Recent synthetic advances in pyridine-based thermotropic mesogens

Investigation of solute-solvent interactions of 4'-alkyl-4-cyanobiphenyl liquid crystals using Raman spectroscopy

Liquid crystal materials possess hybrid liquid and solid-like properties with high response to stimuli. The 4'-alkyl-4-cyanobiphenyls (nCB) are an important class of liquid crystals that are widely used for various applications. In this study, six alkylcyanobiphenyl liquid crystal samples (5CB to 10CB) were examined using Raman spectroscopy in a total of twelve solvents of various polarities. The distinctive bands contributed by the LC sample are from C≡N stretch, C-C aromatic ring breathing, C-C biphenyl linking stretch, and C-H in-plane deformation. These modes are found to be responsive to different solvent environments by shifting positions. For instance, the cyano stretching mode of 5CB is blue-shifted from 2229 cm-1 for the pure sample to 2233 cm-1 when measured in hexane due to the repulsive interaction between the mode and the solvent bath. On the other hand, this mode undergoes a red-shift to 2220 cm-1 in methanol due to hydrogen bond interaction between the mode and solvent molecules. Overall, the shift in the position of the vibrational modes of nCB molecules was correlated with solvent properties such as acceptor number, donor number, and Kamlet-Taft dipolarity constants, which are a measure of solvent polarity. In samples with longer alkyl chains (8CB, 9CB, and 10CB), the wavenumber of the stretching modes was generally shifted to around the same position in all the solvents. Such observations are related to the folding back effect and enhanced dimerization constant with an increase in the chain length.

Light-oriented 3D printing of liquid crystal/photocurable resins and in-situ enhancement of mechanical performance

Additive manufacturing technology has significantly impacted contemporary industries due to its ability to generate intricate computer-designed geometries. However, 3D-printed polymer parts often possess limited application potential, primarily because of their weak mechanical attributes. To overcome this drawback, this study formulates liquid crystal/photocurable resins suitable for the stereolithography technique by integrating 4'-pentyl-4-cyanobiphenyl with a photosensitive acrylic resin. This study demonstrates that stereolithography facilitates the precise modulation of the existing liquid crystal morphology within the resin. Furthermore, the orientation of the liquid crystal governs the oriented polymerization of monomers or prepolymers bearing acrylate groups. The products of this 3D printing approach manifest anisotropic behavior. Remarkably, when utilizing liquid crystal/photocurable resins, the resulting 3D-printed objects are approximately twice as robust as those created using commercial resins in terms of their tensile, flexural, and impact properties. This pioneering approach holds promise for realizing autonomously designed structures that remain elusive with present additive manufacturing techniques.

Calixarene Functionalized Supramolecular Liquid Crystals and Their Diverse Applications

Liquid crystals are considered to be the fourth state of matter with an intermediate order and fluidity in comparison to solids and liquids. Calixarenes are among one of the most versatile families of building blocks for supramolecular chemistry due to their unique vaselike structure that can be chemically engineered to have different shapes and sizes. During the last few decades, calixarenes have drawn much attention in the field of supramolecular chemistry due to their diverse applications in the fields of ion and molecular recognition, ion-selective electrodes for catalysis, drug delivery, gelation, organic electronics and sensors, etc. Imbuing liquid crystallinity to the calixarene framework leads to functionalized calixarene derivatives with fluidity and order. Columnar self-assembly of such derivatives in particular enhance the charge migration along the column due to the 1D stacking due to the enhanced π-π overlap. Considering limited reports and reviews on this new class of calixarene based liquid crystals, a comprehensive account of the synthesis of calixarene liquid crystals along with their mesomorphic behavior and potential applications are presented in this review.

