Experimental and Theoretical Approaches of New Nematogenic Chair Architectures of Supramolecular H-Bonded Liquid Crystals

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.

Fluorination Improves the Electro-Optical Properties of Benzoxazole-Terminated Liquid Crystals in High Birefringence Liquid Crystal Mixtures: Experimental and Theoretical Investigations

Aromatic heterocyclic liquid crystal (LC) materials have received much attention from LC chemists for their high birefringence and large dielectric anisotropy, yet few have reported their properties in LC mixtures. In this work, a series of fluorinated benzoxazole liquid crystal compounds were synthesized to evaluate their electro-optical properties in high birefringence LC mixtures, with the expectation of further establishing the theoretical basis and experimental evidence for their applications in LC photonics. Firstly, the effects of the lateral fluorine substituent positions on the molecular synthetic yield, mesomorphic and solubility properties were comparatively investigated. Afterwards, we focused on the fluorination effects on the core electro-optical properties, including birefringence, dielectric anisotropy and further investigation of the viscoelastic coefficient of high birefringence LC mixtures. Research results showed that the benzoxazole liquid crystal compounds possess low melting points, wide nematic phase intervals and good solubility by appropriate lateral fluorine substitution, which is beneficial to further improve the electro-optical properties of high birefringence LC mixtures. Meanwhile, the theoretical and experimental results corroborate each other to well reveal the structure–property relationship. This study demonstrates that fluorination would promote promising applications of benzoxazole-terminated liquid crystals in emerging LC optical devices.

Experimental and Theoretical Investigations of Three-Ring Ester/Azomethine Materials

New three-ring ester/azomethine homologues series, (E)-4-((4-hydroxybenzylidene)amino)phenyl 4-(alkoxy)benzoate In, were prepared and their properties were investigated experimentally and theoretically. FT-IR, NMR, and elemental analyses were used to confirm the chemical structures of the synthesized compounds. The mesomorphic activities of the planned homologues were evaluated using differential scanning calorimetry (DSC) and polarized optical microscopy. All of the homologous examined were found to have non-mesomorphic properties. Theoretical calculations using the density functional theory (DFT) were used to validate the experimental data and determine the most stable conformation of the synthesized compounds. All calculated conformers' thermal properties, dipole moments, and polarizability were discussed. The results show that the terminal alkoxy chain length affects the thermal parameters of the conformers. The correlations between these parameters' values and the conformer type were demonstrated. The base component was expected to be in two conformers according to the orientation of the N atom of imine-linkage. DFT calculations revealed the more probable of the two possible conformers, and the incorporation of the alkoxy terminal chain in one position affect its geometrical and mesomerphic characteristics.

Polymorphic Phases of Supramolecular Liquid Crystal Complexes Laterally Substituted with Chlorine

New supramolecular complexes, based on H-bonding interactions between 4-(pyridin-4-yl) azo-(2-chlorophenyl) 4-alkoxybenzoates (Bn) and 4-[(4-(n-hexyloxy)phenylimino)methyl]benzoic acid (A6), were prepared and their thermal and mesomorphic properties investigated via differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FT-IR) in order to confirm their H-bonding interactions. The mesophase behavior of each mixture was examined by DSC and polarized optical microscopy (POM). According to the findings of the study, in all of the designed mixtures, the introduction of laterally polar chlorine atom to the supramolecular complexes produces polymorphic compounds possessing smectic A, smectic C and nematic mesophases, in addition, all products have low melting transitions. Thermal stabilities of the associated phases depend on the position and orientation of the lateral polar Cl- atom as well as the length of terminal flexible alkoxy chain. Comparisons were made between the present lateral Cl- complexes and previously investigated laterally-neat complexes in order to investigate the impact of the addition, nature and orientation of polar substituent on the mesomorphic behavior. The investigations revealed that, the polarity and mesomeric nature of inserted lateral substituent into the base component play an essential role in affecting their mesomorphic properties. Furthermore, for current complexes, induced polymorphic phases have been found by introducing the chlorine atom.

