray diffraction peaks. The ordinary nematic phase N and the ferroelectric nematic phase NF are separated by an antiferroelectric liquid crystal phase which has low permittivity and a polarization reversal current exhibiting a characteristic double-peak response. In the polarizing light microscope, this antiferroelectric phase shows characteristic zig-zag defects, evidence of a layered structure. These observations suggest that this is another example of the recently discovered smectic ZA (SmZA) phase, having smectic layers with the molecular director parallel to the layer planes. The diffraction peaks from the smectic layering have not been observed to date but detailed 2D X-ray studies indicate the presence of additional short-range structures including smectic C-type correlations in all three phases-N, SmZA and NF-which may shed new light on the understanding of polar and antipolar order in these phases.

Mesogenic Groups Control the Emitter Orientation in Multi-Resonance TADF Emitter Films.

The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL =492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs.

Towards Nematic Phases in Ionic Liquid Crystals - A Simulation Study.

Ionic liquid crystals (ILCs) are soft matter materials with broad liquid crystalline phases and intrinsic electric conductivity. They typically consist of a rod-shaped mesogenic ion and a smaller spherical counter-ion. Their mesomorphic properties can be easily tuned by exchanging the counter ion. ILCs show a strong tendency to form smectic A phases due to the segregation of ionic and the non-ionic molecular segments. Nematic phases are therefore extremely rare in ILCs and the question of why nematic phases are so exceptional in existing ILCs, and how nematic ILCs might be obtained in the future is of vital interest for both the fundamental understanding and the potential applications of ILCs. Here, we present the result of a simulation study, which highlights the crucial role of the location of the ionic charge on the rod-like mesogenic ions in the phase behaviour of ILCs. We find that shifting the charge from the ends towards the centre of the mesogenic ion destabilizes the liquid crystalline state and induces a change from smectic A to nematic phases.

Observation of a uniaxial ferroelectric smectic A phase.

We report the observation of the smectic AF, a liquid crystal phase of the ferroelectric nematic realm. The smectic AF is a phase of small polar, rod-shaped molecules that form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the ∼10 Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field ∼2 × 105 V/m is observed. The SmAF phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic (N)-smectic ZA (SmZA)-ferroelectric nematic (NF)-SmAF phase sequence, and 7N/DIO, exhibiting an N-SmZA-SmAF phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers.

Dynamic light scattering analysis of size-selected graphene oxide 2D colloids fractioned via liquid crystal phase separation.

Exfoliated platelets of graphene oxide (GO) can be considered as polydisperse 2D colloids that form nematic colloidal liquid crystal phases in aqueous suspension even at very low concentrations thanks to their extremely high aspect ratios. However, with the rapidly emerging scientific interest in these GO-based liquid crystals, it became clear that the precise analysis and control of the GO sheet size distribution is essential, both for their scientific understanding and for potential applications, e.g., in optoelectronic devices, nanocomposites, or catalysis. In this work, we show that the mean effective (hydrodynamic) GO platelet width can be determined from the translational diffusion coefficient with depolarized dynamic light scattering by using a model for circular, infinitely thin disks. We further studied the phase separation process of biphasic isotropic-nematic GO dispersions and developed a simple fractionation protocol, which can be used to prepare relatively monodisperse fractions of GO sheets with widths ranging from 2.0-12.4 µm. Overall, we expect that the combined application of these relatively simple fractionation and analysis methods will advance the fabrication of well-defined and size-selected GO-based systems.

Chasing Self-Assembly of Thioether-Substituted Flavylium Salts in Solution and Bulk State.

Two series of flavylium triflates carrying alkoxy side chains in the A-ring (benzo unit of chromylium salt) and thioethers in the B ring (phenyl unit) (On -Fla-Sm ) as well as thioethers at both A and B ring (Sn -Fla-Sm ) were synthesized in order to understand the effect of thioether functionalization on their self-assembly and electronic properties. Concentration-dependent and diffusion ordered (DOSY) NMR experiments of O1 -iV-Fla-S3 indicate the formation of columnar H-aggregates in solution with antiparallel intracolumnar stacking of the AC unit (chromylium) of the flavylium triflate, in agreement with the solid state structure of O1 -V-Fla-S1 . Thioether substitution on the B ring changes the linear optical properties in solution, whereas it has no effect on the A ring. According to differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction bulk self-assembly of these ionic liquid crystals (ILCs) depends on the total number of side chains, yielding SmA and LamCol phases for ILCs with 2-3 chains and Colro , Colh phases for ILCs with 3-6 chains. Thus, we demonstrated that thioethers are a useful design tool for ILCs with tailored properties.

