Liquid Crystalline Materials Research Articles

Photopolymerisable liquid crystals for additive manufacturing

Liquid crystals are often regard as a fourth state phase of matter alongside solids, liquids and gases. It has been estimated that one out of every ten new synthetic compounds is a liquid crystal, mainly due to the fact that liquid crystals have been used in Liquid Crystalline Displays (LCDs) for over 50 years. Liquid crystals, often referred to as mesogens, exhibit an unique range of ordered mesophases with anisotropic physical properties, such as optical, magnetic and dielectric anisotropy, for example. These highly anisotropic properties and the ability of liquid crystals to self-organize on surfaces, has led to them being, are used in a wide range of non-LCD applications. These applications include photochromics, pressure sensitive paints, optical retarders and optical compensation films, etc. Highly crosslinked, anisotropic polymer networks, derived from photopolymerisable liquid crystals, so-called reactive mesogens (RMs), have enabled solution-fabricated organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic devices (OPVs) for plastic electronics. This review will be focus on the relatively recent application of liquid crystals, i.e., the 3D/4D printing of reactive mesogens and subsequent polymerization to form anisotropic polymer networks in advanced additive manufacturing applications. It will highlight the challenges of the synthesis and use of new liquid crystalline materials in 3D/4D printing for additive manufacturing applications. It will show how the study of the successful development of design strategies for the synthesis of novel liquid crystals for flat panel displays is inspiring the development of novel photopolymerisable liquid crystals for additive manufacturing. It is intended that this review will serve to inspire chemists, with little or no knowledge of liquid crystals, or liquid crystal chemists, to design and synthesize new liquid crystals for 3D/4D printing. Photopolymerisable liquid crystals (reactive mesogens) with advanced functional aromatic cores and two photoreactive endgroups are used to fabricate stimulus-responsive liquid crystalline elastomers (LCEs) using 3D/4D printing techniques, such as fused-deposition modeling (FDM), direct ink writing (DIW), digital light-processing (DLP) and two-photon polymerization (2PP). Reactive mesogens represent a highly promising class of advanced functional for new types of actuators, sensors, soft robotics, drug delivery systems, energy-absorbing and reinforced devices, etc., primarily due to their liquid crystalline properties, such as anisotropic swelling, high elastic modulus and sensitive actuation. • Stimulus-responsive photopolymerisable liquid crystals (PLCs) for additive manufacturing. • Structure-property discussion for PLCs with advanced aromatic cores and photoreactive endgroups. • 3D/4D printing PLCs for application in smart devices (actuators, sensors, soft robotics, etc). • Highly promising PLCs due to anisotropic swelling, high elastic modulus and sensitive actuation.

Advanced Functional Liquid Crystals (Adv. Mater. 23/2022)

Liquid Crystals Liquid-crystalline assemblies of a variety of molecules forming 1D, 2D, and 3D structures lead to new functions. Recent advances in functional liquid-crystalline materials based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are highlighted by Takashi Kato and co-workers in article number 2109063. Design strategies, advanced measurements, computational simulations, and functionalization of these materials and interfaces are discussed.

A Novel Approach for the Creation of Electrically Controlled LC:PDMS Microstructures.

This work presents research on unique optofluidic systems in the form of air channels fabricated in PDMS and infiltrated with liquid crystalline material. The proposed LC:PDMS structures represent an innovative solution due to the use of microchannel electrodes filled with a liquid metal alloy. The latter allows for the easy and dynamic reconfiguration of the system and eliminates technological issues experienced by other research groups. The paper discusses the design, fabrication, and testing methods for tunable LC:PDMS structures. Particular emphasis was placed on determining their properties after applying an external electric field, depending on the geometrical parameters of the system. The conclusions of the performed investigations may contribute to the definition of guidelines for both LC:PDMS devices and a new class of potential sensing elements utilizing polymers and liquid crystals in their structures.

Li3+ ion beam irradiation persuaded enhancement in the thermodynamic parameters of a hydrogen-bonded liquid crystalline material

Effect of ion beam irradiation has been investigated in the thermodynamic parameters of a hydrogen-bonded liquid-crystalline material by differential scanning calorimetry. Thermodynamic data suggest that the phase transition temperatures are decreased for a low fluence of irradiation and thereafter increase with the increase in irradiation fluence. The average transition enthalpies and entropies are increased for a low fluence of irradiation and are decreased for higher fluences of irradiation.

A Ten-Year Perspective on Twist-Bend Nematic Materials.

