Smectic C Research Articles

Azobenzene-based supramolecular liquid crystals: The role of core fluorination

The impact of core fluorination on the phase behaviour of supramolecular hydrogen-bonded liquid crystals (HBLCs) is investigated in detail. Therefore, different types of HBLCs were synthesized using two benzoic acid derivatives as proton donors, namely, 4-octyloxybenzoic acid and octylbenzoic acid. The two acids were combined through intermolecular hydrogen-bonding with alkoxyazopyridine derivatives as proton acceptors. Three different types of azopyridines were used either without fluorine substitution or with one lateral fluorine substituent at different positions. The study proved the importance of using core fluorination as a significant tool to modify the liquid crystalline behaviour of HBLCs, where all azopyridines are non-mesomorphic and almost all their complexes exhibit enantiotropic mesophases. The formation of the hydrogen bond between the complementary components was confirmed using FTIR and 1H NMR spectroscopy, while the liquid crystalline self-assembly of the HBLCs was investigated in detail using polarized light microscope (PLM) and differential scanning calorimetry (DSC). Depending on the type of the terminal chain on the benzoic acid derivative and on the position of the lateral fluorine substituent different types of mesophases including nematic (N), smectic A (SmA) and smectic C (SmC) phases were observed. Finally, under UV light illumination all the prepared HBLCs show cis-trans photoisomerization resulting in tuning the liquid crystalline phases, which is of importance for industrial applications.

Induced Phases of New H-bonded Supramolecular Liquid Crystal Complexes; Mesomorphic and Geometrical Estimation.

New five rings architecture of 1:1 supramolecular hydrogen bonded (H-bonded) complexes were formed between 4-(2-(pyridin-4-yl)diazenyl-3-methylphenyl 4-alkoxybenzoates and 4-n-alkoxyphenyliminobenzoic acids. Mesomorphic and optical behaviors of three systems designed complexes were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). H-bonded interactions were confirmed via FT-IR spectroscopy. Computational calculations were carried out by density functional theory (DFT) estimation for all formed complexes. Experimental evaluations were correlated with the theoretical predictions and results revealed that, all prepared complexes possessing enantiotropic tri-mesophases with induced smectic C (SmC) and nematic temperature ranges. Moreover, DFT predicted for all formed supramolecular complexes possessing a non-linear bent geometry. Moreover, the π–π stacking of the aromatic rings plays an important role in the mesomorphic properties and thermal stabilities of observed phases. The energy changes between frontier molecular orbitals (HOMO and LUMO) and the molecular electrostatic potential (MEP) of the designed complexes were discussed and related to the experimental results.

Optical Switching between Liquid-Crystalline Assemblies with Different Structural Symmetries and Molecular Orders

Abstract Optical switching between one-dimensionally ordered layered smectic C (SmC) and three-dimensionally periodic bicontinuous cubic (Cubbi) liquid-crystalline (LC) phases is a fascinating challenge because of the dramatic changes of structural symmetries and orders as well as physical properties. In this work, we have developed a new light-driven Cubbi phase system consisting of 4′-n-hexadecyloxy-3′-cyanobiphenyl-4-carboxylic acid (ACBC-16) and a structurally analogous azo compound (Azo-2). The employment of the CN side-group in the host compound brought about outstanding responsiveness under ultraviolet (UV) light in the mixtures containing 11 and 19 mol % Azo-2, leading to a UV response time of ∼5 s and excellent repeatability of on-off switching for the conversion between the two LC phases. The modification also enabled us to quantitatively analyze the mechanism of the light-induced transformation between the two LC phases. The evaluation revealed that the presence of ∼4% of bent molecules in the system under UV light destabilizes the SmC layered structure originally constructed by rod-shaped molecules, causing formation of the Cubbi phase at temperatures 10–35 K below the temperature range of the thermally-stable Cubbi phase.

