Enantiotropic Liquid Crystals Research Articles

Harnessing liquid crystal attributes of near-unit photoluminescent benzothioxanthene photosensitizers: Photophysical profiling in solution, solid state, and polymer matrix embedding

Liquid crystals (LCs) have garnered significant attention for their unique optical and electrical properties, making them promising candidates in various technological applications such as smart displays, sensors, telecommunications, biomedicals or wearable electronics. In this study, we explore the potential of several highly emissive benzothioxanthene imide (BTI) derivatives as LC materials with a focus on their robustness and temperature-stable emission behavior. By tailoring the molecular structure of BTIs, we have accomplished exceptional emissive properties while maintaining the inherent advantages of LCs, such as their self-organizing ability and responsive nature. We describe the formation of enantiotropic liquid crystals whose mesomorphic properties dependent on the nature, length, and position of the side chain. Moreover, we have investigated the thermal stability of their emission spectra over a wide range of temperature, highlighting their potential use in demanding conditions where precise optical performances are critical. Our findings underscore the importance of molecular design in achieving highly emissive LC materials with enhanced robustness and temperature stability, opening new avenues for the use of BTI derivatives.

Terminal halogen-containing rod-like liquid crystals: Synthesis, self-assembly, photophysical and mechanochromism properties

Five series of cyanostilbene-based rod-like liquid crystals containing one different terminal atom (H, F, Cl, Br and I) at one end and one terminal aliphatic chain with different numbers of carbon atoms at the other end were reported by Suzuki coupling and Knoevenagel reactions. The influence of terminal halogen atoms and terminal chain length on the self-assembly, AIE behavior, temperature-dependent emission and mechanochromism behavior was explored by POM, DSC, XRD, SEM, absorption spectra and emission spectra. All the compounds are enantiotropic liquid crystals. The lowest non-halogen substituted homologue exhibited solo N phase, but the higher non-halogen substituted homologues exhibited mesogenic transition from SmA phase to N phase upon rising temperature. All the lowest halogen substituted homologues exhibited mesogenic transition from SmA phase to N phase upon rising temperature and all the higher homologues only exhibited SmA. The distinct mesogenic phase transition could be attributed to the intermolecular interaction produced by terminal halogen and the rigidity of the terminal aliphatic chain. All the non-halogen substituted compounds and halogen substituted compounds with smaller terminal halogen atom (F, Cl and Br atom) exhibited AIE behaviors, whereas the iodinated compounds exhibited extremely weak emission in solution and aggregated states due to the heavy atoms effect. These compounds also exhibited distinct solid-state emission with blue or cyan fluorescence, which could be quenched by increasing temperature. The reversible mechanochromism behavior was also achieved in all the compounds. The mechanical force induced quench in emission in non-halogen substituted compounds and halogen substituted compounds with smaller terminal halogen atom (F, Cl and Br atom), whereas enhancement in iodinated compounds. The reversible mechanochromism behavior endowed these compounds with potential applications in rewritable paper and anti-counterfeiting. The interesting properties in these liquid crystals would be attributed to the balance of the halogen-halogen interactions, heavy atom effect, steric-hindrance effect and chain length. These investigations would be helpful to understand the relationship between chemical structures and properties.

Biphenyls embedded Schiff base-bis esters: self-assembling behaviour, impact of additional aromatic ring and ester linkage and their DFT investigations

ABSTRACT In the present investigation, two series of biphenyl embedded Schiff base-bis esters alkyl-(E)-4-(((4’-(decyloxy)-[1,1’-biphenyl]-4-yl)methylene)amino)-benzoate, BPK-(2–18) and alkyl-(E)-4-(((4’-((4-(decyloxy)benzoyl)oxy)-[1,1’-biphenyl]-4-yl)-methylene)amino)benzoate, BPEK-(8–18) containing either terminal alkoxy biphenyl core or central biphenyl core have been synthesised and studied for their self-assembling behaviour. The compounds of both series demonstrated enantiotropic liquid crystal properties, displaying typical calamitic mesophases (Smectic A (SmA), Smectic C (SmC)) across a broad temperature range and with high thermal stability. The bimorphic behaviour of new molecules has been investigated by VT-XRD analysis. The density function theory has been done to illustrate the experimental results of the mesomorphic properties of the studied compounds. Several parameters have been calculated such as molecular geometry, dipole moment, polarisability, aspect ratio, etc. The results showed that more planar compounds will attain more packed structures leading to the formation of more ordered smectic phases.