Preparation and DFT Study for New Three-Ring Supramolecular H-Bonded Induced Liquid Crystal Complexes

Supramolecular three-ring Schiff base novel liquid crystal complexes have been prepared and investigated. Schiff bases of para-substituted aniline derivatives and para-pyridine carbaldehyde have been prepared and then mixed in equimolar quantities with para-alkoxy benzoic acids. On one side, the alkoxy chain length varies from 8 to 16 carbon atoms. On the other side, terminal small compact groups substituting aniline with various polarities are used. Hydrogen-bonding interaction was elucidated by FTIR spectroscopy. The mesomorphic thermal and optical characteristics of the samples were obtained by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). All samples exhibit enantiotropic mesophases. Experimental results obtained for the induced mesophases were correlated with density functional theory (DFT) theoretical calculations. The results revealed that both the polar compact groups' polarity and the alkoxy chain lengths contribute strongly to mesomorphic characteristics and thermal stabilities of the mesophases. Surprisingly, the observed values of enthalpy changes associated with the crystalline mesomorphic transitions lie in the range of 2.2-12.5 kJ/mol. However, the enthalpy changes corresponding to the mesomorphic-isotropic transitions vary from 0.9 to 13.9 kJ/mol, depending on the polarity of para-attached groups to the aniline moiety.

Pyridine-Based Three-Ring Bent-Shape Supramolecular Hydrogen Bond-Induced Liquid Crystalline Complexes: Preparation and Density Functional Theory Investigation

A series of new supramolecular three-ring bent-shape Schiff base liquid crystal (LC) complexes were prepared and studied. On one side, two alkoxy chain lengths of the carboxylic acids were used, namely eight and sixteen carbons. Moreover, on the other side, terminal small compact groups, which substituted aniline, with different polarities were utilized. Furthermore, the hydrogen-bonding interactions in the formed complexes were elucidated by Fourier-transform infrared (FT–IR) spectroscopy. The mesomorphic thermal and optical characteristics of the samples were determined by differential thermal analysis (DSC) and polarized optical microscopy (POM). The complexes exhibited enantiotropic and dimorphic mesophase behaviors. The results indicate that the polarity of the compact groups and the lengths of the alkoxy chains greatly impacted the mesomorphic characteristics and thermal stabilities of the mesophases. The observed values of the enthalpy changes (ΔH) associated with the crystalline smectic-A (TCr-SmA) transitions were extremely small compared with the conventional values that characterize supramolecular hydrogen-bonded liquid crystalline complexes. ΔH, which corresponded to the nematic isotropic transitions (TN-I), varied from 0.13 to 9.54 kJ/mol depending mainly on the polarity of the groups that were para-attached to the aniline moiety. Finally, the theoretical results obtained by density functional theory (DFT) calculations were discussed. The DFT geometrical structures showed non-coplanar structures. The mesomorphic range was correlated with the calculated dipole moment, polarizability and the aspect ratios of the investigated compounds.

Experimental and Computational Approaches of Newly Polymorphic Supramolecular H-Bonded Liquid Crystal Complexes

New 1:2 liquid crystalline supramolecular H-bonded complexes (SMHBCs) were synthesized through double H-bond interactions between 4-(nicotinoyloxy) phenyl nicotinate as the base component and two molecules of 4-n-alkoxybenzoic acid (An). The base component was expected to be in two conformers according to the orientation of the N atom and the carboxylate group, syn conformer (I) and anti-conformer (II). DFT calculations revealed that only one of the two possible conformers of I exists, and the addition of the two molecules of the alkoxy acids (A n) did not affect its conformation. The mesomorphic properties of all of the prepared complexes (I/A n), bearing different terminal flexible alkoxy chains were investigated, and the formation of the H-bonds were confirmed by differential scanning calorimetry (DSC), and the phases were identified by polarized optical microscopy (POM), and FT-IR spectroscopy. Highly thermally stable mesophases possessing broad temperature ranges were observed for all investigated complexes compared to their individual components. Depending on the length of the terminal flexible alkoxy chain, the prepared SMHBCs were shown to exhibit di- or tri-morphic enantiotropic mesophases. The effect of replacing one of the -COO- connecting units by an azo group (-N=N-) in the basic molecule (I), on the mesomorphic properties has been investigated experimentally (via DSC) and theoretically (via DFT). The DFT calculations revealed that the polarizability, the dipole moment, and the aspect ratio of the investigated SMHBCs are lower than those of their corresponding ester/azo analogs. All these factors rationalize the enhanced smectic mesophase ranges of the complexes compared with those of the ester/azo analogs. The high aspect ratios and dipole moments of the SMHBCs of the azo derivative enforces the lateral intermolecular attraction that permits the formation of the more ordered smectic C mesophase with respect to the enhanced polymorphic mesophases of the diester derivative.