New luminescent ordered liquid crystalline molecules with a 3-cyano-2-pyridone core unit

The self-organized states of liquid crystals (LCs) have recently received a lot of attention because of their various applications, especially in the fields of electron transport materials and optoelectronic devices. In most of these applications, molecules containing a donor-acceptor skeleton have been widely employed and generally these molecular frameworks have been designed mainly on the basis of the donor-acceptor concept. Inspired from this concept, we synthesized a series of new donor-acceptor based luminescent molecules, i.e. 4,6-bis(4-alkyloxyphenyl)-2-oxo-1,2-dihydropyridine-3-carbonitrile (Pn-series), carrying variable alkoxy chains [i.e. n = 8, 10, 12, 14, 16]. The structures of the synthesized molecules were confirmed by various spectral analyses. Further, their absorption and emission studies indicated that these molecules show blue light emitting properties. Moreover, the experimentally obtained optical band gap was analogous to the theoretical band gap calculated from the DFT study. The first two members of the Pn-series (n = 8 and 10) are non-mesogens. As the alkyl chain length increased to n = 12 and n = 14, the smectic C phase appeared along with an additional low temperature ordered lamellar phase. When n = 16, the smectic C phase disappeared and the compound exhibited only an ordered lamellar phase. This ordered lamellar phase is mainly due to the face to face alignment which makes these molecules potential candidates for electron transport materials and optoelectronic devices.

Preparation of Substituted Pyridines via a Coupling of β-Enamine Carbonyls with Rongalite-Application for Synthesis of Terpyridines

A Hantzsch-type strategy for the synthesis of 2,3,5,6-tetrasubstituted pyridines via an oxidative coupling of β-enamine carbonyl compounds with rongalite was developed. This method employs rongalite as a C1 unit for the assembly of a pyridine ring at C-4 position, offering a facile method for the preparation of substituted pyridine derivatives with a broad functional group tolerance. In particular, this method allows us to prepare terpyridine derivatives, which are important ligands or structural fragments for catalysts and 3D metal–organic frameworks.

Synthesis, Phase Behavior and Computational Simulations of a Pyridyl-Based Liquid Crystal System

A homologous set of liquid crystalline materials (Tn) bearing Schiff base/ester linkages were prepared and investigated via experimental and theoretical techniques. Terminal flexible groups of different chain lengths were connected to the end of phenylbenzoate unit while the other end of molecules was attached to the heterocyclic pyridine moiety. The molecular structures of the designed molecules were evaluated by FT-IR, NMR spectroscopic analyses, whereas their mesomorphic properties were investigated by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). They all exhibited dimorphic properties with the exception of the members having the shortest and longest terminal flexible chains (n = 6 and 16), which were monomorphic. The T16 derivative was further found possessing purely smectic A (SmA) mesophase while others have their lengths covered by nematic (N) phase. Moreover, the computational evaluation of the azomethine derivatives was carried out using a DFT approach. The polarity of the investigated derivatives was predicted to be appreciably sensitive to the size of the system. Furthermore, the Frontier molecular orbitals analysis revealed various distributions of electron clouds at HOMO and LUMO levels.

Polar Alkoxy Group and Pyridyl Effects on the Mesomorphic Behavior of New Non-Symmetrical Schiff Base Liquid Crystals

A series of non-symmetrical Schiff base liquid crystals were prepared and investigated. Schiff bases of p-alkyloxy aniline derivatives and 4-phenyl pyridine-4′-carbaldehyde were synthesized. The terminal alkoxy groups substituting aniline are of varied chain length, namely C6, C8, and C16. The structures of the compounds were confirmed via 1H NMR and 13C NMR spectroscopy. Different mesophases of the samples were thermally and optically characterized by differential thermal analysis (DSC) and polarized optical microscopy (POM). All samples revealed enantiotropic smectic B (SmB) and smectic A (SmA) mesophases. The results obtained were further correlated with the density functional theory (DFT) theoretical calculations. The results are compared to a series of compounds bearing biphenyl moiety in their mesogens. The thermal stabilities of the different mesophase reduced upon the increment of the alkoxy chain length. The temperature ranges of both the smectic mesophases of new compounds bearing the 4-phenyl pyridine moiety are generally expanded higher than the other series. In addition, the total mesophase range is greater in the new compounds when compared to their biphenyl analogues. The DFT results were investigated in terms of the molecular geometries and the frontier molecular orbitals as well as the charge distribution mapping to show and illustrate the difference in the mesomorphic properties.

Metal-free C–H functionalization of pyrrolidine to pyrrolinium-based room temperature ionic liquid crystals

C–H functionalization of pyrrolidine-enabled synthesis of a new class of ionic liquid crystals