Hydrogen-bonded Liquid Crystals Research Articles

Comments on the thermodynamic and dielectric studies on hydrogen-bonded liquid crystal complexes

A research article titled as “Hydrogen-bonded liquid crystal complexes: A comprehensive study of structure, behaviour and potential applications” has been recently published by Meddeb et al., J. Mol. Liq. 395 (2024) 123838. Authors have reported and analysed thermodynamic and dielectric studies on highly ordered SmB and SmG mesophases in 14OBAF/DABCO complexes. But explanations of the dielectric data reported on the material are flawed based on various facts. Unintended wrong interpretations by the authors in reporting and explaining the thermodynamic and dielectric results are discussed here. Influence of various artefacts on acquired dielectric data has also been discussed in order to avoid such problems in further reports.

New Series of Hydrogen-Bonded Liquid Crystal with High Birefringence and Conductivity.

Liquid crystals with high dielectric anisotropy, low operational thresholds, and significant birefringence (Δn) represent a key focus in soft matter research. This work introduces a novel series of hydrogen-bonded liquid crystals (HBLCs) derived from 4-n-alkoxybenzoic, 4-alkoxy-3-fluorobenzoic derivatives (nOBAF), 4-alkoxy-2,3-fluorobenzoic derivatives (nOBAFF), and 2-fluoro-4-nitrobenzoic acid. The HBLCs were characterized using Fourier transform infrared spectroscopy, and their thermal behavior was evaluated via differential scanning calorimetry. Optical observations were conducted using polarized optical microscopy. The results indicate that mixtures containing benzoic acid with a bilateral fluorine substituent exhibit both SmA and SmC phases, while those with a unilateral fluorine substituent exhibit nematic and SmA phases. Moreover, an increase in the length of the alkoxy chain results in an expanded mesophase temperature range. This study demonstrates that the presence of a fluorine substituent and the incorporation of an NO2 group in the molecular structure result in an increase in dielectric permittivity, DC conductivity, dielectric anisotropy, and birefringence.

Comments on the dielectric spectroscopy results on a new series of fluorinated hydrogen-bonded liquid crystals

Recently, a work entitled “Synthesis, thermal, dielectric and electro-optic properties of new series of fluorinated hydrogen-bonded liquid crystals” is published by M. Derbali et al., in J Mol Liq, 367 (2022) 120510. Authors reported SmA and SmC mesophases in the synthesised liquid crystalline materials. However, dielectric data reported here are erroneous when tested even from some fundamental aspects. Inadvertent omissions by the authors in presenting and explaining the dielectric results in terms of frequency dependent permittivity and loss are discussed here so that appropriate precautions may be taken by the readers to avoid such fundamental gross mistakes. Some problems related to other studies are also discussed in brief.

Phenanthrothiophene-Triazine Star-Shaped Discotic Liquid Crystals: Synthesis, Self-Assembly, and Stimuli-Responsive Fluorescence Properties.

Lipophilic biphenylthiophene- and phenanthrothiophene-triazine compounds, BPTTn and CPTTn, respectively, were prepared by a tandem procedure involving successive Suzuki-Miyaura coupling and Scholl cyclodehydrogenation reactions. These compounds display photoluminescence in solution and in thin film state, solvatochromism with increasing solvent's polarity, as well as acidochromism and metal ion recognition stimuli-responsive fluorescence. Protonation of BPTT10 and CPTT10 by trifluoroacetic acid results in fluorescence quenching, which is reversibly restored once treated with triethylamine (ON-OFF switch). DFT computational studies show that intramolecular charge transfer (ICT) phenomena occurs for both molecules, and reveal that protonation enhances the electron-withdrawing ability of the triazine core and reduces the band gap. This acidochromic behavior was applied to a prototype fluorescent anti-counterfeiting device. They also specifically recognize Fe3+ through coordination, and the recognition mechanism is closely related to the photoinduced electron transfer between Fe3+ and BPTT10/CPTT10. CPTTn self-assemble into columnar rectangular (Colrec) mesophase, which can be modulated by oleic acid via the formation of a hydrogen-bonded supramolecular liquid crystal hexagonal Colhex mesophase. Finally, CPTTn also form organic gels in alkanes at low critical gel concentration (3.0 mg/mL). Therefore, these star-shaped triazine molecules possess many interesting features and thus hold great promises for information processing, liquid crystal semiconductors and organogelators.

