Manoj Kr. Paul

Synthesis and Mesomorphic Properties of N(4-n-alkyloxy salicylidene)4′-n-alkylanilines and their Copper Complexes I

A series of metal (Cu) complexes with bidentate Schiff-bases, i.e., N(4-n-alkyloxy salicylidene)4′-n-alkylanilines obtained by the 1:1 condensation of 4-n-alkyloxy salicylaldehyde and 4-n-alkylanilines, i.e., 4-n-hexylaniline, 4-n-heptylaniline, and 4-n-octylaniline, have been prepared and characterized. The synthesis of the ligands N(4-n-butyloxysalicylidene)-4′-n-alkylanilines as well as the complexes and characterization by thermal microscopy for mesomorphism are presented. The ligands are found to exhibit interesting smetic phases (mostly tilted phases) and polymorphism, while the complexes showed orthogonal smectic A, smectic B, and smectic E phases. The mesomorphic range as well as the polymorphism decreases with increase in chain length. However, the coordination with copper leads to thermally stable metallomesogens possessing larger mesomorphic range but with a reduced number of mesomorphic phases.

A novel smectic liquid crystalline phase exhibited by W-shaped molecules

A novel smectic phase has been discovered in the W-shaped molecular system of N,N′-bis[2-hydroxy-5-(4′-n-tetradecylphenylazo)benzylidene]-4-nitro-1,3-phenylenediamine. On slowly cooling this material from the isotropic phase, fine filaments of a chiral nature appear. When they cover the whole surface, schlieren-like textures with two-brush defects are observed. In the texture, fine stripes formed by the filaments exist, which represent an orientational map. Each filament in a specific region has its own chiral character and forms chiral domains, as identified by uncrossed polarizers. According to X-ray analysis, a layer structure with a periodicity of the molecular length is clearly seen. Several diffuse but distinct wide-angle diffraction peaks are observed in the smectic phase and become sharp in the lower-temperature crystalline phases. Based on these experimental results, we can conclude the existence of a new phase with a chiral nature.

Novel chiral filament in an achiral W-shaped liquid crystalline compound

Anomalous optical birefringence and optical rotation have been observed in a liquid crystal phase comprising of W-shaped liquid crystal molecules with hydrogen bonds. The texture of this phase exhibits characteristic chiral filaments, which form strongly segregated chiral domains similar to the B4 phase of conventional bent-core mesogens, despite of having no chiral carbon. Based on X-ray microbeam diffraction, polarizing and confocal optical microscope observations, the molecular arrangement in the filament is discussed, and the novel structure different from that in B4 is proposed; the smectic layers form concentric cylinder, on which the c-director of the W-shaped molecules form screw-like and/or polar arrangement, inducing herringbone anisotropic structure and chirality in each filament, as observed in the experiments.

The synthesis of liquid crystalline lanthanide complexes of Schiff's base ligands: N -(4- n -alkoxysalicylidene)-4′- n -alkylanilines

The reaction of N-(4-n-alkoxysalicylidene)-4′-n-alkylanilines with freshly prepared lanthanide salts leads to mesomorphic terbium and dysprosium complexes exhibiting the smectic A phase.

Fluorescent lanthanide complexes of Schiff base ligands possessing N-aryl moiety: influence of chain length on crossover (calamitic to discotic) phase behaviour

New lanthanide complexes, Tb(III), Dy(III) and Gd(III) of N-aryl based Schiff bases exhibiting mesomorphism have been synthesised and characterised. The crossover phenomena of calamitic lamellar to discotic columnar phase behaviour in these lanthanidomesogens is found to be controlled by the variation in chain length of the single alkyl substituent on the terminal N-aryl ring, hitherto the first case of a metallomesogen with only one alkyl chain length at each end of the ligand as evidenced by thermal microscopy studies and complemented by X-ray studies. It is also observed as a function of temperature in a single component. The electronic properties of these systems are dominated by the donor–acceptor substituted organic chromophore ligand, and the emission maxima revealed a large Stokes shift of 160 nm. The ligand–ligand charge transfer is reflected in emission spectra and is due to the presence of phenyl rings in the ligands.