Nematogenic Laterally Substituted Supramolecular H-Bonded Complexes Based on Flexible Core

New laterally CH3-substituted series of 1:2 hydrogen-bonded supramolecular complexes (HBSMCs) based on flexible acid core were prepared and mesomorphically investigated. Mixtures were formed through H-bonded interactions between adipic acid (A) and 4-(2-(pyridin-4-yl)diazenyl-(3-methylphenyl) 4-alkoxybenzoate (Bn). Mesomorphic and optical properties were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffraction (XRD), and UV-spectroscopy. HBSMCs formed from 1:2 mol mixture of A:2Bn, where the base component (B) bearing different alkoxy chain lengths from n = 8 to 14. The new HBSMCs (A/2B) interactions were confirmed by Fermi-bands formation via FT-IR spectroscopy measurements. Results revealed that all prepared HBSMCs are enantiotropic, exhibiting induced nematic (N) phase. The XRD pattern confirmed the POM texture results. Moreover, a comparison was made between the new laterally HBSMC series based on flexible core and the previously analyzed laterally neat complexes.

Nematic Phase Induced from Symmetrical Supramolecular H-Bonded Systems Based on Flexible Acid Core

New symmetrical 1:2 supramolecular H-bonded liquid crystals (SMHBLCs) interactions, A/2Bn, were formed between adipic acid and 4-(4′–pyridylazophenyl) 4″-alkoxybenzoates. Optical and mesomorphic behaviors of the prepared SMHBLC complexes were investigated using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). FT-IR spectroscopy was carried out to confirm the H-bond interactions of the prepared complexes via Fermi bands formation. Their photo-physical properties were investigated by UV-spectroscopy and the observed absorbance values were found to be mainly dependent on the length of the terminal alkoxy chain. Mesomorphic behaviour for all A/2Bn complexes revealed that all complexes are dimorphic-exhibiting enantiotropic mesophases with induced nematic phases, except for the long chain terminal complex which exhibits only a monomorphic smectic A phase. In order to investigate the effect of mesogenic core geometry on the mesophase properties, a comparison was established between the mesomeric behaviors of the present complexes and previously reported rigid core acid complexes. Finally, the XRD pattern confirmed the POM and DSC results.

New Rod-Like H-Bonded Assembly Systems: Mesomorphic and Geometrical Aspects

Experimental and geometrical approaches of new systems of mesomorphic 1:1 supramolecular H-bonded complexes (SMHBCs) of five rings are discussed. The H-bonding between 4-alkoxyphenylimino benzoic acids (An, as proton acceptor) and 4-(4′–pyridylazophenyl) 4′′-alkoxybenzoates (Bm, as proton donor) were investigated. Mesomorphic behaviors were analyzed by differential scanning calorimetry (DSC) and mesophase textures were identified by polarized light microscopy (POM). H-bonded assembly was established by FT-IR spectroscopic measurements via Fermi band discussion. Thermal and theoretical factors were predicted for all synthesized complexes by density functional theory (DFT) predictions. The results revealed that all prepared complexes were monomorphic, with a broad range of smectic A phases with a high thermal stability of enantiotropic mesophase. Furthermore, DFT stimulations illustrated the experimental results in terms of the influence of the chain length either of the acid or the base component. Many parameters, such as the calculated stability, the dipole moment and the polarizability of the H-bonded complexes, illustrate how these parameters work together to enhance the smectic mesophases with the obtained stability and range.