The discovery of the twist-bend nematic phase (NTB) is a milestone within the field of liquid crystals. The NTB phase has a helical structure, with a repeat length of a few nanometres, and is therefore chiral, even when formed by achiral molecules. The discovery and rush to understand the rich physics of the NTB phase has provided a fresh impetus to the design and characterisation of dimeric and oligomeric liquid crystalline materials. Now, ten years after the discovery of the NTB phase, we review developments in this area, focusing on how molecular features relate to the incidence of this phase, noting the progression from simple symmetrical dimeric materials towards complex oligomers, non-covalently bonded supramolecular systems.

Advanced Functional Liquid Crystals.

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Photo-switching behaviour in liquid crystalline materials incorporating a non-planar dithienylcyclopentene core and their birefringence properties

ABSTRACT Reversible photochromic phase transition of non-planar dithienylcyclopentene-containing liquid crystalline molecules has been studied in this work. All materials showed the smectic A and nematic phases in the cooling process and had reversible and high interconversion yield >90% between colourless-open/blue-closed isomers in the liquid crystalline state as well as in solution by UV- and visible light-irradiation alternatively, as confirmed by nuclear magnetic resonance spectroscopy, polarised optical microscopy, differential scanning calorimetry, powder X-ray diffraction and UV-visible spectroscopy. The thermal stability of both isomers also ensures that they can maintain liquid crystalline phases during ring-open and ring-closure reactions. Their birefringence is between 0.1 and 0.2, which is comparable to that of standard liquid crystalline materials used in optical applications.

Manipulation of Liquid Crystalline Properties by Dynamic Covalent Chemistry─En Route to Adaptive Materials.

Dynamic covalent bonds bear great potential for the development of adaptive and self-healing materials. Herein, we introduce a versatile concept not only for the design of low-molecular-weight liquid crystals but also for their in situ postsynthetic modification by using the dynamic covalent nature of imine bonds. The methodology allows systematic investigations of structure-property relationships as well as the manipulation of the materials' behavior (liquid crystallinity) and the introduction of additional properties (here, fluorescence) by a solvent-free method. For the first time, the transamination reaction is followed by variable-temperature 19F solid-state NMR in the mesophase, providing insights into the reaction dynamics in a liquid crystalline material. Finally, the application potential for the design of liquid crystalline materials with adaptive properties is demonstrated by a sequential combination of these reactions.

Organic liquid crystals in optoelectronic device applications: Field‐effect transistors, nonvolatile memory, and photovoltaics

AbstractWith the growing demands in flexible electronics, the development of plastic and organic materials has gained increased interest. In this focus review, the design concepts and recent advances of organic liquid crystalline materials in optoelectronic devices were introduced. Thermotropic liquid crystalline materials are categorized into three types: calamitic, discotic, and cholesteric types according to the structures of rod‐like, disk‐like, and chiral rod‐like molecules, respectively. Numerous liquid crystalline materials have been successfully incorporated in organic electronic devices. Notably, smectic liquid crystals hold great promise as semiconducting channel, floating gate electret, and charge transporting interlayer in field‐effect transistors, nonvolatile memory, and photovoltaics, respectively. This review sheds light on the great potential and importance of the organic liquid crystalline material for optoelectronic device applications.

Liquid-Crystalline Thermally Activated Delayed Fluorescence: Design, Synthesis, and Application in Solution-Processed Organic Light-Emitting Diodes.

Realizing both high efficiency and liquid crystallinity in one molecule remains a challenge in thermally activated delayed fluorescence (TADF) emission. Herein, two isomeric compounds─m-DPSAc-LC and p-DPSAc-LC with different connection positions between donor and acceptor moieties─were synthesized and characterized. Diphenylsulfone (DPS) was used as the acceptor, acridine (Ac) was used as the donor, and biphenyl derivatives (LC) were employed as the mesogenic group. Both compounds showed a smectic mesophase evidenced by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and temperature-dependent small-angle X-ray scattering (SAXS). The compound p-DPSAc-LC clearly exhibited thermally activated delayed fluorescence due to the much more distorted geometry, whereas m-DPSAc-LC showed simple fluorescence. Compared to the parent TADF molecules without appended mesogenic groups (DPS-Ac), these liquid-crystalline emitters possessed higher hole mobilities and improved device performance. The OLEDs fabricated via solution processing using the liquid-crystalline compound p-DPSAc showed a maximum external quantum efficiency of ∼15% and as such is the first example of a liquid-crystalline TADF material in an OLED device.

New Advanced Liquid Crystalline Materials Bearing Bis-Azomethine as Central Spacer.

