Rod-like Mesogens Research Articles

Liquid Crystal Promoted Self-Assembly of Statistical Copolymers into Diverse Nanostructures with Precise Dimensions.

In both natural and synthetic systems, the segregation of multicomponent entities is vital for regulating functions and the ultimate usage of materials. To accomplish the desired properties via nanosegregation or microphase separation, great effort is usually demanded in the synthesis. For example, microphase-separated block copolymers rely on the delicate controlled/living polymerization of different monomers in sequence. Here, we demonstrate that a facile one-pot copolymerization can generate statistical side-chain copolymers exhibiting well-defined and diverse nanostructures. Two hemiphasmidic (or wedge-shaped) cyclooctene monomers were designed, differing in the peripheral tails of the wedges (dodecyl vs. tetraethylene glycol), with lengths of ca. 1 nm. When combining the two monomers together, the statistical copolymers can show columnar liquid crystal (LC) phase and microphase-separated structures of the two monomers, including sphere, cylinder, double gyroid, and lamella. To the best of our knowledge, this is the first time the gyroid phase has been achieved in statistical copolymers. We further demonstrate that changing the side chains to calamitic (or rod-like) mesogens or the backbone to less flexible polynorbornene, the statistical copolymers can also undergo microphase separation of the side chains. The intrinsic self-assembly scheme of statistical copolymers with mesogenic side chains, which are chemically accurate, affords the resultant nanostructures with precise periodicities at the 10- or sub-10-nm scale. Given the small chemical difference between the side-chain tails, microphase separation is promoted by the anisotropic packing of mesogens. It is validated that the statistical side-chain LC copolymers can be a versatile platform for creating nanostructured materials with tailored functionalities.

13C NMR studies and orientational order of rod-like mesogens with strongly polar terminal cyano group

ABSTRACT 13C NMR studies of two rod-like mesogens with core units consisting of two and three phenyl rings with strongly polar terminal cyano groups are reported. The two-ring mesogen displayed an enantiotropic smectic A (SmA) mesophase while for the three-ring mesogen, enantiotropic nematic and SmA mesophases were observed. The solution 13C NMR studies of both the mesogens are carried out besides static 13C NMR measurements in the liquid crystalline phase. For the two-ring mesogen, the chemical shift anisotropy (CSA) tensors of the core unit are determined by the 2D-separation of undistorted powder patterns by effortless recoupling (SUPER) experiment. Furthermore, 2D-separated local field (SLF) measurements are realised for both the mesogens in the liquid crystalline phase to determine the 13C-1H dipolar couplings to find the orientational order parameters of the phenyl rings. The main order parameter (Szz) of the two-ring mesogen varies from 0.63 to 0.53 across the SmA phase and for the three-ring mesogen, the Szz values are found to be ~0.73 in the SmA phase and ~0.69 in the nematic phase. The CSA values determined from the SUPER experiment are expected to be a significant utility for molecules with polar terminal nitro/cyano groups as ferroelectric nematic phases are realised in them.

Reconfigurable Visible Light-Driven Liquid Crystalline Network Showing Off-Equilibrium Motions Enabled by Mesogen-Grafted Donor-Acceptor Stenhouse Adducts.

Liquid crystalline network (LCN)-based soft actuators have opened up great opportunities to fabricate emerging and intriguing smart materials, serving as potential building blocks for intelligent soft robotics. Endowing LCN actuators with complex responsive behaviors to enhance their intelligence is both challenging and highly demanded. Herein, Donor-Acceptor Stenhouse Adducts (DASAs) molecules with rod-like mesogen and the polymerizable group are judiciously designed and synthesized, which is strong-colored at linear form and de-coloration at cyclic form after visible light. In the colored state, the DASA presents a striking photothermal effect that is capable of driving the motions of LCN film. Upon visible light irradiation, the DASA becomes colorless, making the diminishing photothermal effect. The light-gated switching of the photothermal effect renders the LCN films to be reconfigurable and perform off-equilibrium motions. The varying glass transition temperature of LCN matrix endowing tunable isomerization rates of DASAs and the equilibrium balance of photo- and thermal-isomerization at different temperatures in LCN-P-DASA film mainly guiding the off-equilibrium or stable motions, providing high adjustability of the novel visible light-driven LCN actuators. The multiply modulated LCN-P-DASA film holds great potential in constructing complex visible light-driven soft actuators based on the synergetic effect and interactions of photochemical and photothermal effects.

Thermotropic "Plumber's Nightmare"-A Tight Liquid Organic Double Framework.

