Supramolecular hydrogen-bonded liquid crystals based on 4-n-alkylthiobenzoic acids and 4,4′-bipyridine: Their mesomorphic behavior with comparative study including alkyl and alkoxy counterparts

First mesomorphic and DFT characterizations for 3- (or 4-) n-alkanoyloxy benzoic acids and their optical applications

New liquid crystalline hydrogen bonded 3- (or 4)-n-alkanoyloxy benzoic acids were synthesized and probed theoretically and experimentally. The molecular structures of these compounds were elucidated by proton NMR, carbon-13 NMR and elemental analyses. Differential scanning calorimetry (DSC) was used to investigate the thermal and mesomorphic properties of all the symmetrical dimers that bearing identical alkanoyloxy chains. Moreover, polarized optical microscopy (POM) was used to determine their mesophases. The findings show that all the designed symmetrical dimers exhibit the smectic mesophase with relative thermal stability that depends on the length of their terminal side chain. Additionally, the experimental findings of the mesomorphic behavior are further supported by DFT calculations. The alkanoyloxy benzoic acid para-derivatives (In) were shown to be more stable than their meta-substituted (IIn) analogues due to stronger hydrogen bonding interactions. The computed reactivity parameters showed that the position of ester moiety has a significant impact on the acids reactivity. The absorbance spectra of both the 3- (or 4)-n-alkanoyloxy benzoic acids revealed a blue shift with the increment of the of alkyl chain size; however, the energy band gaps of 3-n-alkanoyloxy benzoic derivatives were found to be slightly higher than those of the 4-n-alkanoyloxy benzoic acids. Moreover, the photoluminescence spectrum of the prepared materials is rather broad, and exhibited a red shift as the alkyl chain length increases. The fluorescence lifetime shown to rise as alkyl chain length grows longer, and 3-n-alkanoyloxy benzoic acids have slightly longer lifetime compared to their 4-n-alkanoyloxy benzoic analogues.

Effects of alkylthio groups on phase transitions of organic molecules and liquid crystals: a comparative study with alkyl and alkoxy groups

The effects of the alkylthio groups on the phase transition behavior of organic liquid crystal molecules were examined by comparing them with the effects of alkyl and alkoxy groups.

Thermal and Mesomorphic Investigations of 1:1 Supramolecular Assemblies of 4-[(4-(n-Alkoxy)phenylimino)methyl]benzoic Acids Having Symmetrical and Un-Symmetrical Terminal Chain Lengths

Thermal and mesomorphic properties of possible 1:1 supramolecular complexes (SMCs) (Im/In) designed from two members of 4-[(4-(n-alkoxy)phenylimino)methyl]benzoic acid with symmetrical or un-symmetrical alkoxy terminal flexible chains (carbons of m and n = 6, 8 and 16), were analyzed by differential scan-calorimetry (DSC), thermogravemetric (TG) analysis, and their mesophases identified by polarized optical microscopy (POM). The equimolecular mixtures of the two acids possess symmetrical and un-symmetrical terminal lengths. The mesomorphic properties of the binary mixtures were examined as a function of the total alkoxy chain length on both sides. Results revealed that the nematic mesophase temperature range increases as the total terminal length increases for all designed un-symmetrical mixtures. A comparison was constructed between the formed SMCs and of those of the previously prepared 4-n-alkoxyphenylazo benzoic acids as well as the 4-n-alkoxy benzoic acids, to examine the impact of mesogenic core on the mesomorphic properties. The comparison indicated that as the mesogenic portion lengthens the thermal mesophase stability exhibits higher values of phase transition temperatures; whereas, the azo and Schiff base moieties exhibited near thermal properties.

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.

Ether- and Thioether-Linked Naphthalene-Based Liquid-Crystal Dimers: Influence of Chalcogen Linkage and Mesogenic-Arm Symmetry on the Incidence and Stability of the Twist-Bend Nematic Phase

The twist-bend nematic (N_TB ) phase with a heliconical nanostructure of the local director generating symmetry breaking by achiral bent-shaped molecules is a hot topic of current liquid-crystal science. As opposed to the most common methylene-linked dimers, this study demonstrates chalcogen ether- and/or thioether-linked 6-(4-cyanophenyl)-2-naphthyl-based liquid-crystal dimers with symmetric and asymmetric π-conjugated mesogenic-arm structures that exhibit the N_TB phase. Although the symmetric bis(ether)-linked dimer exhibits only the conventional nematic (N) phase, the asymmetric bis(ether)-linked dimer can form the N_TB phase. All thioether-linked dimers form the N_TB phase, wherein the dimers with asymmetric arms vitrify in the N_TB phase on cooling to room temperature. The phase transitions are discussed in terms of the chalcogen linkage combination, mesogenic-arm symmetry, and spacer length. It is revealed that thioether-linked dimers based on asymmetric π-conjugated mesogenic arms with terminal cyano groups are highly beneficial for the realization of materials that form a wide range of N_TB phases and glassy N_TB states at room temperature.

New chair shaped supramolecular complexes-based aryl nicotinate derivative; mesomorphic properties and DFT molecular geometry

A new series of chair-shaped liquid crystalline complexes were formed through 1 : 1 intermolecular hydrogen bonding between 4-(4-(hexyloxyphenylimino)methyl)phenyl nicotinate and 4-alkoxybenzoic acids, with different alkoxy chains.

Twist-bend nematic liquid crystals based on thioether linkage

“Thioether”-based twist-bend nematogens.