Mesogenic Properties Research Articles

New room temperature nematogens by cyano tail termination of alkoxy and alkylcyanobiphenyls and their anchoring behavior on metal salt-decorated surface

ABSTRACTA series of cyano tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesized and mesogenic properties described. Comparison with the K series and M series indicates that the terminal cyano group generally enhances the supercooling of the molecules. Furthermore, several binary LC mixtures formed by the cyano tail-terminated compounds were found exhibiting promising room temperature nematic phases ranges comparable to the commercial quaternary mixture E7. The equimolar binary mixture of CN5OCB and CN7OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which is consistent with GBE values we calculated. As such, these materials are promising candidates for sensor devices which display a rapid response to a variety of analytes.

Synthesis of Dendrimers Liquid Crystalline Materials based on 1,3,5-triphenylbenzene

In this study, new compounds of discotic liquid crystalline dendrimers are prepared which is derived from 1,3,5-triphenylbenzene. The synthesis of these compounds shows the effect of dendrimer unit on the mesogenic properties. The structures of prepared compounds characterized by using spectroscopic method e.g. FT-IR and 1HNMR. The liquid crystalline phases of prepared compounds was identified by using the hot-stage polarizing optical microscope (POM) and differential scanning calorimeter (DSC).

Synthesis, mesogenic and photoisomerization studies of (E)-4-[(4-pentyloxyphenyl)diazenyl]phenyl alkanoates

ABSTRACTHomologous series of esters of (E)-4-[(4-pentyloxyphenyl)diazenyl]phenol and aliphatic carboxylic acids were synthesized, 5O-m for short. Derivatives with alkyl chains longer than seven carbon atoms are described in this article for the first time. The mesogenic properties and phase transition parameters (temperatures and enthalpies) were measured using a polarizing microscopy and a differential scanning calorimetry. Investigated compounds exhibit occurrence of the enantiotropic nematic phase, despite two derivatives with the longest alkyl chains. One can observe two crystalline phases in two derivatives (5O-6, 5O-7). The presence of the azo group in the homologous series of compounds suggests that the possibility of trans–cis isomerization is controlled by light. Such studies have been carried out and their results suggest that the presence of two trans–cis isomerization mechanisms described as the first-order reaction.

Synthesized and mesomorphic properties of cholesterol end-capped poly(ε-caprolactone) polymers

Cholesterol end-capped linear and star-shaped poly(e-caprolactone) (PCL) polymers were prepared by a combination of copper catalyzed azide–alkyne cycloaddition (CUAAC, click chemistry) and ring-opening polymerization (ROP) techniques. The chemical structures of these polymers were determinated by Fourier Transform Infrared (FTIR), 1H NMR, 31P NMR, and Gel Permeation Chromatography (GPC). Mesogenic properties of these polymers were examined by wide and small-angle X-ray scattering (WAXS and SAXS), differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The results revealed that PCLs have focal conic textures of the smectic liquid crystals, encouraging the mesomorphism. The mesomorphic temperature ranges became widen with the increasing amount of cholesterol moiety in the polymers.

Synthesis and Characterization of Chiral azo LCPs

Precursor polymer MP series were synthesized by 4-allyloxybenzoic acid cholesteryl ester (M) and 4- hydroxyphenyl 4- (propionyloxy) benzoate (P). We obtained novel themotropic side-chain liquid crystal polymer MPZ series by esterification between MP series and 4-((4-ethoxyphenyl) diazenyl) benzoic acid (Z). Differential scanning calorimetry (DSC) was used to measure the thermal properties of those polymers, and the mesogenic properties were characterized by polarized optical micrograph (POM), DSC and X-Ray. MP series exhibited yellow, green, blue and purple Grandjean textures. MPZ series also showed colorful Grandjean textures. They exhibited thermotropic liquid crystal properties in a broad mesogenic region.

Synthesis and properties of fluorine tail-terminated cyanobiphenyls and terphenyls for chemoresponsive liquid crystals

ABSTRACTA series of fluorine tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesised and mesogenic properties described. Comparison with the non-fluorinated K series and M series indicates that the terminal fluorine atom generally decreases the transition temperatures and, more interestingly, depresses the formation of a smectic phase. Several binary LC mixtures formed by the fluorine tail-terminated compounds were found exhibiting promising room temperature nematic phases with wide ranges. The mixture F7OCB and F8OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which may have potential application in designing a more sensitive and faster LC system to targeted analytes.

Cholesteryl Liquid Crystals as Oil-Based Lubricant Additives: Effect of Mesogenic Phases and Structures on Tribological Characteristics.