Wide nematic phases induced by hydrogen-bonding

ABSTRACTNew series of hydrogen-bonded supramolecular complexes, In/IIm, were prepared and investigated for their mesophase behaviour. The complexes were prepared through hydrogen-bonding between equimolar amounts of 4-alkoxyphenylazo-benzoic acids as the proton donors and non-mesomorphic 4-alkoxyphenylazo pyridines as the proton acceptors. The length of the terminal alkoxy chains was varied systematically on both terminals of the supramolecular complexes. The formation of 1:1 hydrogen-bonded complexes was confirmed by differential scanning calorimetry (DSC) and FTIR spectroscopy. All of new complexes were characterised by DSC and polarised light microscopy (PLM). It was found that the prepared complexes are dimorphic, possessing smectic C (SmC) and nematic (N) phases with relatively wide ranges. A comparison was made between the present series of complexes and the previously investigated isomers of the simple 4-alkoxybenzoic acids with the 4-(4ʹ-pyridylazophenyl)-4ʹʹ-alkoxybenzoates, revealed the stability of the nematic phase is dependent on the length of the acid component.

On the orientational distribution functions in de Vries-type smectic liquid crystals

ABSTRACTIn de Vries smectic liquid crystals, the transition from the orthogonal smectic A (SmA) to the tilted smectic C (SmC) phase occurs with essentially no decrease in smectic layer thickness. This unusual behaviour is commonly explained assuming a ‘hollow-cone’ or ‘volcano-like’ orientational distribution function (ODF) of rod-like molecules in SmA and the transition to SmC being a pure disorder–order transition in the molecular tilt directions. However, even after 20 years of extensive investigations, the experimental confirmation of this ad hoc model is still inconclusive. While optical and electro-optic studies of many de Vries smectics can be readily explained with the hollow-cone model, X-ray diffraction (XRD) studies as well as many polarised Raman spectroscopy (PRS) studies support a broad Maier–Saupe distribution of a sugarloaf-like shape. We review and summarise X-ray, electro-optic and PRS results on the ODFs in the SmA phases of materials claimed to be of the de Vries type and discuss how seemingly contradicting findings can be true at the same time. In molecules where the core is far from collinear with the long molecular axis, the cores may exhibit a volcano ODF while the molecular axes exhibit a sugarloaf ODF.

Synthesis and mesophase characterization of methacrylate based three phenyl ring core side chain liquid crystalline copolymers

A new series of side chain liquid crystalline copolymers consisting of a mesogenic monomer and a non-mesogenic n-butyl methacrylate are prepared by free radical solution polymerization. The mesogenic monomer is synthesized by a multistep route and is built with side arm core consisting of three phenyl rings linked by an ester and a terminal alkoxy chain at one end and hexamethylene spacer between methacyrlate unit and the phenyl ring core. The copolymers are synthesized with different feed ratios of liquid crystalline monomer and n-butyl methacrylate to realize composition dependent copolymers. The mesogenic monomers are characterized by FT-IR, 1H and 13C NMR while for a representative mesogen solution two-dimensional (2D) NMR experiments are also carried out. The hot-stage optical polarized microscopy, differential scanning calorimetry and variable temperature powder X-ray diffraction investigations revealed the existence of enantiotropic nematic (N) and smectic C (SC) mesophases for the copolymers. The weight average and number average molecular weight of the copolymers are found to be in the range 1.6×104 to 3.7×104g/mol from gel permeation chromatography. Thermal degradation profile of mesogenic copolymers investigated by thermogravimetric analysis revealed that the copolymers are stable till 335°C in nitrogen atmosphere. The copolymer compositions were determined using 1H NMR analysis.

Synthesis and mesomorphic properties of 2,4-bis(4′-n-pentyloxybenzoyloxy)- benzylidine-4″- n-alkoxyaniline

ABSTRACTThe synthesis and mesomorphic properties of a new series of 2,4-bis(4′-npentyloxybenzoyloxy)- benzylidine-4″ -n-alkoxyaniline (DC5An) are reported. The molecular structure of compounds was confirmed by FTIR, 1H-NMR, 13C-NMR, mass spectroscopy and elemental analysis. The mesomorphic properties were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) measurements. All compounds of the series exhibit nematic (N) and smectic C (SmC) phases. The first four homologues (DC5A1-DC5A4) display a N mesophase, whereas the highest homologues (DC5A5-DC5A10) exhibit an enantiotropic dimorphism N and SmC phases. The mesomorphic properties of the present series are compared and discussed with other structurally related series.