Gyroid, double diamond and the body-centred "Plumber's nightmare" are the three most common bicontinuous cubic phases in lyotropic liquid crystals and block copolymers. While the first two are also present in solvent-free thermotropics, the latter had never been found. Containing six-fold junctions, it was unlikely to form in the more common phases with rod-like cores normal to the network columns, where a maximum of four branches can join at a junction. The solution has therefore been sought in side-branched mesogens that lie in axial bundles joined at their ends by flexible "hinges". But for the tightly packed double framework, geometric models predicted that the side-chains should be very short. The true Plumber's nightmare reported here, using fluorescent dithienofluorenone rod-like mesogen, has been achieved with, indeed, no side chains at all, but with 6 flexible end-chains. Such molecules normally form columnar phases, but the key to converting a complex helical column-forming mesogen into a framework-forming one was the addition of just one methyl group to each pendant chain. A geometry-based explanation is given.

The influence of positional isomerism of a cyano group of pyrene-based rod-like mesogens on mesomorphism, dual-state emission and multiple applications

Two novel cyanostilbene-based rod-like mesogens consisting of one terminal pyrene and one terminal alkyl chain were prepared by Suzuki coupling and Knoevenagel reactions. The influence of the positional isomerism of the cyano group of cyanostilbene unit on the liquid crystalline characteristics, photophysical characteristics, and mechanochromism characteristics was studied by using polarized optical microscopy, differential scanning calorimetry, X-ray diffraction, absorption spectra, photoluminescence spectra, density functional theory calculation and time-dependent density functional theory calculation. The mesogen in which the cyano group is adjacent to the phenylpyrene unit exhibits a monotropic smectic C phase, whereas the mesogen in which the cyano group is far away from the phenylpyrene unit exhibits an enantiotropic smectic C phase. The mesogen in which the cyano group is adjacent to the phenylpyrene unit exhibits more distinct positive solvatochromism due to more distinct intramolecular charge transfer. The mesogen in which the cyano group is adjacent to the phenylpyrene unit exhibits aggregation-induced emission behavior, but the mesogen in which the cyano group is far away from the phenylpyrene unit exhibits dual-state emission behavior. Reversible mechanochromism behavior could be achieved in both mesogens due to twisted molecular configurations. In addition, the rewritable luminescent paper and bioimaging applications were also achieved. These results demonstrated that the distinct molecular design could endow organic molecules with multifunctional properties and potential applications.

Optical and calorimetric studies near the smectic-A to smectic-C tricritical point

Detailed heat capacity and birefringence measurements have been performed to investigate the transitional behavior near the orthogonal smectic-A to tilted smectic-C (Sm-A–Sm-C) phase transition on several liquid crystalline mixtures comprising of a rod-like mesogen and a hockey stick-shaped compound having substitution of methyl group in the lateral position. Both probing methods were found to be rather successful in assessing the phase transitional behavior with reasonably good accuracy. The data shows a divergent pretransitional excess above and below the transition temperature. Analysis of the data have been performed in detail with the renormalization-group expression with correction-to-scaling terms. The extracted effective critical exponents were observed in between the tricritical and 3D-XY limit. Detailed investigations carried out in this work reveal the dependence of the Sm-A–Sm-C phase transition on the width of the Sm-A phase and the existence of a broad tricritical range.

Impact of the position of nitrogen atom on the liquid crystal, optical and multi-stimuli response behaviors of rod-like mesogens containing multiple functional building blocks

Two novel rod-like mesogens containing one cyanostyrene, one pyridine and one pyrene as π-conjugate rigid core were designed and synthesized by Suzuki coupling and Knoevenagel reactions. The effect of the position of nitrogen-atom on the thermodynamic behavior, liquid crystalline self-assembly, optical behavior, mechanochromism behavior and acidochromism behavior was investigated. The rod-like mesogen in which the nitrogen atom is adjacent to the pyrene displays a phase transition from smectic A to smectic C by increasing temperature, whereas the other rod-like mesogen can only self-assemble into smectic C phase. This difference in mesophase transition behavior might be attributed to the position of nitrogen-atom which induces the change in molecular configuration. Both mesogens exhibit positive solvatochromism and high fluorescence in both solution state and aggregated state which might be attributed to the intramolecular charge transfer and twisted molecular configurations, respectively. Reversible mechanochromism behavior due to changes in molecular stacking and acidochromism behavior due to protonation and deprotonation were also achieved. These multi-stimuli responsive behaviors of the designed rod-like mesogens demonstrate potential applications in the field of multifunctional materials.