Optical and Geometrical Characterizations of Non-Linear Supramolecular Liquid Crystal Complexes

Nonlinear architecture liquid crystalline materials of supramolecular 1:1 H-bonded complexes (I/II and I/III) were prepared through a self-assembly intermolecular interaction between azopyridine (I) and 4-n-alkoxybenzoic acid (II) as well as 4-n-alkoxyphenylazo benzoic acid (III). The H-bond formation of the prepared supramolecular hydrogen bonded (SMHB) complexes was confirmed by Fourier-transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). Optical and mesomorphic behaviors of the prepared complexes were studied by polarized optical microscopy (POM) as well as DSC. Theoretical calculations were performed by the density functional theory (DFT) and used to predict the molecular geometries of the synthesized complexes, and the results were used to explain the experimental mesomorphic and optical properties in terms of their estimated thermal parameters. Ordinary and extraordinary refractive indices as well as birefringence at different temperatures were investigated for each sample using an Abbe refractometer and modified spectrophotometer techniques. Microscopic and macroscopic order parameters were calculated for individual compounds and their supramolecular complexes.

Characterization of New H-Bonded Liquid Crystalline Complexes Based on Iminophenyl Nicotinate

Two new 1:2 supramolecular H-bonded liquid crystalline complexes (SMHBCs) were prepared through double H-bond interactions between either isophthalic acid (A) or terephthalic acid (B) and a nicotinate Schiff base (I6). The formed complexes were thermally investigated by differential scanning calorimetry (DSC), and their phases were identified by polarized optical microscopy (POM) and UV-spectroscopy. The formation of 1:2 hydrogen-bonded complexes was confirmed through their Fermi-bands observed by FT-IR spectroscopy. The first system (I6/A) was found to possess enantiotropic smectic A and nematic mesophases. Induced broad range of smectic A phase was observed in I6/A complex which is not shown by their individual components. The second complex system (I6/B) was purely nematogenic. Density functional theory (DFT) calculations were applied to predict their geometrical parameters. Theoretical studies revealed that the isophthalic complex adopted the W-shape; while the U-shape was adopted by the terephthalic acid complex (I6/B). The linear geometry of the complex based on the isophthalic acid enhanced the mesomorphic behavior observed by the terephthalic complex; I6/B. Actually; the orientation of the two carboxylic groups was shown to highly affect the softness of the derived complex. On the other hand; the mapping orientation of the charge distribution can be used to explain the mesophase behavior. The photophysical characterization of isophthalic complex (I6/A) is also discussed

Binary Liquid Crystal Mixtures Based on Schiff Base Derivatives with Oriented Lateral Substituents

Binary mixtures of the laterally substituted Schiff base/ester derivatives, namely 4-((2- or 3-) substituted phenyl imino methyl) phenyl-4”-alkoxy benzoates, Ia–d, were prepared and mesomorphically studied by differential scanning calorimetry (DSC) and their mesophases identified by polarized optical microscopy (POM). The lateral group (1-naphthyl, 2-F, 2-Br, 3-F in Ia–d, respectively) is attached to different positions of the phenyl Schiff moiety. The mixtures investigated were made from two differently shaped compounds that differ from each other in the polarity, size, orientation, and relative positions of the lateral group. The results revealed that the binary mixture Ia/Ib (bearing the naphthyl and 2-flouro substituents) exhibited the SmA phase, which covered the whole composition range. For the mixtures Ib/Id (2-F and 2-Br), the isomeric lateral F-group in compound Ib distributed the SmA arrangement of Id. In the Ic/Id mixture bearing two positionally and structurally different substituents, the addition of Ic to Id resulted in solid binary mixtures where its behavior may be attributed to the negligible steric effect of the small electronegative fluorine atom compared to the Br atom. Density functional theory (DFT) theoretical calculations were carried out to estimate the geometrical parameters of individual components and to show the effect of these parameters in the mesophase behavior of the binary system, where the higher dipole moment of Id (6 Debye) may be the reason for its high π–π molecular stacking, which influences its mesophase range and stability.

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.