Synthesis, structural and electrochemical characterization of benzimidazole compounds exhibiting a smectic C liquid crystal phase

Mesomorphic benzimidazole compounds were prepared from a biphenyl benzoate based precursor by substitution reaction of alkyl bromide with 2-mercaptobenzimidazole. Molecular structures of the benzimidazole compounds were characterized by nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy and elemental analysis. Differential scanning calorimetry (DSC) measurements and polarizing optical microscopic (POM) observations revealed that the benzimidazole compounds exhibited a thermotropic smectic C (SC) phase. Temperature dependent X-ray diffraction (XRD) patterns suggested a tilted bilayer smectic structure in which intermolecular hydrogen bonds between benzimidazole moieties formed lamellar arrangement. Electrochemical impedance spectroscopy (EIS) characterization suggested that the SC phase favored anhydrous proton conduction of the benzimidazole compounds and the proton conductivities showed an Arrhenius temperature dependence.

Induction of chiral phases in originally achiral hydrogen-bonded dimer liquid crystals

ABSTRACTWe investigated the seventh homologues of the hydrogen bonded in dimers liquid crystals p,n alkyloxybenzoic acids (7OBA), which in its pristine state displays achiral nematic (N) and smectic C (SC) states. The mixture of this liquid crystal with nanoparticles, expressing various shapes and sizes, where the liquid crystal serves as a matrix, introduces new optical and electrooptical properties of the composite material. We concentrated on the mixture 7OBA with single wall carbon nanotubes (SWCNTs), which exposes a set of chiral and ferroelectric states, including the unique one (smectic CG) state, which do not appear in the pristine achiral 7OBA. On the base of polarization microtextural, thermal and polarization spectroscopic (FT far infra red-FT FIR and microRaman) analyses we present a qualitative model of the smectic CG state in low-molecular dimeric liquid crystals.

Non-symmetrically (1,2,4- and 1,3,4-)oxadiazole homologous: synthesis, characterisation and study the effect of different substituents on their mesophase behaviours

ABSTRACTWe have previously demonstrated that the type of oxadiazole isomers is relatively affected on showing the liquid crystalline properties of the compounds that have the same chemical compositions. In this paper, we presented the synthesis and mesomorphic behaviour of related compounds that possess ester linkage with different substituted groups on para of the phenyl to six derivatives containing 1,3,4-oxadiazole and like another six containing 1,2,4-oxadiazole rings. The formation of these compounds was evidenced by FTIR, 1H NMR, 13C NMR, elemental analysis and mass techniques. Their mesophase behaviours were studied by optical polarised microscopy (OPM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). All of them presented liquid crystalline properties with nematic (N), smectic A (SmA) and smectic C (SmC) phases typically to calamitic compounds. The effect of substituent group nature and the type of oxadiazole isomers on the liquid crystalline properties of these compounds are analysed and discussed.

Studies of reorganization of the molecules during smectic A–smectic C phase transition using infrared spectroscopy and generalized two-dimensional correlation analysis

The FT-IR spectra of the 4-hexyloxybenzylidene-4′-dodecyloxyaniline (6BA12) were studied as a function of temperature to describe reorganization of spatial distribution of molecules during various liquid crystal phase transitions: isotopic liquid (IL) – nematic (N), nematic – smectic A (SmA), smectic A – smectic C (SmC), smectic C – smectic I (SmI). The main focus is on the SmA-SmC and SmC-SmI transitions. Two dimensional correlation analysis (2D-IR) of IR spectra has been used to check changes of the arrangement and specific interactions of molecules during phase transitions. A moving-window autocorrelation analysis has been applied to locate transition points basing on the spectroscopic data.

Novel smectic liquid crystals based on benzo[c]cinnoline: their synthesis, mesomorphism, opto- and electro-chemical properties

ABSTRACTA series of novel calamatic liquid crystals based on a polar benzo[c]cinnoline moiety were efficiently prepared through a facile route. Rich smectic mesophases were induced by the monoalkylated and dialkylated molecular design, including highly ordered smectic mesophases in the rectangular and hexagonal orders. Dialkylated phenylbenzocinnoline derivatives showed a very wide temperature range over 150°C for smectic C (SmC) phase, while the monoalkylated ones only presented the low-ordered mesophases, which exhibited a bilayer structure in crystalline phase. From pronounced reversible redox waves in a cyclic voltammogram and low-lying lowest unoccupied molecular orbital level of about −3.2 eV indicated possible electron-transporting behaviour. In addition, a switching behaviour originating from ferroelectricity in SmC* induced by chiral dopant was observed.