Vapor-to-glass preparation of biaxially aligned organic semiconductors.

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180K below the clearing point TLC-iso (and 50K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

Molecular Conformations of Shape Anisometrically Variant Mesogens in Liquid Crystalline Phase Studied by 13C NMR Spectroscopy.

Mesogens that vary in shape anisometry have been investigated by 13C solid-state NMR in the liquid crystalline phase to inspect the conformations. The molecules examined comprise of (i) rod-like mesogen with three-phenyl ring core and terminal hexyloxy chains, (ii) three-ring core linked to the fourth phenyl ring via a spacer, and (iii) trimesic acid connected to three side arms core units through a spacer. The order parameter (Szz) values for the phenyl rings of the rod-like mesogen are 0.65-0.68, while the mesogen with a three-ring core linked to a phenyl ring via spacer showed dissimilarity. Consequently, for the core unit's phenyl rings, Szz is ~ 0.70, and the terminal phenyl ring showed a low value of 0.12. For the trimesic acid based mesogen, the Szz value for the side arm phenyl rings is ~ 0.53, and for the central phenyl ring, a very low value of 0.11 is witnessed. By considering the ordering of the rod-like mesogen as a yardstick and employing the ratios of Szz values of the phenyl rings, the average conformations of other mesogens are arrived. Accordingly, for the trimesic acid based mesogen, a tripod-like conformation instead of l shape is proposed in the liquid crystalline phase.

Flexible films of main-chain luminescent liquid crystalline polymers containing renewable biobased bifuran moieties

Luminescent liquid crystalline polymers (LLCPs) have attracted great attention due to their potential applications in liquid crystal display, optical memory storage, organic light-emitting diode, and other fields. However, the fluorescence signals of LLCPs bearing conventional luminophores showing aggregation-caused quenching behavior are strikingly diminished or even vanish in the aggregated state. Herein, we have synthesized three main-chain LLCPs with α-cyanostilbene (CS) and bifuran units connected by different flexible spacer lengths via melt polycondensation. On the one hand, the CS unit acts as a rigid rod-like mesogen with aggregation-induced emission (AIE) characteristics. On the other hand, the renewable biobased bifuran moiety has been chosen because of two main reasons. One is it is a perfect alternative to the rigid fossil-based biphenyl group. The other reason is the addition of bifuran moiety is beneficial for the polycondensation and the extension of the chain length, leading to the enhancement of the toughness of LLCPs. Upon 365 nm UV light irradiation, these polymers undergo an irreversible Z/E isomerization in solution, while the [2 + 2] cycloaddition of CS chromophores occurs in the solid film. Importantly, the polymers display a good film-forming ability, so that the flexible thin films can be achieved by ordinary compression molding. Moreover, the fluorescent patterns can be directly printed on the flexible films on the basis of the [2 + 2] cycloaddition behavior, which shows their promising applications ranging from information storage to high-tech anti-counterfeiting.

Fluorinated esters with a very broad temperature range of the antiferroelectric phase

ABSTRACT Antiferroelectricity is a desirable property of liquid crystalline materials. Therefore, we synthesised and studied two new chiral rod-like mesogens with a molecular core based on two biphenyls connected via ester linkage, with the phenyl substituted by a fluorine atom. The studied mesogens are characterised by nuclear magnetic resonance spectroscopy and mass spectrometry analysis. They have methyl heptyl in the chiral chain and exhibit an antiferroelectric phase in a very broad temperature range. We investigated their mesomorphic properties and confirmed the phase identification by differential scanning calorimetry and broad-frequency dielectric spectroscopy measurements. Additionally, we compared the studied mesogens with previously synthesised analogous materials. Two mixtures were formulated using a base mixture and new mesogens. The helical pitch of the synthesised mesogens and formulated mixtures was estimated using the selective reflection method.

Molecular organization and molecular order of two rod-like smectogens in mesophases

ABSTRACT Structurally simple two rod-like mesogens with a three-ring core consisting of biphenyl and phenyl rings are synthesised to investigate the mesophase properties, molecular organisation and the orientational order parameter. In mesogen-1, a dodecyloxy terminal chain is present on the biphenyl ring while the phenyl ring is free from substituent. For mesogen-2, in addition to the dodecyloxy chain, the butyl chain is placed on the phenyl ring. The Hot-stage optical polarising microscopy (HOPM) and differential scanning calorimetry (DSC) measurements show smectic A (SmA), smectic B (SmB), and crystal E (CryE) phases for mesogen-1 and SmA, smectic C (SmC) and smectic F (SmF) phases for mesogen-2. The presence of smectic mesophases is established by X-ray Diffraction (XRD). 1D and 2D 13C solid-state NMR experiments are performed in smectic mesophases to find the orientational order parameters of the phenyl rings of the core. The main order parameter (Szz) of the mesogens in the SmB and SmF phase is in the range of 0.87–0.93. The excellent spectral resolution noticed in 13C NMR spectra in the SmB mesophase (mesogen-1) indicates that the internal rotations of the molecules along the long axis are primarily unhindered.