In this study, a homologous series of novel liquid crystalline compounds bearing the bis-azomethine central linkage (–CH=N-N=CH–), namely ((1E,1′E)-hydrazine-1,2-diylidenebis(methanylylidene))bis(4,1-phenylene) dialkanoate (In), was synthesized, and the mesophase and thermal properties were investigated theoretically and experimentally. The molecular structures of the prepared compounds were determined using elemental analysis, NMR, and FT-IR spectroscopy. The mesophase transitions were detected by differential scanning calorimetry (DSC), and the mesophases were identified using polarized optical microscopy (POM). The results indicated that the derivative with the shortest length (I5) was purely nematogenic, while the other homologues (I9 and I15) possessed SmC mesophases. The optimal geometrical structures of the investigated group were derived theoretically. The estimated results demonstrated that all homologues were mesomorphic, and their type depended on the length of the terminal chains. Computations based on density functional theory (DFT) were used to explain the experimental data. The calculated dipole moment, polarizability, thermal energy, and molecular electrostatic potential all showed that it was possible to predict the mesophase type and stability, which varied according to the size of the molecule.

Optical sensor for butylamine vapour based on the photonic structure infiltrated by liquid crystal

ABSTRACT We propose an optical sensor based on the inverse opal (IOP) photonic structure infiltrated by a typical liquid crystalline material doped by the reactive compounds. Benefiting from the unique photonic property of the IOP structure, the composite sensor shows not only a typical textural change but also a responsive photonic band gap (PBG) upon the exposure to butylamine (BA) vapour, due to the reorientation of the liquid crystalline molecules induced by the chemical reaction between the dopant and the BA vapour. This approach simplifies the measurement process by capturing the PBG signal instead of the polarised optical texture, which does not require high cost or complex instruments and might be instructive for detecting other volatiles.

Determination of optical birefringence and orientational order parameter of octyloxycyanobiphenyl from a voltage-dependent optical transmission technique

ABSTRACT Measurements of optical birefringence have been performed on mesogen 4-cyano-4′-n-octyloxybiphenyl (8OCB) from a voltage-dependent optical transmission technique. The temperature dependence of birefringence has been determined from the voltage dependence of transmitted intensity through homogeneously aligned (planar) plane-parallel liquid crystal-filled cells of different cell-thicknesses. At a particular temperature, a systematic increase of birefringence has been observed with a decrease in cell-thickness. From the optical birefringence data, related orientational order parameter values have also been calculated. The outcomes for orientational order parameter are compared with those reported from other well-known techniques for the similar mesogen. Close agreement among the temperature dependence of orientational order parameter from this study and those from other techniques validates the reliability of the present optical method in precise investigation of physical characteristics of liquid crystalline materials.

Hydrogen‐Bonded Cholesteric Liquid Crystals—A Modular Approach Toward Responsive Photonic Materials

A supramolecular approach for photonic materials based on hydrogen‐bonded cholesteric liquid crystals is presented. The modular toolbox of low‐molecular‐weight hydrogen‐bond donors and acceptors provides a simple route toward liquid crystalline materials with tailor‐made thermal and photonic properties. Initial studies reveal broad application potential of the liquid crystalline thin films for chemo‐ and thermosensing. The chemosensing performance is based on the interruption of the intermolecular forces between the donor and acceptor moieties by interference with halogen‐bond donors. Future studies will expand the scope of analytes and sensing in aqueous media. In addition, the implementation of the reported materials in additive manufacturing and printed photonic devices is planned.

Capillary Flow Characterizations of Chiral Nematic Cellulose Nanocrystal Suspensions.

Studying the flow-induced alignment of anisotropic liquid crystalline materials is of major importance in the 3D printing of advanced architectures. However, in situ characterization and quantitative measurements of local orientations during the 3D printing process are challenging. Here, we report a microfluidic strategy integrated with polarized optical microscopy (POM) to perform the in situ characterization of the alignment of cellulose nanocrystals (CNCs) under the shear-flow condition of the 3D printer's nozzle in the direct ink writing process. To quantify the alignment, we exploited birefringence measurements under white and monochromatic light. We show that the flow-induced birefringence patterns are significantly influenced by the initial structure of the aqueous CNC suspensions. Depending on the CNC concentration and sonication treatment, various structures can form in the CNC suspensions, such as isotropic, chiral nematic (cholesteric), and nematic (gel-like) structures. In the chiral nematic phase, in particular, the shear flow in the microfluidic capillary has a distinct effect on the alignment of the CNC particles. Our experimental results, complemented by hydrodynamic simulations, reveal that at high flow rates (Er ≈ 1000), individual CNC particles align with the flow exhibiting a weak chiral structure. In contrast, at lower flow rates (Er ≈ 241), they display the double-twisted cylinder structure. Understanding the flow effect on the alignment of the chiral liquid crystal can pave the way to designing 3D printed architectures with internal chirality for advanced mechanical and smart photonic applications.