Current Topics in Ionic Liquid Crystals.

Ionic liquid crystals (ILCs), that is, ionic liquids exhibiting mesomorphism, liquid crystalline phases, and anisotropic properties, have received intense attention in the past years. Among others, this is due to their special properties arising from the combination of properties stemming from ionic liquids and from liquid crystalline arrangements. Besides interesting fundamental aspects, ILCs have been claimed to have tremendous application potential that again arises from the combination of properties and architectures that are not accessible otherwise, or at least not accessible easily by other strategies. The current review highlights recent developments in ILC research, starting with some key fundamental aspects. Further subjects covered include the synthesis and variations of modern ILCs, including the specific tuning of their mesomorphic behavior. The review concludes with reflections on some applications that may be within reach for ILCs and finally highlights a few key challenges that must be overcome prior and during true commercialization of ILCs.

Mechanisms and Intermediates in the True Liquid Crystal Templating Synthesis of Mesoporous Silica Materials.

Mesoporous silica materials (MSMs) produced by true liquid crystal templating (TLCT) are often considered as direct inverted replicas of the initial lyotropic liquid crystal (LLC) phase. However, the predictive design of tailor-made MSMs requires the full knowledge of the TLCT process, which is still incomplete. Here, we tackle this issue by monitoring the structural evolution during the templating process by small-angle X-ray scattering, showing that after the addition of the silica source the reaction mixture is first isotropic and then an intermediary liquid crystal phase appears, which is the key to the success of the templating process, namely the formation of ordered MSMs. We analyze the structure and the formation dynamics of this intermediary phase and present a simple theoretical model, which allows us to connect the structural parameters of the initial LLC and the MSM. These results provide an enhanced understanding of the TLCT process and are an important step toward the predictable synthesis of new MSMs in the future.

Interplay between Confinement, Twist Elasticity, and Intrinsic Chirality in Micellar Lyotropic Nematic Liquid Crystals.

Recent studies have shown that lyotropic nematic liquid crystals (LLCs) are exceptional in their viscoelastic behavior. In particular, LLCs display a remarkable softness to twist deformations, which may lead to chiral director configurations under achiral confinement despite the absence of intrinsic chirality. The twisted escaped radial (TER) and the twisted polar (TP) are the two representative reflection symmetry breaking director configurations in the case of cylindrical confinement with homeotropic anchoring. We demonstrate how such reflection symmetry breaking of micellar LLCs under cylindrical confinement is affected by intrinsic chirality, introduced by the addition of a chiral dopant. Similarities and differences between the effects of intrinsic chirality on the defect-free TER configuration, and on the TP configuration incorporating two half-unit twist disclination lines, are discussed. In the TP case, topological constraints facilitate stable heterochiral systems even in the presence of a small amount of chiral dopant, with unusual regions of rapidly reversing handedness between homochiral domains. At moderate dopant concentrations, the TP structure becomes homochiral. At high dopant concentrations, for which the induced cholesteric pitch is much smaller than the diameter of the capillary, the cholesteric fingerprint structure develops.

Micellar Lyotropic Nematic Gels.

Lyotropic liquid crystal (LLC) gels are a new class of liquid crystal (LC) networks that combine the anisotropy of micellar LLCs with the mechanical stability of a gel. However, so far, only micellar LLC gels with lamellar and hexagonal structures have been obtained by the addition of gelators to LLCs. Here, the first examples of lyotropic nematic gels are presented. The key to obtain these nematic gels is the use of gelators that have a non-amphiphilic molecular structure and thus leave the size and shape of the micellar aggregates essentially unchanged. By adding these gelators to lyotropic nematic phases, an easy and reproducible way to obtain large amounts of lyotropic nematic gels is established. These nematic gels preserve the long-range orientational order and optical birefringence of a lyotropic nematic phase but have the mechanical stability of a gel. LLC nematic gels are promising new materials for elastic and anisotropic hydrogels to be applied as water-based stimuli-responsive actuators and sensors.