Hydrogen-bonded liquid crystal complexes: A comprehensive study of structure, behaviour and potential applications

Intermolecular hydrogen bond (HB) interaction becomes an intensively studied topic in supramolecular chemistry and soft matter science. Herein, a new series of hydrogen-bonded liquid crystals (HBLCs) composed of 3-flurobenzoic acid derivatives (nOBAF) and 1,4-diazabicyclo [2.2.2] octane (DABCO) has been prepared and characterized by Fourier-transform infrared spectroscopy. The thermal behavior was determined by differential scanning calorimetry and observed by polarized optical microscopy. All complexes were found to exhibit smectic G (SmG) and smectic B (SmB) phases over a wide temperature range. Furthermore, it was found that the thermal stability of these mesophases increases with increasing the alkoxy chain length. The complex dielectric permittivity was reported as a function of frequency in the range 1 Hz–10 MHz. Lower frequency process attributed to the collective fluctuations of azimuthal angle and higher frequencies, associated to the soft mode were observed in SmB. In the SmG, additional to the soft mode (106 Hz) a higher frequency mode (105 Hz) was detected for the first time. The experimental results obtained for these complexes were correlated with density functional theory (DFT) calculations. Furthermore, these results suggest that nOBAF/DABCO has the potential to be switchable dielectric materials.

Light and Humidity Dual-Responsive Anti-Counterfeiting Films Based on Hydrogen-Bonded Cholesteric Liquid Crystal Polymers with Spiropyran.

There is still significant room for improvement when combining structural color with fluorescence patterns in dual anti-counterfeiting and dynamic anti-counterfeiting labels. In this study, we achieved significant breakthroughs under dual anti-counterfeiting conditions by using the structural color properties of the hydrogen-bonded cholesteric liquid crystal (HBCLC) and combining them with the fluorescence dye spiropyran (SP) to create anti-counterfeiting patterns. The anti-counterfeiting label can only display storage information after meeting the conditions of humidity and ultraviolet light (UV) and has the functions of dynamic encryption and repeated reading. We adjusted the center of the reflection band of the HBCLC film to transition from red to infrared under 40-90% relative humidity (RH) conditions and used it as a background film to draw anti-counterfeiting patterns with SP. Since these fluorescence dyes can switch between merocyanine (MC) (red) and SP (colorless) under UV and visible light conditions, when combined with the HBCLC, orthogonal dynamic encryption was achieved. Additionally, with the adsorption of SP, the reflection band of HBCLC films under the same humidity range increased from around 160 nm to around 260 nm, greatly improving the sensitivity to humidity changes. Furthermore, under UV conditions, it can still emit red fluorescence, demonstrating a polymorphic encryption feature, which greatly increased the complexity of the anti-counterfeiting pattern with significant significance to dynamic anti-counterfeiting and information storage.