The influence of core fluorination on the phase behaviour of rod-like mesogens

ABSTRACT The effect of core fluorination on the mesomorphic properties of rod-like Schiff base-derived liquid crystalline materials was reported. For this purpose, two new series of laterally fluorinated Schiff base liquid crystals are synthesised and investigated in detail. The two series differ from each other’s in the length of the terminal alkoxy chain attached to one end of the aromatic core, where in the first series an octyloxy chain is used and in the second series it is replaced by a longer dodecyloxy chain. At the other end in both series of compounds, in addition to the nonsubstituted end (X = H) four different substituents were used (X = CH3O, C6H13O, CH3 and F). The molecular structures of all synthesised compounds were confirmed using different spectroscopic techniques. The molecular self-assembly was investigated using differential scanning calorimetry (DSC) and polarised optical microscope (POM). Depending on the length of terminal alkoxy chain at one end and nature of the substituent X at the other end, different types of mesophases were observed, including nematic and two different types of smectic phases.

Influence of Thiophene Ring on Molecular Order of Structurally Simple π-Conjugated Smectogens:13C NMR Study.

Structurally simple rod-like π-conjugated mesogens with thiophene directly connected to phenyl, biphenyl, and fluorenone rings with terminal chains are synthesized respectively.The occurrence of smectic A/smectic C phases is concurred by a hot-stage optical polarising microscope (HOPM), differential scanning calorimetry (DSC), and X-Ray diffraction (XRD). The static 1D and 2D13C nuclear magnetic resonance (NMR) studies in the liquid crystalline phase are carried out to find the alignment-induced chemical shifts (AIS) and13C-1H dipolar couplings.The orientational order parameters of the mesogens determined from13C-1H dipolar couplings disclose that the long axis is not only collinear to the C3-C4 bond of the thiophene ring but also for the local axes of phenyl and biphenyl rings.For fluorenone-based mesogen, the molecular biaxiality is found to be high owing to the increased breadth of the molecule. The study unveils that the orientation of thiophene and the phenyl rings is similar in the current mesogens in stark contrast to mesogens, where thiophene is connected to phenyl rings through linking groups.

The Effect of Mesogenic Coronas on the Type and Anisotropy of Gold Nanoparticle Superlattices: When Can the Tail Wag the Dog?

The correlation between the size of nanoparticles, the structure and shape of mesogenic ligands and the ensuing assembly behaviour is not really understood. Closer inspection shows very surprising features. Here, 2- and 4-nm gold nanoparticles (NPs) were synthesized, and grafted with a forked ligand containing two rod-like mesogens in its branches: one cholesterol, the other with azobenzene. The 4-nm NPs also contained n-hexylthiol as co-ligand. They were found to form a FCC cubic superlattice, whereas the 2-nm NPs form hexagonal HCP with weak birefringence, hence with partially oriented ligands. The structures were compared with those of related systems containing a range of different azobenzene-to-cholesterol ratios, all giving body-centred tetragonal superlattices with various degrees of anisotropy. Geometric analysis is presented in terms of the asphericity of the NPs' surroundings, requirement for space-filling and structural anisotropy. Some general rules are derived to help design the soft corona around the NPs in order to obtain superlattices with the desired structure and anisotropy.

Optical anisotropy of retarder film made from mixtures of rodlike and T-shaped reactive molecules: dependence on the chemical structure of T-shaped molecules

Five kinds of T-shaped reactive molecules were newly synthesized for optical retarder films. The retarder films were fabricated using the mixtures of rodlike reactive mesogen (RM) and the T-shaped molecules. The effect of the chemical structure of the T-shaped molecules on the optical anisotropy of the retarder films was investigated. The retarder films with the T-shaped molecules resulted in a greater magnitude of an in-plane retardation. Among the five kinds of the T-shaped molecules, the molecule with five benzene rings along the longitudinal direction and two benzene rings along the lateral molecular direction showed the widest liquid crystalline phase and the largest optical retardation. In addition, the NZ coefficients of the films were greater than 1 implying that the films were negative biaxial.