Impact of the terminal fluorination of bisthienylethene-based liquid crystals on the self-assembly and mechanochromic behaviors

Four novel bisthienylethene-based rod-like liquid crystals containing different numbers of terminal fluorine atoms were designed and the effect of the terminal fluorination on liquid crystalline self-assembly, optical characteristics in solid state and mechanochromic behaviors was explored. The transition from non-liquid crystal via monotropic liquid crystal with smectic A phase to enantiotropic liquid crystal with smectic A phase can be achieved and the temperature ranges of smectic A phase gradually increase by increasing the terminal fluorination, which might be due to the increase in intermolecular interactions by increasing the number of terminal fluorine atom. All the compounds exhibited AIE characteristic and distinct solid-state fluorescence with yellow or orange color. In addition, the non-fluorosubstituted and difluorosubstituted bisthienylethene-based liquid crystals exhibited reversible mechanochromism, whereas the monofluorosubstituted and trifluorosubstituted bisthienylethene-based liquid crystals exhibited extremely weak mechanochromism due to different intermolecular interaction, molecular symmetry and molecular conformation. These investigations implied that the incorporation of fluorine atom could result in interesting self-assemblies and photophysical properties in different states, which also provided the understanding of the relationship between chemical structures and properties.

The interplay of chirality and restricted rotation: stabilisation of chiral, frustrated mesophases over a wide thermal range.

Introducing restricted rotation in a molecule, often achieved through the incorporation of double or triple bonds, constitutes a crucial approach to induce a frustrated mesophase. Furthermore, the inclusion of a chiral moiety, such as cholesterol, serves to enhance the stabilization of chiral frustrated mesophases. This study presents the synthesis and characterization of novel optically active dimers incorporating cholesterol and phenyl 3-phenylpropiolate segments interconnected by an ω-oxyalkanoyloxy spacer with varying lengths and parity. Comprehensive characterization using POM, DSC, and X-ray diffraction techniques reveals that these mesogens exhibit enantiotropic liquid crystal (LC) phases. Due to the restricted rotation caused by the triple bonded system, these dimers stabilized the frustrated chiral mesophase TGBC* over a wide temperature range. The identified phases include BP-I/II, N*, and SmA, with odd-membered dimers commonly displaying N* and SmA phases and one member additionally exhibiting BP-I/II. Even-membered dimers exhibit N* and TGBC* phases, with the latter spanning an impressive thermal range of 37 °C to 84 °C. Notably, the thermal range of the TGBC* phase widens with an increase in the terminal tail length. Across the series, clearing temperatures generally decrease with the elongation of the terminal tail, and even-membered dimers consistently exhibit higher clearing temperatures than their odd-membered counterparts, illustrating the spacer's parity-dependent odd-even effect on the dimers' phase transition behavior.

Synthesis, mesomorphic and fluorescent properties of stilbene-containing 1,3,4-oxadiazole compounds

ABSTRACT A new class of mesogenic 1,3,4-oxadiazole derivatives 2a-2e based on stilbene has been synthesised and characterised by means of nuclear magnetic resonance (NMR), electro spray ionisation-mass spectrometry (ESI-MS) techniques and elemental analyses. The thermal properties were investigated by means of differential scanning calorimetry (DSC), polarising optical microscopy (POM) and thermogravimetric analyses (TGA). All compounds exhibited enantiotropic liquid crystal behaviours with typical calamitic mesophases (Smectic A (SmA), Smectic C (SmC) and Nematic phases) in very wide temperature range with excellent thermal stability. The nature of the mesophase is determined greatly by the polarity of the terminal groups. The electron-withdrawing cyano (for 2a) and fluoro (for 2b) groups tended to form smectic A phase, while the electron-donating methyl (for 2c), methoxy (for 2d) and decyloxy (for 2e) groups facilitated the SmA or SmC and nematic phases. Out of expectation, no banana phases were found in these mesogens. It’s noted that the solutions of all compounds in CH2Cl2 individually displayed one absorption band with λmax values at 340–355 nm, and exhibited an intense emission with λmax values at 428–486 nm and photoluminescence quantum yields of 0.39–0.55. The relationship between the molecular structures and mesogenic properties is discussed briefly in this work.