Azopyridine-based hydrogen-bonded liquid crystals with thioether tail

ABSTRACT The first examples of azopyridine-based hydrogen-bonded liquid crystals (HBLCs) with thioether tail are reported. For this purpose, new HBLCs were designed and synthesised using 4-octylthiobenzoic acid as the proton donor and two different types of 4-alkoxyazopyridines as the proton acceptors. The first group of HBLCs has no lateral substitution, whereas the second has a lateral fluorine atom next to the alkoxy chain attached to the azopyridine derivative. Therefore, it was possible to investigate not only the effect of thioalkyl tail but also the influence of the aromatic core fluorination on the phase behaviour of the newly synthesised HBLCs. Hydrogen bond formation between the proton donor and the proton acceptors was confirmed using FTIR spectroscopy and X-ray diffraction (XRD), while the liquid crystalline behaviour of the HBLCs was investigated using differential scanning calorimetry (DSC) and polarised light microscope (PLM). It was found that all the reported HBLCs are mesomorphic exhibiting smectic A (SmA) and/or smectic C (SmC) phases depending on the length of the alkoxy chain as well as the presence or absence of the lateral fluorine atom. The possibility of photo-switching of the reported HBLCs under UV light irradiation was demonstrated for some selected examples in solution as well as in the bulk state.

Fully (Re)configurable Interactive Material through a Switchable Photothermal Charge Transfer Complex Gated by a Supramolecular Liquid Crystal Elastomer Actuator.

Charge transfer complexes (CTCs) based on self-assembled donor and acceptor molecules allow light absorption of significantly redshifted wavelengths to either the donor or acceptor. In this work, we demonstrate a CTC embedded in a hydrogen-bonded liquid crystal elastomer (LCE), which in itself is fully reformable and reprocessable. The LCE host acts as a gate, directing the self-assembly of the CTC. When hydrogen bonding is present, the CTC behaves as a near-infrared (NIR) dye allowing photothermal actuation of the LCE. The CTC can be disassembled in specific regions of the LCE film by disrupting the hydrogen bond interactions, allowing selective NIR heating and localized actuation of the films. The metastable non-CTC state may persist for weeks or can be recovered on demand by heat treatment. Besides the CTC variability, the capability of completely reforming the shape, color, and actuation mode of the LCE provides an interactive material with unprecedented application versatility.

Physical properties of hydrogen bonded nematic liquid crystals and electro-optical properties in terahertz waves [Invited

4-m-alkyl cyclohexane carboxylic acids and 4-n-alkylbenzoic acids were mixed to form hydrogen bonded liquid crystals (HBLCs), which exhibited a nematic phase at room temperature. Dielectric constants and three elastic constants of HBLC mixtures were investigated. Benzoic acid derivatives with some polar groups were added to the HBLC and the dielectric anisotropy was successfully enhanced. Considering the absorption loss of HBLCs in the terahertz wave region, the electro-optical properties of the homogeneous and twisted nematic cells were numerically evaluated using measured physical properties, and the transmitted polarization state was compared with that of conventional nematic LCs.

Influence of dichroism on the optical properties of a twisted nematic liquid crystal cell in the terahertz region.

Polarization control is essential in terahertz (THz) imaging. Liquid crystals (LCs) have the potential to functionalize tunable polarization-control devices. Here, a twisted nematic (TN) cell using a hydrogen-bonded LC is fabricated, and the influence of dichroism in the THz region is discussed. Our results indicate that the polarization state in the Gooch-Tarry minimum condition is affected by the LC dichroism; a nondichroic LC is required for complete linearly polarized output. The output intensity of the dichroic LC-TN cell changed when electrically switched or when the incident THz wave polarization direction was rotated 90°. These intensity variations disappeared when using the nondichroic hydrogen-bonded LC.

Polar nematic phase in short-chained fluorinated hydrogen-bonded liquid crystals

So as to investigate structure–activity relationships in hydrogen bonded liquid crystals, a new short-chained (7 carbon atoms) fluorinated hydrogen-bonded liquid crystal 4-heptyloxy-2-fluorobenzoic acid, 7OBAF, is prepared. Differential scanning calorimetry, polarized optical microscopy and X-ray diffraction disclose a phase sequence as two crystalline, a nematic and the isotropic phases. Dielectric spectroscopy of the nematic phase in the frequency range of 1 Hz–6 MHz reveals a low frequency mode ascribed to ferroelectric clusters. A high polarization is induced by an external electric field, in agreement with results of dielectric measurements. Such polarization increases as the alkyl chain length is shorter. Results of DFT calculations using the Maier–Meier relationship are consistent with the dielectric measurements; they reveal the parallel molecular distribution and provide the dipole moment, polarization anisotropy, birefringence and dielectric anisotropy. Intermolecular hydrogen bonds linking the fluorine substituent and the aromatic hydrogen atoms are also disclosed.