Synergistic structures in lyotropic lamellar gels.

In this work we present a systematic study on the microstructure of soft materials which combine the anisotropy of lyotropic liquid crystals with the mechanical stability of a physical gel. Systematic small-angle neutron (SANS) and X-ray (SAXS) scattering experiments were successfully used to characterize the lyotropic lamellar phase (Lα) of the system D2O -n-decanol - SDS which was gelled by two low molecular weight organogelators, 1,3:2,4-dibenzylidene-d-sorbitol (DBS) and 12-hydroxyoctadecanoic acid (12-HOA). Surprisingly, a pronounced shoulder appeared in the scattering curves of the lamellar phase gelled with 12-HOA, whereas the curves of the DBS-gelled Lα phase remained almost unchanged compared to the ones of the gelator-free Lα phase. The appearance of this additional shoulder strongly indicates the formation of a synergistic structure, which neither exists in the gelator-free Lα phase nor in the isotropic binary gel. By comparing the thicknesses of the 12-HOA (25-30 nm) and DBS (4-8 nm) gel fibers with the lamellar repeat distance (7.5 nm), we suggest that the synergistic structure originates from the minimization of the elastic free energy of the lamellar phase. In the case of 12-HOA, where the fiber diameter is significantly larger than the lamellar repeat distance, energetically unfavored layer ends can be prevented, when the layers cylindrically enclose the gel fibers. Interestingly, such structures mimic similar schemes found in neural cells, where axons are surrounded by lamellar myelin sheets.

Extremely small twist elastic constants in lyotropic nematic liquid crystals.

Recent measurements of the elastic constants in lyotropic chromonic liquid crystals (LCLCs) have revealed an anomalously small twist elastic constant compared to the splay and bend constants. Interestingly, measurements of the elastic constants in the micellar lyotropic liquid crystals (LLCs) that are formed by surfactants, by far the most ubiquitous and studied class of LLCs, are extremely rare and report only the ratios of elastic constants and do not include the twist elastic constant. By means of light scattering, this study presents absolute values of the elastic constants and their corresponding viscosities for the nematic phase of a standard LLC composed of disk-shaped micelles. Very different elastic moduli are found. While the splay elastic constant is in the typical range of 1.5 pN as is true in general for thermotropic nematics, the twist elastic constant is found to be one order of magnitude smaller (0.30 pN) and almost two orders of magnitude smaller than the bend elastic constant (21 pN). These results demonstrate that a small twist elastic constant is not restricted to the special case of LCLCs, but is true for LLCs in general. The reason for this extremely small twist elastic constant very likely originates with the flexibility of the assemblies that are the building blocks of both micellar and chromonic lyotropic liquid crystals.

Gelation of Lyotropic Liquid-Crystal Phases-The Interplay between Liquid Crystalline Order and Physical Gel Formation.

We present a systematical investigation of gelled lyotropic liquid crystals (LLCs). This new class of soft materials combines the anisotropy of LLCs with the mechanical stability of a physical gel. The studied LLC system consists of sodium dodecyl sulfate as a surfactant, n-decanol as a cosurfactant, and water as a solvent. At room temperature, four liquid crystalline phases (lamellar Lα, nematic Nd and Nc, and hexagonal H1) are formed depending on the composition. We were successful in gelling the lyotropic lamellar phase with the low-molecular-weight organogelator 12-hydroxyoctadecanoic acid (12-HOA). The obtained gelled lamellar phase shows optical birefringence, elastic response, and no macroscopic flow. However, we were not able to obtain gels with hexagonal or nematic structure. These findings can be explained twofold. When gelling the hexagonal phase, the long-range hexagonal order was destroyed and an isotropic gel was formed. The reason might be the incompatibility between the gel fiber network and the two-dimensional long-range translational order of the cylindrical micelles in the hexagonal phase. Otherwise, the lyotropic nematic phase was transformed into an anisotropic gel with the lamellar structure during gelation. Evidently, the addition of the gelator 12-HOA to the lyotropic system considerably widens the lamellar regime because the integration of the surface-active 12-HOA gelator molecules into the nematic micelles flattens out the micelle curvature. We further investigated the successfully gelated Lα phase to examine the impacts of the gel network and the remaining monomeric gelator on both the structure and properties of the gelled lamellar phase. Small-angle X-ray scattering results showed an arrested lamellar layer spacing in the gelled state, which indicates a higher translational order for the gelled lamellar phases in comparison with their gelator-free counterparts.

Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals.

Thermotropic ionic liquid crystals based on the flavylium scaffold have been synthesized and studied for their structure-properties relationship for the first time. The mesogens were probed by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Low numbers of alkoxy side chains resulted in smectic (SmA) and lamello-columnar (LamCol ) phases, whereas higher substituted flavylium salts showed Colro as well as ordered and disordered columnar (Colho , Colhd ) mesophases. Mesophase width ranged from 13 K to 220 K, giving access to room temperature liquid crystals. The optical properties of the synthesized compounds were probed towards absorption and emission properties. Strong absorption with maxima between 444 and 507 nm was observed, and some chromophores were highly emissive with quantum yields up to 99 %. Ultimately, mesogenic and dye properties were examined by temperature-dependent emissive experiments in the solid state.

Molecular Electron Density Distribution and X-Ray Diffraction Patterns of Smectic A Liquid Crystals - A Simulation Study.

X-ray diffraction (XRD) is one of the most important methods to assess the long-range translational order in smectic A (SmA) liquid crystals. Nevertheless, the knowledge about the influence of the molecular electron density distribution (MEDD) on the XRD pattern is rather limited because it is not possible to vary the orientational order, the translational order and the MEDD independently in an experiment. We here present a systematic simulation study in which we examine this effect and show that the MEDD indeed has a major impact on the general appearance of the XRD pattern. More specifically, we find that the smectic layer peaks and the intensity ratios thereof strongly depend on the width of the MEDD. The classic approach by Leadbetter et al. to determine the smectic translational order parameter ∑ from XRD intensities works if the MEDD is quite narrow. In all other cases the influence of the MEDD has to be taken into account.

Self-Assembly and Fluorescence of Tetracationic Liquid Crystalline Tetraphenylethene.

A series of tetraguanidinium tetraphenylethene (TPE) arylsulfonates with different chain lengths was prepared via ionic self-assembly of tetraguanidinium TPE chloride and the respective methyl arylsulfonates. Liquid crystalline properties were studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. Tetraguanidinium TPE arylsulfonates with chain lengths of C8 -C12 displayed hexagonal columnar mesophases over a broad temperature range, while derivatives with longer chains showed oblique columnar phases. In solution all compounds displayed aggregation-induced emission behaviour. Temperature-dependent luminescence spectra of the bulk phase of the tetraguanidinium TPE arylsulfonate with C14 side chains revealed a strong luminescence both in the solid state and the oblique columnar mesophase. The emission behaviour was rationalized by a unique combination of restriction of intramolecular rotation of the TPE core, Coulomb interaction between the guanidinium cations and π-π interactions of the anionic arylsulfonate moieties.

The Influence of Carbosilane Nanosegregation on the Dynamics in 'de Vries-type' Liquid Crystals.

The mesogens QL32-6, QL33-6 and QL-34-6 contain 5-phenylpyrimidine cores and terminal nanosegregating carbosilane end groups of different lengths and are known to exhibit 'de Vries-type' properties of varying strength. We report a systematic study of the influence of the nanosegregating sublayer on the dynamics and rotational viscosities of the collective modes in the smectic A* (SmA*) and smectic C* (SmC*) phase using dielectric spectroscopy. It was found that the dynamics of the Goldstone mode corresponding to phase angle fluctuations are almost not affected while the relaxation time and rotational viscosity of the soft mode are influenced by the degree of nanosegregation. In other words, the nanosegregating sublayer does not influence the dynamics of ferroelectric switching in the SmC* phase, but is critical in inducing 'de Vries-type' properties.