Inside Front Cover: Mesogens with aggregation‐induced emission properties: Materials with a bright future

Shine bright like a liquid crystal: In this review the recent developments in the fabrication of luminescent liquid crystalline materials are summarized and a first glance on potential applications is given.

Hockey-Stick Polycatenars: Network formation and transition from one dimensional to three-dimensional liquid crystalline phases

Photo switchable liquid crystalline (LC) materials are of great interest for optical and photonic applications. Herein we report the design, synthesis, and molecular self-assembly of the first examples of photosensitive hockey-stick (HS) polycatenars. Therefore, two new series of HSLCs derived from 4-cyanoresorcinol bent-core unit connected to a short azobenzene-based side arm with one variable alkoxy chain and a long ester-based wing terminated with two alkoxy chains at 3 and 5 positions of the terminal benzene ring are reported. They differ from each other in the length of the terminal chains connected to the long arm. The LC self-assembly of these HSLCs was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electro optical investigations. Depending on length of terminal chains a transition from one dimensional (1D) tilted and non-tilted smectic phases to three dimensional (3D) achiral bicontinuous cubic phases with Ia3¯d symmetry (Cubbi/Ia3¯d) upon chain elongation is observed. Moreover, achiral isotropic liquid networks were observed for medium and long chain homologues. Most of mesophases are room temperature LCs phases with wide ranges as observed in the cooling cycles, where once they are formed on heating, no sign of crystallization is detected down to ambient temperature. Finally, UV light irradiation results in fast and reversible photoinduced transformation between different types of LCs phases as well as between LC phase and isotropic liquid.

Marangoni convection driven by temperature gradient near an isotropic-nematic phase transition point.

Marangoni flow driven by a temperature gradient was observed near the isotropic-nematic phase transition point. By applying the gradient to a liquid crystalline material in sandwich cells, it was possible to measure the flow field near the air interface using the photobleaching method. In the isotropic phase, the direction of the observed flow was opposite to that in the nematic phase. Moreover, when the measurement was performed in the coexistence state of these phases, the flow direction depended on the coating materials of the cell substrates. These singular flow properties are explained well by the singular changes in surface tension and the shape of the air interface near the transition point.

Tuning the donor-acceptor interactions in phase-segregated block molecules.

The assembly of donor–acceptor molecules via charge transfer (CT) interactions gives rise to highly ordered nanomaterials with appealing electronic properties. Here, we present the synthesis and bulk co-assembly of pyrene (Pyr) and naphthalenediimide (NDI) functionalized oligodimethylsiloxanes (oDMS) of discrete length. We tune the donor–acceptor interactions by connecting the pyrene and NDI to the same oligomer, forming a heterotelechelic block molecule (NDI-oDMSPyr), and to two separate oligomers, giving Pyr and NDI homotelechelic block molecules (Pyr-oDMS and NDI-oDMS). Liquid crystalline materials are obtained for binary mixtures of Pyr-oDMS and NDI-oDMS, while crystallization of the CT dimers occurred for the heterotelechelic NDI-oDMS-Pyr block molecule. The synergy between crystallization and phase-segregation coupled with the discrete length of the oDMS units allows for perfect order and sharp interfaces between the insulating siloxane and CT layers composed of crystalline CT dimers. We were able to tune the lamellar domain spacing and donor–acceptor CT interactions by applying pressures up to 6 GPa on the material, making the system promising for soft-material nanotechnologies. These results demonstrate the importance of the molecular design to tune the CT interactions and stability of a CT material.

Construction and performance of OLED devices prepared from liquid-crystalline TADF materials

The device performance is reported for three compounds which show both thermally activated delayed fluorescence and liquid crystallinity, and use the donor 3,6-bis(3,4-didodecyloxyphenyl)carbazole. Two of the compounds, whose photophysics were reported previously, are based on a terephthalonitrile acceptor. A third and new compound is based on an isophthalonitrile acceptor and shows a more temperature-accessible mesophase and enhanced solution emission quantum yield. Two of the compounds show device external quantum efficiencies of between 2-3% and exhibit very small efficiency roll off. The responses are evaluated in terms of the specific nature of the materials.