Graphene Oxide Induced Sub-structures of Bi-tilted Smectic CG in Dimer Liquid Crystals

A nanocomposite built of the hydrogen-bonded (HB) dimer liquid crystal 4-heptyloxybenzoic acid (7OBA) and Graphene oxide (GO) exhibiting a cascade of phase transitions and phases not typical for pristine 7OBA, was investigated by optical polarization microtexture analysis and Raman spectroscopy. We use the 7OBA/GO nanocomposite beacause of the effective functionalizing properties of GO to study the generation and development of the ferroelectric smectic CG phase in its bi-tilted configuration. For the appearance of the CG phase with its substructures we propose an explanation based on the HB formation between 7OBA dimers in closed and open conformations and the GO sheets, aided by the π–π interaction with graphene hexagons which are free from functional groups. We propose molecular and macroscopic models for the substructures CGcl and CGln and indicate their triclinic C1 symmetry, characterized by a polar vector directed oblique to the layers. This confirms that the bi-tilted CG and its substructures are ferroelectric in the bulk similarly to solid ferroelectrics, thus showing potential as material for controllable photonic functional devices.

Hydrogen-bonded liquid crystals formed from 4-alkoxystilbazoles and chlorophenols

Hydrogen-bonded complexes are formed between two chain lengths of 4-alkoxystilbazoles and six different chlorophenols; four complexes are characterised crystallographically and all but two show liquid crystal behaviour.

INFRARED REFRACTIVE INDICES OF HYDROGEN-BONDED LIQUID CRYSTALS FROM THE ENVELOPE METHOD

The refractive index of liquid crystals at infrared wavelengths is to be measured using a novel technique. This method involves constructing the envelopes for the transmitted intensities using the envelope method on Fourier transform infrared spectral data before doing refractive index measurements. The upper and lower envelopes for the envelope approach are made using the transmitted wavenumber intensities. In this case, we consider hydrogenbonded mesogens, specifically p-n-alkyl benzoic acid : p-(p'-Octyloxybenzylidene)-p-cyanoaniline (nBA: OBCA, where n=6, 8). The outcomes of this research demonstrate that the hydrogen-bonded mesogens nBA:OBCA have higher refractive indices than the pure compounds. Refractive index values in the infrared area drop with decreasing wave number and reach saturation. The proposed technique evaluates samples' refractive indices in the infrared region (4000cm-1 - 400cm-1) which is difficult-to-measure. Molecular interactions of hydrogen-bonded liquid crystals are also investigated.

New series of fluorinated hydrogen-bonded liquid crystals with SmB and nematic phases

ABSTRACT A new series of hydrogen-bonded liquid crystals was prepared by mixing 4-n-alkoxy-2.3-difluorobenzoic acid and 4-pentyl-4-biphenylcarbonitrile (5CB). Thermal and liquid crystal behaviour was established using differential scanning calorimetry and polarizing optical microscopy. The influence of terminal acid donors on mesogenic behaviour was discussed in detail. It was found that all complexes with a (1:1) molar ratio exhibit the smectic B phase. In addition, the binary phase diagram that has been established revealed that the mixtures with lower concentration exhibit a nematic phase at room temperature with large stability. Dielectric, electro-optic and viscoelastic properties of 5CB/9OBAFF with molar fraction x = 0.1 were discussed.