Thermotropic liquid–crystalline properties of 4,4′-dialkoxy-3,3′-diaminobiphenyl compounds and their precursors

ABSTRACTA series of 4,4ʹ-dialkoxy-3,3ʹ-diaminobiphenyl compounds were synthesised by three-step procedure that involves alkylation, nitration and reduction reactions. Their chemical structures were characterised by FTIR, 1H and 13C spectroscopy and elemental analysis. Their thermotropic liquid–crystalline (LC) properties were examined by a number of experimental techniques including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VT-XRD). The 4,4ʹ-dialkoxy-3,3ʹ-dinitrobipheyl compounds, precursors to the diamine compounds, were also examined for their thermotropic LC properties. POM studies of focal conic textures and VT-XRD of the 3,3ʹ-diaminobiphenyl derivatives having flexible alkyl chains (C6–C12) exhibited the smectic A (SmA) phase independent of the length of alkyl chains. Similarly, the 3,3ʹ-dinitrobiphenyl derivatives containing alkyl chains C7, and C9–C11 exhibit the SmA phase, those containing C8 formed the smectic C (SmC) phase and C12 formed both the SmA and smectic B (SmB) phases, respectively. The 3,3ʹ-diaminobiphenyl derivatives had excellent thermal stability in the temperature range of 237–329°C, while those of 3,3ʹ-dinitrobiphenyl derivatives were in the temperature range of 270–321°C. The 3,3ʹ-diaminobiphenyl derivatives emitted UV light both in chloroform and acetonitrile.

Inclusions with finite surface anchoring energies in smectic C and chiral smectic C* free-standing films

ABSTRACTDirector distributions around inclusions in smectic C (SmC) and chiral (SmC*) free standing films with finite surface anchoring are approximately analytically determined using equivalent disclination models with (+1)-wedge disclination in the inclusion center, (−1)-disclination outside the inclusion and image disclinations inside the inclusion. Strength of image disclinations depends on anchoring energies. The position of (−1)-disclination with respect to the inclusion is determined by the product of anchoring energies and inclusion radius. At the inclusion surface (−1)-disclination splits into two (−1/2)-disclinations. In the SmC* the director configuration on the inclusion surface is rearranged in order to eliminate the inclusion surface charge.

Relaxation behavior of supra-molecular hydrogen-bonded liquid crystal phase structures: SA:11OBA

ABSTRACTLow-frequency (20 Hz–10 MHz) dielectric response in liquid crystalline (LC) phases of hydrogen-bonded (HB) complex, SA:11OBA, is studied. Synthesis of SA:11OBA with non-mesogenic succinic acid (SA) and mesogenic p-n-undecyloxy benzoic acid (11OBA) and its spectroscopic confirmation are detailed. Phase transition temperatures (TC) involving nematic, smectic-C (SmC), and smectic-G (SmG) phases by capacitance C(T) and loss tanΔ(T) anomaly agree with the microscopy (polarizing optical microscope) and calorimetry (differential scanning calorimetry) results. Low-frequency dispersion infers two types of reorientation processes, viz. higher frequency (∼MHz) and lower frequency (kHz) processes. Distinct time scale mechanism are presented. Arrhenius behavior infers influence of HB on activation energy (Ea). Off-centered dispersion, viz. ϵ′(ω) with ϵ″(ω), analyzed through the Cole–Cole plots in N, SmC, and SmG phases infers a strong temperature trend for dielectric strength (Δϵ) and distribution (α) parameters. Temperature dependence of α-parameter reveals on LC phases increasing fixture of molecular dipole in LC phase structure. Results are discussed in the wake of the body of data reported in LC phases exhibited by other LC compounds.