Chair- and V-Shaped of H-bonded Supramolecular Complexes of Azophenyl Nicotinate Derivatives; Mesomorphic and DFT Molecular Geometry Aspects

New geometrical architectures of chair- and V-shaped supramolecular liquid crystalline complexes were molded through 1:1 intermolecular hydrogen bonding interactions between 4-(4-(hexyloxy)phenylazo)methyl)phenyl nicotinate and 4-alkoxybenzoic acids. The length of terminal alkoxy acid chains varied, n = 6 to 16 carbons. The mesomorphic behaviour of these complexes was examined through differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Fourier-transform infrared spectroscopy (FT-IR) was carried out to confirm the presence of Fermi bands that appeared for the hydrogen bonding formation. Enantiotropic nematic phases were observed and covered all lengths of alkoxy chains. The geometrical structures of the prepared supramolecular complexes geometries were estimated by Density functional theory (DFT) calculations. The supramolecular complexes I/An are projected to exhibit a nonlinear geometry with V-shaped and chair-shaped geometry. The chair-shaped conformers of I/An were found to be more stable than V-shaped isomeric complexes. Moreover, the effect of the change of the mesogenic core on the mesophase thermal stability (TC) has been investigated by a comparative study of the present azo supramolecular H-bonding LCs (SMHBCs) I/An and our previously reported their Schiff base analogue complexes, II/An. The findings of the DFT illustrated the high impact of CH=N as a mesogenic core on the mesomorphic behavior in terms of the competitive lateral and terminal intermolecular interactions as well as the molecular electrostatic potential (MEP).

New Symmetrical U- and Wavy-Shaped Supramolecular H-Bonded Systems; Geometrical and Mesomorphic Approaches

New mesomorphic symmetrical 2:1 supramolecular H-bonded complexes of seven phenyl rings were prepared between 4-n-alkoxyphenylazobenzoic acids and 4-(2-(pyridin-3-yl)diazenyl)phenyl nicotinate. Mesomorphic studies of the prepared complexes were investigated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Fermi bands of the formed H-bonded interactions were confirmed by FT-IR spectroscopy. Geometrical parameters for all complexes were performed using the density functional theory (DFT) calculations method. Theoretical results revealed that the prepared H-bonded complexes are in non-linear geometry with U-shaped and wavy-shaped geometrical structures; however, the greater linearity of the wavy-shaped compounds could be the reason for their stability with respect to the U-shaped conformer. Moreover, the stable, wavy shape of supramolecular H-bonded complexes (SMHBCs) has been used to illustrate mesomeric behavior in terms of the molecular interaction. The experimental mesomorphic investigations revealed that all complexes possess enantiotropic smectic C phase. Phases were confirmed by miscibility with a standard smectic C (SmC) compound. A comparison was constructed to investigate the effect of incorporating azophenyl moiety into the mesomeric behavior of the corresponding five-membered complexes. It was found that the addition of the extra phenylazo group to the acid moiety has a great increment of the mesophase stability (TC) values with respect to the monotropic SmC phase of the five aromatic systems to the high stable enantiotropic SmC mesophase.

Thermal and Photophysical Studies of Binary Mixtures of Liquid Crystal with Different Geometrical Mesogens

Three binary systems were prepared by mixing of two different mesogenic derivatives, homologues, the first is azo/ester, namely 4-alkoxyphenylazo-4′-phenyl-4″-alkoxybenzoates (IIn+m) and the second is Schiff base/ester, namely 4-(arylideneamino)phenyl-4″-alkoxy benzoates (In+m). The two corresponding analogues from both series in the binary mixtures investigated are of the same terminal alkoxy chain length. Mesomorphic properties were investigated by differential scanning calorimetry (DSC) and phases identified by polarized optical microscope (POM). Photophysical studies were investigated by UV spectroscopy connected to a hot stage. Results were discussed based on constructed binary phase diagrams. All mixtures were found to exhibit eutectic compositions, with linear or slightly linear nematic and smectic A stability/composition dependences. Geometrical parameters were predicted applying density functional theory (DFT) calculations. Twist angle (θ), aspect ratio, dipole moment and the polarizability of the individual compounds were discussed and correlated with the experimental results to illustrate the enhanced the mesophase stability and the mesophase range of the mixture at the eutectic composition compared with those of their individual components.

New wide-stability four-ring azo/ester/Schiff base liquid crystals: synthesis, mesomorphic, photophysical, and DFT approaches

DSC thermograms of some prepared compounds: (a) recorded from second heating and (b) from cooling at a rate of ±10 °C min⁻¹.