13C NMR investigations and order parameters of rod-like molecules with terminal phenyl and thiophene rings in mesogenic core✰

Rod-like mesogens with the rigid core constituted by a combination of phenyl and thiophene moieties reveal interesting orientational ordering behavior. In this article, four rod-like mesogens constructed with a terminal alkoxy chain on only one side of the mesogenic core with the other end terminated with either a phenyl or thiophene ring have been investigated using 13C NMR spectroscopy. The mesogen with the terphenyl moiety in the core exhibited polymesomorphism with nematic, smectic C, and smectic I mesophases with very high clearing temperatures. The replacement of terphenyl with terthiophene moiety resulted in suppression of polymesomorphism, with only nematic mesophase being observed. Mesogens with single terminal thiophene connected to two phenyl rings in the core also showed only the nematic mesophase for both the possibilities of linking the core to the thiophene ring at the 2- and 3- positions. One- and two- dimensional 13C NMR spectra have been obtained in the mesophases from which the alignment induced chemical shifts of the ring carbons and 13C-1H dipolar couplings, as well as the local order parameters, have been obtained. The considerable difference in 13C chemical shifts and the 13C-1H dipolar couplings of thiophene ring with change in position of the linking unit is attributed to the difference in the order parameters of the thiophene moiety between the two cases. The data obtained on the ordering of phenyl and thiophene rings in the core units offered information into the effect of replacing the phenyl ring with the thiophene ring. So also, the change of position of the link to the thiophene moiety provided important insights into the orientational behavior in the liquid crystalline phase and on the molecular structure.

Amphiphilic α-cyanostilbene mesogens with oligooxyethylene polar tails: syntheses and properties

ABSTRACT A series of amphiphilic rodlike α-cyanostilbene mesogens bearing an α-cyanostilbene core, with one/three lipophilic and flexible alkyl chains at one end and a polar oligooxyethylene (EO) at the opposite end, have been synthesized and investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray scattering, scanning electron microscope (SEM), UV-vis spectroscopy and photoluminescence as well as CV measurements. These compounds can self-assemble into photoresponsive liquid crystalline phases and possess AIEE characteristics, mechanofluorochromic properties and Fe3+ recognition properties. The conductivity of the lithium cation complex was also measured to be 2.6 × 10−7 S m−1. Such compounds can be considered as a kind of multifunctional materials.

Simple molecular model for ferroelectric nematic liquid crystals exhibited by small rodlike mesogens.

Nematic liquid crystals (NLCs) are the prime example of a liquid medium with an apolar orientational order. In the past couple of years, the ferroelectric nematic (FN) phase has been discovered in some compounds with small rodlike molecules with large longitudinal dipole moments and very restricted chemical structures, as the temperature is lowered from the NLC. We propose a simple model in which the molecules are idealized as cylindrical rods with longitudinal surface charge density waves. The usually strong electrostatic inter-rod interactions favoring antiparallel structures are shown to be subdued in magnitude, and those of parallel structures enhanced, by reducing the amplitudes of the half-waves at both ends of the rods. By introducing an additional increased amplitude of one interior wave, the energy per rod of a cluster of molecules with a pseudohexagonal order is shown to favor the ferroelectric order compared to the antiparallel order, below some value of the inter-rod separation. The model broadly accounts for the restriction in molecular structures for a compound to exhibit the FN phase. It is suggested that the weakly first-order nature of the NLC to FN transition arises from a coupling of the polar order and the density of the medium.

Ultrastable liquid crystalline blue phase from molecular synergistic self-assembly

Fabricating functional materials via molecular self-assembly is a promising approach, and precisely controlling the molecular building blocks of nanostructures in the self-assembly process is an essential and challenging task. Blue phase liquid crystals are fascinating self-assembled three-dimensional nanomaterials because of their potential information displays and tuneable photonic applications. However, one of the main obstacles to their applications is their narrow temperature range of a few degrees centigrade, although many prior studies have broadened it to tens via molecular design. In this work, a series of tailored uniaxial rodlike mesogens disfavouring the formation of blue phases are introduced into a blue phase system comprising biaxial dimeric mesogens, a blue phase is observed continuously over a temperature range of 280 °C, and the range remains over 132.0 °C after excluding the frozen glassy state. The findings show that the molecular synergistic self-assembly behavior of biaxial and uniaxial mesogens may play a crucial role in achieving the ultrastable three-dimensional nanostructure of blue phases.