Design of New V Shaped Thermotropic Liquid Crystals containing Heterocyclic

The characterization and design of this study of new liquid crystals with a V shape compounds containing thiazolidine-2,4-dione and 1,3-phenylene as a core unite with mesophase properties were reported. Preparation and characterization of chloroacetic acid, water, and thiourea to produce thiazolidine-2,4-dione [I] in the presence of strong hydrochloric acid. The 4-hydreoxybenzaldehyde and n-alkyl bromide were reacted with potassium hydroxide to create the n-alkoxy benzaldehyde., then the compound [I] reacted with [II]n in presence of piperidine to produce compounds [III]n. Also, converted resorcinol to a corresponding compound [IV] by refluxing pyridine and DMF with two moles of chloracetyl chloride, after that interacted the compounds [III]n and compound [IV] with sodium acetate to form compounds [V]n. FT-IR and 1HNMR spectroscopy were really used to determine the structures of the produced substances. Using polarized optical microscopy, these mesogens' mesomorphic characteristics were studied (POM).The synthesized molecules exhibited enantiotropic liquid crystal phases, the substances [V]1-4 exhibited an only the enantiotropic nematic phase.

Heterocyclic compounds-based liquid crystals

A series of liquid crystals comprising a heterocyclics dihydro pyrrole and 1,2,3-triazole rings [VII]-[X] were synthesized by many steps starting from a reaction of 3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diamine with chloroacetyl chloride in a mixture of solutions DMF and TEA to synthesise the compounds [I], then the compounds [I] reacted with malononitrile in 1,4-dioxane and TEA solutions to produce compounds [II], then the first step is repeated with compound [II] where it reacted with chloroacetyl chloride in mixture of DMF and TEA to give compound [III], this compound reacted with sodium azide in the presence of sodium chloride and DMF as solvent to produce the compound [IV], which reacted with acrylic acid by a 1.3 dipolar reaction in solvent THF to give compound [V], this compound which reacted with thionyl chloride to gave carbonyl chlorides compound [VI], finally the last compound reacted with a variety of amines to product compounds [VII]a-d. The structures of the synthesized compounds were actually determined by Fourier-transform infrared spectroscopy FT-IR and Proton nuclear magnetic resonance 1H-NMR, The properties of liquid crystalline were characterized by hot stage Polarising Optical Microscope POM. The synthesized molecules exhibited enantiotropic liquid crystal phases, and the compounds [VII]a-d exhibited an enantiotropic nematic phase only.liquid crystal

Synthesis and characterization of alkylthio-attached azobenzene-based liquid crystal polymers: Roles of the alkylthio bond and polymer chain in phase behavior and liquid crystal formation

We demonstrate for the first time that the alkylthio (or thioether) bonds in the side-chains of liquid crystal (LC) polymers considerably influence the phase-transition behavior, mesomorphic properties, and nanostructures of the LC and crystal (Cr) phases. We synthesized three alkylthio-attached azobenzene-based polymethacrylates; namely, P–OS, P–SO, and P–SS, in which alkylthio bonds were introduced at an outer (terminal) 4-position, at an inner (polymer-chain side) 4-position, and at both the outer and inner 4-positions, respectively, of the azobenzene side chains. In marked contrast to the corresponding pre-monomers and methacrylate monomers, the P–OS, P–SO, and P–SS polymers form well-defined enantiotropic LC phases. Notably, while an outer terminal alkylthio bond renders azobenzene-based polymers (P–OS and P–SS) predominantly smectogenic, a sole inner alkylthio bond renders the P–SO polymer nematogenic. In addition, the outer and inner alkylthio bonds contribute to increasing and decreasing the phase-transition temperatures, respectively. The presence of the alkylthio bonds potentially provides LC polymers with crystallizability. Furthermore, X-ray diffractometry results reveal that the P–OS, P–SO, and P–SS polymers form deeply interdigitated bilayer structures in their SmA, highly ordered, and Cr phases. The specific packing mechanism is discussed in terms of intermolecular attractive interactions involving the sulfur atoms.