Rheological evidence for the existence of subphases in the liquid crystalline 4-n-alkoxybenzoic acids

ABSTRACTThe rheological behaviour of the homologous series, the alkyloxybenzoic acids, has been studied at steady flow and small amplitude oscillatory shear. The temperature dependencies of the viscosity were compared with differential scanning calorimetry data in order to estimate pretransitional phenomena in mesophases. The values of the viscosity and flow activation energy (E) can be indicative of the mesophase state. 4-n-pentyloxybenzoic acid yields a classic nematic (N) phase while the next homologues show a cybotactic nematic phase having a higher E value. The N phase is a Newtonian fluid of lower viscosity than that of the isotropic phase. The cybotactic nematic phase by its rheological properties takes an intermediate position between the N phase and a smectic C (SmC) phase. The SmC phase is a viscoelastic and viscoplastic medium. The SmC phase of 4-n-octyloxy-, 4-n-nonyloxy-, 4-n-decyloxy- and 4-n-dodecyloxybenzoic acid can exist in high- and low-ordered states. The more ordered smectic phase is characterised by higher stiffness, viscosity and yield stress than those of the less ordered. The SmC phase of 4-n-hexadecyloxybenzoic acid is characterised by the only state with unstable value of apparent viscosity. The phase state and the transition temperature can be dependent on the thermal history of the sample and a mechanical shear.

Symmetric 3,5-pyrazole and isoxazole heterocycles comprising a bent core unit: synthesis and mesomorphic characterisation

The synthesis and characterisation of some new liquid crystalline (LC) heteroaromatic compounds containing the five-membered pyrazole/isoxazole rings is reported. Some of the compounds exhibited enantiotropic LC properties. The transition temperatures and LC textures of the mesophases were determined using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray studies. The five-membered heterocyclic compounds with the pyrazole core exhibit smectic C (SmC) phase, whereas isoxazoles show SmC and nematic (N) phases.

Tuning the mesomorphic properties of carbosilane-terminated and conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole liquid crystals as binary mixtures with bimodal length distributions

The phase diagrams of binary mixtures of smectic C (SmC) mesogens – the carbosilane-terminated mesogen 2-(6-chlorohexyloxy)-5-(4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl)-1,3,4-thiadiazole (QL13-6) and one of nine conventional 2-alkoxy-5-(4-alkoxyphenyl)-1,3,4-thiadiazole mesogens (QL14-m/n) – with a molecular length ratio QL13-6/QL14-m/n ranging from 1.24 to 1.46 show the induction of a smectic A (SmA) phase in the mole fraction range 0.1 ≤ xQL13-6 ≤ 0.6. The results show that the temperature range of the induced SmA phase is remarkably sensitive to the molecular length ratio, and that this approach to tuning the mesomorphic properties of SmA–SmC liquid crystal mixtures may provide a powerful new tool in the formulation of mixtures for SSFLC and electroclinic applications.

Supermolecular bent configuration composed of achiral flexible liquid crystal trimers exhibiting chiral domains with opposite handedness.

Chirality's effects on physical properties of materials and how chirality arises have persisted as attractive issues in chemistry. We prepared a homologous series of achiral liquid crystal trimers in which three phenylpyrimidine units are connected via flexible heptamethylene spacers. An equimolecular mixture of a trimer with a nematic (N) phase and that with smectic A (SmA), smectic C (SmC), and smectic B phases was found to exhibit an N phase, a SmC phase, and a B4 phase composed of chiral domains with opposite handedness. The chiral characteristics of the B4 phase were confirmed by uncrossing the polarizers in opposite directions. XRD measurements reveal that both SmC and B4 phases have an interdigitated layer structure. That molecular interdigitation might form a supermolecular bent configuration that can produce saddle splay curvature to drive the B4 phase.

Mesomorphism and Shape-Memory Behavior of Main-Chain Liquid-Crystalline Co-Elastomers: Modulation by the Chemical Composition

The mesomorphic behavior, shape shifting, and shape-memory types of two homologous series of main-chain liquid-crystalline co-elastomers (MC-LCEs) with composition-tunable transition temperatures and mesophases structures are reported. The weakly cross-linked macromolecules were prepared in a one-step procedure by the versatile platinum-catalyzed hydrosilylation reaction between siloxane oligomers (linear spacers and netlike point molecules) and liquid-crystalline binary mixtures, composed of defined proportions of divinylic monomeric mesogens. First, the two sets of alternated, linear copolymers with disiloxane or trisiloxane chain extender, respectively, were investigated. They either exhibit both smectic C (SmC) and nematic (N) phases or a single SmC phase, the nature of which being dictated by the length of the inserted siloxane moieties, and the temperatures ranges regulated by the composition of the mesogenic mixture (mesogens ratio). As anticipated, the thermal behavior of the corresponding co-elas...