Mesomorphism of a banana mesogen: influence of a fluoro substituent in the central core

The results of experiments with stable five-ring banana-shaped molecules consisting of laterally 4-fluoro substituted 1,3-phenylene ring as the central unit in Bis-[4-(4′-n-alkyloxybenzoyloxy)salicylidene]-phenylene-4-fluoro-1,3-diamines are presented. These compounds are thermally and hydrolytically stable due to intermolecular or intramolecular hydrogen bonding and exhibit SmCPA and low-temperature crystalline BX phases. The phases had been characterised by thermal microscopy and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction studies and electric field effects. The influence of a lateral fluoro substituent in the central core is clearly demonstrated with the induction of the SmCPA phase which is absent in the parent compound without the substituent. Further these compounds exhibit a large tilt angle. We also describe a comparison of phase behaviour with similar analogous compounds, taking the position of fluoro substitution, changes in the linking groups and the direction of the linking groups into consideration.

Electro-optic and molecular relaxation behaviour of fluoro substituted achiral unsymmetrical four-ring bent-core mesogen

Electro-optic and molecular relaxation studies in a new achiral unsymmetrical four-ring bent-core mesogen are presented in this study. The compound consists of a laterally fluoro-substituted 3,4′-disubstituted biphenyl unit with an ester linkage between the two phenyl rings as the central unit. The mesomorphic properties were characterised by polarising optical microscopy and differential scanning calorimetry. The compound exhibited enantiotropic B7–Bx phase sequence. The electro-optic studies revealed a bistable switching from planar to a homeotropic state under an electric field in B7 phase. Further memory effect was observed between planar and homeotropic textures after the removal of electric field in B7 phase. Temperature dependent permittivity studies as a function of frequency revealed that the molecular relaxation increases with the increasing temperature in B7 phase than in Bx phase.

Mononuclear and binuclear complexes of salicylidene Schiff bases: synthesis and mesogenic properties

The synthesis of new mononuclear and binuclear copper complexes derived from Schiff bases, namely N(4-n-alkyloxysalicylidene)-4′-n-alkylanilines is carried out and their structure and liquid crystalline properties have been investigated. The ligands exhibit rich polymesomorphism; mononuclear complexes exhibit smectic A, smectic C and smectic E phases, while the binuclear complexes mostly exhibit viscous smectic A phase. The isotropic–liquid crystalline transition temperatures of the binuclear complexes in general are found to be lower than their corresponding mononuclear homologues. It was found that the mononuclear complexes exhibit sharp liquid crystalline–isotropic transition temperatures while the binuclear complexes melts over a temperature range. The liquid crystalline phases were characterised by differential scanning calorimetry (DSC) analysis and optical polarised microscopy. X-ray diffraction studies revealed that the intermolecular distance in binuclear complexes is larger than in the mononuclear complexes.

Coumarin based emissive rod shaped new schiff base mesogens and their zinc(II) complexes: synthesis, photophysical, mesomorphism, gelation and DFT studies

ABSTRACT New homologous series of coumarin Schiff base derived from 6-aminocoumarin and their zinc(II) complexes have been synthesised. The spectroscopic characterisations, photophysical properties, phase transition temperature, characterisation of phase and gelation behaviour are reported. The ligand is non-mesogenic at lower 4-n-alkoxy chain length (n = 4) and mesogenic for longer chains (n > 4). For small chains (n = 5, 7, 8), ligand displays monotropic nematic or nematic-smectic A phase sequences, whereas longer homologues (n = 12, 14, 16 and 18) display only enantiotropic smectic A phase. The ligands and their Zn(II) complexes are fluorescent in nature. Interestingly, ligands exhibit gelation property only in polar solvents, whereas Zn(II) complexes discourage gelation. The effect of 4-n-alkoxy chain length on the gelation properties was also discussed. Density functional theory calculations show broad agreement with observed molecular conformation, dipole moment, molecular orbitals and polarisability of the coumarin Schiff base molecules and their Zn(II) complexes. GRAPHICAL ABSTRACT

Dielectric and Optical Behavior of Two Calamitic Hydrogen-Bonded Mesogens

Temperature variation of dielectric loss, dielectric permittivity and percentage optical transmittance of two calamitic H-bonded mesogens C4Py12Bā and C4Py10Bā have been reported in the temperature range of 80°C to 140°C. The dielectric loss and dielectric permittivity have also been measured with variation of frequency. The dielectric studies have been conducted in the frequency range of 1 kHz to 5 MHz. Transition temperatures obtained by using different techniques are found to be in good agreement with each other.