Synthesis and mesomorphic properties of new azo H-shaped liquid crystal compounds

ABSTRACT Using tartaric acid as the core, six azo H-shaped liquid crystal compounds M1-M6 with different terminal substituents were synthesised. All products were structurally well characterised by elemental analysis, 1HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. M1-M6 were thermotropic enantiotropic nematic liquid crystals. They all showed schlieren texture in the process of heating up, and exhibited spherulite texture in the process of cooling down. M1 and M4 whose terminal substitution was nitro had the widest liquid crystal range in the same series of liquid crystal compounds, which were 135.1°C and 96.1°C, respectively, under heating cycle and 167.4°C and 112.7°C, respectively, under cooling cycle. The results show that the terminal substituents and flexible chain have important influence on the mesogenic phase and range. With the increase of flexibility, the mesogenic range of liquid crystal compounds becomes smaller when the end groups are the same. And when the flexible chain is the same, the greater the induced and polarised force of the terminal substituents, the better the mesogenic properties of the liquid crystal compounds.

Synthesis and study the liquid crystalline properties of new compounds containing 5-methyl-2,4-dihydro-3H-pyrazol-3-one

The new compounds of pyrazolines were synthesized from the reaction of different acid hydrazide with ethylacetoacetate and ethanol under reflux. These compounds were obtained from many sequence reactions. The 4-acetyl-5-methyl-2,4-dihydro-3H-pyrazol-3-one compounds synthesized from the reaction of 5-methyl-2,4-dihydro-3H-pyrazol-3-one with acetyl chloride in calcium hydroxide and 1,4-dioxane. Finaly, Schiff bases were prepared via condensation reaction of products of mono- and tri ketone derivatives[IV]a, b with phenyl hydrazines as presented in (Scheme 1, 2). The synthesized compounds were identification by using FTIR, NMR and Mass spectroscopy (of some of them). The mesomorphic behaviour of these derivatives were examined under OPM. The compound [IV]a exhibited enantiotropic liquid crystal (Smectic A on heating and Nematic on cooling) while the compound [V]c showed Smectic A and nematic phases only on cooling (monotropic). Also the two compounds [V]a and [VI]b exhibited nematic mesophase on cooling. The compound [IV]b, [V]b, d and [VI]a, c, d did not show any mesomorphic properties.

The influence of the structure of terminal groups and cores on the properties of schiff base star-shaped liquid crystals

ABSTRACT A series of star-shaped liquid crystal (LC), respectively, based on phloroglucinol, pentaerythritol and glucose as cores, rod-shaped molecules (MeOSBAA, MeSBAA and ClSBAA) containing a schiff base as mesogenic mesogen were synthesised. The molecular structures were confirmed by FT-IR and 1H-NMR techniques. The LC textures and thermal phase behaviours were investigated by polarising optical microscopy and differential scanning calorimetry. The results showed that MeOSBAA and ClSBAA were thermally enantiotropic liquid crystals. Three-arm star LC macromolecules X-SBA3E and four-arm star LC macromolecules X-SBA4E were found to display enantiotropic nematic liquid crystals behaviour. While the five-arm star-shaped chiral LC molecules X-SBA5E exhibited enantiotropic chiral nematic phase.

Synthesis and characterization of symmetrical liquid crystalline compounds based on oxazole and thaizole rings

Twelve compounds containing a sulphur- or oxygen-based heterocyclic core, 1,3- oxazole or 1,3-thiazole ring with hydroxy, methoxy and methyl terminal substituent, were synthesized and characterized. The molecular structures of these compounds were performed by elemental analysis and different spectroscopic tequniques. The liquid crystalline behaviors were studied by using hot-stage optical polarizing microscopy and differential scanning calorimetry. All compounds of 1,4- disubstituted benzene core with oxazole ring display liquid crystalline smectic A (SmA) mesophase. The compounds of 1,3- and 1,4-disubstituted benzene core with thiazole ring exhibit exclusively enantiotropic nematic liquid crystal phases.