Synthesis, thermal, dielectric and electro-optic properties of new series of fluorinated hydrogen-bonded liquid crystals

Two series of hydrogen-bonded liquid crystals (HBLC) were investigated: 4-alkoxy-2,3-difluorobenzoic acids (nOBAFF) with alkoxy chain lengths of n = 7–12 carbon atoms and mixtures of nOBAFF and 4-octyloxybenzoic acid (8OBA). Their phase behavior has been established using differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. At variance with non-fluorinated and monofluorinated homologues, the difluorinated nOBAFF compounds only form SmA and SmC liquid crystalline phases. An additional nematic phase is obtained upon mixing nOBAFF and 8OBA, showing the sequence: Isotropic liquid–Nematic–SmA–SmC–crystalline solid upon cooling from the isotropic phase. All the transition temperatures decrease with increasing alkoxy chain length of nOBAFF, which reveals a decrease of order parameter for longer chains. The dielectric and electro-optic behaviors of the nematic phase of nOBAFF/8OBA mixtures reveal a decrease of the threshold voltage and response time upon increasing the alkyl chain length of nOBAFF.

Optical, spectral and thermal characterization of nematogens in hydrogen bond liquid crystals

A homologous series comprises of eight mesogenic hydrogen bonded liquid crystal (HBLC) referred as nBAO + 4BU BA are designed, synthesized and characterized. All eight mesogens in nBAO + 4BU BA series are subjected to spectral, thermal and optical characterization. Formation of hydrogen bond in the resultant compounds is evinced by FTIR study. All eight mesogens obtained in this series exhibits only nematic phase in their entire thermal spans which are coined as Nematogens. Essential characteristic studies carried out on these nematogens are Parachromatism as a function of temperature, phase transition temperatures,mesogenic thermal span and enthalpy involved during phase transition respectively.

Hydrogen-bonded ionic liquid crystals based on multi-armed structure: synthesis and characterization

Hydrogen-bonded ionic liquid crystals based on multi-armed structure: synthesis and characterization

Dielectric, electro-optical and viscoelastic properties of nematic phase driven by hydrogen-bonding between cyano group and fluoroalkoxybenzoic acid

ABSTRACT Hydrogen-bonded liquid crystals (HBLC) formed by mixing 4-alcyloxy-2-fluorobenzoic acid (nOBAF) and the 4-pentyl-4-biphenylcarbonitrile (5CB) at (1:1), (1:3) and (3:1) (mol/mol) ratios have been investigated. Four HBLC series are prepared by changing the carbon chain length from 7 to 10 designated 5CB/nOBAF. The fluorobenzoic acids (nOBAF) are the proton donors while the pyridine unit (5CB) is the proton acceptors. Thermal behaviour and phase diagrams of the individual series have been established using differential scanning Calorimetry (DSC) and polarising optical microscopy (POM). It was found that all complexes exhibit nematic (N) and smectic G (SmG) phases. For the nematic phase, the dielectric anisotropy was increased while the thresholds voltage was decreased. This can be explained by the strong longitudinal dipole moment of the nematic phase compared to the nOBAF compounds. Furthermore, as the alkoxy chain length increases, the birefringence, response time, rotational viscosity, and splay elastic constant all decrease by the way of the reduction of the order parameter for the longer chains. In addition, hydrogen-bonded proton transfer between cyano and carboxylic acid groups have been explored by Fourier transform infrared spectroscopy (FTIR) for the first time.

Influences of nanoparticles and chain length on thermodynamic and electrical behavior of fluorine liquid crystals

Hydrogen-bonded polar nematic liquid crystal series with the general formula nOBAF (n = 7–12) is studied. The mesomorphic characterization is demonstrated through differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The complexes with short alkyl chains (n = 7, 8) present a wide nematic range and monotropic smectic F mesophase, whereas the longer alkyl chain (n = 10–12) analogues show high melting and low clearing mesomorphic liquid crystals. The thermal range of the mesophase and the birefringence increase with chain length decreasing. Furthermore, the effect of the nanoparticles (LiNbO3) on the thermal and the electrical behavior of 8OBAF are investigated. The presence of LiNbO3 nanoparticles increases the conductivity and reduces the resistivity of the complex.