Mechanical operation could be seriously affected by friction and controlling it by oil lubrication has been considered as an effective way. Good lubricant additives are very necessary to avoid the friction damages, and to find or design new additives is always a challenge. In this study, a systematic investigation of using cholesteryl liquid crystals (LCs) as lubricant additives to obtain exceptional tribological behaviors was performed. In total, four cholesteryl LC compounds were synthesized targetedly and their thermal and mesogenic properties were studied to see the inherent relationship between the mesogenic phases and antifriction and antiwear performance. Through a series of tribological and related tests, including the UMT TriboLab test, three-dimensional optical microscopy, oil film thickness and viscosity tests, etc., the effect of the mesogenic phases and structures of the synthesized cholesteryl LCs on their tribological properties as lubricant additives was investigated and a related mechanism was analyzed. The result showed that within and close to the mesogenic phase temperature ranges, which we called as effective temperature ranges of LC additives ( TEF), the LCs presented better tribological behaviors, meaning they could be used in special lubrication applications that need to be confined in certain temperature scopes; however, the ester groups with long alky tails could help dissolve in base oils and adsorb onto the friction pairs. Among the four LCs, LC-D with a long perfluoroalkyl tail brought widest mesogenic phase with considerably enhanced lubrication performance and increased oil film thickness, viscosity, and thermal stability, indicating that the perfluoroalkyl group could be well used in the structural modification of LC additives to give unexpected tribological performance. This study, in conjunction with our experimental data, suggested that the liquid crystals may be evaluated as potential friction modifiers for temperature-controllable lubrication and also shed a fresh light on the development of novel liquid crystal lubrication materials.

Photoresponsive Halogen-Bonded Liquid Crystals: The Role of Aromatic Fluorine Substitution

A new strategy for controlling the liquid crystalline and photophysical properties of supramolecular mesogens assembled via halogen bonding is reported. Changing the degree of fluorination at the halogen-bond donor of the supramolecular liquid crystal allows for the fine-tuning of the halogen bond strength and thereby provides control over the temperature range of the mesophase. At least three fluorine atoms have to be present to ensure efficient polarization of the halogen-bond donor and the formation of a mesophase. In addition, it was found that stilbazole acceptors are superior to their azopyridine counterparts in promoting stable liquid crystalline phases. The halogen-bond-driven supramolecular liquid crystals between fluorinated azobenzenes and stilbazole/azopyridine acceptors show a rich variety of photoinduced processes driven by azobenzene photoisomerization, dictated not only by the photochemical properties of the molecular components but also by the difference between the operation temperature ...

Blends of Two Perylene Derivatives: Mesogenic Properties and Application As Emitter Materials in OLEDs

Blends consisting of two components, namely a liquid crystalline perylene ester and a bilaterally extended perylene ester, were studied. The liquid crystalline properties of these blends were investigated in detail by means of polarized optical microscopy and differential scanning calorimetry. The resulting phase diagram was used to explore the potential of these blends as emitter layers in OLEDs, which were prepared via thermal evaporation in a vacuum and spin coating of solutions.

Wide-Range Columnar and Lamellar Photoluminescent Liquid-Crystalline Lanthanide Complexes with Mesogenic 4-Pyridone Derivatives.

A series of liquid crystals with various lanthanide ions (EuIII , SmIII , and TbIII ) was designed and prepared starting from the corresponding lanthanide nitrates and N-alkylated 4-pyridone derivatives bearing mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl=hexyl, octyl, decyl, dodecyl, tetradecyl, or hexadecyl). These new lanthanidomesogens were investigated for their mesogenic properties by a combination of differential scanning calorimetry, polarizing optical microscopy, and temperature-dependent powder X-ray diffraction (XRD). Their thermal stability was assessed by thermogravimetric analysis. All of these complexes show enantiotropic liquid-crystalline behavior with lamellar (SmA) phases in the case of shorter-chain complexes (C6 and C8 ) or hexagonal columnar phases (Colh ) for complexes with longer alkyl chains (C12 , C14 , and C16 ), which were assigned on the basis of their characteristic textures and XRD studies. For complexes with an intermediate number of carbon atoms in the side chains (C10 ), both a lamellar phase at lower temperatures and a Colh phase at higher temperatures were evidenced. In the solid state, all these complexes show characteristic emissions assigned to the corresponding lanthanide ion. In addition, the luminescence decay curves showed single-exponential decays with characteristic times in the millisecond range (0.75-0.90 ms for EuIII , 0.045-0.060 ms for SmIII , and 0.75-1.05 ms for TbIII ).