Novel blue-light emitting fluorescent liquid crystals based on 4, 4′-(2-(tert-butyl)- anthracene-9, 10-diyl)diphenol and their optical behavior

Novel fluorescent liquid crystals based on 4, 4′-(2-(tert-butyl)-anthracene-9, 10-diyl)diphenol moiety were synthesized in good yields and were found to be enantiotropic liquid crystals. Their excitation wavelength was found to be at 265 nm and emission maximum was observed at λmax = 435 nm, which corresponds to the blue region of visible light spectrum. The values of refractive indices, recorded at variable temperatures and wavelengths showed a sudden decline in temperature range in which mesophases were observed. Interaction with DNA, as evaluated by UV–Visible and Fluorescence spectroscopy showed the formation of a complex of compound-hsDNA, mainly through minor grooves.

Alkylthio- and alkyl-substituted asymmetric diphenyldiacetylene-based liquid crystals: phase transitions, mesophase and single-crystal structures, and birefringence

ABSTRACTAsymmetric diphenyldiacetylenes (DPDAs) bearing a fixed hexylthio group and different alkyl groups with carbon numbers (n) of 0–8 and 12 at each termini (or 4- and 4ʹ- positions), abbreviated as 6S–DPDA–n, were synthesised and their phase transition behaviour and birefringence were evaluated. It was found that alkyl-unsubstituted and methyl-substituted analogues (6S–DPDA–0 and 6S–DPDA–1, respectively) do not show any mesophases, whereas 6S–DPDA–n (n ≥ 2) form enantiotropic liquid crystal phases. Wherein, the nematic (N) phase for 6S–DPDA–4 is supercooled to room temperature, and 6S–DPDA–12 forms not only N phase but also highly ordered mesophase under room temperature. In light of the fact that symmetrically alkylthio-possessing 6S–DPDA–S6, 6S–DPDA–0 and 6S–DPDA–1 are not mesogenic, the present results strongly suggest that an alkyl group opposite to an alkylthio group plays a vital role to induce mesophases. Single-crystal X-ray analysis revealed anti-parallel packed molecules in the lattice. 6S–DPDA–7 have higher birefringence than 7–DPDA–7 being the symmetric alkyl counterpart, over the entire range of each N phase, which is supported by the enhanced polarisabilities based on density functional theory calculation.

Mesomorphic behavior of methanesulfonate salts of esters of L-Methionine and their optical properties

A set of methanesulfonate salts of esters of (S)-2-Amino-4-(methylthio)butanoic acid [L-Methionine] and fatty alcohols have been synthesized in good yields by a green protocol involving a one-step and solvent-free approach. All derivatives have been evaluated for their liquid crystalline properties using polarizing optical microscopy and differential scanning calorimetry and found to be enantiotropic liquid crystals. Refractive indices were measured for one of the derivatives at different temperatures and wavelengths. It exhibited a high Abbe number and low dispersive power. Synopsis: To be provided by author

Non-conventional multi-arm ester liquid crystal derived from cholic acid: synthesis, thermal and optical behaviour

ABSTRACTWith cholic acid as the core, five multi-arm ester liquid crystals were synthesised with different terminal substituents. B1–B5 were mesogenic arms which were linked to multifunctional chiral core cholic acid. The effect of electron-withdrawing and electron-donating groups on the mesogenic behaviour of the compounds was discussed. All products were structurally well characterised by elemental analysis, 1HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. B2, B4, B5 and C2, C4, C5 and D2, D4, D5 exhibited typical nematic texture and belonged to thermotropic enantiotropic nematic liquid crystals. B3, C3 and D3 also displayed typical nematic texture of thermotropic monotropic nematic liquid crystal. B1, C1 and D1 did not have mesogenic performance. Wider mesogenic range for 81.6°C of C5 whose terminal substituent is nitro on heating cycle while 121.1°C on cooling cycle. The results indicated that terminal substituents have a pretty important effect on the mesogenic phase and range of multi-arm liquid crystal compounds. The mesomorphic behaviour of compounds with polar groups terminally substituted performance much better than those without polar groups. In this ester multi-arm liquid crystal system, electron-withdrawing groups terminally substituted behave better than those electron-donating groups substituted.

Electronic Structure and Thermodynamic Properties of 4-n- heptyl-4́-cyanobiphenyl: A Computational Study

Abstract Study of structure–property relationship suggests that the number of carbon atoms in the alkyl chain of 4-n-alkyl-4-cyanobiphenyl generates the nematic or smectic phase in the liquid crystal series. The 4-n-heptyl-4-cyanobiphenyl (7CB) is a member of 4-n-alkyl-4-cyanobiphenyl (nCB) homologous series, which is well known for its electro-optical properties. The 7CB compound is an enantiotropic liquid crystal. It exhibits nematic phase at 30.0°C and isotropic phase at 42.8°C. Optimization of geometry of 7CB molecule has been performed using Hartree-Fock and DFT methods with 6-31G(d,p) basis set. Electronic structure of the 7CB nematogen has been analysed. The MEP, HOMO-LUMO surfaces, molecular and thermodynamic properties such as total energy, dipole moment, entropy, enthalpy and Gibbs free energy have been obtained at room temperature. Infrared spectrum of the 7CB molecule has been examined vis-a-vis experimental data available in literature.

Photoluminescent nickel(II)-metallomesogens derived from salphen ligands: influence of halogens at the spacer on mesomorphism and emission properties

ABSTRACTThree new series of photoluminescent nickel(II) metallomesogens, [NiL]; H2L = N,N′-Bis(4-n-alkoxysalicylidene)-4-fluoro/bromo/chloro-1,2-diaminobenzene (n = 12, 14, 16) based on ‘salphen’ ligands have been synthesised and their mesomorphic and photophysical properties explored. The complexes, isolated as orange microcrystalline solids were characterised by elemental analyses, FT-IR, 1H NMR and UV-visible spectroscopy. Thermal studies show all the compounds to be enantiotropic liquid crystals displaying columnar mesophase over a wide temperature range. Electronegativity and steric requirement of the halogen substituent at the ligand’s spacer remarkably influence the 2-D packing of the columns in the lattice in these complexes controlling the supramolecular mesomorphic order and photoluminescence. The mesophase behaviour of the fluoro-substituted complex is characterised by a transition from a columnar oblique (p1) to columnar rectangular (p2mm) phase, former stable till ambient temperature. The chloro and bromo analogues, on the other hand, displayed exclusively columnar rectangular (p2mm) mesophase with the former transforming into a glassy state and latter into a crystalline phase during cooling to ambient temperature. Molecular model based on interdigitated anti-parallel and back to back arrangements in the different columnar mesophase are proposed on the basis of X-ray diffraction (XRD) studies. The complexes emit in the blue region when excited with near UV wavelength.

From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

Herein, we demonstrate that a bottom-up approach, based on halogen bonding (XB), can be successfully applied for the design of a new type of ionic liquid crystals (ILCs). Taking advantages of the high specificity of XB for haloperfluorocarbons and the ability of anions to act as XB-acceptors, we obtained supramolecular complexes based on 1-alkyl-3-methylimidazolium iodides and iodoperfluorocarbons, overcoming the well-known immiscibility between hydrocarbons (HCs) and perfluorocarbons (PFCs). The high directionality of the XB combined with the fluorophobic effect, allowed us to obtain enantiotropic liquid crystals where a rigid, non-aromatic, XB supramolecular anion acts as mesogenic core. X-ray structure analysis of the complex between 1-ethyl-3-methylimidazolium iodide and iodoperfluorooctane showed the presence of a layered structure, which is a manifestation of the well-known tendency to segregation of perfluoroalkyl chains. This is consistent with the observation of smectic mesophases. Moreover, all the reported complexes melt below 100 °C, and most are mesomorphic even at room temperature, despite that the starting materials were non-mesomorphic in nature. The supramolecular strategy reported here provides new design principles for mesogen design allowing a totally new class of functional materials.