Multi-arm azobenzene liquid crystal based on cholic acid: synthesis and mesophase properties

ABSTRACTA new series of multi-arm liquid crystals (D1–D5) based on cholic acid as the chiral core and ω-[4-(p-substituted azobenzeneoxy carbonyl]valeric acid (B1–B5) as the mesogenic arms were designed and synthesised. All the mesogenic arms and the multi-arm liquid crystals were structurally well-characterised using elementary analysis (EA), 1H NMR and FT-IR. The photochemical properties were determined by UV spectra. The optical rotations were measured by Automatic Polarimeter. The phase behaviours were investigated by means of polarised optical microscopy, X-ray diffraction and differential scanning calorimetry, while the effect of the terminal groups of azobenzene on mesogenic and photochemical properties were discussed. The results demonstrated that the mesogenic arms exhibited nematic phase except B3. The multi-arm compounds showed chiral nematic except D3. Cholic acid played an important role in inducing chiral nematic of the multi-arm liquid crystals. Tm or Ti of B1–B5 and D1–D5 are influenced by the effect of electrical effects of the terminal groups and the molecular shapes. Furthermore, the multi-arm compounds exhibited well light sensitivity, owning the photoisomerisation of the tran-form azobenzene to the cis-form azobenzene. λmax of D1–D5 displayed blue shift first and then red shift with the different electrical effects of the terminal groups of mesogenic arms.

Design, synthesis and structure of novel G-2 melamine-based dendrimers incorporating 4-(n-octyloxy)aniline as a peripheral unit.

Background: 4-(n-Octyloxy)aniline is a known component in the elaboration of organic materials with mesogenic properties such as N-substituted Schiff bases, perylene bisimide assemblies with a number of 2-amino-4,6-bis[4-(n-octyloxy)phenylamino]-s-triazines, amphiphilic azobenzene-containing linear-dendritic block copolymers and G-0 monomeric or dimeric dendritic liquid crystals with photochromic azobenzene mesogens. The present ab initio study explores a previously unknown use of 4-(n-octyloxy)aniline in the synthesis, structure and supramolecular behaviour of new dendritic melamines.Results: Starting from 4-(n-octyloxy)aniline, seven G-2 melamine-based dendrimers were obtained in 29–79% overall yields. Their iterative convergent- and chemoselective synthesis consisted of SN2-Ar aminations of cyanuric chloride and final triple N-acylations and Williamson etherifications (→ G-2 covalent trimers) or stoichiometric carboxyl/amino 1:3 neutralisations (→ G-2 ionic trimers). These transformations connected G-1 chloro- and amino-termini dendrons to m-trivalent cores (triazin-2,4,6-triyl and benzene-1,3,5-triyl units) or tripodands (central building blocks), such as N-substituted melamines with 4-hydroxyphenyl or phenyl-4-oxyalkanoic motifs. Owing to the diversity of cores and central building blocks, the structural assortment of the dendritic series was disclosed by solvation effects (affecting reactivity), rotational stereodynamism and self-organisation phenomena (determining a vaulted and/or propeller macromolecular shape in solution). DFT calculations (in solution), (VT) NMR and IR (KBr) spectroscopy supported these assignments. TEM analysis revealed the ability of the title compounds towards self-assembling into homogeneously packed spherical nano-aggregates.Conclusions: The (non)covalent synthesis and step-by-step structural elucidation of novel G-2 melamine dendrimers based on 4-(n-octyloxy)aniline are reported. Our study demonstrates the crucial influence of the nature (covalent vs ionic) of the dendritic construction in tandem with that of its central building blocks on the aptitude of dendrimers to self-organise in solution and to self-assembly in the solid state.

Emergent chirality in achiral liquid crystals: insights from molecular simulation models of the behaviour of bent-core mesogens

ABSTRACTWe report insights into the behaviour of achiral bent-core liquid crystals using (a) atomistic models of the PnOPIMB series of mesogens and (b) simple coarse-grained models that demonstrate the spontaneous transfer of chirality between molecules by preferential selection of chiral conformations. The models explain the unusual phenomena that chiral interactions can lead to an increase in chirality (decrease in pitch) of a bulk cholesteric liquid crystal when doped with an achiral material. We demonstrate also that chiral interactions are sufficiently strong to induce nanophase separation of an achiral mesogen within a liquid crystal phase, into nanodomains of opposite handedness by preferential selection of conformations that are instantaneously chiral (i.e. nanophase separation of and conformations of a molecule that is strictly achiral). We suggest that this unusual behaviour contributes to the templating of chiral structures in bulk phases and, hence, to many of the extraordinary properties of bent-core mesogens that include the formations of helical nanofilaments and the chiral dark conglomerate phase.

Synthesis and Mesogenic Properties of Liquid Crystals Based on Cholesterol and Cinnamic Acid Derivatives

A group of new mesomorphic compounds with cholesterol-based, which have derivative of the cinnamic acid, with different lengths of terminal alkoxy chains, are synthesized and characterized. The mesomorphic properties were investigated by DSC and POM. All target compounds showed a cholesteric phase. For 12-TC and 14-TC besides a cholesteric phase, also revealed a SmA* phase. The experimental results demonstrated that with the increase in the terminal chain lengths, the thermal stability of the smectic mesophases of these liquid crystals was increased, and these compounds shows brilliant color changes in the temperature ranges of cholesteric phase.

Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine alkanoate esters

A novel series of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine alkanoate esters were synthesized and their mesogenic properties were studied using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The impact of changing the tail-core linkage from alkyl or alkoxy to ester is profound. Compared to the alkyl or alkoxy linkages, the ester linkage reduced mesogenic properties. Short-tailed compounds are non mesogenic (4a-4e), while long-tailed compounds (4f-4r) exhibit nematic phases. Unlike the alkyl or alkoxy tail series, none of the 18 presented esters in this series exhibits a smectic phase.

Design, synthesis and liquid crystalline behavior of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoates

ABSTRACTA series of ethyl 4-((4-alkoxyphenyl)diazenyl)benzoate is synthesized and the liquid crystalline behavior studied. Synthesized compounds are characterized by FTIR, 1H-NMR, Mass and single crystal XRD. Differential scanning calorimetry was carried out to study the phase transitions and enthalpy changes. Polarizing optical microscopy revealed the mesogenic properties. These techniques revealed that three compounds (2a, 2b, 2f) exhibit liquid crystalline properties in the range of 80–104°C. POM exhibiting focal conical fan like texture which revealed the presence of smectic phases suggestive of their use in LCD and temperature sensing devices, besides 2c can be used as green emitter in OLEDs.

2-(4-Biphenylyl)-1,3,4-oxadiazoles: synthesis and mesogenic studies

ABSTRACTA series of 2-(4-biphenylyl)-1,3,4-oxadiazoles bearing alkyl or alkoxy substituents at the 4ʹ position, and a hydrogen, or alkyl substituent (linear, branched, fluorinated or functionalised) at the 5 position were synthesised and characterised (51 compounds). Their mesogenic properties were evaluated using differential scanning calorimetry and polarising optical microscopy. Results show that substitution of the 1,3,4-oxadiazole ring for a terminal benzene ring in a conventional p-terphenyl enriched the mesogenic properties. Most of the new oxadiazole compounds exhibit either monotropic or enantiotropic smectic A mesophases with widths ranging from 3 to 50°C. A few compounds (short tailed) exhibit nematic phases and some compounds with branched or fluorinated alkyl substituents at the 5 position are also mesogenic.

Formation of Anisotropic Liquid Crystalline Nanoparticles via Polymerization-Induced Hierarchical Self-Assembly.

Polymeric nanoparticles (NPs) containing liquid crystalline (LC) mesogens with tunable anisotropic morphologies have applications in various fields, but their preparation typically suffers from tedious and low-throughput approaches. Here we present an efficient route to the preparation of anisotropic morphologies of azobenzene-containing block copolymers (BCPs) at high solids content via a polymerization-induced hierarchical self-assembly in ethanol. Various anisotropic NPs, including cuboids, short belts, lamellae, and ellipsoidal vesicles, have been obtained in a remarkably broad range of BCP compositions. The NPs exhibit a smectic phase with ordered stripes when observed under TEM. This internal LC ordering plays a significant role on the formation of these intriguing anisotropic morphologies. Morphological transitions from anisotropic to isotropic spheres can be obtained upon UV illumination due to the photoresponsive properties of the azobenzene mesogens. This work significantly expands the scope of accessible morphologies in PISA and suggests that the under explored LC BCPs may have an impactful role in the PISA field.

Optical and thermal characterization of double hydrogen bonded liquid crystals: Binary mixtures

ABSTRACTMethyl Malonic acid (MM) forms hydrogen bonds with octyloxy benzoic acid (8OBA), nonyloxy benzoic acid (9OBA), decyloxy benzoic acid (10OBA) labeled as MM+8OBA, MM+9OBA, and MM+10OBA respectively, all three mesogens exhibit mesogenic properties. Nine binary mixtures are prepared with MM+8OBA and MM+9OBA by varying their molar proportions in steps of 0.1 to 0.9. Similarly nine more binary mixtures are prepared with MM+8OBA and MM+10OBA. Two binary series namely MM+8OBA & MM+9OBA and MM+8OBA & MM+10OBA are analyzed. Various chemical and physical studies of these binary mixtures are investigated which includes Nuclear Magnetic Resonance spectral studies, FTIR, Polarizing Optical Microscopy and Differential Scanning Calorimetry. Phase diagram of above two binary series are constructed. Thermal studies derived from the DSC data include evaluation of order of phase transition, odd even effect, thermal stability factor, thermal equilibrium and variation of specific heat with temperature.

Effect of alkyl chain and linking units on mesophase transitions and molecular order of rod-like thiophene mesogens:13C NMR investigation

This study provides insight into the influence of alkyl chains and linking units on the mesophase properties of thiophene-based mesogens and on the orientational constraints of the core and terminal alkyl chain.